These models are used to estimate runoff and evaporation for large areas when data regarding soil properties, humidity, leaf area indexes, and other types of information are not available. Since this is a transient calculation, the soil is assumed to be saturated for the initial guess. The soil moisture for each step is calculated using the following equation, where i is the current time step and alpha is a runoff extraction function. The runoff extraction function automatically diverts a portion of the precipitation away from the soil to account for rainfall events where the precipitation rate exceeds the infiltration rate. In addition, if soil moisture exceeds the soil-water holding capacity, the surplus is added to the predicted runoff.
Soil moisture is assumed to be conserved, in order to close the mass balance.
Several issues must be considered when implementing the soil-water balance model. Available data for this project are on a monthly basis. Using a historical record of monthly rainfall will result in a predicted runoff that is significantly lower than observed runoff. The runoff extraction function, alpha, has been included to compensate for this effect. The value of alpha is calculated based on an empirical soil-moisture relationship where alpha increases as soil moisture increases.