Downscaling of American National Aeronautics and Space Administration (NASA) daily air temperature in Sicily, Italy, and effects on crop reference evapotranspiration

Procedures for downscaling and spatial disaggregation of daily air temperatures retrieved from the POWER-NASA archive were proposed and validated for the climate of Sicily, Italy. In particular, a cosine function with three empirical parameters derived from ground elevation was suggested to estimate the spatial and temporal variability of a lapse rate adjustment factor used to downscale and to disaggregate POWER-NASA air temperature. Model calibration was carried out over three years (2006-2008), whilst validation regarded the measurements acquired over the following six years (2009-2014). Downscaled daily air temperature results were consistent and comparable with local ground measurements. The spatial disaggregation technique, aimed to map the distributions of Ta, was then tested by the joint use of the Digital Elevation Model (DEM) and the proposed model. Finally, in order to assess the suitability of downscaled air temperatures, two simplified empirical equations to estimate daily reference evapotranspiration (ET0) from air temperature and solar radiation (Turc, 1961; Hargreaves, 1975) were applied. To this aim, daily ET0 estimated by introducing native and downscaled POWER-NASA air temperature values into Turc and Hargreaves equations were compared to the corresponding estimations obtained with FAO 56 Penman Monteith (FAO 56 PM) equation and climate data measured on ground.