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Surface water resources of Sri Lanka
Surface
Water
Sri Lankan surface water resources account both freshwater
and brackish water resources in the island. Freshwater
resources include river water, tank water, reservoir
water Sri Lanka receives mean annual rainfall of 2000
mm. The highest rainfalls are in the southwest central
highlands, exceeding 5,000 mm (Maliboda, 5,330 mm;
Weweltalawa estate, 5,258 mm; and Kenilworth estate,
5,085 mm). The mean annual rainfall values on the
eastern slopes are less than 3,500 mm and the lowest are
in the northwest and southwest lowlands in the arid
zone. Part of this rainfall returns to the atmosphere as
water vapor by evaporation from soil, open water
surfaces and transpiration from vegetation, together
referred to as evapotranspiration. This accounts
approximately about half of the total volume of water
from the precipitation. However, evapotranspiration
cannot be considered as a loss since it is a vital part
of the hydrological cycle (Figure
7) but optimization is important. At the same time
of the precipitated storm water lost to the atmosphere
by evapotranspiration, part of it infiltrates to the
soil. Soil keeps water as soil moisture and the rest
percolates to groundwater. The residual water remains
results the surface water in the island.
River
System
Rivers begins from the central highlands and flow radailly
towards the sea. There are about 103 distinct river
basins covering 90 percent of the island. The southwest
quarter of the island has seven major basins, Kelani
ganga (2,292 km2), Kalu ganga (2,719 km2),
Maha oya (1,528 km2), Attanagalla oya (736 km2),
Gin ganga (932 km2), Nilwala ganga (971 km2),
and Bentota ganga (629 km2). The largest
basin, that of the 335-km-long Mahaweli River which has
a catchment area of 10,448 km2. Most Sri
Lankan river basins are small. Only 17 of the 103 basins
exceed 1,000 km2. In addition to the Mahaweli
basin, only four others exceed 2,500 km2,
Deduru oya, Kala oya, and Malwatu oya have their entire
catchment are in the dry zone, and only Kalu ganga is in
the wet zone. The total runoff in Sri Lanka is estimated
at 49.2 km3/year.
Rivers are categorized into two; perennial rivers and
seasonal rivers. River watersheds located at the wet
zone of the island have a continuous flow though out the
year is referred to as perennial rivers (Kelani river,
Gin Ganga etc). Eighty percent of the rivers in Sri
Lanka are flowing through the dry zone have their
watersheds located at the dry zone or intermediate zone
are seasonal (Kirindi Oya, Maduru Oya etc). Perennial
rivers are classified again into two based on their flow
velocity; fast flowing (flow through steep gradients in
the country) and slow flowing rivers (flow through flat
lowlands). Belihul Oya is an example for a fast flowing
river while Deduru Oya is considered as a slow flowing
river.
Tank
Cascade System
The early settlements in Sri Lanka were concentrated mainly
to the north, north central and north western parts
where a dry climate is prevalent due to the topographic
suitability for paddy cultivation. Since rain falls
within a limited period of the year in these dry zone
districts the ancient Sinhalese developed their own
water management system now known as the Tank Cascade
System (TCS) to overcome the limited water availability.
The TCS is a chain or a series of small tanks
(reservoirs) that are built at successive locations (Figure
8) with a common water source to reuse the water as
much as possible. Since we do not have many seepage
lakes or reservoirs, where groundwater seeps into, the
majority of the tank water is from the rainfall. There
are about 30,000 tanks which have been built in the land
area of about 40,000 km2 of the dry zone,
resulting in nearly a tank for every square kilometer.
See the report on TCS
Reservoirs
Since the independence from the British, several reservoirs
have been made for the purpose of irrigation and
hydropower generation. Dams in Sri Lanka are classed
according to the materials they use. The Victoria dam,
built under the Mahaweli multipurpose project, is the
highest concrete (double curvature) dam with a height of
106 m and a storage capacity of 0.73 km3. The
gross theoretical hydropower potential in Sri Lanka is
estimated at 8,000 (GWh/year). In 1997, 16 hydropower
plants were in operation with an installed capacity of
1,103 megawatts (MW) and thereafter Kukule Ganga and
Upper Kotmale were built. Other than these large scale
dams several small scale mini hydropower dams are
present in the central highlands especially at tea
plantations. Hydropower accounted for 81 percent of
electricity generation in Sri Lanka during the early
decades however; it has been reduced due to less
rainfall as a result of climate change.
Brackish water bodies
The coast line of Sri Lanka is about 1600 km long.
Estuaries and lagoons are the main coastal water bodies
found along this coast line. Within these the seawater
is diluted with the freshwater from the freshwater
bodies or from terrestrial runoff. Estuaries are
partially enclosed coastal water bodies having a free
connection with the ocean. Two types of estuaries found
in Sri Lanka; a basin estuary where the river discharges
into a basin and then the basin connects with the sea (Bolgoda
lake, Chilaw), while a riverine estuary has the river
directly discharges to the sea (Kalu, Kelani and Nilwala
Ganga).
Lagoons are either permanently separated from the sea or
connect with the sea during a part of the year.
Therefore lagoons represent variable salinity based on
the time of the year. Batticaloa, Puttlum are the
largest lagoons in Sri Lanka. Negambo, Lunawa, Lunama
are some of the small lagoons that can be found along
the coast line.
Both lagoons and estuaries are rich in nutrients and
therefore they are highly productive areas. They serve
as nursery grounds for prawns and fish. However, since
these are located in densely populated areas they are
often get polluted.
Ground water
Water pollution
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