Modelling the impacts of changing land use and climate on sediment and nutrient retention in Lake Tana Basin, Upper Blue Nile River Basin, Ethiopia

Abstract

Natural ecosystems provide many different services such as carbon sequestration, regulation of climate change, biodiversity, and aesthetic value. Ecosystem functions are significantly impacted by both natural and man-made processes, such as sediment and nutrient deposition. The Lake Tana basin faces significant problems with soil depletion, sedimentation, and degradation. Thus, this study is designed to assess the effects of climate and land use change on the export of sediment and nutrients as well as their retention in the basin. This study used different data sources such as satellite data for land use and land cover and watershed preparation, climate data from Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) and Climate Model Intercomparison Project phase five (CMIP5), soil data from Ministry of Water, Irrigation and Electricity (MoWIE), and field data for model calibration. Five different Global Climate Models (GCMs) were employed to produce climate projections using the Long Ashton Research Station Weather Generator (LARS-WG) for the basin. The cellular automata (CA) and Markov chain has then been employed for future projections of changes in land use. Finally, the Integrated Valuation of Ecosystem Services and Tradeoff (InVEST) sediment delivery ratio (SDR) and nutrient delivery ratio (NDR) models have been applied to assess the changes in sediment and nutrient export between the baseline and future time periods. The findings indicated that between 1990 and 2019, there was a change in land use that led to an increase in sediment exports of 60.5% and a decrease in sediment retention of 12.8%. Projecting into the future, the amount of exported and retained sediment increases under the RCP4.5 and RCP8.5 scenarios in the 2040s by 8.4% and 17.8%, respectively due to the combined effects of climate and land use change. Equally, nitrogen and phosphorus exports rose under land use change scenarios but are projected to decrease under climate and integrated climate and land use change scenarios. Out of the 12 sub-watersheds that were designated, five were found to have a high mean annual rate of sediment and nutrient export and were then ranked in terms of priority level for conservation planning. Hence, this research shows that the integration of spatial data with the InVEST model can be used to aid land use planning and decision-making to combat nutrient and sediment exports and prioritize watersheds.