Regional and local calibration of Hargreaves equation for estimating reference evapotranspiration

The FAO-56 Penman-Monteith equation (ETo(pm)) has been recommended by the Food and Agriculture Organization (FAO) of the United Nations as the standard equation for estimating reference evapotranspiration (ETo). The FAO-56 PM equation requires numerous weather data that are not available in most of the stations of Brazil central. On the other hand, the Hargreaves equation is a more simple equation for estimating ETo, and demands only maximum and minimum temperature data. However, this equation requires local calibration. This paper examines the potential of using the Hargreaves equation adjusted to estimate the ETo in Goias State. For this, the Hargreaves empiric parameters, HC (empirical Hargreaves coefficient) and HE (empirical Hargreaves exponent), were adjusted considering two procedures, local adjustment (HGL - Hargreaves local adjustment) and regional adjustment (HGR - Hargreaves regional adjustment). For HGL, the adjustment of empiric parameters was done for each weather station. For HGR, the adjustment of empiric parameters was done considering the data set of all weather stations. The Hargreaves equation adjusted by both processes, local and regional, showed values of 17.95 and 21.93% for ERQM respectively, considering the full range of climatic data. The Hargreaves equation adjusted by both processes is an option to estimate the daily values of ETo in Goias State where there are available data limitation.