The Water Budget

The components of the water balance (infiltration, deep percolation, evaporation from the soil surface, etc.) determine the amount of water available to the crop. Water flows in the soil following the water potential gradient, which can be analyzed by the Richards equation. Infiltration rate decreases with time until a steady-state value. Layered soils have lower infiltration than homogenous soils. Deep percolation can be estimated based on soil properties and water content above Field Capacity, depending also on soil evaporation and transpiration. Runoff is calculated based on the Curve Number and the amount of precipitation. Monthly effective rainfall, i.e. not lost by runoff, can be calculated by the methods of the Food and Agriculture Organization (FAO) and the United States Department of Agriculture-Soil Conservation Service (USDA-SCS).