HYDROLOGIC EVALUATION OF A MEDITERRANEAN WATERSHED USING THE SWAT MODEL WITH MULTIPLE PET ESTIMATION METHODS

The Penman-Monteith (P-M) method suggested by the Food Agricultural Organization in irrigation and drainage paper 56 (FAO-56 P-M) was used in the Soil and Water Assessment Tool's (SWAT) water balance simulation at the outlet of an experimental watershed in Sicily, Italy. A sensitivity analysis determined that the model was more sensitive to this potential evapotranspiration (PET) parameter than to the other six parameters impacting surface runoff in this small Mediterranean watershed. The FAO-56 P-M method was compared to the three existing SWAT PET methods from 1997 to 2003. The watershed's water balance was more realistically simulated by the FAO-56 P-M method than by the other PET methods. The traditional P-M method incorporated into SWAT overestimated total (surface and base flow) runoff volumes observed during the entire period by approximately 50%; however, total runoff volumes were underestimated by only 17% when the FAO-56 P-M method was used. The surface runoff simulation results using the FAO-56 P-M PET equation for calculating daily values was sufficient at the monthly time interval (Nash-Sutcliffe efficiency >0.75) during the calibration and validation periods. The incorporation of the FAO-56 P-M method has broadened the SWAT model's applicability to watersheds that are in semi-arid environments with high-intensity, short-duration rainfall events.