Pengembangan Model Hujan-Limpasan Pada Daerah Aliran Sungai Volkan Tua Dan Volkan Muda (Studi Kasus: Das Sumur Mbandung Dan Das Sempor Daerah Istimewa Yogyakarta)

INTISARI Kajian tentang hubungan antara hujan dengan limpasan pada skala daerah aliran sungai (DAS) sangat penting karena memberikan informasi yang berguna untuk pemanfaatan dan pengelolaan sumberdaya air yang lebih baik dalam DAS tersebut. Penelitian ini bertujuan untuk: (1) mengkaji karakteristik curah hujan, yaitu durasi, tebal dan intensitas, terkait dengan distribusi spasial dan temporalnya dalam wilayah kajian; (2) mengkaji karakteristik limpasan pada daerah aliran sungai (DAS) volkan tua dan DAS volkan muda; (3) mengembangkan model hujan-limpasan berbasis distribusi spasial dan temporal dan menerapkannya pada DAS-DAS kajian. DAS-DAS kajian adalah DAS Sumur Mbandung (volkan tua) dan DAS Sempor (volkan muda) yang berada dalam wilayah administrasi Daerah Istimewa Yogyakarta, Indonesia. Satu sistem sungai kajian berukuran kecil dipilih dalam tiap DAS kajian. Data-data hidrologi yang digunakan bersifat distribusi spasial dan temporal, yaitu geomorfologi, jenis tanah, jenis tutupan dan penggunaan lahan, curah hujan dan aliran. Pengolahan data-data spasial tan temporal menggunakan tekhnologi sistem informasi geografi (SIG). Curah hujan lebihan dihitung menggunakan metode Natural Resources Conservation Service-Curve Number (NRCS-CN). Model hujan-limpasan dikembangkan melalui penerapan konsep kesetimbangan air dalam sel. Algoritma model penelusuran limpasan berbasis sel kisi dibuat berdasarkan prinsip simpan-lepas menggunakan konsep waktu penjalaran. Program komputasi model dibuat menggunakan perangkat lunak Matlab berbasis data ASCII hasil konversi data raster hasil olahan SIG. Hasil penelitian menunjukkan bahwa meskipun sistem-sistem sungai kajian berukuran kecil, namun curah hujan tidak merata di dalamnya. Karakteristik-karakteristik geomorfologi, tanah serta tutupan dan penggunaan lahan pada DAS-DAS kajian berpengaruh pada limpasan yang dihasilkan serta pada proses penjalaran limpasan. Persentase volume limpasan yang terhitung di keluaran sistem sungai kajian pada DAS volkan tua lebih besar dibandingkan dengan persentase pada sistem sungai kajian DAS volkan muda. Algoritma model penelusuran yang diusulkan mampu menyimulasikan hidrograf limpasan langsung pada ke dua DAS dengan nilai-nilai efisiensi Nash-Sutcliffe lebih besar dari 0,75 serta bias volume dan kesalahan relatif debit puncak lebih kecil dari 25%. Nilai koefisien kekasaran Manning (n) pada alur sungai bervariasi terhadap tebal curah hujan rata-rata dalam tiap fraksi waktu curah hujan menurut sangkutan berpangkat. Nilai n pada alur sungai kajian di DAS Sempor cenderung lebih besar dibandingkan dengan nilai n pada alur sistem sungai kajian di DAS Sumur Mbandung untuk tebal curah hujan yang sama. ABSTRACT Investigations of the relation between rainfall and runoff at catchment scales are crucial as it provides information needed for a better water resource planning and management in the catchment. This study aims to: (1) investigate rainfall characteristics (i.e., duration, depth, and intensity) in the study areas in relation to spatial and temporal distribution; (2) investigate runoff characteristics within old and young volcanic catchments; and (3) develop a distributed rainfall-runoff model and applying it to the study catchments. The study catchments are Sumur Mbandung catchment, referred to as an old volcanic catchment, and Sempor catchment, referred to as a young volcanic catchment. Both catchments are located within the administrative region of Yogyakarta, Indonesia. One small experimental catchment was selected in each study catchment. The study based on spatiotemporal hydrological data within the experimental catchment, including geomorphological characteristics, soil types, land use and land cover types, rainfall, and flow. Geographic information system (GIS) technology was used to operate the spatiotemporal data. The Natural Resources Conservation Service-Curve Number (NRCS-CN) method was applied to calculate rainfall excess. The proposed rainfall-runoff model was based on the cells water balance. The cell-based routing algorithm was constructed to simulate direct runoff hydrographs (DRHs) by applying the principle of storage-release and the concept of travel time. Distributed raster data obtained using GIS technology was converted into ASCII data format for scripting the routing algorithm using Matlab programming codes. Results show that, even the experimental catchments are small, rainfall varies spatially and temporarily over the catchments during an event. Geomorphological, soil, and land use and land cover characteristics are of important factors that control the runoff production in the catchments. Percentage of runoff volume at the outlet of the experimental old volcanic catchment is greater than that of the young one. The proposed routing model algorithm simulates DRHs of all selected events in the study areas with excellent performance. All validated model DRHs were satisfying having the Nash-Sutcliffe coefficients of greater than 0.75, and the volume bias and peak discharge errors of less than 25%. The Manning coefficient (n) values in the channel vary with respect to the average rainfall depths in each time fraction according to power relation. The channel at Sempor study catchment tends to give n values greater than that at the Sumur Mbandung study cathment giving that both have the same rainfall depth. Keywords: Old and young volcanic catchments, Rainfall, Runoff, Rainfall-runoff model, Cell-based routing, GIS, Matlab, DRHs.