Spatial variability of evaporation and moisture storage in the swamps of the upper Nile studied by remote sensing techniques

The estimation of spatial variation of evaporation E in a catchrnent is fundamental to many applications in water resources and climate modeling. Evaporation, being the sum of interception, soil evaporation, open water evaporation and transpiration, is a key variable in water balance determinations, but also to estimate the moisture, heat and CO 2 interactions between land and atmosphere (see e.g., Sellers et al., 1996 ). Several climatic studies have indicated that atmospheric circulation and rainfall are significantly affected by the large-scale variation of soil moisture and evaporation (e.g., Savenije, 1996b ; Entekhabi et al., 1999 ). Three methods can be used to estimate the evaporation at a regional scale: by up-scaling point measurements, by remote sensing techniques (e.g., modeling the energy balance at the land surface) and by hydrological modeling (e.g., Savenije, 1997 ). Each of the three methods has its limitations, and an optimal procedure probably would be a combination of the three approaches. To maximally profit from remote sensing and hydrological modeling, data assimilating is gaining terrain in hydrological studies ( Walker et al., 2001 ; Jhorar et al., 2002 ; Schuurmans et al., 2003 ) as well as in climate studies (e.g., Dolman et al., 2001 ). Extensive reviews of remote sensing flux determination methods have been presented by Choudhury ( 1989 ), Moran and Jackson ( 1991 ) and Kustas and Norman ( 1996 ).