Soil Water Budget

Mikhail Sabaev
Yevgeniy Miretskiy

Project Java Webmaster: Glenn A. Richard
Center for High Pressure Research
SUNY Stony Brook

How to Use this Applet

This applet helps you learn to calculate a soil moisture budget based on precipitation and potential evapotranspiration (Ep). Stored soil moisture (St) is to the quantity water that is contained in the pore spaces between soil particles. The water considered in this analysis is above the water table and is available to the roots of plants.

In a soil water budget, a column of soil is considered, and the St is expressed as the depth of the water that would be obtained if it were separated from the sediment. In concept, it is similar to expressing precipitation (P) in inches or centimeters. In this analysis, St is expressed as millimeters of water, and it is assumed that the soil can hold a maximum of 100 millimeters. Under this condition, the soil is said to be saturated. Any water added to the saturated soil is supplus (S) and will either flow away as runoff or result in percolation of surplus water down to the water table, and St will remain at 100 millimeters.

Water is removed from the soil by evapotranspiration. This is a combination of direct evaporation from the soil, and transpiration, which is evaporation from plants. Evapotranspiration reduces the quantity of soil water, which can decrease until it reaches 0. At this point, no more evapotranspiration can take place until precipitation adds more water to the soil.

Potential evapotranspiration (Ep) is the amount of evapotranspiration that could occur if a limitless supply of water were available in the soil. It is based on weather conditions such as wind and temperature and biological factors. In the winter, Ep is low in areas of temperate climate due to low temperatures and lack of leaves on deciduous plants. Under these conditions, precipitation (P) is higher than Ep, so the amount of actual transpiration (Ea) equals Ep. In fact, even in the spring, when Ep begins to exceed P, St is reduced to make up the difference and Ea continues to equal Ep. However, eventually St may become depleted and reach 0. Ea cannot exceed the total of St plus P. Therefore, when Ep exceeds this total, Ea equals St plus P. When Ep exceeds Ea, the difference is quantified as a soil water deficit (D).

To use this applet, enter P and Ep values for each month. Also enter a value for St for the previous December, under Initial Storage. At this point, you have entered all the quantities that are required to calculate the remaining ones. You can enter your own values for the remaining quantities in order to test yourself. If you click the Error mode check box, any erroneous quantities you entered will be highlighted in red. To display all of the the correct quantities, click the Reveal button. This will replace the calculated values that you entered. The Reset button clears all values. To navigate the spreadsheet, you can hold down Ctrl and use the arrow keys. Hitting Enter moves to the next cell to the right, or to the beginning of the next row when the insertion point is in the last column.

Try these sample values for P, Ep, and Initial Storage, which are taken from the Long Island Water Resources Curriculum Activities Guide, Grades 7 - 12, published by the Museum of Long Island Natural Sciences (phone: (516) 632-8230):

  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
P 105 107 125 109 86 63 86 122 94 93 112 117
Ep 1 3 17 41 74 112 141 137 79 51 3 1
Initial Storage: 100  

Mathematical Concepts

Source Code

Mikhail Sabaev:

Yevgeniy Miretskiy:

visitors since May 2, 1997

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