ALFALFA BASAL CROP COEFFICIENTS FOR FAO–56 PROCEDURES IN THE DESERT REGIONS OF THE SOUTHWESTERN U.S.

Abstract

The FAO Irrigation and Drainage Paper No. 56 (FAO56) is expected to provide a universally consistentmethodology for obtaining reliable estimates of crop evapotranspiration (ETc) from standard weather data and cropcoefficient (Kc) information. Yet, for many areas, including the southwestern desert region of the U.S., evaluation of theFAO56 crop coefficient methods for local conditions has been limited. In this study, measurements of daily alfalfa ETc wereused to evaluate the dual crop coefficient approach of FAO56, which separates the single Kc into two coefficients, a basalcoefficient, Kcb (primarily transpiration), and a wet soil evaporation coefficient, Ke. A second objective was to calibratealfalfa Kcb baselines for constructing localized, climateadjusted FAO56 Kcb curves for growth cycles under the seasonalclimatic variations encountered in the semiarid, southwestern U.S. Daily ETc was measured for alfalfa in three weighinglysimeters subjected to varying irrigation regimes for eight cutting cycles in 1985 at a field site in Phoenix, Arizona. DailyKc data were determined from the measured ETc and calculations of the FAO56 grassreference evapotranspiration (ETo)using meteorological data obtained at the site. Daily Kc values were partitioned into the dual crop coefficients (Kcb and Ke)using backcalculations of the FAO56 dual procedures. Examples are given to illustrate how the magnitudes of Kcb, Ke, andKc varied with respect to crop development and wet and dry surface soil conditions. The effects of water stress on ETc wereconsidered in the backcalculations using the FAO56 water stress reduction coefficient (Ks). Linearstyle FAO56 Kcbcurves were then constructed separately for each lysimeter for each of the eight cutting cycles based on the backcalculatedKcb data. Daily ETc based on the constructed curves and FAO56 calculation procedures were highly correlated to the dailymeasured lysimeter ETc (r2 = 0.98), and the mean daily difference between calculated and measured ETc (0.03 mm) was notsignificant (p > 0.7). Comparison of measured ETc for waterstressed and wellwatered alfalfa indicated that calculated Ksdescribed ETc reductions due to soil water stress adequately when atmospheric evaporative demand was considered in theKs calculation. Baseline Kcb values for constructing local climateadjusted FAO56 alfalfa Kcb curves were determined as0.30, 1.22, and 1.05 for the initial, midseason, and end of late season, respectively.