Reducing erosion and eutrophication through soil and water conservation practices in upper Lake Rawapening

Abstract

Agriculture has been one of erosion and eutrophication sources in Lake Rawapening. Former research found massive fertiliser use on agricultural area in Rawapening catchment. Vegetable farming contributed the biggest percentage of Nitrogen and Phosphorus input that are 953.7 and 118 tons/year respectively. These were followed by irrigated paddy field with 845.8-ton Nitrogen/year and 208-ton Phosphorus/year. This lake has nine sub-watersheds: Parat, Sraten, Panjang, Kedungringin, Rengas, Legi, Galeh, Ringin, and Torong in which Panjang has the highest pollutant contribution to the lake. Since Panjang is the most significant sub-watershed and has extensive vegetable farming, therefore research to identify applicable practices in reducing erosion and eutrophication was undertaken in the area. Field surveys and interviews with landowners were employed. Field surveys included observing riparian, measuring catchment size, slopes, documenting existing land uses and ownerships. Interviews involved 52 landowners in Dusun Bendo and Ngonto, Desa Candi, Kecamatan Bandungan, Kabupaten Semarang. Based on existing practices and potential techniques, we propose soil and water conservation practices variables. They can be grouped into three: runoff farming, mini bridges, and terrace strengthening. Runoff farming consists of three techniques: individual/group infiltrating wells, on-site sediment traps, and drainage enhancement. Majority landowners choose infiltrating well for its easiness on obtaining materials and constructing. They also believe that wells have no disturbance on their vegetables. On-site sediment traps are rejected because of inundation concern, but farmers with very small landownership consider this choice as long as the traps has drainage system. Almost all farming land have mini bridges and terraces. They acknowledge that terraces were inherited from their predecessors.