Using Models to Determine Irrigation Applications for Water Management.

Simple models are used by field researchers and production agriculture to estimate crop water use for the purpose of scheduling irrigation applications.  These are generally based on a simple volume balance approach based on estimates of soil water holding capacity, irrigation application amounts, precipitation, and daily estimates of crop evapotranspiration.  Crop ET is estimated from ET of a reference crop, usually based on a version of the Penman-Monteith equation, and a crop coefficient that is commonly based on days or growing degree days since planting.  This approach is common in the Western U.S. and is often referred to as the FAO-56 method.  The approach requires daily data from a weather station, which is available in many irrigated areas in the western states.  Since errors in daily ET accumulate over time, it is recommended that the method be supplimented by periodic soil water content measurements.  The main weakness of the method is determining appropriate crop coefficients for the crop and conditions.