Evapotranspiration measures the quantity of water lost by evaporation from the soil and the plant. It is expressed in mm/day. In semi-arid areas, a main part of precipitation is in fact lost by evapotranspiration. So, this quantity is an important component of the water cycle and it becomes necessary to estimate it and to monitor its temporal evolution with precision. Up date, in Mbuluzi catchment in Swaziland, evapotranspiration is only measured at discrete points where ground measurements stations are available. This paper addresses the problems of computing maps of evapotranspiration with a large coverage and a high temporal frequency, by using only remote sensing data and without additional field and/or meteorological data