Assessment of the impact of climate and land use changes on the flood frequency of river Semliki in its lower reaches in Ntoroko district
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Floods have been and continue to be one of the most frequent natural hazards causing loss of lives and destruction of property in the Semliki river basin. The two main factors affecting flood frequency, land use change and climate change, have attracted the attention of many researchers for at least the past three decades. There are, however, few studies that examine the effects of climate and land-use changes on the frequency of flooding of Ugandan rivers, and even fewer studies that make a quantitative distinction between the two. This study was conducted to assess climate and land use changes impact on flood frequency of river Semliki in its lower reaches in Ntoroko district. A HEC-HMS hydrological model was used to simulate discharges for the historical period (1991-2020) using bias-corrected rainfall and temperature satellite-based data with and without land use change. Discharge simulations were then run for statistically downscaled future climate projections of the GFDL-ESM4 general circulation model under the SSP24.5 and SSP58.5 future climate scenarios for the near future (2021-2050) and mid-future (2051-2080). Flood frequency analyses were then performed on the annual peak discharges from the simulated discharge time series using HEC-SSP. Comparisons of the resulting flood frequency curves indicated that the flood frequency of the river Semliki increases significantly under both land use and climate changes relative to the baseline period (1991-2020). The 20 year (2000-2020) anthropogenic land use change induced an average increase of 4.87% in the magnitude of annual peak discharges for the considered return period floods [100,50,20,10,5,&2years]. Additionally, the highest increments in annual peak discharges induced by GFDL-ESM4 climate projections under the SSP5-8.5 scenario were 34.70% in the near future and 43.72% in the mid-future for the 20 year and 100 year return period floods respectively. Despite the uncertainty associated with its findings, this study provides insights into future changes in the flood risk of river Semliki due to projected climate changes. It is thus recommended that any current and future flood mitigation and management strategies within the Semliki river catchment in Ntoroko district, Uganda should take into account the expected increases in its flood frequency.