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

Reference evapotranspiration (ETo) is widely used for irrigation scheduling, to enhance efficient use of water resources and sustainability of agro-ecosystem productivity as well as to protect the environment. The standard Penman-Monteith formulations, as suggested by ASCE and FAO-56 papers, are generally applied for accurate estimations of ETo, from hourly to daily scale. In absence of detailed meteorological information several simplified equations, using a limited number of variables, have been alternatively proposed (Blaney-Criddle, Hargreaves-Samani, Turc, Makkink and Pristley-Taylor). In this paper, the performance of different reference evapotranspiration equations, at hourly and daily scale, was evaluated on the basis of scintillometer measurements collected during six month, in 2005, in an experimental plot maintained under “reference” conditions (short grass). The plot is located in South West of Sicily, where olive represents the dominant crop. The comparison between estimated and measured fluxes confirmed how FAO-56 Penman-Monteith (PM) standardized equation is characterized by the lowest mean error and the minimum bias. However, the analysis evidenced that even the Pristley-Taylor equation can be considered a valid alternative to the PM method. Daily values of ETo, obtained by the examined methods, were then used as input in FAO-56 water balance model, in order to evaluate, for olive groves, the errors associated to simulated actual evapotranspiration ET.