Fixed versus variable bulk canopy resistance for reference evapotranspiration estimation using the Penman–Monteith equation under semiarid conditions

In this paper, daily ET₀ estimates at two semiarid locations, Zaragoza and Córdoba, were obtained from the Penman–Monteith equation using either fixed (70 s m^?1) or variable r_c values. Variable r_c values were computed with two models, Katerji and Perrier, and Todorovic. Daily ET₀ estimates were computed from 24-h meteorological averages or from the sum of hourly estimates. Daily ET₀ measured values were obtained from a weighing lysimeter (Zaragoza) and an eddy covariance system (Córdoba). There was a good agreement at both locations between estimated and measured ET₀ values using a fixed r_c value and 24-h meteorological averages. Estimates obtained from the sum of hourly estimates were somewhat worse. When 24-h meteorological averages were used, the Katerji and Perrier model for variable r_c slightly improved ET₀ estimates at both locations. But that improvement does not support the effort to locally calibrate that model. When daily ET₀ estimates were obtained from the sum of hourly estimates, the Todorovic model improved the estimation at Zaragoza and, at a lesser degree, at Córdoba. Under the semiarid conditions of the two studied locations, the use of the Todorovic model is recommended to get hourly ET₀ estimates from which daily estimates can be obtained. If 24-h meteorological averages are used, a fixed r_c value as proposed by Allen et al. Crop evapotranspiration: guidelines for computing crop water requirements, FAO Irrigation and Drainage Paper No. 56, FAO, Rome, 1998] should be enough for accurate ET₀ estimates.