The Upper Palani Hills catchment basin drains into the Lake. Rainfall is the chief source of water into the lake. Average annual rainfall in the hill area is 1,500 mm (4.9 ft). The mean day temperature in the coolest months is below 17 °C (62.6 °F) in the higher areas. [13] The lake is formed below the Palani hill ranges at the origin of the Varahanadhi (Varaha River) which is used for purposes of water supply and irrigation. As a part of the World Bank funded Water Resources Consolidation Project (WRCP) of Tamil Nadu, sedimentation condition was assessed for a large number of reservoirs including Berijam reservoir. This indicated that the 1.91 cubic hectometres (1,550 acre·ft) gross storage capacity of the reservoir in 1911 decreased to 1.804 cubic hectometres (1,463 acre·ft) in 1987. The percentage reduction of storage due to siltation is 23%, over a period of 76 years. [14][15] Berijam Lake had stable land conditions, thus erosion was less and the lake received less sediment but more organic matter in the proportion of (5.5?41.8%). The rate of sedimentation is indirectly proportional to the influx of organic matter. [2]
