Analisis Spasial Koefisien Limpasan Terdistribusi Menggunakan Metode Cook Berbasis Grid di Wilayah DAS Ciliwung Hulu

Abstract

DAS Ciliwung Hulu memiliki peran penting dalam sistem hidrologi wilayah, tetapi rentan terhadap peningkatan limpasan permukaan akibat dinamika kondisi fisik dan perubahan penggunaan lahan. Penelitian ini bertujuan mengestimasi distribusi spasial koefisien limpasan dan potensi debit limpasan permukaan menggunakan metode Cook terdistribusi berbasis grid. Analisis dilakukan dengan mengintegrasikan parameter topografi, infiltrasi tanah, tutupan lahan, simpanan permukaan, serta intensitas hujan yang dihitung menggunakan rumus Mononobe berdasarkan waktu konsentrasi DAS. Hasil penelitian menunjukkan bahwa DAS Ciliwung Hulu didominasi oleh kelas koefisien limpasan tinggi dengan nilai 0,50–0,75 seluas 12.163,68 Ha atau 80,01% dari total luas DAS, sedangkan kelas ekstrem dengan nilai lebih dari 0,75 mencakup area seluas 2.817,63 Ha atau 18,54%. Kondisi tersebut dipengaruhi oleh dominasi topografi berbukit hingga curam sebesar 72,84%, infiltrasi tanah lambat akibat tekstur lempung sebesar 90,44%, serta penggunaan lahan non-hutan yang mencapai 56,15% dari total wilayah. Intensitas hujan di wilayah penelitian berkisar antara 9,65–17,53 mm/jam dengan kategori hujan lebat dan konsentrasi tertinggi berada di bagian barat laut DAS. Kondisi tersebut menghasilkan potensi debit limpasan permukaan sebesar 0,001 hingga 0,00395 m³/detik per unit grid berukuran 30 × 30 meter, dengan sebaran debit tertinggi terkonsentrasi di wilayah barat laut hingga tengah DAS. Hasil penelitian menunjukkan bahwa pendekatan metode Cook terdistribusi mampu menggambarkan variasi spasial respon hidrologi DAS secara rinci dan dapat digunakan sebagai dasar dalam penentuan prioritas pengelolaan DAS serta konservasi air untuk mengurangi risiko banjir di wilayah hilir

The Upper Ciliwung Watershed plays an important role in the regional hydrological system but is vulnerable to increasing surface runoff due to physical landscape dynamics and land use changes. This study aimed to estimate the spatial distribution of runoff coefficients and potential surface runoff discharge using a distributed grid-based Cook method. The analysis was conducted by integrating topographic parameters, soil infiltration, land cover, surface storage, and rainfall intensity calculated using the Mononobe equation based on watershed concentration time. The results showed that the Upper Ciliwung Watershed was dominated by a high runoff coefficient class ranging from 0.50–0.75, covering 12,163.68 Ha or 80.01% of the total watershed area, while the extreme class with values greater than 0.75 covered 2,817.63 Ha or 18.54%. These conditions were influenced by the dominance of hilly to steep topography reaching 72.84%, slow soil infiltration caused by clay-textured soils covering 90.44% of the area, and non-forest land use accounting for 56.15% of the total watershed area. Rainfall intensity in the study area ranged from 9.65–17.53 mm/hour and was classified as heavy rainfall, with the highest intensity concentrated in the northwestern part of the watershed. These conditions produced potential surface runoff discharge ranging from 0.001 to 0.00395 m³/second per 30 × 30 meter grid cell, with the highest runoff concentration located in the northwestern to central areas of the watershed. The results indicate that the distributed Cook method is capable of describing spatial variations in watershed hydrological responses in detail and can be used as a scientific basis for watershed management prioritization and water conservation efforts to reduce flood risk in downstream areas