A simple parameterization of bulk canopy resistance from climatic variables for estimating hourly evapotranspiration

This paper examines a model for estimating canopy resistance r(c) and reference evapotranspiration ETo on an hourly basis. The experimental data refer to grass at two sites in Spain with semiarid and windy conditions in a typical Mediterranean climate. Measured hourly ETo values were obtained over grass during a 4 year period between 1997 and 2000 using a weighing lysimeter (Zaragoza, northeastern Spain) and an eddy covariance system (Cordoba, southern Spain). The present model is based on the Penman-Monteith (PM) approach, but incorporates a variable canopy resistance rc as an empirical function of the square root of a climatic resistance r* that depends on climatic variables. Values for the variable r(c) were also computed according to two other approaches: with the r(c) variable as a straight-line function of r* (Katerji and Perrier, 1983, Agronomie 3(6): 513-521) and as a mechanistic function of weather variables as proposed by Todorovic (1999, Journal of Irrigation and Drainage Engineering, ASCE 125(5): 235-245). In the proposed model, the results show that r(c)/r(a) (where r(a) is the aerodynamic resistance) presents a dependence on the square root of r*/r(a), as the best approach with empirically derived global parameters. When estimating hourly ETo values, we compared the performance of the PM equation using those estimated variable r(c), values with the PM equation as proposed by the Food and Agriculture Organization, with a constant r(c) = 70 s m(-1). The results confirmed the relative robustness of the PM method with constant rc, but also revealed a tendency to underestimate the measured values when ET, is high. Under the semiarid conditions of the two experimental sites, slightly better estimates of ET, were obtained when an estimated variable r(c) was used. Although the improvement was limited, the best estimates were provided by the Todorovic and the proposed methods. The proposed approach for r, as a function of the square root of r* may be considered as an alternative for modelling r(c), since the results suggest that the global coefficients of this locally calibrated relationship might be generalized to other climatic regions. It may also be useful to incorporate the effects of variable canopy resistances into other climatic and hydrological models. Copyright (c) 2005 John Wiley & Sons, Ltd.