Ruhunu-Walawe Basin, Sri Lanka
The Ruhuna basins in southern Sri Lanka, as defined by IWMI, encompass three of the main rivers that flow through ancient Ruhuna, including the longest and most important river in the region, Walawe. Today, the Ruhuna basins are important in the broader Sri Lankan context, being the location of a major hydropower plant, irrigation schemes that make a significant contribution to national food production, and important nature reserves.
Average annual rainfall in this part of the basin falling within the South-East dry zone is less than 1500 mm, but varies from 2500 mm in the upper reaches to less than 1000 mm in the lower parts.
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.
It’s a mainly hard rock area with low groundwater potential, with saline groundwater in part of the basin. Water from many tubewells is not suitable for drinking because of the high fluoride or iron content.
The upper part of the Ruhuna basin is mountainous with a fewer number of settlements. The lower part consists mainly of agricultural lands under major, medium and minor irrigation systems and rain-fed farming systems. Tea and rubber are grown in the upper watershed area, while rice is the main crop in the lower part of the basin, albeit with increasing degrees of crop diversification.
According to the 2001 Census of Housing and Population, around 950,000 people inhabit the area.
The uplands are thinly populated with sub-montane wet evergreen forests, grasslands and tea estates. The non-irrigated area in the lower basin is characterized by low shrub vegetation, exploited for shifting cultivation. A 3,400ha 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 by Sri Lankan standards. Crop diversification is one of the main strategies of the irrigation agency to save water and increase farmer incomes.
Irrigated agriculture is the main water user and major irrigation schemes like the Uda Walawe and Lunugamwehera provide water for a large number of farmers involved in rice, banana and other field crop cultivations. While the right bank of Uda Walawe is fully developed, the left bank is being extended to provide irrigation to undeveloped rain-fed areas. The proposed Ruhunupura development plan (urban, port and industrial developments) will increase the demand for water and make the existing situation more complex. With the increasing demand for water from other sectors under the present water short situation in the basin, greater efforts at agency and other stakeholder levels are required to make management decisions and improve water allocation to address issues such as optimization of water utilization and productivity, and environment and health.
Soon after completion of the Uda Walawe dam 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.
Public sector organizations in the area are based on administrative and not hydrological boundaries. Different agencies have overlapping responsibilities over certain functions. This creates problems in collective decision-making, fund allocation, implementation and progress monitoring and evaluation process with regard to the management of the basin as a whole.
Most of the livelihoods issues in the basin relate to reducing poverty. Nearly the entire population of the basin are farmers (their main share of income coming from agriculture). People living in the wet zone area get more income from perennial crops in home gardens, but factors such as changing market forces, and fragmentation of land and resources have a negative effect on the livelihoods. People living in the dry zone engage in slash and burn (chena) rain-fed cultivation and receive only seasonal income. Most of these people also engage in illegal activities like logging, hunting wild animals and toddy making during the dry periods. The impact of inland fishery activity is important in this respect.
Irrigation development has already had negative impacts on the aquatic environment, particularly in the wetlands and the coastal zones. Very limited quantitative knowledge of these impacts exists. No attempt has been made to establish the objective of environmental water management in the basins and also to quantify associated environmental flow requirements or releases.
Uneven distribution and a dearth of hydro-meteorological data collection stations and inaccuracies in existing data sets in the basin are a real constraint for water resources assessment.
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 farmer organizations involved in maintaining canals, planning cropping patterns, adjusting conflicts, and enforcing sanctions.
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.