Daily reference evapotranspiration estimates by the Penman-Monteith equation in Southern Italy. Constant vs. variable canopy resistance

The performance of the Penman-Monteith (PM) equation to estimate daily reference evapotranspiration (ETO) was investigated by attributing three distinct features to the canopy resistance (r(c)): (i) r(c) constant at 70 sm(-1) (Allen et al., 1998; FAO Irrigation and Drainage Paper n. 56), (ii) r(c) variable as linear function of a critical resistance r*, depending on weather variables and empirical parameters relating r(c) to r* (Katerji and Perrier, 1983; Agronomie, 3[6]: 513-521) and (iii) r(c) variable as a mechanistic function of weather variables only (Todorovic, 1999; J. Irrig. Drainage Eng., ASCE, 125[5]: 235-245). Daily weather and grass lysimeter data, measured for a period of seven years at Policoro (Southern Italy), were used. The results confirmed the relative robustness of the PM method with constant r(c) while better estimates were obtained only when variable r(c) was used. The mechanistic approach of Todorovic (1999) provided the best estimates, while the approach of Katerji and Perrier (1983), with empirically derived parameters, has shown to be not conservative enough to be extended to different locations without calibration.