Soil Water Balance for Crop Land

Introduction

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: Dissipate heat that is produced by the plant's metabolic activities.

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. Donate electrons through photolysis for photosynthesis.

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 ). Light H 2 0 -------------> (2 H +) + (2 e -) + (1/2 O 2 ) The released electrons participate in a series of oxidation-reduction reactions that eventually drive the biochemical reactions in photosynthesis. Dissolve minerals and organic compounds in cells.<

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 equation

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 Addition of water to the soil:

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
Removal of water:

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

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.

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: 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.

Example 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.

Suggested reading:

Hillel, D. 1971. Soil and Water: Physical principles and processes. Academic press, New York.

Hanks, R.J . and Ashcroft, G.L . 1980. Applied Soil physics: Soil water and temperature applications. Advanced series in agricultural review 8, Springer verlag, 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: 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.
Example 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.

Suggested reading:

Hillel, D. 1971. Soil and Water: Physical principles and processes. Academic press, New York. Hanks, R.J. and Ashcroft, G.L. 1980. Applied Soil physics: Soil water and temperature applications. Advanced series in agricultural review 8, Springer verlag, New York