Evapotranspiration estimates derived from subsoil salinity data

A method is described for estimating past evapotranspiration (ET) rates from irrigated fields. It is based on the assumption that salt concentrations below the crop root zone are a function of the flux of naturally occurring salts applied by irrigation at the soil surface and the rate of soil water depletion due to ET. Thus, if irrigation histories and subsoil salinity data are available, ET rates may be calculated. For irrigated fields with low leaching fractions, corrections to the ET estimates may be necessary due to chemical precipitation. This is done through modeling of solution chemistry or from the use of observed chloride concentrations in solution. ET estimates for 1976-1980 for cotton at Safford, Arizona were made from subsoil salinity data collected to 3 m in April 1983. The average seasonal ET was found to be 1.04 m, which was only 2% below the average seasonal ET estimated by the modified Blaney-Criddle method. The estimated ET for cotton was on average 0.48 of the seasonal pan evaporation and 0.76 of the annual irrigation plus rain-fall amount. The method appears to be suitable for use in selected semi-arid and arid areas which have relatively uniform soils and deep water tables.