The impacts of land use and climate change on Simiyu river discharge and the riverine sediment dynamics flowing towards lake Victoria

Abstract

This study aimed to trace the dominant sources of riverine sediments and assess climate change's current and future impacts on the river discharge at the critical agroecological region of the Simiyu catchment. Geochemical fingerprinting of the riverbed sediments and potential sediment sources were compared using a Bayesian mixing model (MixSIAR) to attribute the dominant riverine and land-use sources to the Simiyu Mainstem. The mixing model outputs showed that the Simiyu tributary was the dominant sediment source to the Simiyu Mainstem with 63.2%, while the Duma tributary accounted for 36.8%. Cultivated land was shown to be the main land-use source of riverine sediment, accounting for 80 % and 86.4% in the Simiyu and Duma sub-tributaries, respectively, followed by channel banks with 9% in both sub tributaries. The Soil and Water Assessment Tool (SWAT) under RCPs 4.5, 6.0, and 8.5 were also used to project the impacts of climate change on river discharge throughout 2030–2060. The selected three General Circulation Models (GCMs) predicted an increase in the annual average temperature of 1.4°C in 2030 to 2°C in 2060 and an average reduction of 7.8% in rainfall, which causes a decrease in river discharge. The simulated river discharge from the hydrological model under RCPs 4.5, 6.0 and 8.5 revealed a decreasing trend in annual average discharge by 1.6 m3s−1 from 5.66 m3s-1in 2019 to 4.0 m3s-1in 2060. Arbitrary, there will be an increase in frequent flood occurrence in the future (2030–2060) compared to the current period (1990–2019), with extreme discharges of 451.3 m3s−1and 232.8 m3s−1 at exceedance probabilities of 0.01% and 99.99%, respectively. The demonstrated application of sediment source tracing provides an important pathway for quantifying the dominant sediment sources in the rivers flowing towards Lake Victoria. This information is vital for designing catchment wide management plans that should focus on buffering the projected decreases in discharge, reducing soil erosion and sediment delivery from farming areas to the river networks, and ultimately supporting food security and water quality in the Lake Victoria Basin