Improvement of Hargreaves method for reference evapotranspiration in hilly area of central Sichuan Basin

Numerous methods have been proposed for estimating ET sub(0), among which Penman-Monteith (P-M) model recommended by Food and Agriculture Organization of the United Nations (FAO) in 1998 since it provids the highest accurate results across the world wherever in an arid or humid environment. Thus, there is an urgent need to develop a much simpler and more appropriate model in areas with limited data, such as the Hargreaves-Samani (HS) method, which is modified in this paper. To obtain more accurate HS model of ET sub(0) in hilly area of central Sichuan Basin, 3 parameters(including temperature index, temperature coefficient and temperature constant) were calibrated based on Bayesian Theory using daily meteorological data from 1953 to 2002. The trend analysis based on monthly ET sub(0) showed that the trend of ET sub(0) based on modified HS model was the same as that from PM model, which performed as the downwards Quadratic parabola in one year but increasing slightly in years from 2003 to 2013.