Tabbowa Reservoir

Location: 80 6’ 0 N and 790 55’ 0 E to 80 2’ 0 N and 800 0’ 0 E; 895512 N and 380640 E to 888117 N and 389804 E; 15 km from the Puttalam estuary in the Puttalam District, of the North-western Province.

Area: 591 ha at full supply level

Altitude: 15 m above mean sea level

Overview: The Tabbowa reservoir is a medium sized, shallow water, ancient storage reservoir, constructed by damming the Nanneri Oya, a major tributary of the Mi Oya, for irrigation purposes. The reservoir was restored between 1913-1925 and then again in 1969. The wetland area has rich fossiliferous soils that are rare elsewhere in Sri Lanka. The tank and its environs are included within the Karuwalagaswewa Sanctuary, due to its importance as a habitat for elephants and water birds.

Physical features: The reservoir is located in the Mi Oya basin covering an area of 150,000 ha. The catchment area of the reservoir extends to about 39,000 ha and the main water inflow is from the Nanneri Oya, a major tributary of the Mi Oya, which discharges into the Puttalam lagoon. High water levels in the reservoir coincide with peak rainfall in the catchment area. The Jurassic sediments at Tabbowa (known as Tabbowa beds) from the relatively high ground around the Anuradhapura road are estimated to be 500 m in depth. The major soil types common to the area are Reddish-brown earths and Low Humic Gley and Alluvial types. The reservoir is situated in the dry zone of the western low country, where the mean annual rainfall is between 1,000 –1,500 mm. The heaviest rains are experienced during the inter-monsoon months (April and October- November). Average annual relative humidity is 75% and varies between 73.5% in September to 87.5% in December. The annual average temperature is around 240C while it can go up to 320C in June.

Ecological features: The aquatic vegetation consists of phytoplankton and rooted and unrooted macrophytes, while the vegetation in the peripheral areas of the wetland, including its immediate catchment area comprises mainly of tropical dry/mixed evergreen forest, scrubland and grassland savannah communities. The dominant phytoplankton is the blue green algae, Myxophyceae, while the zooplankton community is dominated by small-sized rotifers. 65 macrophytes have been identified from the wetland and its environs. Grasses such as Cynodon dactylon and sedges dominate the submerged vegetation and the drawdown areas. Unrooted vegetation consists of Naja marina and Lippa nodiflora. On either side of the major inlet of the Nanneri Oya, Carex filicine is the dominant species while there is also a strand of trees and shrubs consisting of species such as Acacia nilotica. Nymphoides indicum and Amaranthus sp. can be found in flooded pools in the drawdown area around the reservoir. Tall trees and shrubs such as Terminalia arjuna, Nauclea orientallis, Barringtonia ceylanica, Syzygium makull, Vitex leucoxylon and Caesalphia bonduce occupy the periphery of the reservoir. About 65 species of plants have been recorded from the reservoir area. The abundance of gastropods, bivalves and chironomid larve in the shallow areas might be a reason for high waterfowl diversity in the wetland. 16 fish species, 57 birds, 13 reptiles and 12 mammals have been recorded from the area.

Noteworthy fauna: The freshwater fish are dominated by exotic forms such as Oreochromis spp., and Labeo rohita. Among the freshwater fish, Clarias brachysoma is endemic. Noteworthy aquatic reptiles associated with the reservoir include Crocodylus palustris and serpents such as Xenochrophis asperrimus (endemic) and Gerarda prevostianus. The tank is an ideal feeding habitat of colony nesting large water birds such as Pelecanus philippensis, Threskiornis melanocephalus, Platalea leucorodia, Anastomus oscitans and Mycteria leucocephala. The reservoir functions as an important source of water for large mammals such as Elephas maximus, while locally declining mammals such as Lutra lutra and Prionailurus viverrinus have also been recorded in this area.

Noteworthy flora: Economically valuable timber species around the reservoir include Diospyros ebenum, Tectona grandis, Terminalia arjuna, Azadirachta indica and Manilkara hexandra. A mixture of grasses and sedges thrive on the border areas of the reservoir.

Land use: Wildlife conservation, tank-based subsistence fisheries, irrigated agriculture, homesteads and cattle grazing are the main land and wetland use activities in the area.

Possible changes in land use: Extension of Inginimitiya Irrigation Project will increase the flow through rates to Tabbowa reservoir. Establishment of the proposed hotel complex on the southwestern part of the Tabbowa tank bed might also negatively impact on the reservoir. The spread of the invasive alien Prosopis juliflora could lead to the establishment of uniform stands of this species around the tank.

Hydrological and biophysical values: The water that is stored during the rainy season is released during the Maha season according to the irrigation requirements downstream.

Social and cultural values: Local reeds such as Hambupan (Typha), Thulhiriya and Gallaha (Cyperus spp.) are used in the production of handicrafts. The grasslands in the border areas of the reservoir are used by locals for feeding cattle and buffaloes. There are considerable traces of ancient settlements in the vicinity, which go back to the commencement of the Vijayan dynasty, 543 BC.

Scientific research and monitoring: Research with respect to geology and fisheries have been conducted by the Kelaniya University, while studies into the avifauna of the reservoir have been carried out by the CBC. The DWC is monitoring elephant movements.

Conservation education: Information not available

Recreation and tourism: Limited, especially due to the non-availability of drinking water and accommodation facilities.

Conservation measures taken: The CEA developed a wetland site report and conservation management plan in 1993. The tank and its environs are included within the Karuwalagaswewa Sanctuary (21, 933 ha), which was declared under the FFPO in 2002 to facilitate elephant migration. The reservoir was declared a FMA under the FARA in 2005. DWC undertakes habitat management within the area.

Conservation measures proposed: The wetland site report and conservation management plan produced by the CEA provides details of proposed conservation measures.

Disturbance and threats: Forest cover in the catchment has been reduced due to timber extraction and clearing for chena cultivation and homesteads. These activities have lead to increased siltation of the tank. Reforestation of the area with exotic Eucalyptus camaldulensis has contributed to significant degradation of the vegetation cover in the area. Encroachment of the tank area for agriculture is evident. The spread of invasive alien Prosopis juliflora is a growing conservation issue.

Land tenure: State owned

Management authority and jurisdication: The Sanctuary is under the purview of the DWC while tank management is carried out by the Irrigation Department. Fisheries are managed by the Fisheries Department.

References: CEA/ARCADIS Euroconsult (1993d); DWC (personal communication)