Pollutant Transformation Performance and Model Development in African Wetland Systems: Large Catchment Extrapolation

Two wetland systems in the Lake Victoria Basin, western Kenya, were investigated and monitored to assess buffering capacity and also to develop a model approach to evaluating larger-scale pollutant-buffering capacity of regional wetlands. The Dionosoyiet wetland was located in the highlands of the Rift Valley Province, at approximately 2,000 m altitude. It was located immediately adjacent the Kericho town centre, covered a 34 ha area and was set in a catchment of 23 km². The wetland was located in the upper reaches of the Sondu-Miriu river system which flows into Lake Victoria. The 560 ha Chepkoilel wetland, near Eldoret, had an agricultural catchment area of 210 km² with major inflows to the wetland contributed by the Sergoit-Misikuri river system. The catchment drained areas of mild slopes ranging up to 2,160 m above sea level. Water-quality investigations were undertaken in the wetlands from June 2004 to April 2005 for nutrients and suspended solids. Hydrology and water-quality modelling were performed utilising the LAVINKSWEB model. The model was adapted to incorporate a rainfall-runoff module based on the isochronal histogram technique and a partially stochastic prediction of water quality (TSS, TN and TP) based on incoming flow rates. Using the data gathered from June 2004 to April 2005 for calibration and earlier climatic data, modelling was performed to cover an 11-year period, from January 1994 to December 2004, and indicated that the wetland removed 43% TSS, 41% TP and 20% TN with average areal removal rates of 21.3 TSS, 0.038 TP and 1.03 TN (kg ha^–1 day^–1) for the Dionosoyiet system and considerably greater removal from the Chepkoilel system. The findings and model development show that in addition to being critical ecosystem diversity reservoirs and central community/agricultural-activity resources, the wetlands perform significant functions of water-quality improvement. The preservation of these wetlands and other similar wetlands is important in ensuring sustainable utilisation of water resources in the Lake Victoria Basin.