Simulation of Hydrological Effects of Land Use/ Land Cover, Climate Change, and Effect of Dam at Gilgel Abay River Basin, Ethiopia

Abstract

Water is the most essential resource for survival of living things and it is the most crucial resource associated with land use/ land cover (LU/LC) and climate changes. Hence, it is very important to make evaluations of the expected impact on the hydrology and water resources. Flood is the most chronic and hazardous phenomena all over the world and causes loss of human life, natural resources as well as infrastructures. In addition, dams have been designed and constructed for various purposes. However, dams have effects on water and sediment transport, which determines overall morphology of river. Ethiopia has many dams; one of these dams is Koga dam which was constructed across Koga River, which is tributary to Gilgel Abay River, but information on effects on river hydrology and sediment transport was not evaluated. Therefore, this research was conducted at Gilgel Abay River Basin to address the following objectives; (1) to develop a hydrological model to evaluate the effect of land use/ land cover and climate change over the years on stream flow in the river basin, (2) to simulate stream flow to Lake Tana (3) to estimate future daily annual peak stream flow and flood frequency, and (4) to identify effect of dam on river hydrology and sediment transport. Precipitation Runoff Modeling System (PRMS), which is a modular-design, deterministic, distributed-parametric modelling system was used to evaluate the impacts of various combinations of precipitation, climate, and land use changes on stream flow as well as for predicting future annual daily peak stream flow. System inputs are daily time-series values of precipitation, minimum and maximum air temperature, and parameter files which are generated from Geographical Information System Weasel (GIS Weasel). The methods which were used to evaluate combined effects of LU/LC, vegetation type, vegetation density and climate changes on stream flow were two different periods` LU/LC, vegetation type, vegetation density and climate changes, these were: period one (1990-2000) and period two (2001-2010) of LU/LC, vegetation type, vegetation density and climate changes. These gridded maps as well as soil maps were used in GIS Weasel to generate parameters for PRMS model. Hence, these generated parameters within different time series data fed to PRMS model to simulate stream flow. To estimateii future daily annual peak stream flow and flood frequency, the values for historical climate changes in the basin were adjusted on the basis of changes that are projected for 21st century at Gilgel Abay river basin. Air temperature was adjusted by temperature values of no change, +1.50c and +30c of historical temperatures by adjusting model parameters rather than adjusting input variables. Precipitation was adjusted by two different precipitation values ranging from -10% to 10% of observed precipitation by adjusting input variables. In addition, Effect of Koga dam on river hydrology and sediment transport was evaluated by using hydrograph variations before and after the construction of dam as well as sediment yield at the catchment outlet of Koga river basin before and after the construction of dam by using Revised Universal Soil Loss Equation (RUSLE) and Sediment Delivery Ratio (SDR). As climate and LU/LC, vegetation type and vegetation density changed from period one to period two, stream flow increased by 13.5% and ET decreased by 18.3% compared to baseline period (1993-2000). Future annual daily peak stream flow with 50% and 1% AEPs will increase by 14.3% of historical modeled value of peak stream flow at the end of 21st century when temperature is held constant and precipitation increases by 10%, but for other combinations, there is a decrease of stream flow. There is reduction of 5.9 t/year of sediment yield at the outlet of Koga river due to the construction of Koga dam. Generally, combined effects of LU/LC and climate change are more on stream flow and ET than individual effects, and Future annual daily maximum peak stream flow and flood frequency will decrease by large amount as temperature increases. In addition, construction of dam has an effect on river hydrology and sediment transport.