DAMPAK PERUBAHAN PENGGUNAAN LAHAN TERHADAP EROSI DI DAERAH TANGKAPAN AIR WADUK SERMO

Sermo Reservoir began to operate in 2007 with expected useful life of 50 years. It was designed to intercept ± 25 million cubic water. The main function of reservoir is to supply irrigation, domestik water, and maintenance river. Problems that was found in catchment area were the entry of high rate sediment into the reservoir due to landslides and soil erosion, so it can reduce useful life. Conservation efforts to decrease land erosion and sedimentation have been done by Balai Pelestarian Sumber Daya Air Yogyakarta, surrounding communities, and related institutions. These efforts include making the canal around the dam, making check dams on the rivers which enter the reservoir, green belt, reforestation catchment area, through community development, and training of land conservation. This research aimed to investigate and analyze the land use changes and sheet erosion from land use in 1998 (map RBI), 2004 (Quickbird) and 2010 (image Spot XS), determining changes in estimated useful life of the reservoir services based land-use change and providing business input land conservation Land use changes analysis was done by using input/output methode. This analysis technique is useful to know the broad and large land use changes. The calculation sheet erosion was done by overlying rain erosivity, soil type, slope class, and land use become land unit map. The USLE formula was applied to estimate the average annual sheet erosion. Sheet erosion was compared to the data for the year echosounding to estimate the Sediment Delivery Ratio (SDR). Sheet erosion and SDR is used to predict the useful life of the reservoir with a dead storage method. The research finding showed that land use changing the period 1998 - 2004 covering an area of 28,06 Ha has caused increase of erosion of 10.776,96 tons. It has reduced the useful life of the reservoir services for 3 years from the original 36 years to 33 years. While land use changes period 2004 - 2010 an area of 22,72 Ha has caused increasing erosion of 6.980,36 tons. It has reduced the useful life of the reservoir service for 2 years from the original 33 years to 31 years. The conservation effort to extend reservoir useful life with non-structural improvements terrace and reforestation, structural methode with making gully plug were done to accommodate sediment and scraping regularly the existing dam