ESTIMATING WATER BUDGET IN THE SUDD MARSHES USING GRACE AND MODIS

The Sudd Marshlands, located in South Sudan are an extensive wetlands system which have been heavily impacted by both human and climate forces over the past decades. The Sudd wetlands are highly variable in size, averaging roughly 30,000 km2, but extending to as large as ~130,000 km2 during the wet seasons. These marshlands serve as a buffer for water mass moving down the Nile River system, and are therefore the major source of baseflow to the White Nile. A combination of time series GRACE satellite gravity measurements and MODIS visible images 2003-2015 for the Sudd region were used to determine the time dependent change in surface water area (SWA) in the marshes, marshland extent and variability in total water storage. Combined open water area and vegetation abundance and cover, as determined by MODIS (NDVI and MNDWI), is highly correlated with total mass variability observed by GRACE (RL05 Tellus land grid). Annual variability in the Sudd correlates well with combined SWA and vegetation extent. Variability of vegetation is seen to correlate well on an annual basis with water storage variation, and with a 2 month lag (water mass increases and decreases lead vegetation increases and decreases) when examined on a monthly basis. Combined with Nile flow information, we use this to estimate seasonal varying evapotranspiration (ET) from the wetlands system. Results show that the overall wetlands extent and health is observed to be water limited. Predictions for precipitation variability resulting from climate change (as predicted by CMIP5 suite of models) and human diversions of water through navigation modifications are expected to lower water availability and lower variability in these systems. This will lead to the shrinking of this important regional wetlands system, with resulting loss in habitat and other ecosystem services.