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

Air temperature (Ta) is one of the key factors in agro-hydrological studies including estimation of crop reference evapotranspiration (ET0), which is crucial for irrigation water management and sustainability of agro-ecosystem productivity. Because direct measurements of ET0are difficult, expensive and time consuming, the use of physically based or empirical approaches linked to meteorological information is often preferred. The Prediction of Worldwide Energy Resource project developed by the American National Aeronautics and Space Administration (POWER-NASA) provides daily meteorological information on a 1° latitude by 1° longitude grid. Despite the poor spatial resolution characterizing this archive, the data is global and continuous in time, so that it can be used in particular regions of the globe where weather measurements are not well spatially distributed and/or there are some missing weather data to fill. 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 ET0estimated 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.