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Soil Water Balance for Crop Land Soil water balance computation
The objective of this lesson is to help the user understand the concept of soil water balance. An understanding of water balance is necessary to appreciate the role of various management strategies in minimizing the losses and maximizing the utilization of water, which is the most limiting factor of crop production in semi-arid tropics (SAT). Water is essential for plant life. The important functions of water in the plant are:
Plants continually absorb water from their growth medium and evaporate part of it into the surrounding atmosphere. This is transpiration and takes place mainly through the leaves. Water loss through transpiration helps the plant dissipate heat that has accumulated as a result of metabolism and radiant energy intake.
The photosynthetic process is driven by light energy. Light functions initially to split water into protons (H+), electrons (e-), and molecular oxygen (1/2 O
2 ).
Water is called the universal solvent, because any molecule that is polar will dissolve in it. As ions are formed, they are surrounded by the oppositely charged ions of water. In the cell, water serves as a medium through which solutes move. Water requirements of the plants are met by the supplies from soil, which acts as reservoir for water. The amount of water held by soil depends on the inputs and losses from the system and holding capacity of the soil. Important sources of water in the field are generally rainfall and irrigation. Losses of water include surface runoff from the field, deep percolation out of root zone or drainage, evaporation from the soil surface and transpiration from the crop canopy. 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. As explained earlier, 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. Hence the soil water balance equation in its simplest form of expression is: Change in soil water = Inputs of water - Losses of water 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: Water Inputs = P + I + C
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. Water Losses = ET + D + RO Soil water balance
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.
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. Soil water balance computation Let us work a few examples using the Soil Water Balance Equation to appreciate the usefulness of this model. Example 1:
Given:
Estimate evapotranspiration (ET) from the field during 01 to 31 Aug. Equation: Thus, evapotranspiration which is difficult to be measured could be estimated using the Soil Water Balance Equation. Example 2:
M1 + P + I + C = ET + D + RO + M2 150 + 600+ 0 + 0 = 530 + D + 70 + 60 D = 750 mm - 660 mm = 90 mm
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.
Suggested reading: Hillel, D. 1971. Soil and Water: Physical principles and processes. Academic press, New York.
Soil water balance computation Let us work a few examples using the Soil Water Balance Equation to appreciate the usefulness of this model. Example 1:
M1 + P + I + C = ET + D + RO +M2 Thus, evapotranspiration which is difficult to be measured could be estimated using the Soil Water Balance Equation. Example 2:
M1 + P + I + C = ET + D + RO + M2 150 + 600+ 0 + 0 = 530 + D + 70 + 60 D = 750 mm - 660 mm = 90 mm
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. Suggested reading: Hillel, D. 1971. Soil and Water: Physical principles and processes. Academic press, New York. |