MODELING THE IMPACTS OF CLIMATE VARIABILITY ON DISCHARGE OF THE UPPER RIVER YALA BASIN, KENYA

Abstract

The Upper River Yala, which is a very important tributary of Lake Victoria, has been subjected to a lot of discharge variation owing to the high intensity rains and the growing climate variability. Extreme weathers have changed the streamflow patterns, and there is a high association between tributary inflows and precipitation. In spite of the forecasts of the inter-annual discharge variation, a uniform trend was not yet determined about when the peak or the minimum flow occurs. Past research has looked at rainfall runoff relations but has not estimated the combined effects that climate variability and land use alterations have on discharge especially in the upper Yala catchment. The primary aim of this research was to model the effects of climatic variability on the discharge in the Upper River Yala Basin, Kenya. These objectives were to: simulate the effect of temperature variability on discharge; simulate the effect of rainfall variability on discharge; and to determine the effect of simulated climate variability on discharge in different land use and land cover cases. Based on historical daily temperatures and precipitation records (19902019), the Soil and Water Assessment Tool (SWAT) was installed to model the hydrological responses when different climatic and land cover conditions occurred. River gauging station data was used to calibrate and validate the model (19902006 and 20072019 respectively). It was found that discharge is highly dependent on the temperature and variability of rainfall. Rises in temperature correlated with higher river flows (coefficient = 11.264) and the effect of the maximum temperature was heavy compared to the effect of the minimum temperature. Rainfall became the most dominant factor and it contributed to 94.2% of the discharge variation (correlation coefficient = 0.963). High discharges were seen to occur with high rainfalls and low discharges during dry seasons. Changes in land use especially agricultural development and deforestation were noted to cause a reduction in river flow during the dry season and increase in river flow during the wet season. Dry season discharge decreased at 2.1 m 3 /s and 1.68 m 3 /s and wet season discharge climbed at 1.18 m 3 /s and 5.69 m 3 /s which signifies less water availability and more flows and risk of flood. These results indicate that climatic variability and altered land use have amplified seasonal variations on river discharge, lessening dry season flows and amplifying wet season flows. The temperature change had a major influence on the river flow, and an increase in the temperatures led to a decrease in the dry season discharge and the change in seasonal water supply. Times of change in discharge were also due to rainfall variability where low rainfall caused a decrease in dry season flows and high rainfall events causing a rise in the wet season flows indicating that the basin is sensitive to rainfall variability. Temperature and rainfall fluctuation together with land use alterations like agricultural growth and deforestation aggravated seasonal discharge distributions. This highlights the interactive action of climate and human activities on the dynamics of the rivers. The literature finds that climate variability, especially, rainfall and temperature changes, and land use processes have a significant influence on the pattern of discharges in rivers. It suggests that hydrological models need to be incorporated with temperature and rainfall variability, sustainable land use is needed, and the adoption of an Integrated Water Resources Management (IWRM) framework should be used. Future research should incorporate additional environmental variables, long-term climate scenarios, and ecological impacts, while integrated climate–land use–hydrology models can improve prediction and support sustainable management of the Upper Yala Basin.