The main aim of this study is the use of the satellite ASTER data for assessing the daily actual evapotranspiration by solving the energy balance equation at the surface. The energy balance model used is SEBAL (surface energy balance algorithm for land). It is a one-source model that calculates the net radiation, soil heat flux and sensible heat flux at each remote sensing pixel and estimates the latent heat flux as the residual term in the energy balance. It involves in input the NDVI, surface temperature and albedo. The study area encompassed by ASTER image contains extremes in surface albedo, vegetation cover and surface temperature values. It corresponds to Ksar Chellala plains in Algeria.

To estimate the sensible heat flux, which represents the crucial variable in solving the energy balance equation, we opted for a method based on the use of profiles in the surface layer, with the coupling taking place in fluxes at the convective boundary layer base. We showed that a simplification of this method by ignoring the atmosphere thermal stability functions can give the same results approximately. It is then possible to estimate the sensible heat flux and calculate the latent heat flux as the residual term of the energy balance. We can then asses the actual daily evapotranspiration and some moisture indices: evaporative fraction, Priestley and Taylor parameter and surface resistance to evaporation. These indices allow quantitative monitoring in pixel basis of water status degree.