Comparison of methods for applying the Priestley-Taylor equation at a regional scale

The scenario assumed for this study was that of a region with a complete or first-order weather station surrounded by a network of second-order stations, where only monthly air temperature data were available. The objective was to evaluate procedures to estimate the monthly or parameter of the Priestley-Taylor equation in the second-order stations by adjusting and extrapolating alpha values determined at the first-order station. These procedures were applied in two climatic zones of north-east Spain with semi-arid continental and semi-arid Mediterranean climates, respectively. Procedure A assumed alpha to be constant over each zone for each month (direct extrapolation). Procedure B accounted for differences in vapour pressure deficit and available energy for evapotranspiration between the first- and second-order stations. Procedure C was based on equating the Penman-Monteith (P-M) and Priestley-Taylor (P-T) equations on a monthly basis to solve for cu. Methods to estimate monthly mean vapour pressure deficit, net radiation and wind speed were developed and evaluated. A total of 11 automated first-order weather stations with a minimum period of record of 6 years (ranging from 6 to 10 years) were used for this study. Six of these stations were located in the continental zone and five in the Mediterranean zone. One station in each zone was assumed to be first-order whereas the remainder were taken as second-order stations. Monthly alpha parameters were calibrated using P-M reference crop evapotranspiration (ET0) values, calculated hourly and integrated for monthly periods, which were taken as 'true' values of ET0. For the extrapolation of monthly alpha parameters, procedure A was found to perform slightly better than procedure B in the Mediterranean zone. The opposite was true in the continental zone. Procedure C had the worst performance owing to the non-linearity of the P-M equation and errors in the estimation of monthly available energy, vapour pressure deficit and wind speed. Procedures A and B are simpler and performed better. Overall, monthly P-T ET0 estimates using extrapolated alpha parameters and R-n - C values were in a reasonable agreement with P-M ET0 calculated on an hourly basis and integrated for monthly periods. The methods presented for the spatial extrapolation of monthly available energy. vapour pressure deficit and wind speed from first- to second-order stations could be useful for other applications. Copyright (C) 2001 John Wiley & Sons, Ltd.