Reference crop evapotranspiration derived from geo-stationary satellite imagery: a case study for the Fogera flood plain, NW-Ethiopia and the Jordan Valley, Jordan

Abstract

First results are shown of a project aiming to estimate daily values of reference crop evapotranspiration ET₀ from geo-stationary satellite imagery. In particular, for Woreta, a site in the Ethiopian highland at an elevation of about 1800 m, we tested a radiation-temperature based approximate formula proposed by Makkink (MAK), adopting ET₀ evaluated with the version of the Penman-Monteith equation described in the FAO Irrigation and Drainage paper 56 as the most accurate estimate. More precisely we used the latter with measured daily solar radiation as input (denoted by PMFAO-Rs). Our data set for Woreta concerns a period where the surface was fully covered with short green non-stressed vegetation. Our project was carried out in the context of the Satellite Application Facility on Land Surface Analysis (LANDSAF) facility. Among others, the scope of LANDSAF is to increase benefit from the EUMETSAT Satellite Meteosat Second Generation (MSG). In this study we applied daily values of downward solar radiation at the surface obtained from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) radiometer. In addition, air temperature at 2 m was obtained from 3-hourly forecasts provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). <br><br> Both MAK and PMFAO-Rs contain the psychrometric "constant", which is proportional to air pressure, which, in turn, decreases with elevation. In order to test elevation effects we tested MAK and its LANDSAF input data for 2 sites in the Jordan Valley located about 250 m b.s.l. <br><br> Except for a small underestimation of air temperature at the Ethiopian site at 1800 m, the first results of our LANDSAF-ET₀ project are promising. If our approach to derive ET₀ proves successfully, then the LANDSAF will be able to initiate nearly real time free distribution of ET₀ for the full MSG disk.