STREAM FLOW RESPONSE TO LAND USE/LAND COVER CHANGE-: (THE CASE OF GELANA CATCHMENT, RIFT VALLEY LAKE BASIN, ETHIOPIA)

Abstract

Water resources are significantly affected by land use/land cover change (LU/LCC) at multiple spatial scales. This change is overwhelming in recent years globally. LU/LCC also aggravated in Gelana catchment within space and time. The main objective of this study was to detect the LU/LCC and investigating its effect on stream flow in the Gelana catchment, Rift valley Lake basin, Ethiopia. Spatio-temporal data were gathered and analyzed. Meteorological data from 1987 to 2018 and hydrological data from 1991 up to 2010 were used for characterizing the climate and used as model input. ERDAS 2014 was used to process preliminary data extracting, layer stacking, pan sharpening of images, image classification, and accuracy assessment for the Landsat image of 1988, 2003 and 2018. The Soil and Water Assessment Tool (SWAT) was used to perform stream flow simulation. The model was calibrated and validated on a monthly and daily time scale for a period of 1991 to 2005 and 2006 to 2010 during calibration and validation respectively. The accuracy of LU/LC classification was achieved by overall, user, producer accuracy and kappa coefficient. Results of the LU/LCC detection show that from the year 1988 to 2018 cultivation land increased by 39.7%, grass land increased by 3.7% and, the settlement area increased by 1.3%. While, mixed forest area and shrub land decreased by 38.3% and, 6.1% respectively. The model result depicts that the SWAT model run on a monthly time scale could able to capture the different response mode of the catchment well reasonably as evaluated through the coefficient of determination, Nash Sutcliffe efficiencies and PBAIS of 0.82, 0.77 and -9 during calibration and 0.76, 0.7 and -10.4 during validation respectively. From this study, it was perceived that, expansion of farming land and reduction of forest and shrub land affect stream flow volume of the catchment. The stream flow results indicated that 1-day and 7-day max flow increased in 4.3m3 /s, 5.1 m3 /s and 1-day and 7-day min flows were by 0.12m3 /s and 0.1 m3 /s respectively in comparing 1988 and 2018 simulation period. Beside the volume of annual flow increased by 0.94 m3 /s during from 1988 simulation period to 2018 simulation period. Moreover; past LU/LCC affects the wet season flow it increased in 1.56 m3 /s, and the dry season flow was decreased in 0.32 m3 /s in comparing 1988 and 2018 simulation period. The findings of this study indicate that water resources development in the catchment needs a proper understanding of the LU/LCC and its effect on stream flow. Thus, for sustainable water resources development, catchment interventions aiming at reducing high flows and increasing base flows are recommended.