Sri Lanka is a tropical island country with an average annual rainfall of 900–5,000 mm, which provides 131,230 million m3 of freshwater annually. Zones in its southwest are the wettest, receiving over 2,000 mm/year, while northern (and some southeast) areas are considered dry zones with under 1,500 mm/ year. There are 103 river basins, and the largest river is the 335 km Mahaweli River, covering an area of 10,448 km2. Rivers in the wet zones contribute over 50% of the runoff despite covering only about one third of the land area. There are over 3,400 wetlands, which include ancient irrigation reservoirs, recently constructed multipurpose reservoirs, tanks and lakes.