NERACA LENGAS TANAH

1. NERACA LENGAS TANAH Oleh: IekeW. Ayu, S. PriyonodanSoemarno psl-ppsubnopember 2012

2. LENGAS TANAH • Definisi: • Air yang disimpandalamtanah. • Salahsatufaktor yang sangatpentingdalamprosespedologisdanpertumbuhantanaman. • Adatigamacambentuklengastanah: • Water adhering in thin films by molecular attraction to the surface of soil particles and not available for plants is termed hygroscopic water. • Water forming thicker films and occupying the smaller pore spaces is termed capillary water. Since it is held against the force of gravity it is permanently available for plant growth and it is this type of soil water which contains plant nutrients in solution. • Water in excess of hygroscopic and capillary water is termed gravitational water, which is of a transitory nature because it flows away under the influence of gravity. When the excess has drained away the amount of water retained in the soil is termed its field capacity, when some of its pore spaces are still free of water. • (Source: LANDY / DUNSTE) Diunduh dari: ….. 15/11/2012

3. SOIL WATER BALANCE Plant production involves CO2 intake through stomatal openings in the epidermis. Most water that plants take up from the soil is again lost to the atmosphere by transpiration through the same openings. The daily turnover can be considerable: transpiration from 0.4 cm of water from a crop surface on a clear sunny day corresponds with a water loss from the root zone of than 40.000 kg ha-1 d-1. If soil moisture uptake by the roots is not replenished, the soil will dry out to such an extent that the plants wilt and - ultimately- die. The tenacity with which the soil retains its water is equalled by the suction which roots must exert to be able to take up soil moisture. This suction known as the soil moisture potential or 'matrix suction', can be measured. In hydrology, the potential is usually used and is expressed as energy unit per weight of soil water, with the dimension of length (van Bakel, 1981). An optimum range exists within which the plant takes up water freely. Above or below this level the plant senses stress; it reacts by actively curbing its daily water consumption through partial or complete closure of the stomata. The consequence is evident: this stomatal closure interferes with CO2 intake and reduces assimilation and dry matter production consequently. A crop growth simulation model must therefore keep track of the soil moisture potential to determine when and to what degree a crop is exposed to water stress. This is commonly done with the aid of a water balance equation, which compares for a given period of time, incoming water in the rooted soil with outgoing water and quantifies the difference between the two as a change in the soil moisture amount stored. Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

4. PERSAMAAN NERACA LENGAS TANAH Soil water balance, like a financial statement of income and expenditure, is an account of all quantities of water added, removed or stored in a given volume of soil during a given period of time. The soil water balance equation thus helps in making estimates of parameters, which influence the amount of soil water. Using the soil water balance equation, one can identify periods of water stress/excesses which may have adverse affect on crop performance. This identification will help in adopting appropriate management practices to alleviate the constraint and increase the crop yields. The amount of water in a soil layer is determined by those factors that add water to the soil and those factors that remove water from it. The soil water balance equation in its simplest form of expression is: Perubahan air dalam tanah = Input air – Kehilangan air Diunduh dari: http://www.icrisat.org/what-we-do/learning-opportunities/lsu-pdfs/Soil%20Water%20Balance%20for%20Crop%20Land.pdf….. 12/11/2012

5. PENAMBAHAN AIR KE TANAH Water is usually added to the soil in three measurable ways - precipitation (P), irrigation (I), and contribution from the ground-water table (C). The contribution from the ground water will be significant only if the ground-water table is near the surface. So, the inputs of water can be presented as: Input Air = P + I + C Kehilangan Air dari tanah: Water is removed from the soil through evaporation from soil surface or transpiration through plant together known as evapotranspiration (ET), and deep drainage (D). Further, a part of the rain water received at the soil surface may be lost as surface run-off (RO). The above three factors are negative factors in the equation. The losses of water from soil can then be represented by the following equation. Kehilangan Air = ET + D + RO Diunduh dari: http://www.icrisat.org/what-we-do/learning-opportunities/lsu-pdfs/Soil%20Water%20Balance%20for%20Crop%20Land.pdf….. 12/11/2012

6. NERACA LENGAS TANAH The change in the soil water content which is the difference between the water added and water withdrawn will now read: Change in Soil water = (P + I + C) - (ET + D + RO) Soil water refers to the amount of water held in the root zone at a given time. This amount can be measured. The change in soil water from one measurement to another depends on the contribution of components in the equation. Suppose the amount of water in the root zone at the beginning is M1 mm and at the end of a given period is M2 mm, thus the equation is expressed as : M1 - M2 = P + I + C - ET - D - RO or M1 + P + I + C = ET + D + RO + M2 With the help of this equation one can compute any one unknown parameter in the equation if all others are known. The quantitative data on rainfall (P) evapotranspiration (ET), deep drainage (D) and soil moisture at a given time (M1 or M2) for different locations and for different practices are useful for selecting appropriate water-management strategies. Diunduh dari: http://www.icrisat.org/what-we-do/learning-opportunities/lsu-pdfs/Soil%20Water%20Balance%20for%20Crop%20Land.pdf….. 12/11/2012

7. PENGHITUNGAN NERACA LENGAS TANAH Let us work a few examples using the Soil Water Balance Equation to appreciate the usefulness of this model. Contoh 1: Soil = Vertisol Crop = Sorghum Period = 01 to 31 Aug Area = 2 ha Given: Soil moisture in the profile # on Aug 01 (M1) = 300 mm Precipitation or Rainfall (P) = 70 mm Irrigation (I) = Nil Contribution from ground water (C) = Nil Run-off of 200 cubic m from 2 ha field (R) = 10 mm Deep drainage (D) = Nil Soil moisture in the profile on Aug 31 (M2) = 250 mm Estimate evapotranspiration (ET) from the field during 01 to 31 Aug. Equation: M1 + P + I + C = ET + D + RO +M2 300 +70 + 0 + 0 = ET + 0 + 10 + 250 ET = 370 mm - 260 mm = 110 mm Thus, evapotranspiration which is difficult to be measured could be estimated using the Soil Water Balance Equation. Diunduh dari: http://www.icrisat.org/what-we-do/learning-opportunities/lsu-pdfs/Soil%20Water%20Balance%20for%20Crop%20Land.pdf….. 12/11/2012

8. PENGHITUNGAN NERACA LENGAS TANAH Contoh 2: Soil = Alfisol Crop = Millet Area=1 ha Period = 10 June (sowing date) to 30 Sept (harvesting) given: Soil moisture in the profile on Jun 10 (M1) = 150 mm Precipitation or Rainfall (P) = 600 mm Irrigation (I) = Nil Contribution from ground water (C) = Nil Evapotranspiration (estimated) (ET) = 530 mm Run-off of 200 cubic m from 1 ha field (RO) = 70 mm Soil moisture in the profile on Sep 30 (M2) = 60mm Estimate: Deep drainage (D) losses from the field during crop period. Equation: M1 + P + I + C = ET + D + RO + M2 150 + 600+ 0 + 0 = 530 + D + 70 + 60 D = 750 mm - 660 mm = 90 mm Thus, deep drainage (D) losses in the field which is not easy to measure could be estimated using the Soil Water Balance Equation. We hope that this lesson and the examples have helped you in understanding and computing the various components of the Soil Water Balance Equation. For more detailed treatment please refer any standard textbook on soil physics. Diunduh dari: http://www.icrisat.org/what-we-do/learning-opportunities/lsu-pdfs/Soil%20Water%20Balance%20for%20Crop%20Land.pdf….. 13/11/2012

9. NERACA LENGAS TANAH Evapotranspiration can also be determined by measuring the various components of the soil water balance. The method consists of assessing the incoming and outgoing water flux into the crop root zone over some time period (Figure 6). Irrigation (I) and rainfall (P) add water to the root zone. Part of I and P might be lost by surface runoff (RO) and by deep percolation (DP) that will eventually recharge the water table. Water might also be transported upward by capillary rise (CR) from a shallow water table towards the root zone or even transferred horizontally by subsurface flow in (SFin) or out of (SFout) the root zone. In many situations, however, except under conditions with large slopes, SFin and SFout are minor and can be ignored. Soil evaporation and crop transpiration deplete water from the root zone. If all fluxes other than evapotranspiration (ET) can be assessed, the evapotranspiration can be deduced from the change in soil water content (D SW) over the time period: ET = I + P - RO - DP + CR ± D SF ± D SW Diunduh dari: http://www.fao.org/docrep/X0490E/x0490e04.htm….. 13/11/2012

10. NERACA LENGAS TANAH The estimation of Ks requires a daily water balance computation for the root zone. The root zone can be presented by means of a container in which the water content may fluctuate. The daily water balance, expressed in terms of depletion at the end of the day is: Dr, i = Dr, i-1 - (P - RO)i - Ii - CRi + ETc, i + DPi where : Dr, i root zone depletion at the end of day i [mm], Dr, i-1 water content in the root zone at the end of the previous day, i-1 [mm], Pi precipitation on day i [mm], ROi runoff from the soil surface on day i [mm], Ii net irrigation depth on day i that infiltrates the soil [mm], CRi capillary rise from the groundwater table on day i [mm], ETc, i crop evapotranspiration on day i [mm], DPi water loss out of the root zone by deep percolation on day i [mm]. Diunduh dari: http://www.fao.org/docrep/x0490e/x0490e0e.htm….. 13/11/2012

11. …NERACA LENGAS TANAH Transpor air dalam profil tanah Diunduh dari: http://www.sckcen.be/fr/Media/Images/Our-Research/RD-disposal-of-waste/%28offset%29/40….. 13/11/2012

12. MODEL NERACA LENGAS TANAH Soil-water balance is an accounting procedure for near-surface soil-moisture. It is included in a class of models known as “bucket” models : Properties are averaged for each drainage basin , for example, a watershed has a single water holding capacity Changes in soil moisture are calculated using a mass balance Historical precipitation is known Evaporation can be estimated using the temperature-based (Hargreaves) equation . Runoff is dependent upon soil moisture and precipitation Diunduh dari: http://www.ce.utexas.edu/prof/maidment/grad/martinez/seminar/sld008.htm ….. 13/11/2012

13. MODEL GROUNDWATER The groundwater model is a lumped-parameter type Portions of the formation fed by different catchment areas are assumed to have uniform storage and transmissivity parameters Properties are also averaged over depth This is different from finite element models like MODFLOW or GMS where there is an analysis grid consisting of a large number of cells Recharge functions : Simulate interaction between surface water and groundwater Recharge to aquifer is mostly from channel losses in streams that flow over outcrop Empirical functions were developed by plotting inflows against known recharge determined by USGS Inflow is the stream flow at a gauge above the recharge zone plus intervening runoff; intervening runoff is calculated using an areal scaling ratio assuming that conditions are the same for catchment and intervening regions Used historical pumping data Diunduh dari: http://www.ce.utexas.edu/prof/maidment/grad/martinez/seminar/sld010.htm ….. 13/11/2012

14. WATERBANK: MODEL NERACA LENGAS TANAH Diunduh dari: http://www.dpi.nsw.gov.au/archive/agriculture-today-stories/december-2009/more-time-at-or-below-wilting-point/waterbank ….. 13/11/2012

15. SOIL WATER BALANCE (SWB) The components of the soil water balance (SWB) are depicted in Figure . The water balance can be summarised as in equation :ÄSm = Sm + P – T – E – I – R – D The change in the soil water content (ÄSm) over a period of time depends on the original water content (Sm) plus precipitation (rain and irrigation, P); minus transpiration (loss of water by plants, T); minus evaporation (loss of water from the soil surface, E); minus interception (water held in the plant canopy, I); minus runoff (surface water not penetrating the soil and running away, R) ; minus drainage (water draining away below root zone, D). Diunduh dari: http://www.senwes.co.za/Files/main_Scenario/archive_articles/2006/2006-12-01_effective_irrigation_soil_water_balance.htm ….. 13/11/2012

16. . HASIL-HASIL NERACA AIR The combined results from the water balances have been used to assess how much water systems have; how much is stored; what the variability factors are; and what the connections between resources are. A discussion of the relative contribution of the main components of the balances is provided. Diunduh dari: http://www.water.gov.au/WaterAvailability/Waterbalanceassessments/Waterbalanceresults/index.aspx?Menu=Level1_3_2_3….. 13/11/2012

17. MODEL NERACA LENGAS TANAH In large canal irrigation project areas, integrated management of surface and groundwater resources can improve water use efficiencies and agricultural productivity and also control water logging. Such integrated management requires an estimation of spatial distribution of recharge and ground water flow in the underlying aquifer. Recharge occurs both as percolation losses from fields and seepage losses from the water distribution network. Diunduh dari: http://www.sciencedirect.com/science/article/pii/S0378377403001446….. 13/11/2012

18. NERACA LENGAS TANAH A soil water balance model, if designed to adequately represent the physical processes involved, and if carried out with a short enough (daily) time step, can provide realistic estimates of deep drainage (potential recharge) over long periods. The single store (single layer) mass water balance model applicable to semi-arid areas, which recognises the wetting of the near surface during rainfall, with subsequent availability of water for evaporation and transpiration in the days following rainfall. The model allows for the major hydrological processes taking place at or near the soil-vegetation surface including runoff. Diunduh dari: http://www.sciencedirect.com/science/article/pii/S0016706107000924….. 13/11/2012

19. Schematization of the soil profile in the AWC calculation of the CERU32 program (Le Bas et al., 1997) The soil profile is schematized in three layers (Le Bas et al., 1997): A worked surface layer, defined by ploughing depth. The available water for this layer corresponds to the total available water (water volume between –1500 kPa suction (wilting point) and –5 kPa (field capacity)) estimated by rules using the topsoil input variables. • A subsurface layer, between the ploughing depth and the EAW (Easily Available Water) depth. The latter is user defined. The available water for this layer corresponds to the total available water (water volume between –1500 kPa suction (wilting point) and –5 kPa (field capacity)) estimated by rules valid for the subsoil, using the subsoil input variables. But if the soil layer is less than the depth to textural change the rules for subsoil uses topsoil input variables. • A deeper layer, between the EAW depth and the maximum rooting depth. The available water for this layer corresponds to the easily available water (water volume between –200 kPa suction and –5 kPa (field capacity)) estimated by rules valid for the subsoil, using the subsoil input variables. But if the soil layer is less than the depth to textural change the rules for subsoil uses topsoil input variables. Variabel Topsoil Variabel Subsoil Kedalaman Akar Diunduh dari: http://eusoils.jrc.ec.europa.eu/projects/sinfo/5_2_1_en.htm….. 13/11/2012

20. PERHITUNGAN NERACA LENGAS TANAH Therefore the purpose of soil water balance calculations is to estimate daily value of the actual soil moisture content, which influences soil moisture uptake and crop transpiration. Schematic representation of the different components of a soil water balance Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

21. …KANDUNGAN LENGAS TANAH AKTUAL Actual soil moisture content can be established according to (Driessen, 1986): Where: where : qt : Actual moisture content of the root zone at time step t [cm3 cm-3] Nup : Rate of net influx through the upper root zone boundary [cm d-1] INlow : Rate of net influx through the lower root zone boundary [cm d-1] Ta : Actual transpiration rate of crop [cm d-1] RD : Actual rooting depth [cm] P : Precipitation intensity [cm d-1] Ie : Effective daily irrigation [cm d-1] Es : Soil evaporation rate [cm d-1] SSt : Surface storage [cm] SR : Rate of surface runoff [cm d-1] CR : Rate of capillary rise [cm d-1] Perc : Percolation rate [cm d-1] Dt : Time step [d] Zt : Depth of groundwater table [cm] Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

22. LENGAS TANAH ZONE PERAKARAN The processes directly affecting the root zone soil moisture content can be defined as: Infiltration: i.e. transport from the soil surface into the root zone; Evaporation: i.e. the loss of soil moisture to the atmosphere; Plant transpiration: i.e. loss of water from the interior root zone; Percolation: i.e. downward transport of water from the root zone to the layer below the root zone; Capillary rise: i.e. upward transport into the rooted zone. The textural profile of the soil is conceived homogeneous. Initially the soil profile consists of three layers (zones): The rooted zone between soil surface and actual rooting depth The lower zone between actual rooting depth and maximum rooting depth The subsoil below maximum rooting depth Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

23. SISTEM SWB TANPA EFEK GROUNDWATER The variables of the soil water balance in the actual water-limited production situation are calculated for freely draining soil. No influence from groundwater is assumed and crop water requirements for continuous growth with either drought stress or water excess and a possible reduction of the crop transpiration rate, leading to a reduced growth are quantified. Submodel Lengas Tanah For the rooted zone the water balance equation is solved every daily time step. The water balance is driven by rainfall, possibly buffered as surface storage, and evapotranspiration. The processes considered are infiltration, soil water retention, percolation and the loss of water beyond the maximum root zone. At the upper boundary, processes comprise infiltration of water from precipitation or irrigation, evaporation from the soil surface and crop transpiration. If the rainfall intensity exceeds the infiltration and surface storage capacity of the soil, water runs off. Water can be stored in the soil till the field capacity is reached. Additional water percolates beyond the lower boundary of the rooting zone. Flow rates are limited by the maximum percolation rate of the root zone and the maximum percolation rate to the subsoil. Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

24. SISTEM SWB TANPA EFEK GROUNDWATER The variables of the soil water balance in the actual water-limited production situation are calculated for freely draining soil. No influence from groundwater is assumed and crop water requirements for continuous growth with either drought stress or water excess and a possible reduction of the crop transpiration rate, leading to a reduced growth are quantified. SubmodelLengas Tanah The textural profile of the soil is conceived homogeneous. Initially the soil profile consists of three layers (zones): the rooted zone between soil surface and actual rooting depth the lower zone between actual rooting depth and maximum rooting depth the subsoil below maximum rooting depth Root zone extension from initial rooting depth to maximum rooting depth. Its effect on the soil moisture content is accounted for in this soil water balance calculation. From the moment that the maximum rooting depth is reached the soil profile is described as a two layer system (Driessen, 1986). The lower zone no longer exists. As mentioned earlier, no groundwater influence is assumed and capillary rise is not accounted for. Only downward flow, evaporation from the soil surface and transpiration are estimated. Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

25. …Kandungan Lengas Tanah Initial The initial value of the actual soil moisture content in the root zone can be calculated as: Where qt : Actual soil moisture content in rooted zone [cm3 cm-3] qwp : Soil moisture content at wilting point [cm3 cm-3] Wav : Initial available soil moisture amount in excess of qwp [cm] RD : Actual rooting depth (see Section 5.6.) [cm] The initial actual soil moisture content, qt, cannot be lower than the soil moisture content at wilting point. In case the crop cannot develop airducts, the initial soil moisture content cannot be higher than the soil moisture content at field capacity. If the crop can develop airducts the initial soil moisture content cannot exceed the soil porosity. Wav, the initial available soil moisture amount in excess of qwp should be provided by the user. Multiplying the actual soil moisture content with the rooting depth yields the initial water amount in the rooted zone. Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

26. …LENGAS TANAH DI BAGIAN ZONE BAWAH . The initial amount of soil moisture in the zone between the actual rooting depth and the maximum rooting depth (i.e. lower zone), can be calculated as: Where Wlz : Soil moisture amount in the lower zone [cm] Wav : Initial available soil moisture amount in excess of qwp [cm] RDmax : Maximum rooting depth [cm] RD : Actual rooting depth [cm] qt : Actual soil moisture content in rooted zone [cm3 cm-3] qwp : Soil moisture content at wilting point [cm3 cm-3] The soil moisture content of the lower zone is also limited by the field capacity in case the crop cannot develop airducts, else the soil moisture content is limited by the soil porosity. Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

27. …EVAPORASI . Evaporation depends on the available soil water and the infiltration capacity of the soil. If the water layer on the surface, the so called surface storage, exceeds 1 cm, the actual evaporation rate from the soil is set to zero and the actual evaporation rate from the surface water is equal to the maximum evaporation from a shaded water surface. If the surface storage is less than 1 cm and the infiltration rate of the previous day exceeds 1 cm d-1, the actual evaporation rate from the surface water is set to zero and the actual evaporation rate from the soil is equal to the maximum evaporation from a shaded soil surface. All water on the surface can infiltrate within one day. The value of the variable days since last rain, Dslr, is reset to unity. If the infiltration rate is less than 1 cm d-1, the amount of infiltrated water is considered too small to justify a reset of the parameter Dslr and the evaporation rate decreases as the top soil starts drying. The reduction of the evaporation is thought to be proportional to the square root of time (Stroosnijder, 1987, 1982). The evaporation can be calculated as: where Es : Evaporation rate from a shaded soil surface [cm d-1] Es,max : Maximum evaporation rate from a shaded soil surface (see eq. 6.7) [cm d-1] Dslr : Days since last rain [d] When a small water amount has infiltrated, or rather wetted the soil surface, this amount can be evaporated the same day, irrespective of Dslr. Therefore, the actual evaporation from the soil surface, as calculated according to equation 6.20, should be corrected for this water amount infiltrating the soil. This amount should be added to the actual evaporation rate. However, it should be noted that the actual evaporation can never exceed the maximum evaporation rate. Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

28. …PRESIPITASI Not all precipitation will reach the surface. A fraction will be intercepted by leaves, stems, etc. From the amount of precipitation which reaches the soil surface, a part runs off. Runoff from a field can be 0-20 percent, and even higher on unfavorable surfaces (Stroosnijder & Koné, 1982). It can be assumed that a fixed fraction of the precipitation will not infiltrate during that particular day. This fraction can be reduced in situations with relatively small amounts of rainfall. The reduction factor is defined as function of the rainfall amount (van Diepen et al., 1988). Note that the non infiltrating fraction refers to rainfall only. Irrigation water is assumed to infiltrate freely. Reduction factor of the non infiltrating fraction as a function of rainfall. Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

29. …PERKOLASI If the root zone soil moisture content is above field capacity, water percolates to the lower part of the potentially rootable zone and the subsoil. A clear distinction is made between percolation from the actual rootzone to the so-called lower zone, and percolation from the lower zone to the subsoil. The former is called Perc and the latter is called Loss. The percolation rate from the rooted zone can be calculated as: where Perc : Percolation rate from the root zone to the lower zone [cm d-1] Wrz : Soil moisture amount in the root zone [cm] Wrz,fc : Equilibrium soil moisture amount in the root zone [cm] Dt : Time step [d] Ta : Actual transpiration rate [cm d-1] Es : Evaporation rate from a shaded soil surface [cm d-1] The equilibrium soil moisture amount in the root zone can be calculated as the soil moisture content at field capacity times the depth of the rooting zone: where Wrz,fc : Equilibrium soil moisture amount in the root zone [cm] qfc : Soil moisture content at field capacity [cm3 cm-3] RD : Actual rooting depth [cm] Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

30. …LAJU PERKOLASI . The percolation rate and infiltration rate are limited by the conductivity of the wet soil, which is soil specific and should be given by the user. Note that the percolation from the root zone to the lower zone can be limited by the uptake capacity of the lower zone. Therefore, the value calculated with equation 6.21 is preliminary and the uptake capacity should first be checked. The percolation from the lower zone to the subsoil, the so-called Loss, should take the water amount in the lower zone into account. If the water amount in the lower zone is less than the equilibrium soil moisture amount, a part of the percolating water will be retained and the percolation rate will be reduced. Water loss from the lower end of the maximum root zone can be calculated as: where Loss : Percolation rate from the lower zone to the subsoil [cm d-1] Perc : Percolation rate from root zone to lower zone [cm d-1] Wlz : Soil moisture amount in the lower zone [cm] Wlz,fc : Equilibrium soil moisture amount in the lower zone [cm] Dt : Time step Water loss from the potentially rootable zone, is also limited by the maximum percolation rate of the subsoil, which is soil specific and should be provided by the user. The equilibrium soil moisture amount in the lower zone can be calculated as the soil moisture content at field capacity times the root zone depth: where Wrz,fc : Equilibrium soil moisture amount in the lower zone [cm] qfc : Soil moisture content at field capacity [cm3 cm-3] RDmax : Maximum rooting depth [cm] RD : Actual rooting depth [cm] Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

31. …SIMPANAN LENGAS TANAH . For rice an additional limit of five percent of the saturated soil conductivity is set to account for puddling (a rather arbitrary value, which may be easily changed in the program). The saturated soil conductivity and is calculated using equating with pF= -1.0 (i.e. a hydraulic head of 0.1 cm). The percolation rate from the lower zone to the sub soil is not to exceed this value (van Diepenet al., 1988). As mentioned before, the value calculated with equation, should be regarded as preliminary; the storage capacity of the receiving layer may become limiting. The storage capacity of the lower zone, also called the uptake capacity, is the amount of air plus the loss (van Diepenet al., 1988). It can de defined as: where UP : Uptake capacity of lower zone [cm d-1] RDmax : Maximum rooting depth [cm] RD : Actual rooting depth [cm] Wlz : Soil moisture amount in lower zone [cm] qmax : Soil porosity (maximum soil moisture) [cm3 cm-3] Dt : Time step [d] Loss : Percolation rate from the lower zone to the subsoil [cm d-1] Percolation to the lower part of the potentially rootable zone can not exceed the uptake capacity of the lower zone. Therefore the percolation rate is set equal to the minimum of the calculated percolation rate and the uptake. Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

32. …INFILTRASI AWAL The infiltration rate depends on the available water and the infiltration capacity of the soil. If the actual surface storage is less then or equal to 0.1 cm, the preliminary infiltration capacity is simply described as: where INp : Preliminary infiltration rate [cm d-1] FI : Maximum fraction of rain not infiltrating during time step t [-] CI : Reduction factor applied to FI as a function of the precipitation intensity [-] P : Precipitation intensity [cm d-1] Ie : Effective irrigation [cm d-1] SSt : Surface storage at time step t [cm] Dt : Time step [d] Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

33. LAJU INFILTRASI AWAL . The maximum fraction of rain not infiltrating during time step t, FI can be either set to a fixed value or assumed to be variable by multiplying FI with a precipitation dependent reduction factor CI which is maximum for high rainfall and will be reduced for low rainfall. The user should provide FI. The CI table is included in the model and is assumed to be fixed. The calculated infiltration rate is preliminary, as the storage capacity of the soil is not yet taken into account. If the actual surface storage is more than 0.1 cm, the available water which can potentially infiltrate, is equal to the water amount on the surface (i.e. supplied via rainfall/irrigation and depleted via evaporation): where INp : Preliminary infiltration rate [cm d-1] P : Precipitation intensity [cm d-1] Ie : Effective irrigation [cm d-1] Ew : Evaporation rate from a shaded water surface [cm d-1] SSt : Surface storage at time step t [cm] Dt : Time step [d] However, the infiltration rate is hampered by the soil conductivity and cannot exceed it. Soil conductivity is soil specific and should be given by the user. Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

34. …INFILTRASI YANG TERKOREKSI Total water loss from the root zone can now be calculated as the sum of transpiration, evaporation and percolation. The sum of total water loss and available pore space in the root zone define the maximum infiltration rate. The preliminary infiltration rate cannot exceed this value. The maximum possible infiltration rate is given by: where INmax : Maximum infiltration rate [cm d-1] qmax : Soil porosity (maximum soil moisture) [cm3 cm-3] qt : Actual soil moisture content [cm3 cm-3] RD : Actual rooting depth [cm] Dt : Time step [d] Ta : Actual transpiration rate [cm d-1] Es : Evaporation rate from a shaded soil surface  [cm d-1] Perc : Percolation rate from root zone to lower zone [cm d-1] Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

35. INFILTRASI Infiltrasi mencerminkan kecepatan meresapnya air ke dalam tanah melalui permukaan tanah. Semakin tinggi infiltrasi, semakin banyak air yang akan tersedia bagi tanaman dan semakin sedikit air runoff di permukaan tanah, semakin sedikit pula erosi dan pencucian unsur hara. Seresah tanaman, tumbuhan hidup, atau permuakan yang kasar, akan menghambat aliran air di permukaan tanah, sehingga air mempunyai kesempatan untuk meresap ke dalam tanah. Kerak tanah dapat mereduksi infiltrasi dan dapat diminimumkan dengan jalan membiarkan seresah tumbuhan tetap di permukaan tanah, memperbaiki kandungan bahan organik tanah, dan memacu aktivitas biologis. Diunduh dari: ….. 13/11/2012

36. PENGELOLAAN KAPASITAS LAPANG .. Beberapapraktekpengelolaantanah yang dapatmeningkatkankapasitaslapangdanmemperbaikiinfiltrasi: Pengelolaanbahanorganik. Bahanorganikdapatmeningkatkankemampuantanahmenyimpan air (water-holding capacity, WHC) melaluiduacara. Bahanorganikmampumenyimpandanmenahanbanyak air, dandapatmemperbaikistrukturtanah – meningkatkan total volume danukuranpori yang dapatmenyimpan air danmencegahpembentukankeraktanahdipermukaan. Praktekpengolahantanah. Membiarkanseresahsisapanendipermukaanatanahdapatmemperlambat runoff danmencegahpembentukankeraktanahdipermukaan. Seresahinidapatmendorongperkembanganpopulasicacingtanahdanorganisme lain yang membuatliangdalamtanah, dan air hujandapatdengancepatmeresapkedalamtanahmelaluilubang-lubangtersebut. Pencegahanpemadatan. Pemadatantanahdapatmereduksi WHC, karenaberkurangnyajumlahdanukuranporitanah. Pengendalianerosi. Erosi tanah dapat mereduksi kedalaman tanah (solum tanah menjadi tipis) dan menurunkan WHC. Diunduh dari: ….. 13/11/2012

37. KONSERVASI TANAH - INFILTRASI Praktekkonservasi yang dapatmemperburukinfiltrasi: Pembakarandanpengangkutansisa-sisapanen, membiarkantanahberadanpekaterhadaperosi Metodepengolahantanah yang merusakkoneksiporidenganpermukaantanah, danmencegahakumulasibahanorganiktanah Lalulintasperalatandanternak, terutamapadasaattabnahdalamkondisibawah, yang menyebabkanpemadatandanreduksiporositastanah. Beberapapraktekkonservasimembantumempertahankanataumemperbaikiinfiltrasi air kedalamtanah : Meningkatkantutupanvegetatifdipermukaantanah, Mengelolaresiduvegetatif, dan Meningkatkanbahanorganiktanah. Biasanya, praktek-praktekinimeminimumkangangguantanahdanpemadatantanah, melindungitanahdarierosi, danmendorongperkembanganstrukturtanah yang baikdanruangpori yang kontinyus. Sebagaisolusijangkapendekmengatasiburuknyainfiltrasiadalahmembongkarkerakpermukaandenganmembajaktanah, danlapisantanah yang kompakdapatdibongkardenganpengolahantanahsecaradalam. Diunduh dari: ….. 13/11/2012

38. MEMPERBAIKI INFILTRASI Praktekkonservasi yang memperbaikilajuinfiltrasi: Pergilirantanaman Tanamanpenutuptanah Grazing terkendali Pengelolaan residu dan seresah tanaman, serta Pengolahan tanah Pemanfaatansisapanen. Nilai Maksimum Laju Infiltrasi berbagai tipe tanah . (*) Asumsi tanaman penutup tanah penuh. Laju pada Tanah bera sebesar ½ dari laju pada tanah dengan tumbuhan penutup tanah penuh. Diunduh dari: ….. 13/11/2012

39. …LIMPASAN PERMUKAAN Surface runoff is also taken into account by defining a maximum value for surface storage. If the surface storage exceeds this value the exceeding water amount will run off. Surface storage at time step t can be calculated as: where SSt : Surface storage at time step t [cm d-1] P : Precipitation intensity [cm d-1] Ie : Effective irrigation rate [cm d-1] Ew : Evaporation rate from a shaded water surface [cm d-1] IN : Infiltration rate (adjusted) [cm d-1] Surface runoff can be calculated as: where SRt : Surface runoff at time step t [cm] SSt : Surface storage at time step t [cm] SSmax : Maximum surface storage [cm] SSmax is an environmental specific variable and should be provided by the user. Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

40. SURFACE RUNOFF Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the hydrologic cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source. If a nonpoint source contains man-made contaminants, the runoff is called nonpoint source pollution. A land area which produces runoff that drains to a common point is called a watershed. When runoff flows along the ground, it can pick up soil contaminants such as petroleum, pesticides, or fertilizers that become discharge or nonpoint source pollution. Diunduh dari: ….. 13/11/2012

41. LIMPASAN PERMUKAAN .FaktorMeteorologi yang mempengaruhi runoff: Tipepresipitasi (rain, snow, sleet, etc.) Intensitashujan Jumlahhujan Lamanyahujan Distribution of rainfall over the watershedS Direction of storm movement Antecedent precipitation and resulting soil moisture Other meteorological and climatic conditions that affect evapotranspiration, such as temperature, wind, relative humidity, and season. KarakteristikFisik yang mempengaruhi runoff: Land use Vegetasi Tipe Tanah Drainage area Bentukdaerahtangkapoan air Elevation Kemiringan Topography Direction of orientation Polaaliran Drainage Ponds, lakes, reservoirs, sinks, etc. in the basin, which prevent or alter runoff from continuing downstream. Diunduh dari: ….. 13/11/2012

42. RUNOFF & TIPE TANAH Kapasitas infiltrasi suatu tanah dipengaruhi oleh porositas tanah, yang menentukan kapasitas simpanan air dan mempengaruhi resistensi air untuk mengalir ke lapisan tanah yang lebih dalam. Porositas suatu tanah berbeda dengan tanah lainnya. Kapasitas infiltrasdi tertinggi dijumpai pada tanah-tanah yang gembur, tekstur berpasir; sedangkan tanah-tanah liat dan berliat biasanya mempunyai kapasitas infiltrasi lebih rendah. Bagan-bagan berikut menyajikan beragam kapasitas infiltrasi yang diukur pada berbagai tipe tanah. Kapasitas infiltrasi juga tergantung pada kadar lengas tanah pada akhir periode hujan sebelumnya. Kapasitas infiltrasi aweal yang tinggi dapat menurun dengan waktu (asalkan hujan tidak berhenti) hingga mencapai suatu nilai konstan pada saat profil tanah telah jenuh air. Kurva kapasitas infiltrasi untuk berbagai tipe tanah yang berbeda. Diunduh dari: ….. 13/11/2012

43. Koefisien Runoff Selain faktor-faktor yang bersifat spesifik-lokasi, perlu diperhatikan juga adalah homogenitas kondisi fisik daerah tangkapan air. Meskipun pada sekala mikro, ternyata juga ada variasi kemiringan, tipe tanah, vegetasi penutup dll. Oleh karena itu setiap daerah-tangkapan air mempunyai respon-runoff yang spesifik, dan respon ini juga akan tergantung pada ragam kejadian hujan. Disain sarana pemanenan air memerlukan pengetahuan tentang jumlah runoff yang akan dihasilkan oleh hujan dalam suatu daerah tangkapan. Biasanya diasumsikan bahwa volume runoff sebanding dengan kedalaman (jumlah) hujan. Runoff [mm] = K x Rainfall depth [mm] Dalam kondisi daerah-tangkapan di pedesaan yang tidak ada bagian kedap air, koefficien K, yang mencerminkan persentase runoff dari suatu kejadian hujan, bukanlah merupakan faktor yang konstan. Nilai koefisien ini sangat beragam dan tergantung pada faktor-faktor spesifik lokasi dan karakteristik hujannya. Misalnya, dalam suatu daerah tangkapan tertentu, dengan kondisi initial yang sama (misalnya kadar lengas tanah awal), kejadian hujan selama 40 menit dengan intensitas rataan 30 mm/jam akan menghasilkan persentase runoff lebih kecil dibandingkan dengan kejadian hujan selama 20 menit tetapi dengan rataan intensitas 60 mm/jam, walaupun total hujan keduanya sama. Diunduh dari: ….. 13/11/2012

44. …PERUBAHAN KANDUNGAN LENGAS TANAH The rates of change in the water amount in the root and lower zone are calculated straightforward from the flows found above: where DWrz : Change of the soil moisture amount in the root zone [cm] DWlz : Change of the soil moisture amount in the lower zone [cm] Ta : Actual transpiration rate [cm d-1] Es : Evaporation rate from a shaded soil surface [cm d-1] IN : Infiltration rate [cm d-1] Perc : Percolation rate from root zone to lower zone [cm d-1] Loss : Percolation rate from lower zone to sub soil [cm d-1] Dt : Time step [d] Due to extension of the roots into the lower zone, extra soil moisture becomes available, which can be calculated as: where RDt : Rooting depth at time step t [cm] RDt-1 : Rooting depth at time step t-1 [cm] RDmax : Maximum rooting depth [cm] Wlz : Soil moisture amount in the lower zone [cm] DWrz : Change of the soil moisture amount in the root zone [cm] DWlz : Change of the soil moisture amount in the lower zone [cm] Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

45. JUMLAH LENGAS TANAH DI ZONE AKAR The actual water amount in the root zone and in the lower zone can be calculated according to: where Wrz,t : Soil moisture amount in the root zone at time step t [cm] Wlz,t : Soil moisture amount in the lower zone at time step t [cm] Wrz,t-1 : Soil moisture amount in the root zone at time step t-1 [cm] Wlz,t-1 : Soil moisture amount in the lower zone at time step t-1 [cm] DWrz : Rate of change of the soil moisture amount in the root zone [cm] DWlz : Rate of change of the soil moisture amount in the lower zone [cm]. Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

46. …KANDUNGAN LENGAS TANAH AKTUAL The actual soil moisture content can now be calculated according to: where qt : Actual soil moisture content at time step t [cm3 cm-3] Wrz,t : Soil moisture amount in the root zone at time step t [cm] RD : Actual rooting depth [cm] Diunduh dari: http://www.treemail.nl/download/treebook7/soil/chapt6.htm….. 12/11/2012

47. KANDUNGAN LENGAS PROFIL TANAH Modeled soil water content over the 10-m profile on 30 Mar. 1997 (maximum soil water storage), 22 Aug. 1997 (beginning dry season), and 8 Jan. 1998 (minimum soil water storage). Diunduh dari: https://www.soils.org/publications/sssaj/articles/67/6/1672 ….. 13/11/2012

48. SOIL WATER CONTENT  The distribution of soil water content in the 0–200 cm soil profile for two shrubs. (a) The driest profile for C. korshinkii, (b) The wettest profile for C. korshinkii, (c) The driest profile for S. psammophila, (d) The wettest profile for S. psammophila. The vertical dashed lines represent soil water content at the permanent wilting point of 0.064 cm3 cm− 3. Diunduh dari: http://www.sciencedirect.com/science/article/pii/S0016706111003648 ….. 13/11/2012

49. POROSITAS TANAH Patterns of vertical distribution of bulk density (a), porosity (b), and organic matter content (c) within the soil profile above the landslide. The different position of the black layer zone in c reflects an excavation slightly upslope of the pit where bulk density/porosity cores were collected (a and b). Diunduh dari: http://www.sciencedirect.com/science/article/pii/S0016706109002274 ….. 13/11/2012

50. POROSITAS TANAH Air-filled porosity for the switch plow treatment of the bare fallow and residue covered field as a function of the soil profile. Values are means, n = 4. Values in each profile layer followed by the same letter are not significantly different at the P < 0.05 level. Diunduh dari: http://www.sciencedirect.com/science/article/pii/S0167198706000493….. 13/11/2012