Presentasi berjudul: "IEKE W.A., WIYONO, S. PRIYONO, dan SOEMARNO 2012"— Transcript presentasi:
1 IEKE W.A., WIYONO, S. PRIYONO, dan SOEMARNO 2012 PERILAKU AIRDALAM TANAHIEKE W.A., WIYONO, S. PRIYONO, dan SOEMARNO 2012
2 What is Soil Moisture? Lengas Tanah? Soil moisture is difficult to define because it means different things in different disciplines.For example, a farmer's concept of soil moisture is different from that of a water resource manager or a weather forecaster.Secara umum, lengas tanah adalah air yang ditahan dalam ruang pori tanah.Surface soil moisture is the water that is in the upper 10 cm of soil, whereas root zone soil moisture is the water that is available to plants, which is generally considered to be in the upper 200 cm of soil.Diunduh dari: …… 11/11/2012
3 Soil moisture – Lengas Tanah Lengas tanah merupakan air yang idtahan dalam pori tanah dalam zone perakaran tanaman, biasanya dalam profil tanah hingga kedalaman 200 cm.Water storage in the soil profile is extremely important for agriculture, especially in locations that rely on rainfall for cultivating plants. For example, in Africa rain-fed agriculture accounts for 95% of farmed land.Water storage is a term used within agriculture to define locations where water is stored for later use. These range from natural water stores, such as groundwater aquifers, soil water and natural wetlands to small artificial ponds, tanks and reservoirs behind major dams.Diunduh dari: 11/11/2012
4 SOIL WATER CONTENT – Kadar Air (Lengas) Tanah Kadar air tanah (lengas tanah) adalah jumlah air yang ada di dalam tanah.Water content is used in a wide range of scientific and technical areas, and is expressed as a ratio, which can range from 0 (completely dry) to the value of the materials' porosity at saturation. It can be given on a volumetric or mass (gravimetric) basis.Diunduh dari: …… 11/11/2012
5 w = 100 Mw/Ms KADAR LENGAS TANAH The water content in soil is also known as moisture content and can be expressed asw = 100 Mw/Ms Where: w = moisture content (%)Mw = mass of water in soil (kg, lb)Ms = dry mass of soil (kg, lb)The water content test according ASTM D consists of determining the mass of the wet soil specimen and then drying the soil in an oven hours at a temperature of 110oC. Diunduh dari: …… 11/11/2012
6 NERACA AIR – NERACA LENGAS The water balance is an accounting of the inputs and outputs of water.The water balance of a place, whether it be an agricultural field, watershed, or continent, can be determined by calculating the input, output, and storage changes of water at the Earth's surface.The major input of water is from precipitation and output is evapotranspiration.The geographer C. W. Thornthwaite ( ) pioneered the water balance approach to water resource analysis.He and his team used the water-balance methodology to assess water needs for irrigation and other water-related issues. Diunduh dari: 11/11/2012
7 (After Strahler & Strahler, 2006) NERACA AIRThe soil water balance(After Strahler & Strahler, 2006)Precipitation (P).Precipitation in the form of rain, snow, sleet, hail, etc. makes up the primarily supply of water to the surface. In some very dry locations, water can be supplied by dew and fog. Diunduh dari: …… 11/11/2012
8 Actual Evapotranspiration (AE). NERACA AIRActual Evapotranspiration (AE).Evaporation is the phase change from a liquid to a gas releasing water from a wet surface into the air above. Similarly, transpiration is represents a phase change when water is released into the air by plants.Evapotranspiration is the combined transfer of water into the air by evaporation and transpiration. Actual evapotranspiration is the amount of water delivered to the air from these two processes. Actual evapotranspiration is an output of water that is dependent on moisture availability, temperature and humidity. Think of actual evapotranspiration as "water use", that is, water that is actually evaporating and transpiring given the environmental conditions of a place. Actual evapotranspiration increases as temperature increases, so long as there is water to evaporate and for plants to transpire.The amount of evapotranspiration also depends on how much water is available, which depends on the field capacity of soils. In other words, if there is no water, no evaporation or transpiration can occurDiunduh dari: …… 11/11/2012
9 Potential evapotranspiration (PE). NERACA AIRPotential evapotranspiration (PE).The environmental conditions at a place create a demand for water. Especially in the case for plants, as as energy input increases, so does the demand for water to maintain life processes. If this demand is not met, serious consequences can occur. If the demand for water far exceeds that which is actual present, dry soil moisture conditions prevail. Natural ecosystems have adapted to the demands placed on water.Potential evapotranspiration is the amount of water that would be evaporated under an optimal set of conditions, among which is an unlimited supply of water. Think of potential evapotranspiration of "water need". In other words, it would be the water needed for evaporation and transpiration given the local environmental conditions. One of the most important factors that determines water demand is solar radiation. As energy input increases the demand for water, especially from plants increases. Regardless if there is, or isn't, any water in the soil, a plant still demands water. If it doesn't have access to water, the plant will likely wither and die.Diunduh dari: …… 11/11/2012
10 Soil Moisture Storage (ST). Change in Soil Moisture Storage (ΔST). NERACA AIRSoil Moisture Storage (ST).Soil moisture storage refers to the amount of water held in the soil at any particular time. The amount of water in the soil depends soil properties like soil texture and organic matter content. The maximum amount of water the soil can hold is called the field capacity. Fine grain soils have larger field capacities than coarse grain (sandy) soils. Thus, more water is available for actual evapotranspiration from fine soils than coarse soils. The upper limit of soil moisture storage is the field capacity, the lower limit is 0 when the soil has dried out.Change in Soil Moisture Storage (ΔST).The change in soil moisture storage is the amount of water that is being added to or removed from what is stored. The change in soil moisture storage falls between 0 and the field capacity.Diunduh dari: …… 11/11/2012
11 NERACA AIRDeficit (D)A soil moisture deficit occurs when the demand for water exceeds that which is actually available .In other words, deficits occur when potential evapotranspiration exceeds actual evapotranspiration (PE>AE). Recalling that PE is water demand and AE is actual water use (which depends on how much water is really available), if we demand more than we have available we will experience a deficit. But, deficits only occur when the soil is completely dried out. That is, soil moisture storage (ST) must be 0. By knowing the amount of deficit, one can determine how much water is needed from irrigation sources.Diunduh dari: …… 11/11/2012
12 NERACA AIRSurplus (S)Surplus water occurs when P exceeds PE and the soil is at its field capacity (saturated). That is, we have more water than we actually need to use given the environmental conditions at a place.The surplus water cannot be added to the soil because the soil is at its field capacity so it runs off the surface. Surplus runoff often ends up in nearby streams causing stream discharge to increase.A knowledge of surplus runoff can help forecast potential flooding of nearby streams. Diunduh dari: …… 11/11/2012
13 Computing a Soil - Moisture Budget NERACA AIRComputing a Soil - Moisture BudgetThe best way to understand how the water balance works is to actually calculate a soil water budget. A knowledge of soil moisture status is important to the agricultural economy of this region that produces mostly corn and soy beans.To work through the budget, we'll take each month (column) one at a time. It's important to work column by column as we're assessing the moisture status in a given month and one month's value may be determined by what happened in the previous month. Diunduh dari: …… 11/11/2012
14 Water Budget (location of Rockford, Illinois). Field Capacity = 90 mm Water Budget - Rockford, IL Field Capacity = 90 mmWater Budget - Rockford, IL Field Capacity = 90 mmWater Budget - Rockford, IL Field Capacity = 90 mmNERACA AIRWater Budget (location of Rockford, Illinois). Field Capacity = 90 mmJFMASONDYearP50496678100106888486735645881PE54012314512685448531P-PE613816-17-57-4212948ΔST175712ST9030AE6342633258Diunduh dari: …… 11/11/2012
15 Soil Moisture Recharge . Field Capacity = 90 mm Soil Moisture Recharge - Rockford, IL Field Capacity = 90 mmNERACA AIRSoil Moisture Recharge . Field Capacity = 90 mmJFMASONDYearP50496678100106888486735645881PE54012314512685448531P-PE613816-17-57-4212948ΔST-1612ST9030AE6342633258Diunduh dari: …… 11/11/2012
16 Soil Moisture Recharge - Rockford, IL Field Capacity = 90 mm NERACA AIRWe'll start the budget process at the end of the dry season when precipitation begins to replenish the soil moisture, called soil moisture recharge, in September. At the beginning of the month the soil is considered dry as the storage in August is equal to zero. During September, 86 mm of water falls on the surface as precipitation. Potential evapotranspiration requires 85 mm. Precipitation therefore satisfies the need for water with one millimeter of water left over (P-PE=1). The excess one millimeter of water is put into storage (ΔST=1) bringing the amount in storage to one millimeter (August ST =0 so 0 plus the one millimeter in September equals one millimeter). Actual evapotranspiration is equal to potential evapotranspiration as September is a wet month (P>PE). There is no deficit during this month as the soil now has some water in it and no surplus as it has not reached its water holding capacity.During the month of October, precipitation far exceeds potential evapotranspiration (P-PE=29). All of the excess water is added to the existing soil moisture (ST (September) + 29 mm = 30 mm). Being a wet month, AE is again equal to PE.Calculating the budget for November is very similar to that of September and October. The difference between P and PE is all allocated to storage (ST now equal to 78 mm) and AE is equal to PE.Diunduh dari: …… 11/11/2012
17 NERACA AIR Soil Moisture Surplus During December, potential evapotranspiration has dropped to zero as plants have gone into a dormant period thus reducing their need for water and cold temperatures inhibit evaporation. Notice that P-PE is equal to 45 but not all is placed into storage.Why? At the end of November the soil is within 12 mm of being at its field capacity. Therefore, only 12 millimeters of the 45 available is put in the soil and the remainder runs off as surplus (S=33).Given that the soil has reached its field capacity in December, any excess water that falls on the surface in January will likely generate surplus runoff. According to the water budget table this is indeed true. Note that P-PE is 50 mm and ΔST is 0 mm. What this indicates is that we cannot change the amount in storage as the soil is at its capacity to hold water. As a result the amount is storage (ST) remains at 90 mm. Being a wet month (P>PE) actual evapotranspiration is equal to potential evapotranspiration. Note that all excess water (P-PE) shows up as surplus (S=50 mm).Diunduh dari: …… 11/11/2012
18 Soil Moisture Surplus . Field Capacity = 90 mm NERACA AIRSoil Moisture Surplus . Field Capacity = 90 mmJFMASONDYearP50496678100106888486735645881PE54012314512685448531P-PE613816-17-57-4212948ΔST-1612ST9030AE6342633258Diunduh dari: …… 11/11/2012
19 Surplus Lengas TanahGiven that the soil has reached its field capacity in December, any excess water that falls on the surface in January will likely generate surplus runoff. According to the water budget table this is indeed true. Note that P-PE is 50 mm and ΔST is 0 mm. What this indicates is that we cannot change the amount in storage as the soil is at its capacity to hold water. As a result the amount is storage (ST) remains at 90 mm. Being a wet month (P>PE) actual evapotranspiration is equal to potential evapotranspiration. Note that all excess water (P-PE) shows up as surplus (S=50 mm).Similar conditions occur for the months of February, March, April, and May. These are all wet months and the soil remains at its field capacity so all excess water becomes surplus. Note too that the values of PE are increasing through these months. This indicates that plants are springing to life and transpiring water. Evaporation is also increasing as insolation and air temperatures are increasing. Notice how the difference between precipitation and potential evapotranspiration decreases through these months. As the demand on water increases, precipitation is having a harder time satisfying it. As a result, there is a smaller amount of surplus water for the month.Surplus runoff can increase stream discharge to the point where flooding occurs. The flood duration period lasts from December to May (6 months), with the most intense flooding is likely to occur in March when surplus is the highest (61 mm).….Diunduh dari: …… 11/11/2012
20 Soil Moisture Utilization. Field Capacity = 90 mm NERACA AIRSoil Moisture Utilization. Field Capacity = 90 mmJFMASONDYearP50496678100106888486735645881PE54012314512685448531P-PE613816-17-57-4212948ΔST-1612ST9030AE6342633258Diunduh dari: …… 11/11/2012
21 Soil Moisture Utilization By the time June rolls around, temperatures have increased to the point where evaporation is proceeding quite rapidly and plants are requiring more water to keep them healthy. As potential evapotranspiration is approaching its maximum value during these warmer months, precipitation is falling off. During June P-PE is -17 mm. What this means is precipitation no longer is able to meet the demands of potential evapotranspiration. In order to meet their needs, plants must extract water that is stored in the soil from the previous months. This is shown in the table by a value of 17 in the cell for ΔST (change in soil storage). Once the 17 m is taken out of storage (ST) it reduces its value to 73.The month of June is considered a dry month (P<PE) so AE is equal to precipitation plus the absolute value of ΔST (P + |ΔST|). When we complete this calculation (106 mm + 17 mm = 123 mm) we see that AE is equal to PE. What this means is precipitation and what was extracted from storage was able to meet the needs demanded by potential evapotranspiration. Note that there is no surplus in June as the soil moisture storage has dropped below its field capacity. There is still no deficit as water remains in storage. The calculations for July is similar to June, just different values. Note that by the time July ends, water held in storage is down to a mere 16 mm.Diunduh dari: …… 11/11/2012
22 NERACA AIR Soil Moisture Deficit J F M A S O N D Year P 50 49 66 78 100106888486735645881PE54012314512685448531P-PE613816-17-57-4212948ΔST-1612ST9030AE6342633258Diunduh dari: …… 11/11/2012
23 August, like June and July, is a dry month. Soil Moisture DeficitAugust, like June and July, is a dry month.Potential evapotranspiration still exceeds precipitation and the difference is a -42 mm.Up until this month there has been enough water from precipitation and what is in storage to meet the demands of potential evapotranspiration. However, August begins with only 16 mm of water in storage (ST of July). Thus we'll only be able to extract 16 mm of the 42 mm of water needed to meet the demands of potential evapotranspiration So, of the 42 mm of water we would need (P-PE) to extract from the soil. In so doing, the amount in storage (ST) falls to zero and the soil is dried out.What happens to the remaining 26 mm of the original P-PE of 42? The unmet need for water shows up as soil moisture deficit. In other words, we have not been able to meet our need for water from both precipitation and what we can extract from storage. AE is therefore equal to 100 mm (84 mm of precipitation plus 16 mm of ΔST).….Diunduh dari: …… 11/11/2012
24 Soil Moisture Seasons Recharge Four soil moisture seasons can be defined by the soil moisture conditions.RechargeThe recharge season is a time when water is added to soil moisture storage (+ΔST). The recharge period occurs when precipitation exceeds potential evapotranspiration but the soil has yet to reach its field capacity. SurplusThe surplus season occurs when precipitation exceeds potential evapotranspiration and the soil has reached its field capacity. Any additional water applied to the soil runs off. If this water runs off into nearby streams and rivers it could cause flooding. Thus, the intensity (amount) and duration (length of season) of surplus can be used to predict the severity of potential flooding.Diunduh dari: …… 11/11/2012
25 NERACA AIR Deficit Utilization The utilization season is a time when water is withdrawn from soil moisture storage (-ΔST). The utilization period occurs when potential evapotranspiration exceeds precipitation but soil storage has yet to reach 0 (dry soil).DeficitThe deficit season occurs when occurs when potential evapotranspiration exceeds precipitation and soil storage has reached 0. This is a time when there is essentially no water for plants. Farmers then tap ground water reserves or water in nearby streams and lakes to irrigate their crops. Thus, the intensity (amount) and duration (length of season) of deficit can be used to predict the need for irrigation water.Whether a place experiences all four seasons depends on the climate and soil properties. Wet climate and those places with soils having high field capacities are less likely to experience a deficit period. Likewise the duration and intensity of any season will be determined by the climate and soil properties. Given equal amounts of precipitation, coarse textured soils will generate runoff faster than fine textured soils and may experience more intense surplusDiunduh dari: …… 11/11/2012
27 Molekul air bersifat dipolar: STRUKTUR &CIRIH2OMolekul air terdiri atas atom oksigen dan dua atom hidrogen, yang berikatan secara kovalenAtom-atom tidak terikat secara linear (H-O-H), tetapi atom hidrogen melekat pada atom oksigen seperti huruf V dengan sudut 105o.Molekul air bersifat dipolar:Zone elektro positif+H H105oZone elektro negatif-
28 Lingkaran Tanah-Air-Tanaman LTAT mrpk sistem dinamik dan terpadu dimana air mengalir dari tempat dengan tegangan rendah menuju tempat dengan tegangan air tinggi.Air kembali ke atmosfer(evapo-transpirasi)Hilang melalui stomata daun (transpirasi)Air dikembalikan ke tanah melalui hujan dan irigasiPenguapanSerapan bulu akar
29 The model predicts transpiration (E) as a function of the inputs. SISTEM TANAH-TANAMANStructure of water transport model for the soil-leaf continuum, with the inputs outlined in boxes.Root and shoot components are represented by a resistance network, each component of which varies according to the inputted K(y) function from vulnerability curves of xylem.Layers of roots reach to different soil depths according to an inputted root area profile. Canopy layers reflect an inputted leaf area and Y profile.Soil is modeled as a rhizosphere resistance connecting roots to bulk soil of an inputted y and K(y).The model predicts transpiration (E) as a function of the inputs.
30 AIR TANAHKekuatan ikatan antara molekul air dengan partikel tanah dinyatakan dengan TEGANGAN AIR TANAH. Ini merupakan fungsi dari gaya-gaya adesi dan kohesi di antara molekul - molekul air dan partikel tanahKohesiAdesiH2OPartikel tanahAir terikatAir bebas
32 ).Fine textured soils with small pores can hold the greatest amounts of PAW.Coarse textured sandy soils with large pores can hold the least amounts of PAW.
33 Status Air TanahPerubahan status air dalam tanah, mulai dari kondisi jenuh hingga titik layuJenuh Kap. Lapang Titik layuPadatan Pori100g air gtanah jenuh air100g g udarakapasitas lapang100g g udarakoefisien layu100g g udarakoefisien higroskopis
34 TEGANGAN &KADAR AIRPERHATIKANLAH proses yang terjadi kalau tanah basah dibiarkan mengering.Bagan berikut melukiskan hubungan antara tebal lapisan air di sekeliling partikel tanah dengan tegangan airBidang singgung tanah dan airKoef Koef Kapasitaspadatan tanah higroskopis layu lapang10.000atm31 atm atm /3 atmatm Mengalir krn gravitasiTegangan air1/3 atmtebal lapisan air
35 Representasi bola air yang menyelubungi partikel padatan tanah
36 JUMLAH AIR DALAM TANAHThe amount of soil water is usually measured in terms of water content as percentage by volume or mass, or as soil water potential. Water content does not necessarily describe the availability of the water to the plants, nor indicates, how the water moves within the soil profile. The only information provided by water content is the relative amount of water in the soil.Soil water potential, which is defined as the energy required to remove water from the soil, does not directly give the amount of water present in the root zone either. Therefore, soil water content and soil water potential should both be considered when dealing with plant growth and irrigation.The soil water content and soil water potential are related to each other, and the soil water characteristic curve provides a graphical representation of this relationship.
37 Kurva tegangan - kadar air tanah bertekstur lempung TEGANGAN vskadar airKurva tegangan - kadar air tanah bertekstur lempungAir kapilerAir Air tersediahigros-kopis Lambat tersedia Cepat tersedia Air gravitasiZone optimumTegangan air, bar31 Koefisien higroskopisKoefisien layuKapasitas lapangKap. Lapang maksimumpersen air tanah
38 Air tersedia bagi tanaman Tegangan air tanah (bar / atm Air GravitasiTegangan air tanah (bar / atmTitikLayuKapasitas lapangKadar air volumetrik, %Hubungan antara kadar air tanah dan tegangan air tanah untuk tekstur lempung
39 STRUKTUR & CIRI POLARITAS Molekul air mempunyai dua ujung, yaitu ujung oksigen yg elektronegatif dan ujung hidrogen yang elektro-positif.Dalam kondisi cair, molekul-molekul air saling bergandengan membentuk kelompok-kelompok kecil tdk teratur.Ciri polaritas ini menyebabkan plekul air tertarik pada ion-ion elektrostatis.Kation-kation K+, Na+, Ca++ menjadi berhidrasi kalau ada molekul air, membentuk selimut air, ujung negatif melekat kation.Permukaan liat yang bermuatan negatif, menarik ujung positif molekul air.Kation hidrasi Tebalnya selubung air tgtpd rapat muatan pd per-mukaan kation.Rapat muatan =Selubung air muatan kation / luas permukaan
40 STRUKTUR & CIRI KOHESI vs. ADHESI IKATAN HIDROGENAtom hidrogen berfungsi sebagai titik penyambung (jembatan) antar molekul air.Ikatan hidrogen inilah yg menyebabkan titik didih dan viskositas air relatif tinggiKOHESI vs. ADHESIKohesi: ikatan hidrogen antar molekul airAdhesi: ikatan antara molekul air dengan permukaan padatan lainnyaMelalui kedua gaya-gaya ini partikel tanah mampu menahan air dan mengendalikan gerakannya dalam tanahTEGANGAN PERMUKAANTerjadinya pada bidang persentuhan air dan udara, gaya kohesi antar molekul air lebih besra daripada adhesi antara air dan udara.UdaraPermukaan air-udaraair
41 ENERGI AIR TANAHRetensi dan pergerakan air tanah melibatkan energi, yaitu: Energi Potensial, Energi Kinetik dan Energi Elektrik.Selanjutnya status energi dari air disebut ENERGI BEBAS, yang merupakan PENJUMLAHAN dari SEMUA BENTUK ENERGI yang ada.Air bergerak dari zone air berenergi bebas tinggi (tanah basah) menuju zone air berenergi bebas rendah (tanah kering).Gaya-gaya yg berpengaruhGaya matrik: tarikan padatan tanah (matrik) thd molekul air;Gaya osmotik: tarikan kation-kation terlarut thd molekul airGaya gravitasi: tarikan bumi terhadap molekul air tanah.Potensial air tanahKetiga gaya tersebut di atas bekerja bersama mempengaruhi energi bebas air tanah, dan selanjutnya menentukan perilaku air tanah, ….. POTENSIAL TOTAL AIR TANAH (PTAT)PTAT adalah jumlah kerja yg harus dilakukan untuk memindahkan secara berlawanan arah sejumlah air murni bebas dari ketinggian tertentu secara isotermik ke posisi tertentu air tanah.PTAT = Pt = perbedaan antara status energi air tanah dan air murni bebasPt = Pg + Pm + Po + …………………………( t = total; g = gravitasi; m = matrik; o = osmotik)
42 Hubungan potensial air tanah dengan energi bebas Energi bebas naik bila air tanah berada pada letak ketinggian yg lebih tinggi dari titik baku pengenal (referensi)+Poten-sial positifEnergi bebas dari air murniPotensial tarikan bumiMenurun karena pengaruh osmotikPotensial osmotik (hisapan)Poten-sial negatifPotensial matrik (hisapan)-Menurun karena pengaruh matrikEnergi bebas dari air tanah
43 POTENSIAL AIR TANAH POTENSIAL TARIKAN BUMI = Potensial gravitasi Pg = G.hdimana G = percepatan gravitasi, h = tinggi air tanah di atas posisi ketinggian referensi.Potensial gravitasi berperanan penting dalam menghilangkan kelebihan air dari bagian atas zone perakaran setelah hujan lebat atau irigasiPotensial matrik dan OsmotikPotensial matrik merupakan hasil dari gaya-gaya jerapan dan kapilaritas.Gaya jerapan ditentukan oleh tarikan air oleh padatan tanah dan kation jerapanGaya kapilaritas disebabkan oleh adanya tegangan permukaan air.Potensial matriks selalu negatifPotensial osmotik terdapat pd larutan tanah, disebabkan oleh adanya bahan-bahan terlarut (ionik dan non-ionik).Pengaruh utama potensial osmotik adalah pada serapan air oleh tanamanHisapan dan TeganganPotensial matrik dan osmotik adalah negatif, keduanya bersifat menurunkan energi bebas air tanah. Oleh karena itu seringkali potensial negatif itu disebut HISAPAN atau TEGANGAN.Hisapan atau Tegangan dapat dinyatakan dengan satuan-satuan positif.Jadi padatan-tanah bertanggung jawab atas munculnya HISAPAN atau TEGANGAN.
44 Cara Menyatakan Tegangan Energi Tegangan: dinyatakan dengan “tinggi (cm) dari satuan kolom air yang bobotnya sama dengan tegangan tsb”.Tinggi kolom air (cm) tersebut lazimnya dikonversi menjadi logaritma dari sentimeter tinggi kolom air, selanjutnya disebut pF.Tinggi unit Logaritma Bar Atmosferkolom air (cm) tinggi kolom air (pF)
45 KANDUNGAN AIR DAN TEGANGAN KURVA ENERGI - LENGAS TANAHTegangan air menurun secara gradual dengan meningkatnya kadar air tanah.Tanah liat menahan air lebih banyak dibanding tanah pasir pada nilai tegangan air yang samaTanah yang Strukturnya baik mempunyai total pori lebih banyak, shg mampu menahan air lebih banyakPori medium dan mikro lebih kuat menahan air dp pori makroTegangan air tanah, Bar10.000LiatLempungPasir0.0110 Kadar air tanah, % 70
47 Hubungan antara kadar air tanah dengan tegangan air tanah
48 Kapasitas air tersedia dalam tanah yang teksturnya berbeda-beda
49 Gerakan Air Tanah Tidak Jenuh Gerakan tidak jenuh = gejala kapilaritas = air bergerak dari muka air tanah ke atas melalui pori mikro.Gaya adhesi dan kohesi bekerja aktif pada kolom air (dalam pri mikro), ujung kolom air berbentuk cekung.Perbedaan tegangan air tanah akan menentukan arah gerakan air tanah secara tidak jenuh.Air bergerak dari daerah dengan tegangan rendah (kadar air tinggi) ke daerah yang tegangannya tinggi (kadar air rendah, kering).Gerakan air ini dapat terjadi ke segala arah dan berlangsung secara terus-menerus.Pelapisan tanah berpengaruh terhadap gerakan air tanah.Lapisan keras atau lapisan kedap air memperlambat gerakan airLapisan berpasir menjadi penghalang bagi gerakan air dari lapisan yg bertekstur halus.Gerakan air dlm lapisan berpasir sgt lambat pd tegangan
50 Gerakan Jenuh (Perkolasi) Air hujan dan irigasi memasuki tanah, menggantikan udara dalam pori makro - medium - mikro. Selanjutnya air bergerak ke bawah melalui proses gerakan jenuh dibawah pengaruh gaya gravitasi dan kapiler.Gerakan air jenuh ke arah bawah ini berlangsung terus selama cukup air dan tidak ada lapisan penghalangLempung berpasir Lempung berliatcm15 mnt 4 jam3060jam jam12024 jam jam15030 cm cmJarak dari tengah-tengah saluran, cm
51 Pola Penetrasi dan Pergerakan Air pada tanah Berpasir dan tanah Lempung-liat
53 Pengaruh struktur tanah terhadap pergerakan air tanah ke arah bawah
54 PERKOLASIJumlah air perkolasiFaktor yg berpengaruh:1. Jumlah air yang ditambahkan2. Kemampuan infiltrasi permukaan tanah3. Daya hantar air horison tanah4. Jumlah air yg ditahan profil tanah pd kondisikapasitas lapangKeempat faktor di atas ditentukan oleh struktur dan tekstur tanahTanah berpasir punya kapasitas ilfiltrasi dan daya hantar air sangat tinggi, kemampuan menahan air rendah, shg perkolasinya mudah dan cepatTanah tekstur halus, umumnya perkolasinya rendah dan sangat beragam; faktor lain yg berpengaruh:1. Bahan liat koloidal dpt menyumbat pori mikro & medium2. Liat tipe 2:1 yang mengembang-mengkerut sangat berperan
55 LAJU GERAKAN AIR TANAHKecepatan gerakan air dlm tanah dipengaruhi oleh dua faktor:1. Daya dari air yang bergerak2. Hantaran hidraulik = Hantaran kapiler = daya hantari = k.fdimana i = volume air yang bergerak; f = daya air yg bergerak dan k = konstante.Daya air yg bergerak = daya penggerak, ditentukan oleh dua faktor:1. Gaya gravitasi, berpengaruh thd gerak ke bawah2. Selisih tegangan air tanah, ke semua arahGerakan air semakin cepat kalau perbedaan tegangan semakin tinggi.Hantaran hidraulik ditentukan oleh bbrp faktor:1. Ukuran pori tanah2. Besarnya tegangan untuk menahan airPada gerakan jenuh, tegangan airnya rendah, shg hantaran hidraulik berbanding lurus dengan ukuran poriPd tanah pasir, penurunan daya hantar lebih jelas kalau terjadi penurunan kandungan air tanahLapisan pasir dlm profil tanah akan menjadi penghalang gerakan air tidak jenuh
56 Gerakan air tanah dipengaruhi oleh kandungan air tanah Penetrasi air dari tnh basah ke tnh kering(cm)18Tanah lembab, kadar air awal 29%Tanah lembab, kadar air awal 20.2%Tanah lembab, kadar air awal 15.9%Jumlah hari kontak, hariSumber: Gardner & Widtsoe, 1921.
57 GERAKAN UAP AIRPenguapan air tanah terjadi internal (dalam pori tanah) dan eksternal (di permukaan tanah)Udara tanah selalu jenus uap air, selama kadar air tanah tidak lebih rendah dari koefisien higroskopis (tegangan 31 atm).Mekanisme Gerakan uap airDifusi uap air terjadi dlm udara tanah, penggeraknya adalah perbedaan tekanan uap air.Arah gerapan menuju ke daerah dg tekanan uap rendahPengaruh suhu dan lengas tanah terhadap gerapan uap air dalam tanahLembab Dingin Kering DinginKering Panas Lembab Panas
58 RETENSI AIR TANAH KAPASITAS RETENSI MAKSIMUM adalah: Kondisi tanah pada saat semua pori terisi penuh air, tanah jenuh air, dan tegangan matrik adalah nol.KAPASITAS LAPANG: air telah meninggalkan pori makro, mori makro berisi udara, pori mikro masih berisi air; tegangan matrik bar; pergerakan air terjadi pd pori mikro/ kapilerKOEFISIEN LAYU: siang hari tanaman layu dan malam hari segar kembali, lama-lama tanaman layu siang dan malam; tegangan matrik 15 bar.Air tanah hanya mengisi pori mikro yang terkecil saja, sebagian besar air tidak tersedia bagi tanaman.Titik layu permanen, bila tanaman tidak dapat segar kembaliKOEFISIEN HIGROSKOPISMolekul air terikat pada permukaan partikel koloid tanah, terikat kuat sehingga tidak berupa cairan, dan hanya dapat bergerak dlm bentuk uap air, tegangan matrik-nya sekitar 31 bar.Tanah yg kaya bahan koloid akan mampu menahan air higroskopis lebih banyak dp tanah yg miskin bahan koloidal.
59 Klasifikasi Air Tanah Klasifikasi Fisik: 1. Air Bebas (drainase) 2. Air Kapiler3. Air HigroskopisAir Bebas (Drainase):a. Air yang berada di atas kapasitas lapangb. Air yang ditahan tanah dg tegangan kurang dari atmc. Tidak diinginkan, hilang dengan drainased. Bergerak sebagai respon thd tegangan dan tarika gravitasi bumie. Hara tercuci bersamanyaAIR KAPILER:a. Air antara kapasitas lapang dan koefisien higroskopisb. Tegangan lapisan air berkisar atmc. Tidak semuanya tersedia bagi tanamand. Bergerak dari lapisan tebal ke lapisan tipise. Berfungsi sebagai larutan tanahAIR HIGROSKOPIS :a. Air diikat pd koefisien higroskopisb. Tegangan berkisar antara atmc. Diikat oleh koloid tanahd. Sebagian besar bersifat non-cairane. Bergerak sebagai uap air
60 Agihan air dalam tanahBerdasarkan tegangan air tanah dapat dibedakan menjadi tiga bagian: Air bebas, kapiler dan higroskopisKoef. Higroskopis Kap. Lapang Jml ruang porikurang lebih 31 atm kurang lebih 1/3 atmLapisan olahAir higros Air Kapiler Ruang diisi udarakopik Peka thd gerakan Biasanya jenuh uap airHampir tdk kapiler, laju pe Setelah hujan lebatmenunjukkan nyesuaian me sebagian diisi air,sifat cairan ningkat dg me tetapi air cepat hi-ningkatnya ke lang krn gravitasilembaban tanah bumi Lapisan bawah tanahKarena pemadatan ruangpori berkurangStrata bawah (jenuh air)Kolom tanah Jumlah ruang pori
61 Klasifikasi Biologi Air tanah Klasifikasi berdasarkan ketersediaannya bagi tanaman:1. AIR BERLEBIHAN: air bebas yg kurang tersedia bagi tanaman. Kalau jumlahnya banyak berdampak buruk bagi tanaman, aerasi buruk, akar kekurangan oksigen, anaerobik, pencucian air2. AIR TERSEDIA: air yg terdapat antara kap. Lapang dan koef. Layu.Air perlu ditambahkan untuk mencapai pertumbuhan tanaman yang optimum apabila % air yg tersedia telah habis terpakai.Kalau air tanah mendekati koefisien layu, penyerapan air oleh akar tanaman tdk begitu cepat dan tidak mampu mengimbangi pertumbuhan tanaman3. AIR TIDAK TERSEDIA: AIR yg diikat oleh tanah pd TITIK LAYU permanen, yaitu air higroskopis dan sebagian kecil air kapiler.KH KL KP % pori31 atm atm /3 atmAir Air Ruang udara danHigroskopis Kapiler air drainaseTdk tersedia Tersedia BerlebihanDaerah Optimum
62 Faktor yg mempengaruhi Air Tersedia Faktor yg berpengaruh:1. Hubungan tegangan dengan kelengasan2. Kedalaman tanah3. Pelapisan TanahTEGANGAN MATRIK : tekstur, struktur dan kandungan bahan organik mempengaruhi jumlah air yg dapat disediakan tanah bagi tanamanTEGANGAN OSMOTIK: adanya garam dalam tanah meningkatkan tegangan osmotik dan menurunkan jumlah air tersedia, yaitu menaikkan koefisien layu.Persen air Sentimeter air setiap 30 cm tanah10Kap. LapangAir tersediaKoef. Layu 56 Air tidak tersediaPasir Sandy loam Loam Silty-loam Clay-loam LiatTekstur semakin halus
63 KEHILANGAN UAP AIR DARI TANAH HADANGAN HUJAN OLEH TUMBUHANTajuk tumbuhan mampu menangkap sejumlah air hujan, sebagian air ini diuapkan kembali ke atmosfer.Vegetasi hutan di daerah iklim basah mampu menguapkan kembali air hujan yg ditangkapnya hingga 25%, dan hanya 5% yg mencapai tanah melalui cabang dan batangnya.Awan hujanPembentukan AwanpresipitasitranspirasievaporasiinfiltrasiRun offTanah permukaanperkolasiGroundwaterSungai - lautBatuan
64 Pengendalian Penguapan MULSA & PENGELOLAANMulsa adalah bahan yg dipakai pd permukaan tanah untuk mengurangi penguapan air atau untuk menekan pertumbuhan gulma.Lazimnya mulsa spt itu digunakan untuk tanaman yang tidak memerlukan pengolahan tanah tambahanMULSA KERTAS & PLASTIKBahan mulsa dihamparkan di permukaan tanah, diikat spy tdk terbang, dan tanaman tumbuh melalui lubang-lubang yg telah disiapkanSelama tanah tertutup mulsa, air tanah dapat diawetkan dan pertumbuhan gulma dikendalikanMULSA SISA TANAMANBahan mulsa berasal dari sisa tanaman yg ditanam sebelumnya, misalnya jerami padi, jagung, dan lainnyaBahan mulsa dipotong-potong dan disebarkan di permukaan tanahCara WALIK DAMI sebelum penanaman kedelai gadu setelah padi sawahMULSA TANAH Pengolahan tanahEfektivitas mulsa tanah dalam konservasi air-tanah (mengendalikan evaporasi) masih diperdebatkan, hasil-hasil penelitian masih snagat beragam
65 Olah Tanah vs Penguapan Air Tanah Alasan pengolahan tanah:1. Mempertahankan kondisi fisika tanah yg memuaskan2. Membunuh gulma3. Mengawetkan air tanah.Pengendalian Penguapan vs Pemberantasan GulmaPerlakuan Hasil jagung (t/ha) Kadar air tanah (%)hingga kedalaman 1 mTanah dibajak dg persiapan yg baik1. Dibebaskan dari gulma2. Gulma dibiarkan tumbuh3. Tiga kali pengolahan dangkalPersiapan Buruk4. Dibebaskan dari gulmaSumber: Mosier dan Gutafson, 1915.Pengolahan tanah yg dapat mengendalikan gulma dan memperbaiki kondisi fisik tanah akan berdampak positif thd produksi tanamanPengolahan tanah yg berlebihan dapat merusak akar tanaman dan merangsang evaporasi, shg merugikan tanaman
66 KAPASITAS SIMPANAN AIR TANAH Soil "holds" water available for crop use, retaining it against the pull of gravity.This is one of the most important physical facts for agriculture.If the soil did not hold water, if water was free to flow downward with the pull of gravity as in a river or canal, we would have to constantly irrigate, or hope that it rained every two or three days.There would be no reason to pre-irrigate. And there would be no such thing as dryland farming.
67 Potensial air tanah (-bar) Hubungan antara Potensial Air Tanah dnegan Air Tersedia pada tiga macam tekstur tanahKapasitas lapangAir tersedia (%)Titik layu permanenPotensial air tanah (-bar)
68 The soil's ability to hold water depends on both the soil texture and structure. Texture describes the relative percentages of sand, silt, and clay particles.The finer the soil texture (higher percentage of silt and clay), the more water soil can hold.Gravity is always working to pull water downwards below the plant's root zone.To counteract the pull of gravity, soil is able to generate its own forces, commonly called "matric forces" ("matric" because of the soil "matrix" structure that forms the basis for the forces).
69 An important fact about the soil's water-holding forces is that as the level of soil moisture goes down, the soil generates more force.This is the reason that some water will move up into the root zone from a shallow ground water table. As the plant extracts water in the root zone, the soil pulls water up from the area with more water to the area with less.As you would expect, the rate at which the water-holding forces go up with decreasing soil moisture is different for different soils. In a coarse soil, they will go up slowly.This means that plants can extract a great amount of water from coarse soils before they stress. In contrast, these forces rise quickly in finer soils.
70 HUBUNGAN TANAH – AIR - TANAMAN Lapisan olahLapisan olah dalamLapisan subsoilLapisan bahan induk
72 The role of soil in the soil-plant-atmosphere continuum is unique. HUBUNGAN TANAH - AIRThe role of soil in the soil-plant-atmosphere continuum is unique.It has been demonstrated that soil is not essential for plant growth and indeed plants can be grown hydroponically (in a liquid culture).However, usually plants are grown in the soil and soil properties directly affect the availability of water and nutrients to plants.Soil water affects plant growth directly through its controlling effect on plant water status and indirectly through its effect on aeration, temperature, and nutrient transport, uptake and transformation.The understanding of these properties is helpful in good irrigation design and management.
73 The soil system is composed of three major components: solid particles (minerals and organic matter), water with various dissolved chemicals, and air.The percentage of these components varies greatly with soil texture and structure.An active root system requires a delicate balance between the three soil components; but the balance between the liquid and gas phases is most critical, since it regulates root activity and plant growth process.
74 Jumlah air tersedia dipengaruhi tekstur tanah Inchi Air per foot tanah The amount of soil water is usually measured in terms of water content as percentage by volume or mass, or as soil water potential.Water content does not necessarily describe the availability of the water to the plants, nor indicates, how the water moves within the soil profile.The only information provided by water content is the relative amount of water in the soil.AirTersediaKapasitas LapangPersen AirInchi Air per foot tanahTitik LaytuAirTidak Tersedia
75 Potensial air tanah (MPa) Air dalam tanah (% berat kering) Soil water potential, which is defined as the energy required to remove water from the soil, does not directly give the amount of water present in the root zone either.Therefore, soil water content and soil water potential should both be considered when dealing with plant growth and irrigation.The soil water content and soil water potential are related to each other, and the soil water characteristic curve provides a graphical representation of this relationship.Kapasitas lapangPotensial air tanah (MPa)Titik layu permanen-1.5MPaAir dalam tanah (% berat kering)
76 The nature of the soil characteristic curve depends on the physical properties of the soil namely, texture and structure. Soil texture refers to the distribution of the soil particle sizes.The mineral particles of soil have a wide range of sizes classified as sand, silt, and clay.The proportion of each of these particles in the soil determines its texture.All mineral soils are classified depending on their texture. Every soil can be placed in a particular soil group using a soil textural triangle .For example a soil with 60% sand and 10% clay separates is classified as a Sandy loam
77 KAPASITAS LAPANGThere are limits on the amount of water that soil holds for crop use. The upper limit is termed "field capacity".During an irrigation, or whenever excess water is added to soil, water drains down through the soil due to the pull of gravity.At first, this internal drainage is relatively rapid.However, it soon slows to almost nothing.(The increasing soil water-holding forces finally start to counteract gravity.) At this point we would say the soil is at field capacity.
78 At some point it will essentially stop dripping. You can demonstrate field capacity using a visualization of a sponge (like soil, a porous material that will hold water).Using a pan of water, hold a sponge under water until it is saturated. Now, pull the sponge out of the water.It will immediately start to drip water, quickly at first, then slower and slower.At some point it will essentially stop dripping.The internal drainage has stopped and the sponge is at field capacity.It is very important to note that you can soak more water into soil that is already at field capacity.There will be open soil pores that will take the water. However, the excess water will not be held.It will just drain down until the soil moisture returns to field capacity.
79 KAPASITAS LAPANG Field capacity is a soil-based concept. That is, it depends on the texture and structure of the soil as well as the physical conditions in the field.Coarse soils have lower field capacities than fine soils.If there is a high water table or severe stratification that would restrict drainage, the field capacity would be higher than normal.
80 AIR TERSEDIA & ZONE AKAR The water held by the soil between field capacity and permanent wilting point is termed the "available water holding capacity" of the soil.It is water that is "available" for the plant to use. Water added to the soil in excess of field capacity will drain down, below the active root system.Water held by the soil that is below the permanent wilting point is of no use, the plant has died.As a crop manager you are concerned with the soil moisture throughout the depth of the plant's active root system, the "effective root zone".
81 The effective root zone is that depth of soil where you want to control soil moisture (just as you control fertility and weed/pest pressures).The effective root zone may or may not be the actual depth of all active roots. It may be shallower because of concerns for crop quality or development (as with many vegetable crops).For example, with cotton you may estimate the effective root zone as 6 feet for a preirrigation, 2 feet for the first seasonal irrigation, 4 feet for the second seasonal, and 6 feet thereafter. For an almond orchard, you may estimate the effective root zone as four feet for the entire season. With onions, the major concern is with the top 2 feet.
82 HUBUNGAN AIR & TANAHThe soil is composed of three major parts: air, water, and solids . The solid component forms the framework of the soil and consists of mineral and organic matter.The mineral fraction is made up of sand, silt, and clay particles. The proportion of the soil occupied by water and air is referred to as the pore volume.The pore volume is generally constant for a given soil layer but may be altered by tillage and compaction. The ratio of air to water stored in the pores changes as water is added to or lost from the soil. Water is added by rainfall or irrigation, as shown in Figure 2.Water is lost through surface runoff, evaporation (direct loss from the soil to the atmosphere), transpiration (losses from plant tissue), and either percolation (seepage into lower layers) or drainage.
83 Saturated (wet) soil. All pores (light areas) are filled with water Saturated (wet) soil. All pores (light areas) are filled with water. The dark areas represent soil solids.
84 Water distribution in a soil at field capacity Water distribution in a soil at field capacity. Capillary water (lightly shaded areas ) in soil pores is available to plants. Field capacity represents the upper limit of plant-available water.
85 Water distribution in a soil at thw wilting point Water distribution in a soil at thw wilting point. This water is held tightly in thin films around soil particles and is unavailable to plants. The wilting point represents the lower limit of plant-available water.
86 HUBUNGAN ANTARA AIR-TERSEDIA DAN DISTRIBUSI AIR DALAM TANAH .
88 Air dalam tanah (in/ft) Jumlah air tanah pada tiga macam tekstur tanah
89 Jumlah air tersedia dalam tanah yang teksturnya berbeda-beda
90 AIR TANAH & STRES TANAMAN Kalau tanaman menyerap air dari tanah , jumlah air tersedia yang tersisa dalam tanah menjadi berkurang.The amount of PAW removed since the last irrigation or rainfall is the depletion volume.Irrigation scheduling decisions are often based on the assumption that crop yield or quality will not be reduced as long as the amount of water used by the crop does not exceed the allowable depletion volume.The allowable depletion of PAW depends on the soil and the crop. For example, consider corn growing in a sandy loam soil three days after a soaking rain.Even though enough PAW may be avai1able for good plant growth, the plant may wilt during the day when potential evapotranspiration (PET) is high.
91 AIR TANAH & STRES TANAMAN Evapotranspiration merupakan proses hilangnya air tanah ke atmosfer, melalui evaporasi dari permukaan tanah dan proses transpirasi dari tanaman yang tumbuh di tanah .Potential evapotranspiration is the maximum amount of water that could be lost through this process under a given set of atmospheric conditions, assuming that the crop covers the entire soil sur- face and that the amount of water present in the soil does not limit the process.Potential evapotranspiration is controlled by atmospheric conditions and is higher during the day. Plants must extract water from the soil that is next to the roots.As the zone around the root begins to dry, water must move through the soil toward the root (Figure 7). Daytime wilting occurs because PET is high and the plant takes up water faster than the water can be replaced.
92 Hubungan antara distribusi air dalam tanah dan konsep jadwal irigasi ketika 50 percent air tersedia telah habis
94 Jumlah air tanah tersedia dalam berbagai tipe tanah
95 Pengaruh Potensial Air tanah thd konduktivitas hidraulik tanah Efek potensial air tanah thd konduktivitas hidraulikKonduktivitas hidraulik tanahTititk layu permanenKapasitas lapangPotensial air tanahPengaruh Potensial Air tanah thd konduktivitas hidraulik tanah
96 Pola penyerapan air oleh tanaman yang tumbuh pada profil tanah yang tidak mempunyai lapisan penghambat dan suplai air tersedia cukup di seluruh zone perakaran tanaman
97 There are three types of soil water (ie. water in the soil). AIR DALAM TANAHSoil is made up of soil particles in crumb-form (peds), and pore spaces around the soil crumbs.In a well-structured soil, these crumbs are nice and stable....but in a poorly structured soil, the crumbs are unstable which often limits pore-space.The pore-spaces are necessary for holding water, and for the free gaseous exchange of oxygen and carbon dioxide between the plant roots and the soil surface (respiration process).There are three types of soil water (ie. water in the soil).
98 AIR GRAVITASI AIR KAPILER AIR -TANAHAIR GRAVITASIThis is the water which is susceptible to the forces of gravity. It exists after significant rainfall, and after substantial irrigation. This is the water which fills all the pore-space, and leaves no room for oxygen and gaseous exchange. In "light" soils, this tends to drain away quickly. In heavy soils, this can take time.AIR KAPILERThis is the water which is held with the force of SURFACE TENSION by the soil particles, and is resistent to the forces of gravity. This is the water which is present after the gravitational water has drained away, leaving spaces free for gaseous exchange. When the soil is holding it's MAXIMUM capillary water (after the gravitational water has drained), this is called FIELD CAPACITY. At this point, the plant is able to take up water easily, and has the oxygen that it needs in the root zone.
99 AIR HIGROSKOPISAdalah air yang diikat sedemikian kuat (oleh tegangan permukaan) ke partikel tanah sehingga akar tanaman tidak dapat menyerapnya.Sehingga air ini tidak tersedia bagi tanaman.At this stage there's generally sufficient oxygen, but there just isn't enough available water.The plant wilts, and will eventually die if it doesn't get water.When the plant wilts and is unable to recover, this is called the TITIK LAYU PERMANEN
100 TITIK LAYU PERMANENThe closer to the soil particle the water is held, the tighter it's held. And the further from the particle, the looser it's held. It takes little energy for the plant roots to take up the water that's far from the particle and is present at the field capacity point. By contrast, as the water is used up (or evaporates), it takes more and more energy for the plant to take up water.I often use the analogy of drinking through a straw. A short straw, ie. when a cup is 15 cm away from you, is easy to use. A one-metre long straw takes a lot of energy to suck up a drink. A twenty-metre straw is impossible to use. It works much the same with plants. The more the soil dries out, the more energy the plant needs to output in order to get a decent drink.The effect of increased soil salinity (due to high soil salinity, high soil-water salinity, or both) has basically the same effect as a soil drying out. Salt in the soil has as osmotic effect, and causes the water to be held more tightly around the soil particles.Semakin tinggi tingkat salinitas tanahnya, tanaman semakin sulit menyerap air, meskipun air itu ada dalam tanah.
101 Representasi ketersediaan air dalam tanah bagi pertumbuhan tanaman Kadar air tanah (%; mm/100mm)Tegangan air tanah (kPa, sekala log)Representasi ketersediaan air dalam tanah bagi pertumbuhan tanaman
102 AIR –TANAH TERSEDIAIn other words, Plant Available Water (PAW) is the amount of water held in a soil between the limits of Field Capacity and Permanent Wilting Point.However, only the water near to Field Capacity may be Readily Available Water (RAW).This is particularly so for fine textured, clayey soils because a high proportion of PAW is held in small pores and as thin films and plants need to 'do more work' to extract this fraction of water from soils.
103 AIR –TANAH MUDAH TERSEDIA Not all PAW is equally available to plants. As soils dry out and PAW approaches PWP, plants will come under water-stress and wilt. It is the objective of irrigators to avoid this situation.They prefer to irrigate when the soil water content is about 50% of FC or about 100kPa.These limits, however, are set by the irrigator to suit the business enterprise. For example, if growth rates are to be restricted then the trigger for an irrigation event may be 300kPa.As the name suggests, Readily Available Water or RAW is the amount and availability of water in soils that is readily available to plants.
104 Poorly drained soils, however, are less suitable for irrigation. AIR –TANAH TERSEDIAFollowing rainfall, or irrigation, all the pores in soil will be filled with water; this is the Saturation Water Content (SWC). With time the water in the largest pores will drain to depth due to gravitational forces.In coarser textured, sandy and loamy soils this drainage will take place in less than a day and will, therefore, be unavailable to plants.Fine-textured, clayey soils, however, may be somewhat poorly drained and all pores may remain filled with water for several days.In these cases some of the SWC may be available for EvapoTranspiration and would need to be considered in calculations of soil water balances and irrigation scheduling.Poorly drained soils, however, are less suitable for irrigation.They are difficult to manage and may be waterlogged for times that can cause damage to plants for reasons of anaerobic root environments.
105 PERGERAKAN AIR TANAHDuring long-continued heavy rains, infiltration of soil water continues under the force of gravity, carrying the water down to successively greater depths. Soil pores become filled with water, with only a small amount of free air remaining entrapped in bubbles.The soil may, for a time, become almost completely saturated with water. Downward percolation continues beyond the soil water belt into the intermediate belt, a zone too deep to be reached by plat roots. Water may ultimately reach the ground-water zone below .After the rain has ceased, water continues to drain downward under the influence of gravity, but some remains held in the soil, clinging to the soil grains in thin films, by the force of capillary tension.This is the same force that causes ink to be drawn upward in a piece of blotting paper and which permits small water droplets to cling to the side of a vertical pane of glass. Films of capillary water in the soil remain held in place until gradually dissipated by evaporation or drawn into root systems.
106 PERGERAKAN AIR TANAHAfter soil has been saturated by prolonged rains and then drains until no more water moves downward under the force of gravity, the soil is said to be holding its field capacity of water. Most excess water drains out in a day’s time; usually not more that two or three days are required for gravity drainage to cease.Soil-moisture content can be stated in terms of the equivalent depth in inches of water in a given thickness of soil. At field capacity, soil-moisture content ranges from 1 to 4 inches per foot of soil, depending upon soil texture .Sandy soils have low field capacity, which is rapidly reached because of the ease with which the water penetrates the large openings (macro pores). Clay soils, on the other hand, have a high field capacity, but require much longer periods to attain it because of the slow rate of water penetration due to the much smaller openings (micro pores).A comparable, but lower value of soil moisture is the wilting point, below which foliage wilts because of the inability of the plants to extract the remaining moisture .
107 Air tanah pada berbagai kondisi kelengasan (kadar air)
109 Proses Simpanan lengas dalam tanah. Rain water can also be stored in the ground. Soils consist of particles and pores. Those pores can be filled with air but also with water. The amount of pores is a soil is different for different types of soil. The pores in a clay soil account for 40% to 60% of the volume. In fine sand this can be 20%–45%The soil particles have small pores in them where water can enter (soil water) and between the particles are larger pores that can be filled. The soil is filled with water up a certain level. This level goes up and down with changing weather conditions. This water level is the ground water level.The process of water entering the soil is called infiltration. When the soil has taken up all the water it can, we say that it is saturated. If you walk over a saturated soil, you feel that it is wet and soggy, like biscuits dipped in tea.Part of the water that infiltrates, will move on. It will go to underground storage reservoirs or to underground rivers and may, through ground water flows, eventually reach a river or a lake. Another part will be used by plants or will evaporate.Diunduh dari: …… 11/11/2012
110 Proses Simpanan lengas dalam tanah. Diunduh dari: …… 11/11/2012
111 KAPASITAS SIMPANAN LENGAS TANAH For irrigation the soil water storage (SWS) capacity is defined as the total amount of water that is stored in the soil within the plant’s root zone. The soil texture and the crop rooting depth determine this. A deeper rooting depth means there is a larger volume of water stored in the soil and therefore a larger reservoir of water for the crop to draw upon between irrigations.Only a portion of the total soil water is readily available for plant use. Plants can only extract a portion of the stored water without being stressed. An availability coefficient is used to calculate the percentage of water that is readily available to the plant. The maximum soil water deficit (MSWD) (also referred to as the management allowable deficit) is the amount of water stored in the soil that is readily available to the plant.The crop should be irrigated once this amount of moisture has been removed from the soil. Once depleted this is the amount that must be replenished by irrigation. It is also the maximum amount that can be applied at one time, before the risk of deep percolation occurs. However, in some cases leaching of salts is desirable and extra irrigation would be desired.Diunduh dari: …… 11/11/2012
112 HOW TO DETERMINE THE SOIL WATER STORAGE AND THE MAXIMUM SOIL WATER DEFICIET Step 1 Determine the crop rooting depth, RD (m)Step 2 Determine the available water storage capacity of the soil, AWSC (mm/m), Table 2Step 3 Calculate the total soil water storage, SWS (mm)SWS (mm) = RD (m) x AWSC (mm/m) …………… (Equation 1)Step 4 Determine the availability coefficient of the water to the crop, AC (%)Step 5 Calculate the maximum soil water Deficit, MSWD (mm)MSWD = SWS (mm) x AC (%) …………….. (Equation 2)Diunduh dari: …… 11/11/2012
113 Effective Rooting Depth of Mature Crops for Irrigation System Design Diunduh dari: …… 11/11/2012
114 A Guide to Available Water Storage Capacities of Soils Diunduh dari: …… 11/11/2012
116 Infiltration and Soil Water Storage Pidwirny, M. (2006). "Infiltration and Soil Water Storage". Fundamentals of Physical Geography, 2nd Edition.InfiltrationInfiltration refers to the movement of water into the soil layer. The rate of this movement is called the infiltration rate. If rainfall intensity is greater than the infiltration rate, water will accumulate on the surface and runoff will begin.Movement of water into the soil is controlled by gravity, capillary action, and soil porosity. Of these factors soil porosity is most important. A soil's porosity is controlled by its texture, structure, and organic content. Coarse textured soils have larger pores and fissures than fine-grained soils and therefore allow for more water flow. Pores and fissures found in soils can be made larger through a number of factors that enhance internal soil structure. For example, the burrowing of worms and other organisms and penetration of plant roots can increase the size and number of macro and micro-channels within the soil.The amount of decayed organic matter found at the soil surface can also enhance infiltration. Organic matter is generally more porous than mineral soil particles and can hold much greater quantities of water.Diunduh dari: 11/11/2012
117 Infiltration and Soil Water Storage Infiltration The rate of infiltration normally declines rapidly during the early part of a rainstorm event and reaches a constant value after several hours of rainfall.A number of factors are responsible for this phenomena, including:The filling of small pores on the soil surface with water reduces the ability of capillary forces to actively move water into the soil.As the soil moistens, the micelle structure of the clay particles absorb water causing them to expand. This expansion reduces the size of soil pores.Raindrop impact breaks large soil clumps into smaller particles. These particles then clog soil surface pores reducing the movement of water into the soil.Diunduh dari: 11/11/2012
118 SIMPANAN LENGAS TANAHWithin the soil system, the storage of water is influenced by several different forces. The strongest force is the molecular force of elements and compounds found on the surface of soil minerals.The water retained by this force is called hygroscopic water and it consists of the water held within millimeters of the surface of soil particles. The maximum limit of this water around a soil particle is known as the hygroscopic coefficient. Hygroscopic water is essentially non-mobile and can only be removed from the soil through heating.Matric force holds soil water from to 0.06 millimeters from the surface of soil particles. This force is due to two processes: soil particle surface molecular attraction (adhesion and absorption) to water and the cohesion that water molecules have to each other. This force declines in strength with distance from the soil particle. The force becomes nonexistent past 0.06 millimeters.Diunduh dari: 11/11/2012
119 SIMPANAN LENGAS TANAHCapillary action moves this water from areas where the matric force is low to areas where it is high. Because this water is primarily moved by capillary action, scientists commonly refer to it as capillary water.Plants can use most of this water by way of capillary action until the soil wilting point is reached.Water in excess of capillary and hygroscopic water is called gravitational water.Gravitational water is found beyond 0.06 millimeters from the surface of soil particles and it moves freely under the effect of gravity.When gravitational water has drained away the amount of water that remains is called the soil's field capacity.Diunduh dari: 11/11/2012
120 TEGANGAN AIR TANAHThe relationship between the thickness of water film around soil particles and the strength of the force that holds this water. Force is measured in units called bars. One bar is equal to a 1000 millibars.The graph also displays the location of hygroscopic water, the hygroscopic coefficient, the wilting point, capillary water, field capacity, and gravitational water along this line.Diunduh dari: 11/11/2012
121 Soil Water DynamicsO'Geen, A. T. (2012) Soil Water Dynamics. Nature Education Knowledge 3(6):12.Stored water in soil is a dynamic property that changes spatially in response to climate, topography and soil properties, and temporally as a result of differences between utilization and redistribution via subsurface flow.Changes in soil moisture storage can be generalized with a mass balance equation , as a result of the difference between the amount of water added and that which is lost (Hillel 1982).Conceptual diagram of a soil profile illustrating the multiple flow paths through which water moves through soil (Modified from O’Geen et al. 2010)Diunduh dari: 11/11/2012
122 Change in soil moisture storage = inputs – outputs. Water content increases (positive change in storage) when inputs including precipitation or irrigation exceed outputs. Water content decreases (negative change in storage) when outputs such as deep percolation, surface runoff, subsurface lateral flow, and evapotranspiration (ET) exceed inputs.Water storage and redistribution are a function of soil pore space and pore-size distribution, which are governed by texture and structure. Generally speaking, clay-rich soils have the largest pore space, hence the greatest total water holding capacity. However, total water holding capacity does not describe how much water is available to plants, or how freely water drains in soil. These processes are governed by potential energy. Water is stored and redistributed within soil in response to differences in potential energy. A potential energy gradient dictates soil moisture redistribution and losses, where water moves from areas of high- to low-potential energy (Hillel 1982).When at or near saturation, soils typically display water potentials near 0 MPa.Negative water potentials arise as soil dries resulting in suction or tension on water allowing the soil to retain water like a sponge.Diunduh dari: …… 11/11/2012
123 .Water storage and redistribution are a function of soil pore space and pore-size distribution, which are governed by texture and structure.Water content and water potential at saturation, field capacity and permanent wilting point.The difference in water content between field capacity and permanent wilting point is plant available water.Drainable porosity is the amount of water that drains from macropores by gravity between saturation to field capacity typically representing three days of drainage in the field.Diunduh dari: …… 11/11/2012
124 Influence of Texture and Structure. Texture and structure determine pore size distribution in soil, and therefore, the amount of PAW.Coarse textured soils (sands and loamy sands) have low PAW because the pore size distribution consists mainly of large pores with limited ability to retain water. Although fine textured soils have the highest total water storage capacity due to large porosity values, a significant fraction of water is held too strongly (strong matric forces/low, negative water potentials) for plant uptake.Fine textured soils (clays, sandy clays and silty clays) have moderate PAW because their pore size distribution consists mainly of micropores. Loamy textured soils (loams, sandy loams, silt loams, silts, clay loams, sandy clay loams and silty clay loams) have the highest PAW, because these textural classes give rise to a wide range in pore size distribution that results in an ideal combination of meso- and micro-porosity.Soil structure can increase PAW by increasing porosity.Diunduh dari: …… 11/11/2012
125 PERMEABILITAS TANAHSoil structure is highly relevant to water management in soils because it is subject to change either through deterioration by improper management, or to improvement through additions of soil organic matter. In contrast, it is usually infeasible to change texture.Permeability ClassPermeability (cm/hr)Textural classVery slow<0.13claySlow0.13–0.5sandy clay, silty clayModerately slow0.05–2.0clay loam, sandy clay loam, silty clay loamModerate2.0–6.3very fine sandy loam, loam, silt loam, silty clay loam, siltModerately rapid6.3–12.7sandy loam, fine sandy loamRapid12.7–25.4sand, loamy sandVery Rapid>25.4coarse sandDiunduh dari: …… 11/11/2012
126 Total Soil Water Storage Capacity The total soil water storage capacity refers to when all the soil pores or voids are filled with water. This occurs when the soil is saturated or flooded.A peat soil usually has the highest total soil water storage capacity of around 70 to 85% by volume. Sands and gravels will have the lowest total porosity of around 30 to 40% by volume.Total porosity for silt soils ranges from 35 to 50%, and clay soils typically range from 40 to 60%.Restricted drainage conditions can cause the soil to attain its total porosity water content, at which time free water is observed and perched water tables develop (in layered soils) or the apparent water table is found near the surface.When the total soil water storage capacity is reached, air is pushed out of the pores or void spaces and oxygen and other gaseous diffusion in the soil is severely restricted.Most agricultural plants cannot tolerate this condition very long (usually no more than a day or two) as plant root respiration requires some oxygen diffusion to the roots.Without air-filled pores, the concentration of carbon dioxide and other gases like ethylene increase, producing toxic conditions and limiting plant growth.Diunduh dari: 11/11/2012
127 Soil Water Storage.Soil water storage is a function of the surface area of soil particles (i.e., particle-size) and the amount of porosity occurring between these particles (i.e., soil structure). Soil pores occur across a wide range of diameters and are often categorized as macropores (>60 µm) and micropores (<60 µm).Water is present in macropores following precipitation events and is drained by the force of gravity. after water has freely drained due to the force of gravity, the soil is at field capacity and has a soil water potential generally between 0.01 to 0.03 mpa. water in macropores is not available for plant use because it freely drains from the soil profile and is lost from the rooting zone. water held in very small micropores (<0.2 µm) is held so tightly that plants are not able to extract if for use.The permanent wilting point is the soil water potential to which plants can effectively utilize water and corresponds to a soil water potential of approximately 1.5 mpa. thus, the pores in the diameter range 0.2 to 60 µm are the primary storage pores for plant available water (i.e., water held between approximately 0.01 and 1.5 mpa). the distribution of pore sizes is primarily a function of the soil texture and structure. the amount of water storage as a function of soil texture is illustrated in this figure.Diunduh dari: 11/11/2012
128 Soil Water and its Availability. Figure 2.33 indicates the availability of soil water. A soil is at saturation or near saturation following a heavy irrigation or rainfall in which most or all of the spaces between soil particles are filled by water. The force of gravity is greater than the force with which soil particles hold water, so between saturation and field capacity (see below), water is free to drain through the soil by the force of gravity.Field capacity (FC) is the amount of water that a soil can hold against drainage by gravity.Permanent wilting point (PWP) is the moisture content in a soil at which plants permanently wilt and will not recover.Available water (AW) is the water content that the soil can hold between field capacity and wilting point.Diunduh dari: …… 11/11/2012
129 SIMPANAN LENGAS TANAHThe soil water storage or soil water content can be quantified on the basis of its volumetric or gravimetric water content.The volumetric water content is the volume of water per unit volume of soil, expressed as a percentage of the volume.The gravimetric water content is the mass of water per unit mass of dry (or wet) soil.The volumetric water content is equal to the gravimetric water content times the soil's bulk density (on a dry soil basis).Factors that affect the soil water storage are:Total Porosity or Void SpacePore-size and Distribution and ConnectivitySoil Water Pressure Potential or Energy Status of the Soil WaterDiunduh dari: …… 13/11/2012
130 SIMPANAN LENGAS TANAHThe total porosity or void space ultimately establishes the upper limit of how much water can be stored in a given volume of soil. When all the pores are filled with water the soil is saturated, and cannot store any more water. The total porosity is a function of the soil's particle size, particle uniformity and packing or structure because the void space that remains between the solid particles determines the extent and distribution of pore sizes and their connectivity.If one fills the same volume with sand and clay sized particles, the total porosity of the clay is somewhat higher, about 50-55% of the volume compared to about 35-40% for sand. The spaces between the sand particles will have larger voids, but there will be fewer of them. The total porosity of medium textured loamy soils is generally around 50% because the smaller silt and clay particles fill some of the voids between the larger sand particles. Soils with good structure will have somewhat higher total porosity than soil that has been compacted (i.e., where the soil particles are forced closer together).The important influence of pore-size and distribution on soil water storage is in regards to how different pore sizes respond to energy forces or the soil water pressure potential. Under saturated conditions, large pores drain more easily in response to gravity potential. Also, when the soil is unsaturated, large pores are less subject to capillary (or matric potential) forces.In unsaturated soil conditions, the soil water pressure potential becomes negative (suction), and the degree to which this occurs greatly influences the soil water storage (retention) or water content in different sized pores.Diunduh dari: …… 13/11/2012
131 SIMPANAN LENGAS TANAH.The soil water characteristic (retention) curve defines the relationship between the soil water pressure potential or energy status (matric or suction potential) and the soil water content.It's important to note that soil water moves in direct response to the energy or pressure potential forces acting upon it (i.e., moving from a higher to lower energy status), and not necessarily in response to different soil moisture contents (i.e., from higher to lower soil moisture content).Diunduh dari: …… 13/11/2012
132 SIMPANAN LENGAS TANAH.Sumber:The soil water characteristic curve(s) and definitions are used to establish and further refine and quantify the general availability of soil water which is often referred to as :Gravitational water (water subject to drainage),Capillary water (water available to plants), andHygroscopic water (water that is not available to plants).Diunduh dari: …… 13/11/2012
133 PERGERAKAN LENGAS TANAH. Water movement is directly related to the size of pores in the soil. In the small pores of clayey soils, water slowly moves in all directions by capillary action. The lack of large pore space leads to drainage problems and low soil oxygen levels. On sandy soils with large pores, water readily drains downwards by gravitational pull. Excessive irrigation and/or precipitation can leach water-soluble nutrients, like nitrogen, out of the root zone and into ground water.Diunduh dari: …… 13/11/2012
134 Sumber: http://iowacedarbasin.org/runoff/showMan.php?c1=2E-1 SIMPANAN LENGAS TANAHThe water table is defined as the upper surface of groundwater (saturated zone) or that level in the ground below the soil surface where the water is at (and in equilibrium with) atmospheric pressure. At the water table reference, the pressure potential is set equal to zero.Thus, below the water table, the pressure potential becomes positive, and above the water table the pressure potential becomes negative. This negative pressure in unsaturated soil is termed matric, tension or suction pressure potential so as not to confuse it with positive pressures.Water infiltration through the soil-water unsaturated zone and into the water tableSumber:Diunduh dari: …… 13/11/2012
135 Available Water Capacity SOIL WATER STORAGE.Available Water CapacityThe total available water (holding) capacity is the portion of water that can be absorbed by plant roots. By definition it is the amount of water available, stored, or released between field capacity and the permanent wilting point water contents.The soil types with higher total available water content are generally more conducive to high biomass productivity because they can supply adequate moisture to plants during times when rainfall does not occur. Sandy soils are more prone to drought and will quickly (within a few days) be depleted of their available water when evapotranspiration rates are high. For example, for a plant growing on fine sand with most of its roots in the top foot of soil, there is less than one inch of readily available water.A plant transpiring at the rate of 0.25 inches per day will thus start showing stress symptoms within four days if no rainfall occurs. Shallow rooted crops have limited access to the available soil water, and so shallow rooted crops on sandy soils are particularly vulnerable to drought periods. Irrigation may be needed and is generally quite beneficial on soils with low available water capacity.Diunduh dari: …… 13/11/2012
136 SOIL WATER STORAGE. Soil Type Total Available Water, % Total Available Water, in/ftcoarse sand50.6fine sand151.8loamy sand172.0sandy loam202.4sandy clay loam161.9loam323.8silt loam354.2silty clay loamclay loam182.2silty clay222.6claypeat506.0Diunduh dari: …… 13/11/2012
137 Total Soil Water Storage Capacity The total soil water storage capacity refers to when all the soil pores or voids are filled with water. This occurs when the soil is saturated or flooded.A peat soil usually has the highest total soil water storage capacity of around 70 to 85% by volume. Sands and gravels will have the lowest total porosity of around 30 to 40% by volume.Total porosity for silt soils ranges from 35 to 50%, and clay soils typically range from 40 to 60%. Restricted drainage conditions can cause the soil to attain its total porosity water content, at which time free water is observed and perched water tables develop (in layered soils) or the apparent water table is found near the surface.When the total soil water storage capacity is reached, air is pushed out of the pores or void spaces and oxygen and other gaseous diffusion in the soil is severely restricted.Diunduh dari: …… 13/11/2012
138 Total Soil Water Storage Capacity Most agricultural plants cannot tolerate this condition very long (usually no more than a day or two) as plant root respiration requires some oxygen diffusion to the roots.Without air-filled pores, the concentration of carbon dioxide and other gases like ethylene increase, producing toxic conditions and limiting plant growth.Root cells switch to anaerobic respiration, which is much less efficient than aerobic respiration in converting glucose molecules to ATP (adenosine triphosphate, the chemical energy within cells for metabolism and cell division).Diunduh dari: …… 13/11/2012
139 Total Soil Water Storage Capacity As anaerobic (reduced) conditions develop in the soil, nitrification ceases and denitrification is enhanced.Corn plants will quickly yellow in response to this saturated soil state as nitrogen becomes limiting, and the plant tries to adjust by producing more adventitious roots.Prolonged anaerobic conditions in the soil starts to reduce manganese, iron (causing phosphorus to be more soluble), sulfur (producing hydrogen sulfide), and eventually methane gases.Hydrophytic (wetland type) plants are adapted to saturated soils because they are able to obtain oxygen through other forms of plant structure adaptations (i.e. pneumataphores, lenticels, aerenchyma).Diunduh dari: …… 13/11/2012
140 Consider a soil that is saturated with the water table at the surface. SOIL WATER STORAGE..Drainable porosity is the amount of water that drains from macropores by gravity between saturation to field capacity typically representing three days of drainage in the field.Drainable Porosity The drainable porosity is the pore volume of water that is removed (or added) when the water table is lowered (or raised) in response to gravity and in the absence of evaporation.Consider a soil that is saturated with the water table at the surface.If this soil has a subsurface drainage pipe (tile) buried several feet down and it is discharging to the atmosphere at some lower elevation, the drainable porosity water content will be released to the tile drain until the water table is lowered to the depth of the drain. Diunduh dari: 13/11/2012Diunduh dari: …… 13/11/2012
141 SOIL WATER STORAGE. Drainable Porosity Any nutrients or pesticides dissolved or suspended in this readily drainable pore space will also be carried along with this water, either flowing to the tile drain or continuing downward to the water table via deep percolation if no drainage restriction exists.In large pores, nutrients that might otherwise adsorb to the soil particles (ammonium or phosphate) will bypass the soil because of limited time for contact and chemical reactions to occur with the soil surface area.Soils with a wide range of different pore sizes (sandy loams) or soils with mostly small sized pores are better at filtering nutrients and pesticides as they leach through the soil profile. Diunduh dari: …… 13/11/2012
142 SOIL WATER STORAGE. Drainable Porosity The combined aspect of low available water holding capacity and high drainable porosity for sandy soils causes these soils to have a high leaching potential.It will not take much rain or irrigation (or application of liquid manure) to replenish the available soil water and to raise the soil water content to a drainable state.Applying the proper amount (depth) of irrigation to these soils will both conserve water and enhance irrigation and nutrient use efficiency.The variability of drainable porosity with soil texture and structureSoil TextureField Capacity (% by vol.)Wilting Point (% by vol.)Drainable Porosity (% by vol.)clays, clay loams, silty clays30-50%15-24%3-11%well structured loams20-30%8-17%10-15%sandy10-30%3-10%18-35%Diunduh dari: ………..13/11/2012Diunduh dari: …… 13/11/2012
143 SOIL WATER STORAGE.The soil texture and structure fundamentally determines the number and sizes of soil pores, which will influence the fate and transport of air (gas) and water exchange.The figure provides an illustration of how various parameters of soil water storage may be influenced by different texture and structure aspects(From H.M. van Es).Diunduh dari: …… 13/11/2012
144 SOIL POROSITY Saturation: all soil pores are filled Gravitational water:drainable water 0 to -33 kPa.Field capacity(after 1-2 days of drainage)-33 kPa, usually 1/2 of saturation water.Permanent wilting point:-1500 kPaPlant available water:difference in water% at FC and PWP.Diunduh dari: 13/11/2012
145 SOIL POREA well aggregated soil has a range of pore sizes. This medium size soil crumb is made up of many smaller ones. Very large pores occur between the medium size aggregates.The ideal soil can be found in a well aggregated medium-textured loam soil.Such a soil has enough large pore spaces between the aggregates to provide adequate drainage and aeration during wet periods, but also has adequate amounts of small pores and water-holding capacity to provide sufficient water to plants and soil organisms between rainfall or irrigation events.Diunduh dari: 11/11/2012
146 Types of soil poresMacropores (d>0.08mm) occur between aggregates (interped pores) or individual grains in coarse textured soil (packing pores) and may be formed by soil organisms (biopores). They allow ready movement of air and the drainage of water and provide space for roots and organisms to inhabit the soil.Micropores (d<0.08mm) occur within aggregates. They are usually filled with water and are too small to allow much movement of air. Water movement in micropores is extremely slow and much of the water held by them is unavailable to plants. Pore space can be filled with either water or air. The volume of soil water and soil air, however, cannot exceed the total soil porosity. As soil water increases, soil air must decrease and vice versa. Diunduh dari: …… 13/11/2012
147 SOIL STRUCTURESoil structure is the arrangement of pores and fissures (porosity) within a matrix of solid materials (soil particles and organic matter).The solid materials bond and aggregate to give the pores and fissures.The quantity, distribution and arrangement of pores determines water holding capacity, infiltration, permeability, root penetration, and, respiration.Only about 50% of soil is solid material. The remainder is pore space.Small pores within the aggregates provide storage and refuge. The larger pores (and fissures) between the aggregates are the pathways for liquids, gases, roots and organisms.Diunduh dari: 13/11/2012
148 SOIL POROSITY.Porosity of cultivated structured soil (schematic): 1--thin, predominantly capillary pores in aggregates, which fill with water on wetting; 2--medium-sized pores (cells, channels), upon wetting they will fill with water for a short period and subsequently, after the resorption of water, with air; 3--capillary pores; 4--large pores, between aggregates almost always filled with air (according to Kschinakii, 1956);Visible porosity of soil aggregate (reproduction from a microsection). Thin chernozem (southern): 1--micro-aggregates; 2--visible pores (according to Kachinskii et al., 1950). The total porosity is determined according to its volume and specific gravity. It is determined by the following formula:P = (1 - vol/sp) x 100where P--porosity, sp--the specific gravity of the solid phase, vol--weight by volume.Diunduh dari: 11/11/2012
149 Soil management practices that promote good soil structure include: SOIL AGGREGATIONSoil management practices that promote good soil structure include:Minimizing tillage; Timing tillage for optimum moisture conditions (approximately field capacity or a little drier); Maintaining plant litter/residues on the soil surface; Incorporating a constant supply of decomposable organic material; Using sod crops whenever possible; Using green manure and cover crops whenever possible; Applying gypsum and soil conditionersDiunduh dari: …… 13/11/2012
150 AGGREGATE STABILITYRelationship between aggregate stability and soil organic matter in some selected soils from the Cornell University research sites in NY.Diunduh dari: …… 13/11/2012
151 SOIL POROSITY.Porosity refers to the amount of space between the solid soil particles. Pore space can be filled with either water or air.Smaller pores tend to be filled with water.The amount of water-filled pores is referred to as capillary porosity. Large pores are typically filled with air. These air-filled pores are referred to as non-capillary porosity.When a growing media has approximately equal amounts of water-filled and air-filled pore space, the soil is said to have balanced porosity.Soils text books indicate that growing media with balanced porosity provide an ideal environment where beneficial microbes, nutrients, water, and air can interact and thrive.This provides advantageous conditions for desirable soil gas exchange, good mineral/water holding, vigorous root growth, and healthy plants.Diunduh dari: 13/11/2012
152 SOIL POROSITY.A vertical cross sectional view of a highly structured soil. The largest soil units shown are macroaggregates (~ 2 mm diameter).They are composed of microaggregates (~ 0.1 mm) and sand grains, as shown in the center left of the macropore.Four hierarchical classes of soil pore space are illustrated: (1) macropore, (2) intermacroaggregate, (3) intermicroaggregate (includes intramacroaggregate space, see arrow) and (4) tramicroaggregate space. (Illustration by S. L. Rose).Diunduh dari: 13/11/2012
153 SOIL POROSITY.Water is retained in many of these pores when the soil is at field capacity and pore space is large enough to be inhabited by nematodes.The pores between microaggregates but within macroaggregates are large enough to accommodate small nematodes and protozoa and may be the chief habitat of fungi.The smallest class of pores, those within microaggregates, may be only about 1 mm, maximally, and may be inhabited mostly by bacteria (Kilbertus 1980).Diunduh dari: 13/11/2012
154 Soil Pore spacesSoil particles do not fit together snugly. There are spaces between particles. These spaces are called pore spaces and contain water and air.The pore spaces provide the route for the downward movement of water and allow roots to grow into them. They also provide air space, which is essential for plant growth.The larger the pore spaces the better the drainage of water and the less water retained in the soil. Conversely, the smaller the pore spaces the less water drains away and the more water is retained in the soil.Diunduh dari: 13/11/2012
155 Water holding capacity. After a soil has been completely soaked by a downpour of rain all its pore spaces are filled with water and it is regarded as being saturated. Any air in the pore spaces is forced out and the soil is said to be waterlogged.Once the rain stops and the water has a chance to drain away, the total amount of water that remains in the soil, against the force of gravity, is called the field capacity of the soil. Due to capillary forces, water will drain away from the large pore spaces first and remain in some of the smaller pore spaces. So, at field capacity, the larger pores will contain air whilst the smaller pores hold water.Plant roots in the soil are able to suck the water out of many of the small pores. However, eventually water in the small pore spaces is held too tightly by capillary forces for the roots to take it up. This stage, when the turf plants are unable to extract any more water from the soil and begin to wilt, is called the permanent wilting point.Due to differences in particle and pore space size, soils differ in their field capacity and permanent wilting point.Have a look at the different water holding capacities of sand, loam and clay.Diunduh dari: 13/11/2012
156 What is water holding capacity of soil?. Water holding capacity of soil is just that, the specific ability of a particular type of soil to hold water against the force of gravity. Different types of soils have difference capacities, for example a sand soil had a lower capacity to hold water when compared to a clay soil. The nature of the soil, composition of the soil, amount of organic component and size of the soil particles determine its ability to retain water. Water molecules are held closely to the individual soil particles by forces of cohesion. The maximum amount of water a soil can hold before it is saturated and starts to loose water by gravity is known as "field capacity“.Diunduh dari: …… 13/11/2012
157 A plant's available water holding capacity for soils with different textures. The texture of a soil is important for soil water availability because it controls not only how well a soil can hold water but also how well water is absorbed into the soil.Any water that infiltrates into a soil does so primarily through large pores in the soil called “macropores” that are created by plant roots, microorganisms, and physical processes such as freezing and thawing and drying and wetting.Diunduh dari: …… 13/11/2012
158 WHC = WATER HOLDING CAPACITY General relationship between soil texture and available water-holding capacity. As clays increase in a soil, so does water-holding capacity. Typically, clay loam soils hold more than twice as much water as sandy textured soils.The presence of humus in topsoil increases water-holding capacity of loams and sandy loams at a rate of 2.25% water to each percent rise in soil humus (Jenny 1980) which equates to approximately 0.75% increase in water for every 1% increase in organic matter.Diunduh dari: …… 13/11/2012
159 Mitigating for Textures with Low Water-Holding Capacities Organic Amendments — Incorporation of organic amendments (e.g., compost) can increase the water-holding capacity of a soil.Because the water-holding capacity of each type of organic matter varies by composition and degree of weathering, the effect on soil water-holding capacity by any organic matter being considered must be assessed prior to application.Sandy textured soils benefit most from organic matter additions, especially those with plant available water of 9% or less (Claassen 2006), which are typically sands, loamy sands, and sandy loam soils.Testing several different rates of incorporated organic matter on soil moisture-holding capacity should be done to prior to selecting the source and the amount of material to apply.Diunduh dari: …… 13/11/2012
160 Mitigating for Textures with Low Water-Holding Capacities Clay — The water-holding capacity of sandy textured soils can be increased by incorporating clay loam, sandy clay loam, and silty clay loam textures in the soil.The addition of clays should be at rates that result in new soil textures similar to loams, silt loams, or sandy clay loams.Higher rates of clay addition are not recommended. It is always important to test the additions of any soil to another to understand what the effects on water-holding capacity and structure might be.Ideally this should be in the field in small plots.Diunduh dari: …… 13/11/2012
161 Soil moisture retention curve The soil moisture retention curve (pF curve) gives the relation between soil moisture suction and soil moisture content.A soil is at F.C. (field capacity) or has a pF-value of 2, some 2 to 3 days the soil has been saturated by rainfall or irrigation.When the soil becomes dry and plants cannot take up water anymore the soil is at W.P (wilting point) or has a pF=4.2.The amount of water held by a soil in the root zone between F.C. and W.P. and which can be used by plants is described as available water. (F.C.- W.P.= available water) For sand, loam and clay the values are 6, 20 and 17 volume percent respectively.Diunduh dari: …… 13/11/2012
162 Limitations to the concept of plant available water: Plant available water is considered the amount of water held between field capacity and the permanent wilting point.Limitations to the concept of plant available water:Roots are not distributed throughout the profile.Water content is not uniform throughout the profile.Not all plants have the same wilting point.Water may not be as ‘easily’ obtained (i.e. available) as the soil dries andpotential decreases toward the permanent wilting point.Diunduh dari: …… 13/11/2012
164 Struktur Tanah & Agregasi Soil may be a loose assemblage of individual and random particles, or consist of distinctly structured aggregates of distinctive size and shape; the particular arrangement of which is called soil structure.Most methods of measurement are indirect, and measure various properties that are dependent or at the least influenced by specific structural properties; e.g., total porosity, pore size distribution, liquid retention/ transmission, and infiltration.Diunduh dari: …… 13/11/2012
165 Struktur Tanah & Agregasi Soils may be non-structured (e.g., single grain or massive) or consist of naturally formed units known as peds or aggregates.The initial stage in the formation of soil structure is the process of flocculation.Individual colloids typically exhibit a net negative charge which results in an electrostatic repulsion.Diunduh dari: …… 13/11/2012
166 Struktur Tanah & Agregasi Reduction of the forces of electrostatic repulsion allows the particles to come closer together.FlocculationThis process allows other forces of attraction to become more dominant. The formation of these “flocs” in suspension represents the early stages of aggregation.Diunduh dari: …… 13/11/2012
167 Struktur Tanah & Agregasi As this process continues, the flocs become larger and larger forming the more refined structural units.On their own, these units are pretty fragile and the process is easily reversed. But in the presence of natural or artificial binding become more strongly cemented together forming stable soil aggregates.Bahan perekat (pengikat) dapat berupa :Inorganic – Fe & Al oxides, carbonates, amorphous gels and sols; orOrganic – polysaccharides, hemicellulose, and other natural or manufactured organic polymers.Diunduh dari: …… 13/11/2012
168 Struktur Tanah & Agregasi The arrangement or organization of individual soil particles (soil separates) into a specific configuration is called “soil structure”. Soil structure is developed over a geologic time frame, is (or can be) naturally fragile, and is affected by changes in climate, vegetation, biological activity, and anthropogenic manipulation.Soil structure influences the mechanical properties of soil such as stability, porosity and compaction, as well as plant growth, hydrologic function, and erosion.Diunduh dari: …… 13/11/2012
169 Struktur Tanah & Agregasi There are three broad categories of soil structure; single grained, massive, and aggregated.When particles are entirely unattached the structure is completely loose and such soils are labeled single grained. When packed into large cohesive blocks the structure is called massive.Neither have any visible structural characteristics.Between these two extremes particles are present as aggregates or peds.Diunduh dari: …… 13/11/2012
170 Platy: Horizontally layered, thin, flat aggregates similar to wafers. The observable shapes of soil structure in the field are classified as:Platy: Horizontally layered, thin, flat aggregates similar to wafers.Spherical: Rounded aggregates generally < 2.0 cm in diameter that are often found in loose condition called “granules or crumbs”.Blocky: Cube-like blocks, sometimes angular with well-defined sharp faces or sub-angular with rounded faces up to 10cm in size.Columnar or Prismatic: Vertically oriented pillars up to 15cm in diameter.Diunduh dari: …… 13/11/2012
171 Platy and spherical soil structure is common to the surface soil horizons, blocky and columnar/prismatic are associated with the deeper subsurface soil horizonsDiunduh dari: …… 13/11/2012
172 Struktur Tanah & Agregasi StructuredPlaty: horizontal & flatSpherical (Grannular): rounded and <2.0 cmBlocky: cubes up to 10 cm that are angular (sharp edges) or subangular (rounded)Prismatic (Columnar): longer than wide, often 6 sided, sharp or rounded, < 15 cmNon-StructuredSingle GrainMassiveDiunduh dari: …… 13/11/2012
173 Struktur Tanah & Agregasi Aggregate size distribution also influences the pore size distribution.Macropores: Inter-aggregate cavities that influence infiltration, drainage, and aeration.Micropores: Intra-aggregate capillaries important to water and solute retention.Mesopore: Inbetween.Diunduh dari: …… 13/11/2012
174 DISTRIBUSI UKURAN AGREGAT Similar to particle size distribution, the aggregate size distribution also is determined by sieving.An index known as the Mean Weight Diameter (X) based on the size and weight distribution of aggregates is derived by weighing the mass of aggregates within the respective size classes, and characterizing the overall size distribution.(MWD) X = ∑ xiwixi = mean diameter wi = dry mass fractionDiunduh dari: …… 13/11/2012
175 Stabilitas AgregatSince aggregation and stability is time dependent, another useful characterization is that of “aggregate stability”.Aggregate stability expresses the resistance of individual soil aggregates to disruptive forces such as mechanical, wind, and water erosion; freezing/thawing; wetting/drying; and air entrapment.The level of stability is assessed by determining the fraction of the original aggregate mass which has withstood disruptive forces. The laboratory approach uses wetting (misting and/or from bottom up with de-aired water) followed by sieving.Diunduh dari: …… 13/11/2012
176 TIPE-TIPE STRUKTUR TANAH Diunduh dari: …… 13/11/2012
178 AGREGAT TANAHSoil aggregates are formed and stabilized by clay-organic complexes, microbial polysaccharides, fungal hyphae and plant roots.Diunduh dari: …… 13/11/2012
179 AGREGAT TANAHSoil aggregates are associated with relatively large inter-aggregate pore spaces that range from um to mm in diameter. Each aggregate also has intra-aggregate pore spaces that are very small, ranging from nm to um in diameter.Intra-aggregate pores can exclude bacteria (called micropore exclusion). However, after a spill, contaminants can slowly diffuse into these pores. This creates a long-term sink of pollution as the contaminants will slowly diffuse out again.Diunduh dari: …… 13/11/2012
180 Berapa banyak “Pori” dalam tanah? Assume a soil aggregate that is 2 x 2 x 2 mm. Further assume that the volume of the aggregate is 50% pore space. How many pores of diameter 15 um does the aggregate have? How many pores of 50 um?(the volume of a sphere is: 4/3π r3)2 mm2 mm2 mmCalculation for 15 um pores:The volume of the aggregate is 2 mm x 2 mm x 2 mm = 8 mm3Pore space is 50% of 8 mm3 = 4 mm3A pore of 15 um diameter has volume = 4/3 π (7.5 um)3 = 1.77 x 103 um34 mm3 (1000 um)3 / 1.77 x 103 um3 = 2.3 x 10 6 pores of 15 um per aggregate!mm poreDiunduh dari: …… 13/11/2012
181 Where are the bacteria?In soil 80 to 90% of the bacteria are attached to surfaces and only 10-20% are planktonic. Cells have a patchy distribution over the solid surfaces, growing in microcolonies. Colony growth allows sharing of nutrients and helps protect against dessication and predation or grazing by protozoa.Diunduh dari: …… 13/11/2012
182 Pergerakan dan Potensial Lengas Tanah Soil water potential depends on how tightly water is held to a soil surface. This in turn depends on how much water is present.Surface forces have water potentials ranging from –10,000 to –31 atm.Capillary forces have water potentials ranging from –31 to –0.1 atm. Optimal microbial activity occurs at approximately -0.1 atm.At greater distances there is little force holding water to the surface. This is considered free water and moves downward due to the force of gravity.Diunduh dari: …… 13/11/2012
183 Estimating effects of compaction on pore size distribution of soil aggregates by mercury porosimeter J. Lipiec, M. Hajnos, R. Świeboda. Geoderma. Volumes 179–180, June 2012, Pages 20–27The aim of this study was to describe quantitatively the effect of vehicular traffic on pore size distribution (PSD) of topsoil (0.05–0.15 m) and subsoil (0.25–0.35 m) aggregates (3 mm and 8 mm) of silty loam.The total aggregate porosity, average pore radius and volume of larger pores, > 1–3 μm at 0.05–0.15 m depth, and > 0.3–0.4 μm at 0.25–0.35 m decreased with increasing soil compaction, mostly from NC to MC. At 0.25–0.35 m depth this decrease was accompanied by an increase in the volume of smaller pores (< 0.3 μm) mostly from MC to SC. As a consequence, the volume of pores retaining plant available water (0.1–15 μm radius) decreased in compacted soil.The differential pore curves exhibited peaks at the pore throat radius of 1–6 μm. At 0.05–0.15 m depth the peaks under SC were lower than under NC and MC, whereas at 0.25–0.35 m depth they were lower under MC and SC than NC.At all compaction treatments and aggregate fractions the volume of larger pores > 1–3 μm was greater at 0.05–0.15 m depth than at 0.25–0.35 m depth and the inverse was true for smaller pores (< 0.3 μm). The observed changes in pore size distribution in the subsoil are considered as almost irreversible and thus long-lasting or even permanent.Diunduh dari: 13/11/2012
184 Estimating effects of compaction on pore size distribution of soil aggregates by mercury porosimeter J. Lipiec, M. Hajnos, R. Świeboda. Geoderma. Volumes 179–180, June 2012, Pages 20–27Cumulative curve of pore volume vs. equivalent pore radius of 3–5 mm aggregates.Diunduh dari: 13/11/2012
185 Estimating effects of compaction on pore size distribution of soil aggregates by mercury porosimeter J. Lipiec, M. Hajnos, R. Świeboda. Geoderma. Volumes 179–180, June 2012, Pages 20–27Cumulative curve of pore volume vs. equivalent pore radius of 8–10 mm aggregates.Diunduh dari: 13/11/2012
186 Estimating effects of compaction on pore size distribution of soil aggregates by mercury porosimeter J. Lipiec, M. Hajnos, R. Świeboda. Geoderma. Volumes 179–180, June 2012, Pages 20–27Differential curve of pore volume vs. equivalent pore radius of 3–5 mm aggregates.Diunduh dari: 13/11/2012
187 Aggregate Stability Index Crop-Pasture Rotation for Sustaining the Quality and Productivity of a Typic ArgiudollGuillermo A. Studdert, Hernan E. Echeverría, Elda M. CasanovasAggregate Stability IndexThe relationship between SOC and ASI. It can be seen that ASI values under cropping were relatively constant across all the explored range of SOC ( g kg-1).On the other hand, ASI increased with increases in SOC (r = 0.64, P < 0.01) when periods under pasture were analyzed.These results agree with the fact that aggregate stability and SOC are closely related (Greenland, 1981; Oades, 1984).It appears that SOC was not enough by itself to explain ASI variations because ASI values were different at the same SOC under cropping or under pasture, respectively.Diunduh dari: …… 13/11/2012
188 PENDUGAAN LENGAS TANAH GEOLISTRIKPENDUGAAN LENGAS TANAH
189 ciri-ciri fisik air tanah (suhu, kerapatan, dll). GEOLISTRIK.Geolistrik merupakan metoda geofisik yang mempelajari sifat aliran listrik di dalam bumi dan bagaiman cara mendeteksinya di permukaan bumi.Dalam hal ini meliputi pengukuran potensial, arus dan medan elektromagnetik yang terjadi baik secara alamiah ataupun akibat injeksi arus ke dalam bumi.Ada beberapa macam metoda geolistrik, antara lain : metoda potensial diri, arus telluric, magnetotelluric, IP (Induced Polarization), resistivitas (tahanan jenis) dan lainnya.Penyelidikan air tanah dilakukan untuk memperkirakan tempat terjadinya airtanah, kedalaman muka pembentukan (kerikil, pasir, dan lain-lain), sertaciri-ciri fisik air tanah (suhu, kerapatan, dll).Penyelidikan air tanah dapat dilakukan dari permukaan tanah maupun dari bawah permukaan tanah.Penyelidikan air tanah yang biasa dilakukan dari permukaan tanah adalah dengan menggunakan metode Geolistrik.Sumber: Penggunaan Metode Geolistrik Untuk Mendeteksi Keberadaan Air Tanah . Eva Rolia . TAPAK Vol. 1 No. 1 Nopember /11/2012
190 PENDUGAAN GEOLISTRIK.Geolistrik merupakan salah satu metode geofisika yang mempelajari sifat aliranlistrik di dalam bumi dan untuk mengetahui perubahan tahanan jenis lapisanbatuan di bawah permukaan tanah dengan cara mengalirkan arus listrik DC (directcurrent) yang mempunyai tegangan tinggi ke dalam tanah. Metode ini lebihefektif jika digunakan untuk eksplorasi yang sifatnya dangkal, contohnyapenentuan kedalaman batuan dasar, pencarian reservoir air, dan juga digunakandalam eksplorasi geothermal.Tujuan survey geolistrik tahanan jenis adalah untuk mengetahui resistivitas bawah permukaan bumi dengan melakukan pengukuran di permukaan bumi. Resistivitas bumi berhubungan dengan mineral, kandungan fluida dan derajat saturasi air dalam batuan.Metode yang bisa digunakan pada pengukuran resistivitas secara umum yaitu dengan menggunakan dua elektroda arus (C1 dan C2), dan pengukuran beda potensial dengan menggunakan dua elektroda tegangan (P1 dan P2), dari besarnya arus dan beda potensial yang terukur maka nilai resistivitas dapat dihitung menggunakan persamaan:Dengan k adalah faktor geometri yang tergantung penempatan elektrodapermukaan.Sumber: Penggunaan Metode Geolistrik Untuk Mendeteksi Keberadaan Air Tanah . Eva Rolia . TAPAK Vol. 1 No. 1 Nopember /11/2012
191 Metode Geolistrik Resistivitas Pendugaan potensi air tanah menggunakan Metode Geolistrik untuk mengetahui nilai resistivitas batuan dan menentukan potensi atau kandungan air tanah. Dari nilai resistivitas batuan dan potensi atau kandungan air tanah, maka akan diketahui adanya air tanah di Suatu lokasi.Metode geolistrik resistivitas atau tahanan jenis adalah salah satu dari kelompok metode geolistrik yang digunakan untuk mempelajari keadaan bawah permukaan dengan cara mempelajari sifat aliran listrik di dalam batuan di bawah permukaan bumi. Metode resistivitas umumnya digunakan untuk eksplorasi dangkal, sekitar 300 – 500 m. Prinsip dalam metode ini yaitu arus listrik diinjeksikan ke alam bumi melalui dua elektrode arus, sedangkan beda potensial yang terjadi diukur melalui dua elektrode potensial. Dari hasil pengukuran arus dan beda potensial listrik dapat diperoleh variasi harga resistivitas listrik pada lapisan di bawah titik ukur.Diunduh dari: …… 13/11/2012
192 GEOLISTRIK.Geolistrik merupakan metode geofisika yang cukup efektif untuk digunakan dalam mendeteksi keberadaan air tanah dengan memanfaatkan sufat batuan yang mampu mengalirkan arus listrik.Geolistrik merupakan alat alternatif yang dapat digunakan dalam kegiatan teknik sipil untuk mengetahui lapisan tanah di dalam bumi, selain dengan menggunakan metode hand bor, sondir, dan metode lain dalam ilmu teknik sipil.Geolistrik memiliki cara kerja yang efisien karena mudah dioperasikan, mudah dibawa, murah, dan akurasi data yang dapat diandalkan.Sumber: Penggunaan Metode Geolistrik Untuk Mendeteksi Keberadaan Air Tanah . Eva Rolia . TAPAK Vol. 1 No. 1 Nopember /11/2012
193 METODE GEOLISTRIKMetoda geolistrik adalah salah satu metoda geofisika yg didasarkan pada penerapan konsep kelistrikan pada masalah kebumian. Tujuannya adalah untuk memperkirakan sifat kelistrikan medium atau formasi batuan bawah-permukaan terutama kemampuannya untuk menghantarkan atau menghambat listrik (konduktivitas atau resistivitas). Aliran listrik pada suatu formasi batuan terjadi terutama karena adanya fluida elektrolit pada pori-pori atau rekahan batuan. Oleh karena itu resistivitas suatu formasi batuan bergantung pada porositas batuan serta jenis fluida pengisi pori-pori batuan tsb. Batuan porous yg berisi air atau air asin tentu lebih konduktif (resistivitas-nya rendah) dibanding batuan yg sama yg pori-porinya hanya berisi udara (kosong).Diunduh dari: 13/11/2012
194 METODE GEOLISTRIKTemperatur tinggi akan lebih menurunkan resitivitas batuan secara keseluruhan karena meningkatnya mobilitas ion-ion penghantar muatan listrik pada fluida yg bersifat elektrolit. Cara kerja metoda geolistrik secara sederhana dapat dianalogikan dengan rangkaian listrik. Jika arus dari suatu sumber dialirkan ke suatu beban listrik (misalkan kawat seperti terlihat pada gambar) maka besarnya resistansi R dapat diperkirakan berdasarkan besarnya potensial sumber dan besarnya arus yg mengalir.Dalam hal ini besaran resistansi tidak dapat digunakan untuk memperkirakan jenis material karena masih bergantung ukuran atau geometri-nya. Untuk itu digunakan besaran resistivitas yg merupakan resistansi yg telah dinormalisasi terhadap geometri.Dalam prakteknya pengukuran geolistrik dilakukan dengan mengalirkan arus ke dalam tanah melalui 2 elektroda (C1 dan C2) dan respons-nya (beda potensial) diukur melalui 2 elektroda yg lain (P1 dan P2).Berdasarkan konfigurasi elektroda dan respons yg terukur maka sifat kelistrikan medium bawah-permukaan tersebut dapat diperkirakan.Diunduh dari: 13/11/2012