Modeling the Impact of Land-Cover and Rainfall Regime Change Scenarios on the Flow of Mara River, Kenya

The 13,750 km² drainage area of the Mara River basin includes the agricultural and forested areas in the upper basin, the open pastureland in the middle portion of the basin and the Masai Mara Game Reserve and the Serengeti National Park in the lower portion. Hydrometeorological analysis of the basin has shown a decline in the dry season flow and increase peak flood frequency in recent years. Changes in the precipitation pattern (distribution and volume), deforestation in the upper basin and increase water use activity in the large scale agricultural areas have been topics of discussion. This study applies the Soil Water Assessment Tool (SWAT) model to study the hydrologic response of the basin to the major hydrological input scenarios: land cover and precipitation regime changes for the upper portion of the basin. The adapted SWAT model was calibrated using daily flow (1980–90) data from the Amala and Nyangores gauging stations. Two scenarios (rainfall volume reduction by 20% for 2010–2030 period and conversion of the Mau Forest area to agriculture) were developed and used in the hydrological modeling. The model output indicated that, a 20% reduction in the rainfall volume translates to an average 46% reduction in annual flow at the Amala gauging station for the period considered. The forest-to-agriculture conversion of the Mau Forest, the water tower of the Mara River, yields only a 3.2% decline in the combined Amala and Nyangores river annual flow. Examining the low flows has shown that the base flow reduced during the dry seasons with the second scenario and this reduction was significant when rainfall decline was used in the simulation.