Identification of the main meteorological factors for the trend of reference evapotranspiration in Sicily, Italy

Climate change has potential impacts on the hydrological cycle components, especially those strongly related to temperature, such as evapotranspiration. Assessing trends of the reference evapotranspiration (ETo) and of the related climatic factors is essential for improving water resource management especially with reference to watershed  hydrology and agricultural uses. In this research, we aim to analyze the trend of ETo and of its influencing climatic factors.  More specifically, we examine the sensitivity of ETo for different climatic factors and the contribution of climatic factors for the trend of ETo in the study area.  The study considered decadal observations of climatic data from meteorological stations in Sicily, and reference evapotranspiration was estimated through the FAO Penman-Monteith method.  The Mann-Kendall test, with verification of the Trend-free prewhitening (TFPW) method, has been applied for the trend and sensitivity analyses. The Sen’s slope has also been used to examine the magnitude of the trend. Results, relative to a pilot gauging station in Piazza Armerina, indicate that the ETo has decreasing trend only in November with a decrease of 0.790 mm per year. The solar radiation (November and Autumn) and rainfall (Autumn) showed decreasing trends. While other climatic factors (minimum temperature, maximum temperature, mean temperature, wind speed and relative humidity) showed increasing trend both monthly and seasonally in the study area. Furthermore, the sensitive analysis shows that ETo is mostly sensitive to relative humidity and least sensitive to wind speed in the study area. Similarly relative humidity contributes the most to the trend of ETo (44.59% decreasing contribution), while wind speed has the least contribution (0.9% increasing contribution) in the study area.  These results can find application for irrigation scheduling and water related development project in the study area.