研究報文 農業土木学会論文集 Trans. of JSIDRE No.240, pp.117 ～ 124(2005. 12) Appropriate Watershed Runoff Modeling in a Developing Watershed in Indonesia HERYANSYAH Arien*, GOTO Akira**, MIZUTANI Masakazu**, YANUAR Muhammad JP*** * United Graduate School Tokyo University of Agriculture and Technology (Utsunomiya University), Minemachi 350, Tochigi, 321-8505, JAPAN ** Department of Environmental Engineering, Utsunomiya University, Minemachi 350, Tochigi, 321-8505, JAPAN *** Department of Soil Water Engineering, Bogor Agricultural University, INDONESIA Abstract Proper watershed modeling is one of the most essential parts of watershed management. In this study, the performance of Cidanau watershed runoff models based on the Tank Model was evaluated, in view of the models' applicability to water pollution issues. Watershed division for the application of the Tank Model was examined, and the water movement scheme was taken into consideration. The watershed model was expected to perform as a base model for water quality modeling, following which water quality components will be added to the base model structure. Key words: Watershed Modeling, Tank Model, Sub-Catchment Application, Indonesia 1. INTRODUCTION In Indonesia, the need for water resources for human life, agricultural activity, and industry is greatly increasing, because of high population pressure and recent economic growth. On the other hand, population growth and changes in life styles in catchments have caused water pollution. Furthermore, recent developments in the catchments have had a negative impact on watershed functions such as water storage and purification. Proper watershed water quality modeling is essential for avoiding these environmental problems in the future. Since water quality processes are associated with hydrological runoff, a watershed water quality model is performed by the integration of both a runoff model and water quality components. Considering non-point pollutant sources as dominant factors in a watershed consisting of broad rural land and rural villages, a runoff model designed for use as a basis for water quality modeling should have capabilities to represent the spatial variability of water quality in the watershed. That watershed model should also have compatibility with available data. This study tries to establish a watershed runoff model, considering flow directions in sub-catchments as a base model for water quality modeling. The model is expected to simulate hydrologic processes associated with spatial water quality processes inside the watershed. 2. STUDY AREA Cidanau Watershed (221.1 km2) is one of the priority watersheds in Indonesia to be given special handling and management attention. It is located at 6•‹7'-6•‹18' South and 105•‹51'-106•‹3' East, and surrounded by densely populated areas of Banten Province, such as the Cilegon Industrial Area, and the Anyer Beach, Serang, and Pandeglang Districts. The Cidanau watershed is dominated by paddy field and a plantation area (Fig.1), and there is also a swamp protection area (Rawa Danau) in the middle of the watershed. Land slopes are steep (>25%) in the southern part, while in the center are flat, undulating, and gently rolling (0-5%). Considering soil types, Alluvial and Latosol are the majority soil types, as shown in Table 1. The catchment areas of Rawa Danau also have high population densities, as well as many agricultural activities. They supply pollutants to Rawa Danau trough rivers flow. The outlet of Rawa Danau becomes the Cidanau River that flows to KTI Weir and then to the sea. Rawa Danau has an important function such as water purifier, since some of the water quality parameters at the Rawa Danau outlet are better than at other points.. Runoff discharges were observed on a daily basis at seven points in the watershed during the 1995-2004 period, although not all of the points were continuously observed. The stream and rain gauge stations in the Cidanau Watershed are presented in Fig.2. 農 土論 集240(73-6) 657