Modeling the effect of land use change on runoff generation using hydrological models in urban and semi urban areas ( case study of Hawassa town, Ethiopia)

Abstract

For the past few decades, land use and land cover changes occur highly in Awassa catchment due to urbanization, expansion of agricultural land and deforestation. Urbanization effect is one of the most significant land use change that occur in Awassa catchment mainly due to fast growing of Awassa town. The major aim of this study was to assess the effect of land use change on runoff generation using semi distributed models which the approach followed is having three land use and land cover maps of the catchment, analysis was done using Arc-GIS analysis tool. The rate of change in land use is identified in catchment level for two periods and two semi distributed hydrological models, HEC-HMS/Hec-GeoHMS and HBV, utilized to simulate hydrological responses. Stream flow data were used for calibration and validation and comparison of models in simulating land · use change was evaluated based on model performance evaluation criteria. The result of remote sensing and GIS analysis on LULC of the catchment shows that natural vegetation, grass land and water bodies decreased by 6, 57 and 13% respectively for period of 1965-1998 where as agricultural land and urban area increases by 43 and 53%. On similar manner, for period of 1998 - 2006, the area for natural vegetation, grass land and water bodies deceases by 7, 7 and . ?% where as agricultural land and urban area increases by 7 and 91 % respectively From simulation result of semi distributed hydrological models for land use change of the catchment, HEC-HMS model simulation for land use period of 1965-1998 and 1998-2006 shows an increment of 49 and 37 % in peak discharge and 29 and 27% in total volume of runoff. Similarly, HBV model shows the total volume of runoff at outlet of the catchments is 23176 , 27283.13 and 28554.7MM which increases by 15 and 4% for two periods respectively. Model performance criteria indicate HBV model is better than HEC-HMS for simulating land use change with Nash Sutcliffe efficiency of 0.71 and 0.74 in calibration and validation period. On the other hand, HEC-HMS model overestimates runoff in the catchment while HBV underestimate runoff due to l u mp ed nature of the parameters. September 2011 Page iv Modeling the effect of land use change on runoff generation using semi distributed models in urban and semi urban area For future land use of year 2020, analysis of land use change shows that there is significant change in urbanization and agricultural land with value of 81 % and 8 % respectively. For this period, model simulation result on hydrological response of land use change indicates the volume of runoff and peak discharge increases by 19 and 18%. Based on the results, this study concludes that Awassa town needs greater attention while formulating policy for expansion of urban areas and is recommended that with development of town hydrological consequences for land use change should be considered as well as expansion of town should follow management and structural options with minimum possible adverse effect on hydrological respon�se of thecatchment .

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