Impacts of expansion of agriculture and land use change on flow regime of Thiba river, Kenya

Soil and Water Assessment Tool (SWAT) was used to model the impacts of land cover changes on stream flow regime in the Thiba River basin a sub-catchment of Tana River Basin covering a surface area of 1648 km2 in central region of Kenya. The basin is characterized by intensive agricultural activities including the largest rice irrigation scheme in Kenya. A study was undertaken to test the capability of the model in predicting stream flow response under changing land use conditions in a typical tropical river basin. Classified land use maps of 1984, 2004 and 2014 were analyzed to investigate land use changes in the basin. Field based survey, National Irrigation Board (NIB), Kenya Meteorological Department and Water Resources Management Authority (WRMA) provided hydro-meteorological data for the study. The results of the study shows that forest cover in the Thiba River basin has decreased by 18.39 % between 1984 and 2014 while area under rice cultivation increased by 9.38 % in the same period. About 35% of dry season flow and 3% of wet season flow was found to have been directly abstracted for irrigation purposes from the Thiba River between 2007 and 2014. The SWAT Model results showed that there is a significant relationship between the observed and simulated average monthly stream flows in the Thiba River Basin. The Nash-Sutcliffe Efficiency (NSE) and coefficient of determination (R2) during calibration period (1983-1988) were 0.82 and 0.9, respectively, while for the validation period (1989-1993) they were 0.79 and 0.87, respectively. The average monthly stream flows increased by 6.01 m3/s during the wet season and decreased by 1.92 m3/s during the dry season. The changes in stream flow were attributed to the land cover change and rainfall variability. The results implies that future expansion of irrigated agriculture would result to increase in flooding during the wet season and low base flow during the dry season. This can consequently result to water conflicts between upstream and downstream water users. The recommends construction of water storage reservoirs to harvest the high runoff during the wet months for agricultural use during the dry season when base flow is low. Also, it's important that further research on impact of land use change on the water quality.