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Ruhuna-Walawe Basin, Sri Lanka
Size: 2,442 km2
Location: Southern part of Sri Lanka
Average annual precipitation:
Parts of the upper catchment fall within the wet zone of Sri Lanka with average annual rainfall of more than 3,000 mm. The largest part of the basin is in the dry zone. Average annual rainfall of the entire basin is 2,050 mm.
Annual water quantities:
Inflow into the Uda Walawe reservoir ranges between 545 and 1658 million m3 per year, with an average of 1060 million m3. Average discharge into the sea is 34 million m3.
Aquifer:
Mainly hard rock area with low groundwater potential. Saline groundwater in part of the basin. Water from many tubewells is not suitable for drinking because of high fluoride or iron content.
Topography:
Walawe River originates in the central highlands of Sri Lanka (highest point in the basin 2,395 m above sea level). Largest part of the basin is a downward sloping plain towards the Indian Ocean.
Population:
Mostly rural population of approximately 300,000.
Land uses:
Uplands are thinly populated with sub-montane wet evergreen forests, grass lands and tea estates. The non-irrigated area in the lower basin is characterized by low shrub vegetation, exploited with shifting cultivation. A 3,400 ha large reservoir mobilizes water for the Uda Walawe irrigation system. The area around the reservoir is the Uda Walawe National Park with an important elephant population. The Walawe Irrigation System is a success story in crop diversification. In addition to paddy cultivation and sugar cane plantations, about 35% of the irrigated area has non-rice field crops, especially bananas. This is a very high percentage for Sri Lankan standards. Crop diversification is one of the main strategies of the irrigation agency to save water and increase farmers income. Detailed land use maps in digital format are available.
Infrastructure:
The major water regulatory structures are the Uda Walawe reservoir (1967) and the Samanalawewa Dam (1992). The Samanalawewa Dam is only used for power-generation and the tailrace water comes to Uda Walawe.
Water users in the basin:
- Drinking water in the Uda Walawe irrigation system is mainly obtained from shallow wells and tubewells. The quality of water from tube wells is often poor, with high saline, fluoride, and iron content. This is less a problem in the shallow wells. However, the availability of drinking water in the shallow wells seems to be negatively affected by ongoing canal rehabilitation efforts. Recharge of shallow wells might be reduced in areas where canals are concrete lined and this is currently being studied. A few rural areas and the main town, Embilipitiya have a piped water supply with water from the reservoirs.
- Irrigation development in the south of Sri Lanka has had important negative impacts on the environment. A number of brackish water lagoons have been severely affected, either by reduced water flows, or by increased flows of fresh drainage water from irrigated areas, resulting in reduction in salt levels in the lagoons. This has resulted in a major loss of prawn production, which previously sustained a large fishing community. Some of the lagoons are of international importance for its water fowl populations and other wildlife.
- Hydropower generation takes place in the two major dams in the basin.
- The Uda Walawe reservoir supports a large inland fishing community.
- Industrial uses are limited and include garment, cement, and small food-processing factories.
- Limited information is available about water uses and users upstream of the Uda Walawe reservoir.
Existing and forecasted uses and concerns:
Major irrigation development started in the 1960s with the completion of the Uda Walawe dam. Soon after completion it became obvious that there were serious problems with the design and sustainability of the system and major rehabilitation efforts were needed, which are still ongoing. Two major new construction works have started. The Irrigation Department is constructing a canal that will divert water from the Walawe Basin to the adjacent Malala Oya Basin to irrigate more land. The Japanese Bank for International Cooperation (JBIC) is supporting the expansion of the irrigated area on the left bank with 6,000 ha. However, there are serious concerns that not enough water is available for these irrigation expansions.
Water policy and management issues include:
- What are the irrigation management options to conserve the downstream wetlands?
- How can safe drinking water be supplied in areas where deep groundwater is of poor chemical quality and shallow wells fall dry because of changes in irrigation management and infrastructure?
- Is there enough water to expand the irrigated area, as planned?
- Expanding the irrigated area will lead to loss of more than 5,000 ha of secondary forest, an important wildlife habitat. Already existing human-elephant conflicts are expected to aggravate.
- How can farmers' organizations be strengthened and all stakeholders participate in system management in order to improve the sustainability of irrigation?
Institutional structure:
The Mahaweli Authority of Sri Lanka (MASL) built up a strong administration and management structure with responsibilities for all kinds of rural development activities. However, under the World Bank supported restructuring exercise, staff numbers have been reduced drastically and the MASL is now changing from a multipurpose development agency to a basin management agency. In a few pilot areas there have been successful farmers' organizations, involved in maintaining canals, planning cropping patterns, adjusting conflicts, and enforcing sanctions.
Professional capabilities:
Good professional capabilities at central government agencies in the capital Colombo, at about five hours drive from the main town Embilipitiya. Very good professional capability at several Sri Lankan universities and research institutes.
Data and models:
Large data sets are available with the MASL and other government agencies. A water balance model was developed by IWMI for the MASL, supported by JBIC. Preliminary GIS is available with basic infrastructure, land use, soils.
Current Bechmark Basins :