IMPACT OF URBANIZATION ON STORM WATER RUNOFF VOLUME AND QUALITY: THE CASE OF HAWASSA CITY, SOUTHERN ETHIOPIA

Abstract

Land use and land cover change results from various interactions between human being and environment. This study aims to identify and compare changes in land use and land cover occurred in the last 30 years in Hawassa city and its subsequent effect on runoff volume and quality. Information on land use and land cover changes that occurred from 1986 to 2016 in the study area was compared using remote sensing and geographic information system (GIS) with field verifications; Land-sat and spot satellite image were used to analyze the change. Three sets of remotely sensed Land-sat TM 1986, 2000 and 2016 were used to produce the LULC maps at different periods through a supervised classification of the satellite imageries using ERDAS IMAGINE and Arc GIS software. The Soil Conservation System (SCS) runoff volume calculation method that best fit for Ethiopia is used. Urban runoff water samples were gathered from the major runoff outfalls. The runoff sample collected manually using PVC scoop that received the entire flow and where then transferred to sampling bottles. The preparation of composite sample for one rainfall event is done by calibrating the sampling bottle with equal volume for three grab samples and preparing one event composite sample on site for that specific runoff event and sampling station. There was a significant expansion and encroachment of land use and land cover in Hawassa city between 1986 and 2016. In the study period the buildup area increased by 181.8% between 1986 and 2000. It also increased by 184.2% between 2000 and 2016. The total increment during the study period between 1986 and 2016 was about 700%. The second most changed LULC was vegetation cover that declined by 53.5% between 1986 and 2016, by 27.8% between 1986 and 2000 and finally by 37.2% between 2000 and 2016. Cultivated land was the least changed LULC. It declined by 12.2% from 1986 to 2016 but increment between 1986 and 2000 by 5.2% but decreased by 16.5% between 2000 and 2016. Bare land area also decreased by 50.8% from 1986 to 2016. The total storm water runoff depth was 35 mm and total volume was 5,833,460 m3. The storm water runoff depth indicates that 70.8% of the total rain fall changed to runoff water. During this time the contribution of the buildup area contribution was 3.8% and that of non-buildup area was 96.2% area wise. The runoff water quality laboratory results indicate that TN, TP, TSS, TDS, COD, BOD and Bacteria in the sample water were in excess of the level of permit discharge defined by the US environmental regulation