Calibrating Priestley-Taylor Coefficient to Estimate Free Water Surface Evaporation (Case Study: Mahabad Dam Reservoir)

Evapotranspiration is one of the important components of basin water balance which cannot be measured directly at the basin scale. Therefore, it is inevitably estimates through indirect methods. In this regard, the Advection Aridity model, one of the widely used models of complementary relationship has attracted a lot of attentions. Due to the existence of Priestley-Taylor equation in the Advection Aridity model, it is required to calibrate Priestley-Taylor coefficient to increase the accuracy of the model. The current research aims at calibrating Priestley-Taylor coefficient in estimation of potential evaporation through Penman method to apply it in the Advection Aridity model in the studied area of Mahabad dam reservoir, Iran. The required data were collected for a period of 26 years (1986-2012) from Mahabad 1st order meteorological station which is located at short distance from Mahabad reservoir. The results showed that Priestley-Taylor coefficient undergoes monthly changes during a year and decrease during the warm months of the year. Therefore, it is better to use its monthly values in calculations, its minimum and maximum averages were 1.01 and 1.68, respectively. Moreover, the long term average of this coefficient during a period of 26 years has been calculated 1.25.