Thalangama Tank

Location: 60 54’ 30 N and 790 55’ 0 E to 60 53’ 0 N and 790 58’ 0 E; 762830 N and 380312 E to 760975 N and 385833 E; in the Colombo District, of the Western Province.

Area: The tank consists of an irrigation area of 95 acres and a non-irrigable area of 111 acres. The gross capacity of the tank is 50 Ac. Ft.

Altitude: Information not available

Overview: Thalangama Tank was built during the era of King Parakramabahu VI (1551-1547 AD) for paddy cultivation and is situated close to the ancient city of Kotte. The areas between Akurugoda and Thalangama north use the runoff water for paddy cultivation. The tank is an important habitat for water birds, functioning as an urban biodiversity refuge, as it is surrounded by densely populated human settlements of the Colombo District. The site is situated in the Madiwela catchment area within the Kelani river basin, which includes the Colombo flood detention area.

Physical features: The area is part of the Pre-cambrian southwestern group, and the geological formation of the coastal plain consists of gneisses and granite, predominantly of the Vijayan complex of the Pre-cambrian era. The soil is composed mainly of the Aruvi and Boralu series. Half-bog soils are found in the paddy area as well as the catchment area of the tank. The tank and its environs are situated within the Madiwela catchment area, which is influenced by 3 hydrological zones and is an important component for water drainage in the city of Colombo. The total catchment area is 33.34 sq. miles, and the upper reaches extend up to Pannipitiya and Kottawa. The gross capacity of the tank is 50 Ac. ft., and it has two spills, one of which is controlled by a steel gate. The tank is situated in the low country wet zone, with a mean annual rainfall between 2,000 – 2,500 mm, derived mainly from the Southwest monsoon. The area is characterized by high relative humidity and a hot climate with a mean annual temperature of 27.4o C fluctuating between 23.9 –30o C and a relative humidity of 82%.

Ecological features: The Thalangama tank and its surrounding habitats serve as an important flood retention area. Major habitat types associated with the tank include seasonally flooded grassland/paddy fields (between ridge bund and Hokandara–Talawatugoda road), freshwater swamp forests (western boundary of the tank, dominated by Annona glabra, Cerbera odollam and Pandanus odoratissima), aquatic macrophytic vegetation (floating plants and rooted plants with floating leaves) like Eichhornia crassipes and Nelumbo sp. and Annona glabra and Pandanus spp. which occupy the tank edge, home scrublands dominated by species such as Lantana spp., Macaranga peltata and Syzygium caryophyllatum. Large trees like introduced Alstonia spp. are also found in abundance within home gardens along the tank margin. Forty-one plant species have been recorded in the area. 90 bird species (13 are migrants), 12 species of reptiles, 10 species of mammals and 15 freshwater fish species have also been recorded from the tank and its environs.

Noteworthy fauna: Freshwater fish in the tank comprise of a mix of exotic and indigenous species. Among the native fish, Puntius singhala, Etroplus maculates and Esomus thermoicos are among the noteworthy species that are found in the tank. The bird fauna of the tank is very significant when considering its small size. Locally declining aquatic birds include Pelecanus philippensis and Rostratula benghalensis. Noteworthy reptiles include Lissemys punctata, and Xenochrophis piscator, while remnant populations of the endemic primate Trachypithecus vetulus, and other rare mammals such as Prionailurus viverrinus and Lutra lutra have been recorded around the tank.

Noteworthy flora: The flora of this semi-urban area is highly modified, with many exotic species. The floating aquatic plants such as Nelumbium spp. and Nymphaea spp. form a prominent component of the aquatic plants in the tank.

Land use: The surroundings are used for human settlements, home gardens, paddy cultivation, and for livestock rearing while the wetland is used for small-scale fisheries.

Possible changes in land use: Catering for expanding human population needs through development of new housing schemes and other infrastructure is expected to affect the tank and its environs in the long term. New housing development and infrastructure development might impact the catchment area of the tank.

Hydrological and biophysical values: The Thalangama tank and its surrounding paddy fields are important for flood detention in the Greater Colombo area. In 1996, a diversion canal was built connecting the tank with the Kelani River under the storm water-draining project to prevent flooding in the vicinity of the parliament

Social and cultural values: The Thalangama tank is a unique remnant of the historic past. The tank has been used for paddy cultivation for a long time, and historic evidence indicates that the surrounding area had been continuously and extensively cultivated. The northern boundary of the tank lies along the bund, which serves as a connecting road between Thalangama and Akurugoda.

Scientific research and monitoring: FOGSL has conducted several bird surveys in the area. Research work has also been carried out on hydrological aspects, monitoring of water flow and flood rates in the surrounding area under the Greater Colombo Flood Control and Environment Improvement Project.

Conservation education: Universities and an International School in the area use the site for educational purposes. Additionally, in 2005, IUCN conducted an education programme at this site on World Wetlands Day.

Recreation and tourism: Private tour operators use the site as a part of their packaged bird watching tours for tourists. A luxury hotel is located within the tank vicinity.

Conservation measures taken: In 2006, the Overseas School of Colombo in association with FOGSL organised a walk to raise funds for wetland protection, which ended at this site.

Conservation measures proposed: This tank is a proposed EPA under the NEA of the CEA. Following the visit of the Minister of Environment and Natural Resources to this site in 2005, the Kaduwela DS has been instructed to implement projects to enhance the environmental standards around the tank area.

Disturbance and threats: Due to rapid urbanisation of the area, the land value has increased significantly leading to excessive land filling and the reclamation of paddy lands for housing, despite the fact that this activity is illegal. The highly concentrated human population in the Jayawadanagama housing scheme has resulted in dumping of garbage and other solid waste within the tank environs and the release of wastewater in to the tank. Waste from a nearby poultry farm is also released to the tank causing deterioration in water quality. Poaching of water birds such as purple coots and painted storks as well as terrapins is affecting the biodiversity while the spread of invasive plant species such Annona glabra, Eichhornia crassipes and Salvinia molesta are also threatening the wetland.

Land tenure: The surrounding area is mainly privately owned

Management authority and jurisdication: The tank falls under the legal jurisdiction of the Irrigation Department, and the CEA.

References: CEA/ARCADIS Euroconsult (1995a); FOGSL (2005); Rathnayake and Kotagama (2002)