The movement and storage of water in surface soils is important for economic and environmental reasons. The ability of a soil to store water in the root zone determines its ability to support plant growth and crop production. Water moving through the root zone has the potential of carrying chemicals downward to the underlying aquifers. Rainfall and irrigation water entering the soil tends to replenish the soil storage reservoir and extend its ability to support plant life. If infiltrating water exceeds the storage capacity of the soil root zone, the excess water tends to move downward through the soil. This deep drainage contributes to recharging the aquifer below. The challenge to farm managers is to provide sufficient water for producing a crop while limiting the anount of water and dissolved chemicals moving deeply into the soil.

This applet uses the water balance concept to estimate water available for plants and water drainage below the root zone. The basic concept is basically a statement of the principle of conservation of mass. It can be represented by the equation

where Infiltration represents the amount of water entering the soil surface from rainfall or irrigation, Evapotranspiration represents the amount of water leaving the soil by surface evaporation or through plants growing in the soil, Drainage represents the amount of water moving below the root zone, and Storage represents the amount of water stored in the soil root zone.
In this applet the water balance is calculated on a daily basis. Historical daily rainfall amounts along with user-specified irrigation strategies are used to estimate infiltration. Potential evapotranspiration amounts calculated from historical weather data along with time dependent crop coefficients are used to estimate evapotranspiration. The soil water-holding capacity and known storage amount on a specific date are used to characterize the water stored in the soil. If the amount of water entering the soil on a particular day can be stored in the root zone, the the drainage amount is zero for that day. If the infiltration exceeds the storage capacity of the soil the excess is lost as drainage.

Although the water balance concept is useful for some management  purposes, it greatly simplifies the soilwater processes and is not suitable for all uses. Clearly the model is not capable of providing information for time steps of less than one day. Also the model ignores soil water dynamics. It treats the infiltration, storage, and drainage processes as essentially instantaneous processes. This may lead to underestimating water storage and overestimating drainage. Another simplification in this model is the assumption that all rainfall enters the soil. Runoff is ignored. This too will may tend to overestimation of drainage.

In addition to simplifications in the model, we have uncertainty because we do not know the future weather distributions. Thus we cannot estimate the future water balance with certainty. To help overcome this problem, the applet allows the user to view predictions based on a number of past weather records. By viewing predictions collectively for all of these years, the user can gain an understanding of the possible range of predictions that may occur. In more advanced models, more weather records are used and results are presented as probability distributions.

Last Modified: January 16, 2008.