Water-Use Production Functions of Selected Agronomic Crops in Northwestern New Mexico-Phase III

The water requirement for growth of the crops alfalfa, corn, spring barley and pinto beans was investigated in northwestern New Mexico. Results strongly suggest that the level of nitrogen fertility does not alter the seasonal transpiration requirement associated with the production of a given level of yield. The seasonal evapotranspiration requirement, however, required to produce that yield, may differ in response to factors affecting the evaporation component of evapotranspiration. Crop coefficients based on various methods of calculating potential evapotranspiration were found to vary as much as 50 percent between years for identical yield levels. Evidence presented suggests that to adequately predict the evapotranspirational requirement of a targeted yield level, soil-water evaporation will have to be separated from transpiration experimentally, and the factors affecting each component will have to be identified and quantified. Consequently, relying solely on crop coefficients to predict seasonal and intraseasonal water requirements for full and deficit irrigation under conditions requiring frequent light irrigation in an environment of high atmospheric evaporative demand, is subject to error and caution is needed when using this approach.