Cacaban dam is located at Tegal district Central Java Province. The dam is the strategic asset that play a crucial role in providing water for supplying water irrigation (to irrigate 7439 ha of land), flood control and fisheries. Total area of the upper watershed of Cacaban dam is 60.66 km2. Storage capacity of the dam is 90 million m3.
The current condition, land degradation in the upper area of the dam has increased. The degradation caused by excessive farming activity. Sedimentation flowing from the upper watershed to the dam has increased significantly. It implies water storage capacity of the dam become decreasing. In the same time, water demand for irrigating land is getting high especially in the second planting date (PD-2) where the season is moving from the wet season to the dry season and in the third planting date (PD-3) in dry season.
Using high variety of rice by the farmers with shorter age and higher water consumptive use comparing the usual variety of rice gives contribution in increasing water demand of irrigation. Therefore, recalculation of water balance of the Cacaban dam irrigation system is needed.
Hydrologic analysis was used to recalculate water balance at Cacaban dam as base to make a good rule in water resources management. The result shown that cumulative discharge is 60 million m3. Design water storage capacity of the dams is 90 million m3. It is clear that the actual water storage capacity is less than the designed capacity. This fact proved that the water flowing to the dam never been reached overflow through spillway since the dam was constructed in 1958.
There are two period of deficit of water (-) from the second week of October (Oct II) to second week of December (Dec II) and from the first week of March (Mar I) to second week of July (Jul II). Surplus (+) of water happens from the first week of December to the first week of March. Recalculating of the water balance was also conducted for other three Groups. The results also gave that water availability at the dam still not enough to supply water for irrigating total common service area of Cacaban dam system. It is recommended to increase water supply using inter watershed management.
Sukirno and Sahid Susanto. Application of Simple Hydrologic Model for Recalculating Water Balance of Cacaban Dam Irrigation System. Proceeding of International Symposium on Dams in Global Environmental Challenges, ICOLD, Bali 2014.