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 ET<inf>o</inf> 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 a<inf>pt</inf>. The results showed that the surface coverage of the Italian territory, with acceptable ±10% difference using a<inf>pt</inf>=1.26, was 24.9% in Spring, 41.4% in Summer, 34.3% in Autumn and 11.9% in Winter. The recalculation of a<inf>pt</inf> 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 a<inf>pt</inf> is explained by the spatial variability of vapour pressure deficit DE (positive correlation). The rate of a<inf>pt</inf> variation per unit DE was found significantly different between seasons and it was negatively correlated to net solar radiation R<inf>n</inf> and consequently temperature. The general trends of the a<inf>pt</inf> coefficient led to the conclusion that colder-drier environments due to low net radiation and high vapour pressure deficit tend to increase its values.