ASSESSING GROUNDWATER-SURFACE WATER INTERACTIONS OF THE SUDD WETLAND, SOUTH SUDAN, USING NDVI/NDWI TIME SERIES AND MONTHLY GRACE ANOMALIES

The Sudd wetland, South Sudan, is a tropical floodplain wetland along the White Nile. Part of the Nile River Basin, the dominant inputs are flow downstream from southern lakes and seasonal precipitation to local tributaries. Maximum wetland extent lags behind these seasonal precipitation events by 3-4 months. Estimates state that over 80% of the wetland will disappear before the next rainy season, much of it through evapotranspiration. This dramatic seasonal inundation pattern supports a unique ecosystem of native and migratory species. Regional water scarcity and increasing climatic extremes highlight the importance of balancing local water needs with regional development plans. Responsible wetland management requires understanding the entire ecosystem, including how the Sudd wetland interacts with basin-wide hydrology and subsurface aquifer resources.

The Sudd’s remote location and ongoing conflict in the region makes collecting in situ hydrologic data complicated and expensive. Extending our previous one-year analysis, Moderate Resolution Imaging Spectrometer (MODIS) imagery acquired between February 2000 and May 2019 was analyzed using Google Earth Engine (GEE) to quantify the Sudd’s surface inundation. Wetland extent was determined by combining areas with positive Normalized Difference Water Index (NDWI) and high Normalized Difference Vegetation Index (NDVI). Gravity Recovery and Climate Experiment (GRACE) mascon data was also analyzed for total water storage anomalies (TWSA), which track the transport of water through the wetland. TWSA represent the entire water column, including both surface and groundwater. To identify surface water-groundwater interactions, we compared the 15 year overlapping record between MODIS NDVI and NDWI surface water extent data and total water storage anomalies acquired from the Gravity Recovery and Climate Experiment mission (2002-2017). The time lag between the combined NDVI/NDWI extent and GRACE anomalies suggests that the vegetation in the Sudd area is water-limited and relies on surface and very shallow groundwater.