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This study is part of the SUDMED project from IRD (Research Institute for Development), CESBIO, University of Marrakech and Moroccan administrations in charge of agriculture, forestry and water management. The objective of this project is the hydro-ecological modeling of hydrological resources on the Marrakech region. In this context, it is important to characterize the snow cover and the melting dynamics as it is the main water source for the plain. Evaluate the snow cover using satellite images at a short temporal scale is a first step to estimate water quantity available during the melting season, in spring. The objective of this study is to calculate the snow cover area and to follow its evolution during the winter season using both high and low satellite imagery. For this purpose, several snow cover indices (SCI) calculated from low resolution images SPOT/VEGETATION (pixel size of 1 km/sup 2/) have been compared with the snow cover percentage (SCP) calculated derived from high spatial resolution sensors (in particular LandSat-TM). Particular attention was given to the classification procedure and the co-registration problem. Then, pixel-based regressions between SCP and SCI have been calibrated for several dates (when both VEGETATION and TM images were available) with the objective of finding the most robust relationship. The previous relation is applied to a time series of 30 SPOT/VEGETATION images acquired from December 1998 to May 1999. The dynamics of snow cover is compared with rainfalls and flow chronicles observed on three mountainous watersheds. The main interest of this study is to show that low resolution precision can be easily improved using high resolution imagery to obtain reliable quantitative information on snow cover. This last information is an important input of snowmelt runoff models.