The main feature of the ancient irrigation systems of Sri Lanka were intricate networks of small to gigantic reservoirs (wewa or tanks) connected through a series of feeder canals that brought water for year-long rice (Oryza sativa L.) cultivation in the dry zone. Irrigation systems with large number of interconnected reservoirs have evolved since the third century bc. These ancient irrigation systems still function as a crucial element in supplying water for agriculture in the dry zone of Sri Lanka, and they constitute one of the richest sources of wetland biodiversity in the country. An intriguing feature of the tank systems is their sheer density: About 30,000 tanks have been built in a land area of about 40,000 km2 of the dry zone (Mendis 2003). The density and the long-term existence (more than 1,000 years in many cases) make these tanks an important component in the environment and ecosystems of the region.