Weerawila-Tissa-Debara-Yoda Tanks

Location: 60 24’ 0 N and 810 12’ 0 E to 60 8’ 0 N and 810 20’ 0 E; 707426 N and 522117 E to 677955 N and 536881 E; between 5-10 km inland from the southeast coast around the town of Tissamaharama, in the Hambantota District, of the Southern Province.

Area: Surface area at full supply level 1,590 ha (Pannangamuwa – 116 ha; Weerawila - 621 ha; Debara – 78 ha; Tissa - 285 ha; Yoda - 490 ha)

Altitude: 0 – 125m elevation above sea level

Overview: The Weerawila, Debara, Tissa, Pannangamuwa and Yoda wewa tanks form a multi-purpose, ancient, shallow, freshwater tank cluster ranging from small to medium in size. Located in the old area of the Kirindi Oya Irrigation and Settlement Project (KOISP), the tanks are of major importance for buffer storage of irrigation water and are surrounded by densely populated, intensively managed agricultural lands. The tanks provide an alternative series of freshwater habitats for water birds in the network of predominantly brackish to saline coastal wetlands in southeast Sri Lanka. The Weerawila tank is considered an IBA, due to its high richness of aquatic birds. Part of the tank system falls within the Weerawila-Tissa wildlife sanctuary and remnants of Sri Lanka’s ancient Ruhuna kingdom are also found within the wetland cluster.

Physical features: The tank cluster lies within the southeast low country dry zone and receives 1,100 – 1,200 mm rainfall annually. Most of the rainfall occurs within March-May and October-January periods. Annual average temperature is around 270C with minimal monthly fluctuations. The southwest monsoons (May to September) bring desiccating winds, which cause severe drought conditions locally. These tanks, being part of a cascade, are hydrologically interlinked. The main canals of the KOISP scheme intersect the natural catchments of the tanks, while most water entering the tanks originate from the Lunugamvehera reservoir. Due to the predominance of paddy cropping on highly percolated soils, there is marked water shortage and as a result most tanks dry out regularly due to competition for these scarce water resources. Principal soil classes in the valley are low humic gley soils and poor to imperfectly drained alluvial soils.

Ecological features: The tanks are bordered by seasonally inundated fringing vegetation and catchment forests consisting of dry-mixed evergreen forests. The aquatic vegetation types of the tank cluster depend on the depth of the tank and the degree to which the tank dries up during the dry season. Shallow, quick drying water bodies such as Pannangamuwa and Debara Wewa (maximum depth around 2.5 m) possess dense bottom-rooted emergent lotus plants (e.g. Nymphaea spp.). Yoda Wewa with a maximum depth of 3.7 m and large areas of shallow water supports clumps of emergent vegetation in the wet seasons. Tissa and Weerawila tanks are deeper (4.7 m and 4.1 m) and have more open surface water with scattered patches of floating weeds (e.g. Salvinia, Pistia) and supports no emergent vegetation as they dry out more rarely. In many places of the tank bunds large Albizia saman trees are present. Tank bed perimeter vegetation types indicate heavy grazing pressure. The tank shores are grassy with dominant species such as Cynodon dactylon and Cyperus rotundas. Diminutive natural vegetation remains in the uplands, due to clearance for paddy and chena cultivation and home gardens. There are few remnant patches of thorn scrub and woodland to the south of Weerawila tank and at the northern end of the Yoda wewa and Beraliyahela hill. In terms of wildlife habitat, the chena and scrub areas on the eastern shores of Yoda Wewa form part of a continuous belt of sparsely populated thorn scrub vegetation, stretching east and southeast through Nimalawa Sanctuary and the Palatupana Lewaya area to Yala National Park. 78 plant species have been identified from the area. Of the fauna, 25 fish species, 8 reptiles, 3 amphibian species and 5 mammals have been recorded from the tank environs.

Noteworthy fauna: The freshwater fish in the tanks are dominated by exotic species such as Oreochromis spp., Catla catla, and Labeo rohita. The aquatic reptiles in the tanks include Crocodylus palustris, Melanochelys trijuga and Lissemys punctata. The tanks are home to several large water birds (e.g. Ardea cinerea, Threskiornis melanocephalus, Anastomus oscitans, Mycteria leucocephala, Pelecanus philippensis), large flocks of waterfowl (Anas querquedula, Dendrocygna javanica) and cormorants (Phalacrocorax spp.). These tanks also function as important water sources for large mammals such as Elephas maximus.

Noteworthy flora: The seasonally inundated vegetation is dominated by trees such as Terminalia arjuna, Diospyros malabarica and Vitex leucoxylon. The catchment forests consist of timber species such as Manilkara hexandra, Dryptes sepiaria and Chloroxylon sweitenia. The native aquatic vegetation of the tanks includes Nymphaea pubescens, Utricularia spp., Ceratophyllum demersum, Polygonum spp. and Ludwigia adscendens.

Land use: The tank system serves as a major water supply scheme for irrigation, domestic use and fisheries. The surrounding land area is mainly used for human settlements, home gardens, paddy cultivation and as a grazing ground. Man-made infrastructure includes main motorways as well as numerous secondary roads. The Weerawila Air Force airstrip lies south of Weerawila tank. The most common agriculture activity is irrigated paddy cultivation while scattered chena farms (vegetables) are also found. Fishing is done with gill and cast nets from out-rigger canoes in July –August when water levels are low. 90% of fish catches consist of Tilapia. In the two smaller tanks, Debarawewa and Pannangamuwa wewa, fishing is mainly done for home consumption, because the water surfaces of these tanks are almost completely covered with lotus and other vegetation. Clay is extracted from the Weerawila tank bed for brick manufacturing.

Possible changes in land use: Planned expansion of KOISP can increase the water flow to the tank cluster.

Hydrological and biophysical values: Important for buffer storage of irrigation waters within the Lunugamvehera KOISP scheme. Maximum water storage occurs during the rainy season and the water is released for irrigation activities (paddy cultivation).

Social and cultural values: The area forms part of the old remnants (> 2000 years) of the Kingdom of Ruhuna, which includes ancient tanks and 4 dagobas that are of national archaeological significance. Most of the people in the area are poor and depend on government assistance. A large number of families raise cattle and buffaloes. Around 25 families are involved in extracting lotus flowers from the Debara Tank as a livelihood.

Scientific research and monitoring: Studies conducted in and around the tank system include bird censuses conducted by the CBC, various fisheries programmes undertaken by universities and NARA, water quality studies undertaken by the Agricultural Experimental station, Weerawila and more recently several research activities have been undertaken by IWMI.

Conservation education: Information not available

Recreation and tourism: The area surrounding Tissa tank is a popular tourist spot, due to the easy access to visitor facilities. Boat trips can be made in the tank and the site is a popular stopover place for tourists visiting Bundala and Yala National Parks as well as Kataragma.

Conservation measures taken: The 4,170 ha Weerawila-Tissa Sanctuary enclosing Weerawila, Debara, Tissa and Pannangamuwa tanks was established in 1938. In 1994, the CEA developed a wetland site report and a conservation management plan for the wetland cluster under the wetland conservation project. Weerawila Tank has been identified as an IBA, according to the selection criteria of Birdlife International. The Irrigation Department maintains these tanks with the help of the local farmer organisations.

Conservation measures proposed: Some recommendations were proposed in the management plan produced by the CEA which included the following; review the status of the Sanctuary; develop a zoning plan to accommodate fisheries activities; provision of stricter protection. Take measures to control water quality and quantity as well as the distribution of water.

Disturbance and threats: Inflow of drainage water from the KOISP has increased the salinity levels of the tanks as well as the risk of eutrophication due to agrochemicals and cattle dung. Continuous clearing of vegetation for chena and encroachment of paddy cultivation and human settlements into the tank reservation area is a major problem. Spread of invasive alien species such as Salvinia molesta, Pistia stratiotes and Eichhornia crassipes has affected the native aquatic vegetation in tanks. Other concerns include heavy grazing and trampling of tank border area by livestock, excessive extraction of lotus flowers and uncontrolled expansion of clay mining. The tanks are heavily silted due to improper land use activities in the catchment. Discharge of effluents from hotels to the Tissa Tank and pollution by the large number of pilgrims and jeeps being washed in the tank are also major problems. The bund of the tank gets damaged when tourist boats turn near the bund. The biggest problem for all these tanks is the lack of regular maintenance.

Land tenure: The tanks, tank-edge reserves and the Kirindi Oya reserves are under state ownership while the settlements, home gardens and paddy lands are under private ownership.

Management authority and jurisdication: The Weerawila-Tissa wildlife sanctuary is under the purview of the DWC. Tank waters are under the control of the Irrigation Department.

References: Bakker (2000); BirdLife International (2004); CEA/ARCADIS Euroconsult (1994j); Matsuno et al. (2000)