The temporal variation of evaporation from the Sudd wetland and its bio-physical and hydrological interpretation

This paper describes a new methodology to describe the water use from Africas largest wetland (Sudd) without specifying its habitats. The evaporation of the Sudd is estimated by means of satellite measurements and the Surface Energy Balance Algorithm for Land (SEBAL) that converts spectral radiances into fluxes during specific cloud free days. The surface albedo, surface emissivity, leaf area index, roughness height, aerodynamic resistance and surface resistance have been derived from remote sensing technique. The annual average values for these parameters are 0.19 (-), 0.94 (-), 0.44 (-), 0.025 m, 61 s/m and 224 s/m respectively. The bio-physical properties from remote sensing data are used in conjunction with the Penman-Monteith equation to assess the evaporative depletion for longer time periods. For 3 selected years of different rainfall conditions (1995, 1999, 2000), the evaporation for an area of 3.8 million ha was found to be 56.5, 73.9, 63.1 Gm3/yr respectively. Since there is no possibility to measure evaporation in this marshland (physically and politically in-accessible), the bio-physical properties of the swamp vegetation derived from satellite measurements are compared against literature values. The values are found to lay within the expected ranges. During the summer floods, saturated soil conditions are dominant, while in the winter unsaturated soils and decaying vegetation persists. It is demonstrated that the variation of the atmospheric demand in combination with the inter-annual fluctuation of the groundwater table due to a pronounced seasonality in rainfall and inundation by the Nile floods - can be hold responsible for a quasi-constant evaporation rate in the Sudd and its neighboring swamps throughout the year.