Contribution of Climate Variability and Land Use Change to Streamflow Variations in Thika Dam Watershed

Abstract

Climate variability and human activities are the main causes of change in streamflow. Decoupling the respective contribution of climate variability and human activities in changes in streamflow is an important activity of watershed management. It helps in designing appropriate mitigation measures against the change in streamflow.  The main objective of this study was to assess and quantify the change in streamflow respectively caused by climate variability and land use change in Thika River watershed, Kenya. Mann-Kendall analysis was used to assess streamflow data trend while sequential Mann-Kendall analysis was used to establish the breakpoint. The Wang-Tang model was applied to separate the effects of human-induced land use changes and climate variations on the streamflow. Drought analysis was done to investigate the intra-annual effect and transition of these changes to streamflow change. Standard Precipitation-Evapotranspiration Index (SPEI), representing Meteorological Drought (MD), and the Standardized Runoff Index (SRI), representing the Hydrological Drought (HD), were used to study climate and streamflow variability respectively. It was found that average annual Thika River streamflow had decreased significantly in the last 50 years, with a breakpoint in 2003. Climate variability was identified as responsible for 46% of this streamflow change, while the remaining 54% was attributed to land use changes. Furthermore, drought analysis indicated a post-2003 increase in both types of droughts, affirming reduced rainfall and streamflow. However, HDs outnumbered MDs, highlighting the compounding impact of land use change alongside climate variability on streamflow reduction. This comprehensive approach combining the Budyko methodology with drought analysis offers valuable insights into the intra-annual dynamics and transitions of climate variability and land use change effects on streamflow. The study recommends the integration of interventions targeting proper land use and climate-resilient water management for informed decision-making in watershed planning and management, underscoring their importance in addressing evolving hydrological challenges.