PERILAKU AIR DALAM TANAH

1. PERILAKU AIR DALAM TANAH IEKE 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. Secaraumum, lengastanahadalah air yang ditahandalamruangporitanah. 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. Diunduhdari: http://wwwghcc.msfc.nasa.gov/landprocess/lp_home.html …… 11/11/2012

3. Soil moisture – Lengas Tanah Lengastanahmerupakan air yang idtahandalamporitanahdalam zone perakarantanaman, biasanyadalamprofiltanahhinggakedalaman 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. Diunduhdari: http://en.wikipedia.org/wiki/Water_storage…… 11/11/2012

4. SOIL WATER CONTENT – Kadar Air (Lengas) Tanah Kadar air tanah (lengastanah) adalahjumlah air yang adadidalamtanah. 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. Diunduhdari: http://en.wikipedia.org/wiki/Water_content …… 11/11/2012

5. KADAR LENGAS TANAH The water content in soil is also known as moisture content and can be expressed as w = 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 2216-92 consists of determining the mass of the wet soil specimen and then drying the soil in an oven 12 - 16 hours at a temperature of 110oC.  Diunduhdari: http://www.engineeringtoolbox.com/soil-water-content-d_1643.html …… 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 (1899-1963) 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.  Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html…… 11/11/2012

7. NERACA AIR The 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.  Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

8. NERACA AIR Actual 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 occur Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

9. NERACA AIR Potential 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. Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

10. NERACA AIR Soil 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. Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

11. NERACA AIR Deficit (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. Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

12. NERACA AIR Surplus (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.  Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

13. NERACA AIR Computing a Soil - Moisture Budget The 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.  Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

14. NERACA AIR Water Budget - Rockford, ILField Capacity = 90 mm Water Budget - Rockford, ILField Capacity = 90 mm Water Budget - Rockford, ILField Capacity = 90 mm Water Budget (location of Rockford, Illinois). Field Capacity = 90 mm Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

15. NERACA AIR Soil Moisture Recharge - Rockford, IL Field Capacity = 90 mm Soil Moisture Recharge . Field Capacity = 90 mm Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

16. NERACA AIR Soil Moisture Recharge - Rockford, IL Field Capacity = 90 mm We'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. Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 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). Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

18. NERACA AIR Soil Moisture Surplus . Field Capacity = 90 mm Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

19. Surplus Lengas Tanah 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). 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). …. Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

20. NERACA AIR Soil Moisture Utilization. Field Capacity = 90 mm Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 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. Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

22. NERACA AIR Soil Moisture Deficit Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

23. Soil Moisture Deficit August, 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).…. Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

24. Soil Moisture Seasons Four soil moisture seasons can be defined by the soil moisture conditions. Recharge The 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. Surplus The 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. Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

25. NERACA AIR 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). Deficit The 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 surplus Diunduhdari: http://www4.uwsp.edu/geo/faculty/ritter/geog101/textbook/hydrosphere/water_balance_1.html …… 11/11/2012

26. AIR DALAM TANAH

27. STRUKTUR & CIRI H2O Molekul air terdiri atas atom oksigen dan dua atom hidrogen, yang berikatan secara kovalen Atom-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 H 105o Zone 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 irigasi Penguapan Serapan bulu akar

29. SISTEM TANAH-TANAMAN Structure 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 TANAH Kekuatan 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 tanah Kohesi Adesi H2O Partikel tanah Air terikat Air bebas

31. Air Tersediauntukpertumbuhantanaman

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 Tanah Perubahan status air dalam tanah, mulai dari kondisi jenuh hingga titik layu Jenuh Kap. Lapang Titik layu Padatan Pori 100g air 40g tanah jenuh air 100g 20g udara kapasitas lapang 100g 10 g udara koefisien layu 100g 8g udara koefisien higroskopis

34. TEGANGAN & KADAR AIR PERHATIKANLAH proses yang terjadi kalau tanah basah dibiarkan mengering. Bagan berikut melukiskan hubungan antara tebal lapisan air di sekeliling partikel tanah dengan tegangan air Bidang singgung tanah dan air Koef. Koef. Kapasitas padatan tanah higroskopis layu lapang 10.000 atm 31 atm 15 atm 1/3 atm 10.000 atm Mengalir krn gravitasi Tegangan air 1/3 atm tebal lapisan air

35. Representasi bola air yang menyelubungi partikel padatan tanah

36. JUMLAH AIR DALAM 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. 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. TEGANGAN vs kadar air Kurva tegangan - kadar air tanah bertekstur lempung Air kapiler Air Air tersedia higros- kopis Lambat tersedia Cepat tersedia Air gravitasi Zone optimum Tegangan air, bar 31 Koefisien higroskopis Koefisien layu Kapasitas lapang 0.1 Kap. Lapang maksimum persen air tanah

38. Air tersediabagitanaman Air Gravitasi Tegangan air tanah (bar / atm Titik Layu Kapasitaslapang Kadar air volumetrik, % Hubunganantarakadar air tanahdantegangan air tanahuntukteksturlempung

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 tgt pd rapat muatan pd per- mukaan kation. Rapat muatan = Selubung air muatan kation / luas permukaan

40. STRUKTUR & CIRI IKATAN HIDROGEN Atom hidrogen berfungsi sebagai titik penyambung (jembatan) antar molekul air. Ikatan hidrogen inilah yg menyebabkan titik didih dan viskositas air relatif tinggi KOHESI vs. ADHESI Kohesi: ikatan hidrogen antar molekul air Adhesi: ikatan antara molekul air dengan permukaan padatan lainnya Melalui kedua gaya-gaya ini partikel tanah mampu menahan air dan mengendalikan gerakannya dalam tanah TEGANGAN PERMUKAAN Terjadinya pada bidang persentuhan air dan udara, gaya kohesi antar molekul air lebih besra daripada adhesi antara air dan udara. Udara Permukaan air-udara air

41. ENERGI AIR TANAH Retensi 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 berpengaruh Gaya matrik: tarikan padatan tanah (matrik) thd molekul air; Gaya osmotik: tarikan kation-kation terlarut thd molekul air Gaya gravitasi: tarikan bumi terhadap molekul air tanah. Potensial air tanah Ketiga 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 bebas Pt = 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 positif Energi bebas dari air murni Potensial tarikan bumi 0 Potensial osmotik (hisapan) Menurun karena pengaruh osmotik Poten-sial negatif Potensial matrik (hisapan) Menurun karena pengaruh matrik - Energi bebas dari air tanah

43. POTENSIAL AIR TANAH POTENSIAL TARIKAN BUMI = Potensial gravitasi Pg = G.h dimana 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 irigasi Potensial matrik dan Osmotik Potensial matrik merupakan hasil dari gaya-gaya jerapan dan kapilaritas. Gaya jerapan ditentukan oleh tarikan air oleh padatan tanah dan kation jerapan Gaya kapilaritas disebabkan oleh adanya tegangan permukaan air. Potensial matriks selalu negatif Potensial osmotik terdapat pd larutan tanah, disebabkan oleh adanya bahan-bahan terlarut (ionik dan non-ionik). Pengaruh utama potensial osmotik adalah pada serapan air oleh tanaman Hisapan dan Tegangan Potensial 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 Atmosfer kolom air (cm) tinggi kolom air (pF) 10 1 0.01 0.0097 100 2 0.1 0.0967 346 2.53 0.346 1.3 1000 3 1 10000 4 10 9.6749 15849 4.18 15.8 15 31623 4.5 31.6 31 100.000 5 100 96.7492

45. KANDUNGAN AIR DAN TEGANGAN KURVA ENERGI - LENGAS TANAH Tegangan air menurun secara gradual dengan meningkatnya kadar air tanah. Tanah liat menahan air lebih banyak dibanding tanah pasir pada nilai tegangan air yang sama Tanah yang Strukturnya baik mempunyai total pori lebih banyak, shg mampu menahan air lebih banyak Pori medium dan mikro lebih kuat menahan air dp pori makro Tegangan air tanah, Bar 10.000 Liat Lempung Pasir 0.01 10 Kadar air tanah, % 70

46. Teksturtanahdan air tersedia

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 air Lapisan 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 penghalang Lempung berpasir Lempung berliat cm 0 15 mnt 4 jam 30 60 90 1 jam 24 jam 120 24 jam 48 jam 150 30 cm 60 cm Jarak dari tengah-tengah saluran, cm