Kalawewa Tank

Location: 80 3’ 0 N and 800 30’ 0 E to 70 57’ 0 N and 800 36’ 0 E; 889859 N and 444906 E to 878792 N and 455915 E; northwest of Dambulla in the Anuradhapura District, of the North-central Province.

Area: 2,590 ha at full supply level

Altitude: 130 m above mean sea level

Overview: Kalawewa is a large, ancient irrigation tank, which was built by King Dhathusena (455-473 AD) by damming the Kala Oya. This ancient tank was restored in 1887 and again in 1939. Kalawewa plays a pivotal role in the modern Mahaweli irrigation scheme, as it is one of the main storage tanks in North-central Sri Lanka. There is a thriving inland fishery in the Kalawewa – Balaluwewa system. The vegetation, particularly the grasses in the drawdown area, presents the primary source of fodder for sizable herds of cattle and buffaloes in these areas.

Physical features: Kalawewa is the largest water storage tank in the Kala Oya basin, with an active surface storage capacity of 123 mcm. The command area of the Kalawewa is 23,800 ha. Since the implementation of the Mahaweli Development Project, the tank receives water through the Dambulu Oya, a main headwater tributary of the Kala Oya. Kalawewa is located within the dry zone of Sri Lanka, the area receives rainfall mainly during September – November (northeast monsoon), with an average annual rainfall of 1,219 mm. Mean monthly temperature is around 27.90C while the mean monthly relative humidity varies from 60% (March) to 80% (December). During the southwest monsoon period strong, dry winds blow constantly over the plain, making the area drier. The soil in this area consists of reddish-brown earths and low humic gley soils typical of the northern lowland region, with some alluvial soils being found in the river valleys.

Ecological features: The aquatic vegetation comprises mainly of phytoplanktons, while rooted, floating and submerged macrophytes are also present. Terminalia arjuna and Nauclea orientalis dominate the seasonally inundated plant communities associated with the tank fringes of the Kalawewa. The undergrowth in this area is not dense and it is ideal habitat for wildlife. Flagship species such as Asian elephants are frequently recorded in this area. Herbaceous flora mainly comprises of annuals with a decrease in diversity towards the waterline. The grass Cynodon dactylon is the only species found in the shallow areas along the waterline. The surrounding landscape includes natural vegetation types such as dry mixed evergreen forests and man-made habitats such as chena cultivations, paddy fields and home gardens.

Noteworthy fauna: The freshwater fish are dominated by exotic species such as Oreochromis mossambicus, Labeo rohita, and other carp species. Indigenous species such as Etroplus spp, Puntius spp and Channa spp. have been 110 recorded from Kalawewa. Aquatic reptiles include Crocodylus palustris, Lissemys punctata and Melanochelys trijuga. This is also an ideal habitat for large colonies of water birds including Pelecanus philippensis, Phalacrocorax niger and Anastomus oscitans, Phalacrocorax fuscicollis. Raptors such as Haliaeetus leucogaster and Ichthyophaga ichthyaetus are also notable species that are found at Kalawewa. Mammals that visit the tank include Elephas maximus, Prionailurus viverrinus and Lutra lutra.

Noteworthy flora: Dry mixed evergreen forest patches are found distributed around the tank, which harbour valuable timber species such as Manilkara hexandra and Vitex pinnata. The seasonally inundated swamps are dominated by Terminalia arjuna.

Land use: Fishing activities are done using bottom set nets, cast nets and drift nets. The surrounding forest is a source of medicinal plants while paddy cultivations and chena cultivations are also found. Animal rearing is a common livelihood particularly in the middle basin.

Possible changes in land use: Allocation of more land to agriculture and increased conversion of land for the Mahaweli Irrigation Scheme are potential methods of land use change.

Hydrological and biophysical values: Kalawewa acts as an important water storage tank in the Kala Oya basin. It stores Mahaweli irrigation water from upstream and releases the water for agriculture in the Maha season according to downstream irrigation requirements. The water level of the perennial tank fluctuates, depending on the amount of water released for irrigation hence the tank has an extensive draw down area, which gets exposed during periods of low water level.

Social and cultural values: Locally found reeds are utilized for the production of handicrafts. Brick making also takes place to some extent using the sediment from the draw down area.

Scientific research and monitoring: MASL has initiated a pilot study in order to prepare a basin management plan within the Kala Oya basin. Kalawewa is included in this study. The Institute of Fundamental Studies has undertaken numerous water quality studies in the tank.

Conservation education: Information not available

Recreation and tourism: As the tank is situated within the culturally important and touristically popular Anuradhapura District, a large number of tourists visit the area annually. Elephant and bird watching are popular activities among the tourists. There is a new hotel under construction in the area.

Conservation measures taken: Part of the tank catchment and fringing region falls under the Kahalle-Pallekelle Sanctuary.

Conservation measures proposed: Information not available

Disturbance and threats: The invasive alien species Lantana camara is extensively found in this area. Other disturbances include the deforestation of the seasonally inundated forest and clearing of land for expanding cultivations.

Land tenure: State owned

Management authority and jurisdication: The Kalawewa area comes under the mandate of the Ipologama DS Division, which comprises of 32 Grama Niladhari divisions. The tank comes under the jurisdiction of the MASL. The DWC has jurisdiction over the Kahalle-Pallekelle Sanctuary.

References: Dissanayake (2000); MASL (2005); Silva (1996)