Assessing the impacts of land use and land cover change on hydrology of watershed: a case study on Gigel-Abbay Watershed, Lake Tana Basin, Ethiopia

Dissertation submitted in partial fulfillment of the requirements for the Degree of Master of Science in Geospatial Technologies.

The population growth for the last 16 years caused changes in land cover of the Gilgel Abbay watershed, Lake Tana basin, Ethiopia. The effects of the land cover changes have impacted on the stream flow of the watershed by changing the magnitude of surface runoff and ground water flow. This study is mainly focusing on the assessment of the impacts of the land cover changes on the stream flow by changing SURQ and GWQ for the wet months (June, July, August) and dry months (January, February, March) through satellite Remote Sensing (RS) and Geographic Information System (GIS) integrated with the SWAT model. ArcGIS used to generate land use and cover maps from Landsat TM and ETM+ acquired, respectively, in 1986 and 2001. The land cover maps were generated using the Maximum Likelihood Algorithm of Supervised Classification. The accuracy of the classified maps was assessed using Confusion Metrics. The result of this analysis showed that the cultivated land has expanded during the study period of 1986-2001. Using the two generated land cover maps, two SWAT models set up were run to evaluate the impacts the land use and cover changes on the stream flow of the study watershed. The performance of the SWAT model was evaluated through sensitivity analysis, calibration, and validation. Ten flow parameters were identified to be sensitive for the stream flow of the study area and used for model calibration. The model calibration was carried out using observed stream flow data from 01 January 1987 to 31 December 1994 and a validation period from 01 January 1995 to 31 December 2001. Both the calibration and validation results showed good match between measured and simulated stream flow data with the coefficient of determination (R2) of 0.93 and Nash-Sutcliffe efficiency (ENS) of 0.95 for the calibration, and R2 of 0.91 and ENS of 0.90 of the validation period. The result of this analysis indicated that the mean monthly stream flow increased by 16.26m3/s for the wet months while for the dry months decreased by 5.41 m3/s. Generally, the analysis indicated that flow during the wet months has increased, while the flow during the dry months decreased. The SURQ increased, while GWQ decreased from 1986 to 2001 due to the increment of cultivated lands. The model results showed that the stream flow characteristics changed due to the land cover changes during the study period.