The Impacts of Climate Change and Land Use Dynamics on the Hydrology of Upper Awash River Basin.

Abstract

Land use and climate are two important factors influencing hydrological conditions. Land use change can result in change of flood frequency, severity, base flow, and annual mean discharge, while climate variability can change the flow routing time, peak flows and volume. The main objective of this study was to assess the mechanisms associated with physical and dynamical processes influencing the hydrology of the Upper Awash River basin, especially under a changing land cover and climate. Specifically the study tried to quantify the past changes in land use and future climate change and their implication to catchment hydrology. The past changes in land use were determined by classifying the Landsat images of the years 1972, 1985, 1995, and 2005. Bias corrected CORDEX RCM data was used to analyze future climate conditions. Power transformation and linear regression techniques were used to correct the biases of precipitation and temperature respectively. The Soil and Water Assessment Tool (SWAT) model was used to investigate the impact of land cover and climate change on stream flow. The model was calibrated and validated using historical flow data. Land cover change analysis has shown an increase in urban land use and cultivation by 82.6% and 16% respectively and decrement in the other land uses like forest, grassland, water body, wetland, and shrub land by 1.7%, 313%, 11%, 91.31%, and 360% respectively. The climate scenario has indicated that the rain fall will increase by 15.8% and 26.7% for the wet months in the short (2011-2040) and long (2071-2100) time horizon scenario respectively; But it will decrease by 5.07% during the medium time horizon scenario (2041-2071). The average maximum annual temperature will increase for all the short, medium and long time horizon scenarios. The SWAT model was calibrated using flow data from 1987 to 1999 and validated from 2000 to 2009. The R 2 and Nash‐Sutcliffe Efficiency (NSE) values for the watershed were 0.82 and 0.83 for calibration, and 0.74 and 0.7 for validation, respectively. Simulation of catchment response for land cover dynamics has indicated a 3.5%, 5%, and 9.6 % increase in the mean wet monthly flow for 1985, 1995, and 2005 land covers respectively; but a decrease in the dry average monthly flow by 5.2%, 8.4% and 4.5% for land cover of 1985, 1995 and 2005 land covers respectively. Simulation of SWAT model for the future climate scenarios has shown that the average annual runoff will increase by 9.2% and 10.6% for the short (2011-2041) and long (2071- 20100) time horizon scenarios. Finally, the study also recommends different adaptation and mitigation strategies should be developed to deal with the consequences of these environmental changes.