Transformasi hujan menjadi limpasan, erosi, dan sedimentasi di sub das berhutan dan tidak berhutan (studi kasus di hutan pendidikan gunung walat, Sukabumi Jawa Barat)

Abstract

Land use changes impacts of a watershed could be showed by hydrological behaviors, such as surface run off and discharge changes, surface erosion and sedimentation changes. Researches related to rainfall transformation, erosion, and sedimentation in the different land uses are necessary as an effort to control surface run-off, erosion, sedimentation and flood. The aim of this research is to know the differences of rainfall transformation, surface erosion, and sediment load of river in the forested watershed and non forested (bare land) watershed also to know the difference of USLE and SDR methods to estimate the soil loss (erosion). The research was conducted in Cipeureu sub watershed and Cibadak sub watershed in Sukabumi, West Java in the period of Mei-1 Agustus 2010. Collected data were daily rainfall, discharge, sediment concentration, physical soil properties, slope data, forest land use area and land conservation, also the data of boundaries of catchment area. The data were obtained from rainfall station, using ring samples for soil properties, digital analysis for slope, interview, and literature studies for additional data. The research shows that discharge fluctuation of Cipeureu sub watershed is smaller than Cibadak sub watershed, the guess result of surface erosion with USLE method in Cibadak sub watershed is bigger than Cipeureu sub watershed, those are in Cibadak sub watershed is about 2857,46 ton/ha/year and in Cipeureu sub watershed is about 3,49 ton/ha/year. The guess result of erosion with sediment rate measurement and SDR in Cipeureu sub watershed is about 6,57 ton/ha/year and 3,43 ton/ha/year, while in Cibadak sub watershed is about 9,46 ton/ha/year and 6,36 ton/ha/year. This research concludes that forest sub watershed transforms rainfall to surface run off is about 35%, sediment rate smaller (0,0015 mm/day), surface erosion is in the very small class. While non forest sub watershed transforms rainfall to surface run off is about 52%, sediment rate is about 0,02 mm/day, and surface erosion is in the heavy class. Besides, the guess result of surface erosion using USLE method, that compared with sediment rate, and guess result using SDR approach is overestimate.

Dampak perubahan penggunaan hutan di suatu DAS dicerminkan oleh perilaku hidrologi seperti perubahan laju aliran permukaan dan debit sungai, erosi dan sedimentasi. Penelitian transformasi hujan, erosi, dan sedimentasi akibat perubahan penggunaan lahan dipandang perlu sebagai upaya pengendalian aliran permukaan, erosi dan sedimentasi, serta banjir. Penelitian ini bertujuan untuk mengetahui transformasi hujan menjadi limpasan, laju erosi permukaan, dan muatan sedimen aliran sungai di Sub DAS berhutan (Sub DAS Cipeureu) dan tidak berhutan (Sub DAS Cibadak) serta mengetahui perbedaan hasil pendugaan erosi permukaan menggunakan pendekatan USLE dan persamaan SDR. Penelitian ini dilaksanakan di Sub DAS Cipeureu dan Sub DAS Cibadak pada bulan Mei sampai dengan 1 Agustus 2010. Data yang dikumpulkan berupa data curah hujan, data debit aliran sungai, data konsentrasi sedimen aliran, data sifat fisik tanah, data kemiringan lereng, data penggunaan lahan dan konservasi tanah, serta data batas wilayah Sub DAS yang masing-masing diperoleh dari stasiun curah hujan, didapat dengan cara pengambilan sampel air sungai, pengambilan sampel tanah dengan ring sample, analisis digital, dan studi literatur. Hasil penelitian menunjukkan bahwa fluktuasi debit Sub DAS Cipeureu lebih kecil dibandingkan dengan Sub DAS Cibadak, hasil pendugaan erosi permukaan dengan metode USLE di Sub DAS Cibadak lebih besar dari Sub DAS Cipeureu, yaitu di Sub DAS Cibadak sebesar 2857,46 ton/ha/thn dan di Sub DAS Cipeureu sebesar 3,49 ton/ha/thn. Hasil pendugaan erosi dengan perhitungan laju sedimen dan SDR di Sub DAS Cipeureu yaitu sebesar 6,57 ton/ha/thn dan 3,43 ton/ha/thn, sedangkan di Sub DAS Cibadak sebesar 9,46 ton/ha/thn dan 6,36 ton/ha/thn. Kesimpulan dari penelitian ini yaitu Sub DAS berhutan mentransformasikan hujan menjadi limpasan sebesar 35%, laju sedimen relatif lebih kecil (0,0015 mm/hari), erosi permukaan sangat ringan (SR). Sedangkan Sub DAS tidak berhutan, mentransformasikan hujan menjadi limpasan sebesar 52%, laju sedimen 0,02 mm/hari, dan erosi permukaan termasuk kelas berat (B). Hasil pendugaan erosi permukaan menggunakan pendekatan USLE, yang dibandingkan dengan laju sedimen, dan hasil pendugaan menggunakan SDR cenderung overestimate.