The hydrology of the Sudd : hydrologic investigation and evaluation of water balances in the Sudd swamps of southern Sudan

This dissertation describes the hydrology of the Sudd swamps in southern Sudan, comprising a hydrologic investigation and evaluation of its water balance. The study improves historical interpretations and unveils so far unknown conditions and interdependencies in the swamp and floodplain system, considering the area morphology, vegetation dynamics, water balance, flow conditions and hydrodynamic processes in the system. The assessment is based on field works carried out in the swamps over a three year period between 2004 and 2006 as well as historical and remote sensing data. Interdependencies between morphology and hydrology are described considering the effect of ground slopes and morphological features on spill and flooding of the seasonally flooded grasslands. Bathymetric depth profiles and cross sectional depth and flow distributions have been established for the Sudds inland delta and the question of spill into the Bahr el Ghazal basin has been assessed. Vegetation dynamics in the swamp have been described and quantified assessing the extent of changes in the channel and lagoon system of the Sudd using Landsat satellite images for water body delineation. Changes in-between years and as well compared to Lake Victoria outflow data have been established and quantified; correlation was found for the largely water level dependent lagoon system. Variations in the channel system were interpreted to happen in a certain pattern but the extent of changes could not be correlated to the outflow data as they are influenced by additional, here not considered factors like wind drift and channel blockages by vegetation. The water balances of the floodplains, while controlled by river levels, were found to be dependent on a complex interaction between soil, vegetation, topography and seasonal trends in rainfall and evapotranspiration. Based on field measurements, these components have been assessed in detail and evaluated regarding their function in the seasonal cycle of flooding and drying. An analysis of the soil and evapotranspiration conditions as well as the interaction with vegetation and meteorological conditions using field and laboratory experiments was conducted. Sources, processes, flow directions and the fate of the floodwaters on both the river fed seasonal floodplains and the rainfed grasslands have been established showing that river spill is responsible for area flooding while no return flow occurs and drying is caused by evapotranspiration alone. Additionally it was found that rainfall can only cause temporary flooding in extreme events. To obtain missing flow data for important but ungauged locations, methods have been established to derive these, combining upstream flows from Lake Albert and torrential runoff derived from the Collaborative Historical African Rainfall Model (CHARM) rainfall data in the catchment between Lake Albert and Mongalla. The results provide previously unavailable flow data at Mongalla, the entry to the Sudd swamp, with a high level of confidence; data which are essential for detailed hydrological assessments of the swamp system. In addition to evaluations based on measurements, a numerical hydrodynamic assessment has been carried out with the DHI MIKE21 model. The study has established water level gradients, flow directions and velocities in the swamp as well as on the seasonal floodplains.