Water management

River basins and water resources

Sri Lanka's radial network of rivers begins in the central highlands. There are about 103 distinct river basins covering 90 percent of the island. The southwest quarter of the island has seven major basins with catchment areas ranging from 620 to 2 700 km². They are: Kelani ganga (2 292 km²), Kalu ganga (2 719 km²), Maha oya (1 528 km²), Attanagalla oya (736 km²), Gin ganga (932 km²), Nilwala ganga (971 km²) and Bentota ganga (629 km²). An exception to the above radial pattern is the largest basin, that of the 335-km-long Mahaweli River which has a catchment area of 10 448 km². After leaving the central highlands, it runs almost north for 90 km from Minipe to Manampitiya and a then further 70 km through several distributaries as far as Verugal and Mutur on the east coast. Most Sri Lankan river basins are small. Only 17 of the 103 basins exceed 1 000 km². In addition to the Mahaweli basin, four others exceed 2 500 km². Three of these (Deduru oya, Kala oya and Malwatu oya) have their entire catchment are in the dry zone, and only Kalu ganga is in the wet zone. The total runoff in Sri Lanka is estimated at 49.2 km³/year.

Groundwater resources have been extensively used since ancient times for domestic purposes using shallow open wells in almost all parts of the country. Sri Lanka's largest aquifer extends over 200 km in the northwestern and northern coastal areas. There are about 15 000 tube-wells in the country. The quality of the groundwater is generally fairly good and relatively constant throughout the year. However, in some parts of the country (northern and northwestern coastal areas) excessive concentrations of iron and nitrates (due to agrochemicals and fertilizers) have been reported. Furthermore, due to uncontrolled abstraction of groundwater for domestic and agricultural uses, brackish water intrusion has occurred in the coastal areas. In 1985, the internal renewable groundwater resources were estimated at 7.8 km³, most (estimated at 7 km³/year) returning to the river systems and being included in the surface water resources estimate. In 1991, the total internal renewable water resources of the country were estimated at 50 km³/year.

Economic development, population pressure and growing demands for food production, electric power and adequate water and sanitation services are placing increasing pressure on water resources. It is predicted that by 2000 the demand for water will outstrip supply particularly in the country's dry zone where most irrigation schemes are located.

More information
(http://www.fao.org/nr/water/aquastat/countries/sri_lanka/index.stm)