Modeling Impacts of Agricultural Land Expansion on the Streamflow Regime: Case Study of Thiba River Watershed in Kenya

Abstract

The continuous increase of agricultural water demand in Thiba River Watershed as a result of the agricultural expansion especially in the Mwea Irrigation Scheme and other parts of the watershed has led to over abstraction of water from the scheme’s main water supplier, Thiba River, hence reducing its flows. Thiba River supplies water to Kamburu dam, a hydroelectric dam, on its downstream and it is also depended upon by various livelihoods. This study sought to find out the potential impacts the expansion of agricultural lands would have on the streamflow regime of Thiba River and how it would affect the downstream users. The research was carried out by determining the streamflow response to agricultural water abstraction using Spearman’s rank correlation method. A relationship between streamflow and rainfall variability using regression analysis was also established. In order to understand how agricultural expansion had occurred, comparison was made between the 2004 and 2014 land use maps for the watershed. The HEC-GeoHMS and HEC-HMS models were then used to model the impacts of agricultural expansion on the streamflow. Different streamflow scenarios were simulated and their effects and impacts on the downstream users assessed. The results of the study showed that there was a very significant decline in the streamflow in the drier months compared to the wetter months. The regression analysis of the variability between streamflow and rainfall provided a moderately strong but significant relationship between the two variables. In addition, results showed that agricultural area had increased by over 6% between 2004 and 2014. Results from the model calibration showed a satisfactory efficiency with an NSE value of 0.50. Simulations of three different future scenarios indicated that there would be a significant decline in the future streamflow as a result of land use and population changes due to expanded agricultural lands. The results also showed that increase in precipitation would have very little impact on the streamflow. The study therefore concluded that increased agricultural expansion would lead to a tremendous decline in the streamflow. This would therefore call for proper water management and adaptation mechanisms to be put in place in order to maintain future water supply from Thiba River. This study is useful to water professionals and managers in developing a robust integrated water and land management system. In addition, the findings from this study would inform policy makers to make informed decisions on river water resource management.