Seite - 469 - in Book of Full Papers - Symposium Hydro Engineering

3. THE CITARUM RIVER HYDROLOGICAL REGIME. Indonesia in the tropics in general the main characteristics of the tropical climate is the uniformity of temperature, solar radiation, humidity, wind speed. While the main climate parameter that varies in time and space is rainfall. Factors that control rainfall are (van der Weert, 1994): equatorial double rainy seasons, transfers (monsoons), and local influences. Presents regional variations of rainfall in Indonesia. It is generally seen that seasonal variations are highly visible in Java, Nusa Tenggara, East Timor, and South and Southeast Sulawesi. Annual runoff depends on rainfall and evapotranspiration. In general, the evapotranspiration in the catchment area is relatively small compared to the existing rainfall, so it is not surprising that the annual rainfall is closely related to runoff (van der Weert, 1994). The runoff component is divided into base flow, interflow and surface runoff. Determination of surface flows for irrigated agricultural areas, mixed gardens, settlements, and plantations is a correlation between soil types, slope and slope length as well as flow management practices. Citarum River is one of the rivers that have long discharge data. In 1922, the discharge post was installed in Palumbon which was in the upper reaches of Jatiluhur reservoir with an area of 4133 km2 of DAS. In January 1944, during the period of independence war, observation data were interrupted and resumed from May 1962 to August 1987 when the station was dismantled because of the flooding of the Cirata dam construction. Based on the recording of all long-term data in the Citarum River basin using Isohyet method, the mean annual rainfall is estimated to be 2610 mm. From the comparison of precipitation in the pre-independence period (Sep-22 to Aug-29) with after independence (Sep-79 to Aug-86) the annual rainfall is almost 2454 mm / year and 2470 mm / year but the annual flow is increased by 10% from 1137 mm / year to 1261 mm / year (van der Weert, 1994). In addition, from the observation data of debit at the Palumbon station fluctuations of flow debits between the period before independence (1923-1943) and after independence (1963-1984) were higher. Based on the Citarum cascade reservoir operation data, there is a decrease in annual flow and also higher fluctuations as “Fig.4”. Changes from this hydrological regime could be attributed to land use change. Unfortunately, there are no statistics on land use data prior to the independence war that can be used as a comparison. Changes in the hydrological regime include the tendency of seasonal leap, longer dry season, shorter rain time with greater intensity as an indication of climate change as “Fig.5”. Under such conditions, at the operational level of the Citarum cascade reservoir the question arises, "How to treat the reservoir reservoir and Citarum cascade reservoir operation to anticipate the trends?" 469