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Total internal natural renewable water resources are around 2 018.3 km3/year (Table 2 and Table 3). Surface water resources are an estimated 1 972.6 km3/year and groundwater resources 457.4 km3/year. Most of the groundwater, an estimated 90 percent or 411.7 km3/year, returns as baseflow to the rivers. It is assumed that only 30 percent of groundwater resources, or 137.2 km3/year, are consumable, called ‘safe yield’ (Table 4) (Bakosurtanal, 2001). Over-abstraction of groundwater in Jakarta has caused saline groundwater to reach about 10 km inland from the coastline and has led to land subsidence at a rate of 2-34 cm/year in east Jakarta.
Although water resources are abundant, the seasonal and spatial variation in the rainfall pattern and lack of adequate storage create competition and conflicts among users. Municipal and industrial wastewater is discharged virtually untreated into the waterways causing rapid deterioration in the quality of river water.
Most of the lakes in Indonesia are of volcanic origin. Lake Toba is the largest volcanic lake in the world with an average surface area of 1 100 km2 and an average volume of 1 258 km3.
In 2006, total dam capacity reported was 22.49 km3, but total capacity is higher because the capacity was unknown for some dams. In 1995 total dam capacity was an estimated 15.83 km3. The dams with a capacity of over 1 km3 are Jatiluhur (2.89 km3), Siruar (2.82 km3), Cirata (2.17 km3), Pongkor (1.95 km3), Batu Bokah (1.67 km3), Kotopanjang (1.55 km3) and Riam Kanan (1.20 km3).
By developing large dams, Indonesia has progressively been able to extend its water resources utilization to support 2 200 MW of hydropower generation, representing 20 percent of the national generating capacity.