Comparison of hourly and daily Penman-Monteith grass- and alfalfa-reference evapotranspiration equations and crop coefficients for maize under arid climatic conditions

The practical estimation of actual crop evapotranspiration (ET_a) frequently involves calculating a reference crop evapotranspiration (ET_ref), and then applying suitable crop coefficients (K_c). Two standardized calculation procedures based on the Penman-Monteith equation are commonly used as ET_ref estimations (ET_o for grass and ET_r for alfalfa) across diverse environments. However, not much is known about the difference between ET_o and ET_r and their corresponding K_c values for maize in the arid environment of Northwest China with maize being the dominant crop. Estimates from the two procedures were comprehensively compared at both hourly and daily time steps for 2013 and 2014 growing seasons. The sum-of-hourly ET_r and ET_o (summed from hourly values over 24 h periods) values were averagely lower than their corresponding daily ET_r and ET_o values (calculated from daily average meteorological data) by about 1% and 5% in 2013, and 2% and 7% in 2014, respectively. The ratios of ET_r to ET_o varied seasonally with a mean value slightly lower than the generalized value (1.35) for arid conditions, due primarily to the differences in responsiveness of both standardized PM equations to the trends and interations among input parameters. The K_c curves of maize based on grass-reference (K_co) and alfalfa-reference (K_cr) were experimentally derived using daily ET_o and ET_r combined with ET_a measured by eddy covariance method. Results suggest the benefit and potentially improved accuracy for ET_a estimation when applying alfalfa reference equation combined with the timestep-specific K_c curves under the arid conditions. These results provide an acceptable accuracy and precision for estimation of ET_a of maize field use using the standardized Penman-Monteith equations, and offer the possibility of extrapolating new sets of K_co and K_cr to this region or other areas with similar climatic and crop management conditions.