There is a potential significance for improving the monitoring and management of irrigation water needs in semi-arid areas in Mediterranean countries like Greece. This study deals with the estimation of actual evapotranspiration (ET_a) above selected cotton fields located in the semi-arid region of Thessaly, Greece, for the growing season of 2022. The described methodology estimates daily crop evapotranspiration values using a combination of Sentinel-2 and Sentinel-3 satellite images and meteorological data derived from the Weather Research and Forecast (WRF) model. The methodology consists of seventeen separate steps for the estimation of energy parameters and the final estimation of actual daily evapotranspiration values at a 20 × 20 m spatial resolution. The basic idea of the process is based on the Two-Source Energy Balance (TSEB) methodology. The Sen-ET SNAP graphical user interface developed by the European Space Agency is used to estimate ET_a, with the use of Sentinel 2 and Sentinel 3 satellite data. One of the innovations of the study is the use of the WRF model instead of the initially proposed ERA-5 for the retrieval of the relevant meteorological data. Another innovation is the computation of the regional adjustment factor, which leads to realistic values for daily ET_a. The results are very promising; however, this study still needs validation through in situ experiments.