Using earth observation data and CROPWAT model to estimate the actual crop evapotranspiration

The aim of this paper is to estimate and compare the daily actual evapotranspiration (ETc) for the maize crop, using Earth Observation data and the CROPWAT model, in the agrometeorological conditions of Romania. For the purpose of this paper, the CROPWAT model was run for the specific weather conditions of the year 2000, at two test-areas situated in the main agriculture production zone of Romania: Craiova and Alexandria. The method used in this study for the computation of ETc is based on the energy balance of the surface, expressed in a simplified from. The method uses the connection between the evapotranspiration, the net radiation and the difference between surface and air temperature, measured around 14:00 h. local time of the satellite passage. The NOAA–AVHRR cloud-free satellite images on the Romanian territory were selected from the image database in the maize vegetation period (April–September 2000). Applying the method based on the energy balance of the surface, expressed by the two simplified versions, for the maize crop in the specific conditions of the Romanian Plain, the daily crop evapotranspiration values have been obtained. These values were compared with the outputs resulted from the CROPWAT model. The comparative analysis showed that the ETc values estimated by the method based on the energy balance of the surface which uses NOAA–AVHRR satellite-derived data, are generally greater than those simulated by the CROPWAT model, the differences being in the range +0.45 to −1.9 mm/day. The results of the study showed that the ETc values estimated from both simplified versions of the method based on the energy balance of the surface are quite similar and in a good correlation with the simulated values obtained by the CROPWAT model, with relative errors of ±10–15%. The results of the study emphasized that the satellite data could bring an important contribution for comparisons and validation of the model outputs for some parameters, like the crop daily evapotranspiration.