Using scintillometry to assess reference evapotranspiration methods and their impact on the water balance of olive groves

Reference evapotranspiration (ET0) is widely used for irrigation scheduling, to promote an efficient use of water resources for a sustainable agro-ecosystem productivity, as well as to manage water quality and to face other environmental concerns. As suggested by ASCE-EWRI and FAO, standard Penman-Monteith methods are generally applied for an accurate estimation of ET0 from hourly to daily scale. In absence of detailed meteorological information several simplified equations, using a limited number of variables, have been proposed as alternative. In this paper, the performance of different reference evapotranspiration methods, at hourly (Penman-Monteith, Pristley-Taylor, Makkink and Turc) and daily scale (Penman-Monteith, Blaney and Criddle, Hargreaves, Pristley-Taylor, Makkink and Turc), was evaluated against scintillometer measurements collected during six month in 2005 in an experimental plot maintained under "reference" conditions (alfalfa crop). The daily values of ET0 obtained with the examined methodologies were then used as input in the FAO-56 agro-hydrological model, in order to evaluate, for an olive grove in a Mediterranean environment, the impact on simulated actual evapotranspiration. The experiment was carried out in South-West Sicily, in an area where olive groves are the major crop. The comparison between estimated and measured fluxes confirmed that FAO-56 Penman-Monteith (PM) standardized equation is characterized by the lowest mean bias error (-0.15 mm d-1 and 0.06 mm d-1 using daily or hourly data, respectively). Additionally, the analysis also highlighted that the Pristley-Taylor equation can be considered a valid alternative for an accurate estimation of ET0 (mean bias error of 0.35 mm d-1 and 0.43 mm d-1 using daily or hourly data, respectively). The application of the FAO-56 water balance model on the investigated olive grove evidenced that the best estimations of actual evapotranspiration are obtained when the Pristley-Taylor ET0 data are used as input, confirming that this approach can be considered a valid alternative to the standard Penman-Monteith.