Water quality modelling of the Lake Victoria basin and impact investigation of climate change on the quality of the Lake

Over 30 million inhabitants depend on Lake Victoria (LV) in East Africa. The lake has a complex shoreline structure comprising of gulfs and bays. Inner Murchison bay (IMB) is the source of water supply to Kampala city and also recipient of its wastes. Consequently, water treatment costs by National Water & Sewerage Corporation (NWSC) have trebled in the past decades; thus the need to invest in alternative options. There is lack of insight on the long-term variation of water quality in the IMB, hence the need to study the IMB water quality and link the influence of management actions and climate change on the IMB. This study helps to understand the past and future changes in IMB water quality and support decision making for sustainable economic development.

This research analyzed water quality patterns in the IMB. It explained the long-term variations in the IMB water quality over the past decade by applying a combined empirical – model based approach. It uses lake level variations to study the IMB hydrodynamics and consequently provides a plausible water quality model to study and obtain a better understanding of the impacts of pollution and climatic conditions. The improved understanding and model were applied to study effects of climate change on the IMB water quality and to analyze the effect of selected management practices. The Delft3D-delwaq module was applied for the IMB water quality model and climate change impacts studied for the RCP4.5 and RCP8.5 scenarios. The impact of management scenarios in terms of dredging and regulating effluent quality to improve water quality in the IMB was studied.

Water quality in the IMB deteriorated exponentially in 2001-2014 due to increased pollution and high residence time of water. The worst water quality occurred in 2010 and was attributed to increased diffuse pollution due to improved drainage in the city and the declining wetland effect. Climate change is projected to translate into potentially strong but highly uncertain lake level changes (2040s: -12 to +5 & 2075s: -15 to +4); with annual precipitation increasing by 7 - 18%. The increased lake levels may lead to frequent saturation and increased organic matter degradation especially for low BOD concentrations (<15 mg/l). A 30% reduction of wastes in the IMB (at the hotpots) could result to more than 60% improvement of ammonium downstream of the IMB while ammonium levels downstream of the IMB  could improve by more than 50% by regulating the effluent quality to national standards. However organic matter levels in the IMB have accumulated to levels too high to assume effective dilution/flushing by the main lake.