Hydrological responses to land use and land cover Changes of ribb river watershed, upper blue nile, Ethiopia

Abstract

The main objective of this study is to assess the impact of land cover changes on the hydrology of Ribb river basin. Specifically, the study analyzed the present land covers that have taken place in the catchment and its effect on the hydrological responses of the catchment. Land cover change scenarios were used to determine the potential effect that will happen on the catchment hydrology. The Soil and Water Assessment Tool (SWAT2009) model was used to investigate the impact of land cover change on hydrological responses of the study area. The model was set up using readily available spatial and temporal data, and calibrated against measured discharge and sediment concentration. Sensitivity analysis result shown SCN curve number (CN), Soil Evaporation Compensation Factor (ESCO), Soil Depth (m) (Sol_Z), Threshold water depth in the shallow aquifer for flow (GWQMN), Base flow alpha factor (Alpha_Bf), (REVAPMN) and Soil Available Water Capacity (SOL_AWC) were found the most influential parameters affecting flow and USLE equation support practice (USLE_P),Linear parameter for maximum sediment yield (SPCON), Exponential parameter for maximum sediment yield in channel sediment routing (SPEXP),Cropping practice factor (USLE_C),channel cover factor (CH_COV1),channel erodiability factor (CH_ERODMO) were the most sensitive parameters affecting sediment yield of the catchment respectively. The model was calibrated from 1996-2008 and validated from 2009-2014 for both flow and sediment at Ribb river gauging station. The performance of the model was evaluated on the basis of performance rating criteria, graphical method, water balance, coefficient of determination (R2) and Nash Sutcliff efficiency (NSE). The R2 and NSE values for the catchment were (0.79, 0.78) for flow calibration, (0.7, 0.68) for flow validation, (0.77, 0.71) for sediment calibration and (0.72, 0.72) for sediment validation respectively. Three land use/cover change scenarios were developed to analyze the impact of land use/cover changes to the hydrological regime. Base scenario: current land use practices has cultivated land, grass land, shrub and bush land, forest land, built up area and water body, scenario1: shrub and bush lands completely changed to forest land and scenario2: Grass land changed to cultivated land. The result for different land use scenarios show that: conversion of shrub land to forest area reduced surface runoff, reduced the amount of sediment transported out and increase base flow but conversion of grass land in to cultivated land areas increased surface runoff during wet seasons and reduced base flow during the dry seasons and also as the peak flow increases it is suspected of carrying more sediment.. In general, from the result of land use scenario, the changes in stream flow characteristics could be related to the change of the land use.