A predictive model for quantifying the combined effect of land use change and climatic variability on sedimentation in Malaba sub catchment.

Abstract

This research aimed at using SWAT as a predictive model to assess the impacts of historical, current, future climate change and land use land cover changes on sedimentation in Malaba sub catchment. Four scenarios were modelled in SWAT during this study, the first scenario focused on historical (1990 to 2005) sedimentation and flow in the catchment. The second scenario focused on the current (2006 to 2020) sediment yield and flow in the catchment. Calibration and validation were done using SWAT CUP software version 5.1.6. The calibrated and validated model was then used to further simulate two future scenarios. The two future scenarios used projected land use data and projected weather data. Land use data was projected using Clerk Labs TerrSet software while weather data was projected using the Statistical Downscaling Model (SDSM)version 4.2 considering RCP 4.5 and RCP 8.5. The third scenario focused on flow and sediment yield of 2023 to 2035. The fourth scenario focused on flow and sediment yield of 2036 to 2050. The results obtained from climate change analysis indicated un even changes to occur from 2020 to 2050, rainfall over the catchment is expected to increase by an average of 19.6% (RCP 8.5) and 18% (RCP 4.5), maximum temperature is expected to increase by 1.02°C (RCP 8.5) and 0.85°C (RCP 4.5), minimum temperature is expected to increase by an average of 0.5°C (RCP4.5) and 0.75°C (RCP 8.5) Land use change results indicated that from 2020 to 2050 cropland is expected to increase from 65% to 72.32% of the total catchment area by 2050 while built-up area is expected increase from 2.26% to 3.67%. Forests are expected to reduce from 6.5% to 1.9% of the catchment area while water bodies (streams, wetlands and swamps) are expected to reduce from 3.3% to 1.42%. Average annual Sediment yield is expected to generally increase from 2431.4 tons/km2/year (1993 to 2020) to an average annual sediment yield of 3147.6 tons/km2/year from 2020 to 2050, that is to say an increase of 29.5%. The average rate of annual sediment yield increase is estimated at 1.056% per year. Therefore, mitigation measures should be adopted to ensure sustainable management of the catchment. The study findings of this research shall be relevant for planning, design and management of reservoirs, dams, irrigation systems and sustainability of eco systems in the catchment.