Revisiting the Priestley-Taylor method for the assessment of reference crop evapotranspiration in Italy

Aim of the study is to test the Priestley-Taylor (P-T) method for the assessment of reference crop evapotranspiration ETo in the Italian territory using as a base the ASCE standardized Penman-Monteith method (ASCE). Monthly averages of daily mean climatic data which cover the period 1950-2000 were used in this study. Analysis was performed on the spatial variability of the seasonal difference between ASCE and P-T method using the typical value 1.26 for the advection coefficient apt. The results showed that the surface coverage of the Italian territory, with acceptable ±10% difference using apt=1.26, was 24.9% in Spring, 41.4% in Summer, 34.3% in Autumn and 11.9% in Winter. The recalculation of apt using the ASCE method showed high spatial and temporal variability of the coefficient. Regression analysis showed that more than 90% of the spatial variability of the seasonal apt is explained by the spatial variability of vapour pressure deficit DE (positive correlation). The rate of apt variation per unit DE was found significantly different between seasons and it was negatively correlated to net solar radiation Rn and consequently temperature. The general trends of the apt coefficient led to the conclusion that colder-drier environments due to low net radiation and high vapour pressure deficit tend to increase its values.