Walawe is located in three, locally defined, climatic zones: dry, intermediate and wet. These groups are identified using long-term average annual precipitation and classed as: less than 1500 mm, dry; between 1500 mm and 2500 mm, intermediate; more than 2500 mm, wet. Defining an area as dry, while annual rainfall is still 1500 mm, must be considered in the context that reference evapotranspiration is about 1700 mm. Especially during the inter-monsoon periods, when rainfall is limited, severe water shortages can occur, resulting in reduced, or no yield, insufficient supply of drinking water and energy crisis. Temperatures in Walawe are constant, with temperatures in the lowlands ranging from 25 to 28°C and in the upper elevations from 23 to 25°C throughout the year.
The topography of Walawe Basin can be considered as a classical one: mountainous, relatively wet, catchment areas with limited development and downstream flat areas with developed water resources. These lowlands consist of rolling or undulating plain dotted with few isolated hills, while the rivers originate from the southern slopes of the central highland massif at elevations of up to 2000 m. Three geomor-phic regions can be recognized across the basin. The highland region, which is made up of a complex of hill and valley landforms, and a major scarp at an elevation of more than 1000 m. The upland region is made up of a highly dissected plateau and has an elevation of between 300 and 1000 m. The lowland region, which accounts for about 70% of the basin, has developed water resources, including complex systems of linked reservoirs.
The dominant land type in Walawe is Chena, the traditional shifting cultivation system in Sri Lanka where, after one or two crops, land is abandoned for several years. Rice is grown in the lowlands covering an area of about 30,000 ha. The land use type 'garden' is often referred to as Other Field Crops (OFCs) and includes all crops except rice. OFC is sometimes irrigated, but mostly outside the official command areas. Tea can be found only in the highland region.
Water management in Walawe is based on a complex system of reservoirs, locally called tanks, which are linked in parallel as well as serially. A substantial number of minor tanks have been abandoned, some smaller tanks are used and managed by farmer groups and some major tanks are operated by the irrigation department. The total amount of storage is estimated at 600 X 106 m3, covering about 5000 ha. Samanalwewa and Uda Walawe are the biggest in the basin. Both of them are used for irrigation as well as power generation, with an installed capacity of 120 MW (Samanalwewa) and 6 MW (Uda Walawe). This hydropower generation is of paramount importance for the country, as about 70% of electricity consumed is supplied by hydropower.
An overview of some characteristics describing the state of surface water resources can be found in Table 10.1. Water seems to be abundant in Walawe, as can be seen from the total annual average rainfall of 1860 mm. At the same time, only 33% of this water reaches the surface water system and the remainder is evaporated directly by open water bodies, soil evaporation and plant transpiration. Water availability expressed per capita is often used as a measure of water stress in a country or region. A true minimum human need for water can be defined as the amount of water to maintain human survival and is approximately 3—5 l of clean water for drinking. Increasing this amount to about 20 l of clean water per capita per day can realize improvements in human health substantially (Esrey and Habicht, 1986). A recommended basic water requirement for human domestic needs is set at 50 l of clean water per capita per day (Gleick, 2000). Besides these water requirements for
Table 10.1. Key characteristics describing the state of surface water resources. Data represent the entire Walawe Basin and are based on long-term averages.
Area (km2) Population
Precipitation (mm/year) Precipitation (106 m3/year) Surface runoff (106 m3/year) Surface runoff fraction (%) Outflow to sea (106 m3/year) Outflow fraction from precipitation (%) Outflow fraction from surface runoff (%) Rainfall per capita (m3/year) Surface runoff per capita (m3/year)
2,500 637,000 1,840 4,600 1,500
basic needs, water is required for other services. Most important are water for food and water for nature. An average of 1000 m3 has been proposed as reasonable to meet these requirements (Gleick, 2000). At the same time, the UN has proposed that areas having less than 1700 m3 per capita water supply per year should be considered as experiencing water stress (Revenga et al., 2000). Data from Table 10.1 indicate that water stress in Walawe is moderate, since the total water availability, based on precipitation in the basin, is 2666 m3 and based on surface water resources is 888 m3 per capita.
Shallow groundwater is of paramount importance to many people in the basin for their domestic needs. It is interesting that seepage losses from canals, reservoirs and paddy fields are indispensable for maintaining water levels in shallow wells, and several cases have been studied where canal lining resulted in less water in these shallow domestic wells. Only about one-third of the population has access to a piped water supply, indicating the dependency of so many people on this fragile linkage between surface water management and shallow groundwater.
Walawe has two major nature reserves: Uda Walawe National Park, just north of the Uda Walawe reservoir, and the downstream-located wetlands including Kalapuwa lagoon. Besides these two designated areas of natural importance, many other areas of the basin serve as biotypes for wild species: plants, birds, reptiles and mammals. The paddy fields, for example, are wetlands with a high number of bird species and should be protected. In terms of water resources and sensitivity to changing environments, including climate change, special attention should be given to the Uda Walawe National Park and the Kalapuwa lagoon. Some detailed studies for adjacent lagoons have been published recently, indicating the sensitivity of these lagoons to changes in upstream water management (Stanzel et al., 2002).
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