Abstract
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). 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. 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.
Highlights
Nowadays the method proposed by Allen et al (1998) to determine evaporative demands of agricultural crops has become generally recognized as the most accurate and has been adopted as a standard approach
ET0 determined from LANDSAF solar radiation data and26 using MAK compares well with the ground truth PMFAO
Sion of the PMFAO formula where net radiation is observed directly is not taken as a reference
Summary
Nowadays the method proposed by Allen et al (1998) to determine evaporative demands of agricultural crops has become generally recognized as the most accurate and has been adopted as a standard approach. It is a concept in which the reference evapotranspiration, ET0, is introduced and in which the water requirements of a particular crop (ETcrop) are obtained with ETcrop = Kc ET0 (1). ET0 is the evapotranspiration (mm d−1) under the given meteorological conditions from a reference surface, notably, an extensive, hypothetical grass reference crop with specific characteristics. The reason is obvious: it is very difficult to maintain grass in the required conditions in the dry season
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