Evaluation of FAO-56 Alfalfa Crop Coefficients in an Arid Southwestern U. S. Climate

The ASCE-EWRI standardized Penman-Monteith equation was used in conjunction with a soil water balance model to estimate crop ET for five years in an irrigation scheduling program for a 28,000-hectare farm in northern New Mexico. The calculated crop ET was then compared to measured crop ET to evaluate the FAO-56 tabulated alfalfa crop coefficients. Calculated ET_c estimates exceeded measured ET_c determined by water balance by 23% in the first 80 days, resulting in a 7.5 cm cumulative difference between calculated and measured ET. Possible causes for this disparity were examined and corrected. Errors in the near surface neutron probe measurements were found and corrected, local crop growth stages were applied to the data, weather data were evaluated and corrected, code was added to eliminate possible aridity, a user defined wind limit was added, application efficiency was derived and applied, and the FAO-56 suggested climatic correction was added. The combination of corrections did not completely reduce the cumulative difference. The tabulated FAO-56 Kcb coefficients were adjusted to further reduce the cumulative difference. With all positive corrections, the cumulative ASM difference was reduced from 7.5 cm to ~1 cm.