Regionalization of the Hargreaves Coefficient for the Assessment of Distributed Reference Evapotranspiration in Southern Italy

The Hargreaves equation (HE) is the most reliable and diffused temperature-based method for the estimate of the reference crop evapotranspiration ETo in areas with limited meteorological data. Several studies proposed its further enhancement by adopting correction equations for recalibrating its parameters, specifically the empirical Hargreaves coefficient, C = 0.0023. In this paper, a local recalibration for southern Italy is shown, taking as a reference the Food and Agriculture Organization of the United Nations (FAO) Penman-Monteith (PM) equation applied over 137 (34 coastal and 103 inland) meteorological stations, allowing the estimation of 357,920 daily ETo values in the period 1994-2011. Through a regression analysis, the adjusted Hargreaves coefficients (AHC), minimizing the errors for each individual station between the HE and the FAO Penman-Monteith equation, and providing better performances than the original HE applied at the monthly scale, were correlated to the available daily meteorological data; reliable regionalization equations of the AHCs were found determined by either the average daily temperature range (Delta T) over bar or the ratio between the average daily temperature (T) over bar and (Delta T) over bar, with similar coefficients to analogous relationships proposed in the literature. Specifically, the (Delta T) over bar dependent model proved to be very reliable over the whole study area for all climate types, from the shore up to 1,928 m. The improvement in the performances of the regional model with respect to the original HE was on average 22%, and it also proved able to simulate the yearly cumulative ETo evolution in several meteorological stations. The strength of the model was positively tested through a jackknife resampling procedure and a sensitivity analysis, taking into account possible trends for the average daily temperature range. DOI: 10.1061/(ASCE)IR.1943-4774.0000547. (C) 2013 American Society of Civil Engineers.