Assessing reference evapotranspiration by the Hargreaves method in north-eastern Italy

The Food and Agriculture Organization of the United Nations proposed the (FAO)-56 Penman–Monteith equation (FAO-56 PM) as the standard method for estimating reference evapotranspiration (ET0). This equation requires weather data which are not available in most stations or present wide gaps and/or inaccuracies in their measurement. To solve this problem, the Hargreaves equation (HARG) is recognized by FAO and is often used. This equation is based on average, minimum and maximum air temperature and extraterrestrial radiation. It tends to overestimate ET0 in humid conditions and requires a local calibration. This paper examines the possibility for calibrating the HARG equation in Veneto region (north-eastern Italy) according to different criteria. For this study, full weather data sets of daily values collected along the period 1994–2006 from 35 agro-meteorological stations located in the Veneto plain were used. Ten stations were selected for calibration of the adjusted HARG equations, in order to represent the different areas of the plain, while the other 25 were used for validation. The median daily FAO-56 PM ET0 was 1.93 mm and the original HARG overestimated this value by 18.9%. Adopting a common calibrated value of 0.0020 instead of 0.0023 as a constant value (HA) in the HARG formula, the overestimation was reduced to 2.6%. Other calibrations, as specific for each site or deriving HA from a relationship between T/!T, did not produce further estimation improvement.