Analisis Karakteristik Hujan dan Penggunaan Lahan terhadap Debit Aliran Sungai DAS Ciliwung Hulu

Abstract

This research was aimed to 1) analyze rainfall characteristics (daily, monthly, and yearly), 2) analyze rainfall erosivity (EI30), and 3) assess the influence of rainfall and landuse againts the water level of the upstream Ciliwung watershed. The characteristics of the local rainfall were obtained by the use of Thiessen’s polygon method. The applied data about daily rainfall covered the period of 1985 – 2010. The local rainfall erosivity was obtained through the data analysis of daily rainfall pias (2007 – 2010) at Citeko station. Multiple regression analysis was used to determine the corelation between landuse and rainfall to changes of the water level of upstream Ciliwung watershed. The criteria of rainfall in the upstream Ciliwung watershed area had 100 – 200 mm (rather wet) within three months, the period ranged from June to August. Rainfall at ≥ 200 mm (wet months), occured within the period of September to May. The highest daily rainfall occurred in May and October, about 28 mm and 30 mm per one day respectively, while the lowest from about 22 mm per one day occurred in July. Rainfall at the Ciliwung watershed area was normal (N) with average percentage of 36 %. The highest erosivity of daily rainfall occurred on April 19th, 2009 which had the figure of 737.977 ton.m / ha cm / hour, while the lowest occured on May 18, 2007 figuring about 0.003 ton.m / ha cm / hour. In the months of October, November, December, January, February, March, April, and May often occured rainfall which resulted high erosivity of rainfall. Rainfall with high intensity did not always result high erosivity of rainfall. Equation model resulted from multiple regression analysis within the changes of land use and rainfall againts the changes of water level of the upstream Ciliwung watershed: Y = – 996,63+0,94 X1–0,21 X2+0,41 X3+0,92 X4–0,17 X5+0,15 X6 where in X1 = annual rainfall (mm), X2 = dense forest (ha), X3 = mixed farms (ha), X4 = settlement (ha), X5 = rice field (ha), and X6 = agricultural dry land (ha). Based on the equation, it can be interpreted that the greater the rainfall volume was and the less density of forest area, the water level of river became higher. The more settlement and mixed farm areas increasing, greater of water level flow it produced. The increase of water level of the upstream Ciliwung watershed every year was caused by the increase of rainfall yearly within the whole period of 1985 – 2010, and the increase of the dense forest conversion settlement and agricultural land.

Penelitian ini bertujuan untuk mengananalisis karakteristik curah hujan (CH harian, bulanan, dan tahunan), menganalisis erosivitas hujan (EI30), dan untuk mengkaji pengaruh curah hujan dan penggunaan lahan terhadap debit aliran sungai DAS Ciliwung hulu. Karakteristik hujan wilayah diperoleh dengan menggunakan metode poligon Thiessen. Data curah hujan harian yang digunakan selama periode (1985 – 2010). Erosivitas hujan wilayah diperoleh melalui pengolahan data pias hujan harian (2007 – 2010) dari stasiun Citeko. Untuk mengetahui hubungan antara penggunaan lahan dan curah hujan terhadap perubahan debit aliran sungai digunakan analisis regresi berganda. Kriteria hujan di kawasan DAS Ciliwung hulu memiliki periode curah hujan 100 – 200 mm (bulan lembab) sebanyak tiga bulan yaitu dari bulan Juni – Agustus. Periode curah hujan 200 mm (bulan basah) sebanyak sembilan bulan yaitu bulan September – Mei. Curah hujan harian tertinggi terjadi pada bulan Mei dan Oktober, yaitu sebesar 28 mm/hari dan 30 mm/hari, sedangkan curah hujan harian terendah terjadi pada bulan Juli, yaitu sebesar 22 mm/hari. Curah hujan di wilayah DAS Ciliwung hulu bersifat normal (N) dengan rata – rata presentase sebesar 36 %. Erosivitas hujan harian tertinggi terjadi pada tanggal 19 April 2009 sebesar 737,977 ton.m/ha cm/jam, sedangkan erosivitas hujan harian terendah terjadi pada tanggal 18 Mei 2007 sebesar 0,003 ton.m/ha cm/jam. Bulan Oktober, November, Desember, Januari, Februari, Maret, April, dan Mei merupakan bulan dengan banyak kejadian hujan menghasilkan erosivitas hujan tinggi. Hujan dengan intensitas tinggi tidak selalu menghasilkan erosivitas hujan tinggi. Model persamaan yang dihasilkan dari analisis regresi berganda antara peubah penggunaan lahan dan peubah curah hujan terhadap perubahan debit aliran sungai adalah: Y = – 996,63+0,94 X1–0,21 X2+0,41 X3+0,92 X4–0,17 X5+0,15 X6 dimana X1 = curah hujan tahunan (mm), X2 = hutan lebat (ha), X3 = kebun campuran (ha), X4 = pemukiman (ha), X5 = sawah (ha), dan X6 = tegalan atau lading (ha). Berdasarkan persamaan di atas dapat diartikan bahwa, semakin besar volume curah hujan dan semakin berkurang luasan hutan lebat, maka debit aliran sungai akan semakin besar. Semakin bertambah luas pemukiman dan kebun campuran, maka debit aliran yang dihasilkan juga akan semakin besar. Meningkatnya debit aliran sungai DAS Ciliwung hulu tiap tahunnya dikarenakan curah hujan tahunan yang meningkat selama periode (1985 – 2010) dan meningkatnya konversi hutan lebat menjadi lahan pemukiman (terbangun) dan lahan pertanian.