Using the ERA5 dataset of atmospheric variables to estimate daily reference evapotranspiration in Sicily, Italy.
Authors: Provenzano Giuseppe , Ippolito Matteo 
Published: March 2021

&lt;p&gt;Crop evapotranspiration (ET) plays a key role in many hydrological processes involving the soil-plant-atmosphere system. The concept of reference crop evapotranspiration (ET&lt;sub&gt;0&lt;/sub&gt;) was introduced to estimate the atmosphere evaporation demand independently of crop type, development stage and management practices. Among the available methods to estimate ET&lt;sub&gt;0&lt;/sub&gt;, the Penman-Monteith equation proposed by the Food and Agriculture Organization of the United Nations (FAO56-PM), is considered one of the most accurate, so that it is assumed as a reference to calibrate other simplified procedures. In several regions of the world, the limited availability of meteorological observations to estimate ET&lt;sub&gt;0&lt;/sub&gt; can be overcome by using gridded reanalysis dataset created by data assimilation of weather observations. Different datasets with relatively high spatial resolution but different in terms of Spatio-temporal resolution have been generated and are freely downloadable at the global scale. The latest ERA5-Land product released in 2019 is characterized by a spatial grid to 0.1&amp;#176; latitude and 0.1&amp;#176; longitude. The database provides several land variables at hourly time-step including, among others, air temperature, dew point temperature and solar radiation at 2.0 m above the soil surface, as well as the wind speed components at 10 m height.&lt;/p&gt;&lt;p&gt;The objective of the research was to assess the suitability of ERA5-Land dataset of climate data to predict daily reference evapotranspiration in Sicily, Italy. For the period 2006-2015, the performance of the reanalysis data to capture the local climate variables was assessed based on the comparison with the corresponding ground data measured by a network of 39 climate stations in Sicily belonging to the Agrometeorological Information Service (SIAS). After evaluating the statistical errors associated with each climatic variables retrieved from the ERA5-Land, the comparison between daily ET&lt;sub&gt;0&lt;/sub&gt; values obtained with the FAO56-PM and considering both the dataset was carried out.&lt;/p&gt;&lt;p&gt;The analysis showed that air temperature, solar radiation and wind speed retrieved by the ERA-5 dataset resulted in quite good agreement with the corresponding measured on the ground, with an average root mean square error (RMSE) equal respectively to 1.8&amp;#176;C, 2.9 MJm&lt;sup&gt;-2&lt;/sup&gt;d&lt;sup&gt;-1&lt;/sup&gt;, and 1.3 ms&lt;sup&gt;-1&lt;/sup&gt; and corresponding mean bias errors (MBE) of -0.4&amp;#176;C, 1.0 MJm&lt;sup&gt;-2&lt;/sup&gt;d&lt;sup&gt;-1&lt;/sup&gt;&amp;#160; and -0.1 ms&lt;sup&gt;-1&lt;/sup&gt;. On the other hand, relative air humidity was characterized by average values of RMSE and MBE respectively equal to 10.3% and 5.6%. When considering all the examined climate stations, the RMSE and MBE values associated with ET&lt;sub&gt;0&lt;/sub&gt; ranged from 0.4 to 1.3 mm d&lt;sup&gt;-1&lt;/sup&gt;, and -1.0 and 0.0 mm d&lt;sup&gt;-1&lt;/sup&gt;, supporting the possibility to consider the ERA-5 data to obtain suitable estimations of crop reference evapotranspiration even for other Mediterranean countries where measured climate data are not available.&lt;/p&gt;