BWI’s blog post invites you to explore some of the key river basins of Sri Lanka, often referred to as the “Pearl of the Indian Ocean” or “Teardrop of India”. Discover their importance, unique features, and challenges they encounter. Read more and gain a deeper understanding of their connection to the island’s water cycle.
BWI’s blog post “Hydrology in Sri Lanka” offers a comprehensive exploration of the island’s most significant river basins. These river networks have been vital to the local population, supporting a hydraulic civilization with roots that stretch back to the 5th century B.C. Even today, Sri Lanka’s water resources remain essential for sustaining the nation’s socio-ecological balance. In this post, BWI introduces you to the island’s landscape and explores five chosen river basins, discussing their importance, unique characteristics, and the challenges they face.
Sri Lanka is a tropical island located in the Indian Ocean, spanning a total area of 65,525 square kilometers. Rainfall in the region sustains a radial network of rivers. These originate in the central highlands and flow towards the plateaus in other parts of the country. As a result, the central region serves as the primary watershed, while the plateaus offer various opportunities for irrigated agriculture. The island is also home to 103 distinct rivers, covering an area of approximately 59,217 square kilometers.
The geographical positioning of the main river basins enabled early settlers to establish an extensive irrigation network as early as the 5th century B.C. The highland watersheds were easily conserved, and the settlers developed agricultural systems on the plateaus by storing water through a well-developed network of reservoirs. Consequently, these river networks played a crucial role in the lives of the local population, fostering a long history of hydraulic civilization.
The island’s sole water source is rainfall, with 75% of precipitation occurring between October and December during the northeast monsoons. The majority of water resources in these basins are allocated to agriculture and hydropower, with agriculture being the largest consumer. For instance, in 2020, agriculture contributed approximately 8% to the national GDP. Furthermore, rice is the island’s most important crop, producing 4.1 million tonnes in 2019-2020 (enough to feed the entire population). It accounts for 1.8% of the GDP, representing 42% of all agricultural land. Since rice is the primary food source, much of the country’s land is devoted to paddy cultivation.
Additionally, the complex river network is closely intertwined with the island’s water cycle:
Sri Lanka has three climatic zones:
There are also two monsoon seasons:
Hydrology in Sri Lanka
Location
The Mahaweli river is the longest in Sri Lanka. It stretches across 335 km and drains approximately 16% of the country’s land area. The river basin spans around 10,448 km², with annual rainfall varying significantly across its upper and lower sections. The Mahaweli river traverses the wet, intermediate, and dry climatic zones of Sri Lanka. It collects an annual rainfall of about 2680 mm, with 9 × 10⁹ m³ discharged into the sea. The basin’s rainfall distribution is highly dependent on time and space, impacting water availability throughout the region.
Significance
The Mahaweli Ganga river is crucial for Sri Lanka’s agriculture and energy sectors. It irrigates 3,650 km² of paddy fields through a vast network of canals. Being the highest provider of irrigation water, it carries one-seventh of the annual runoff. The river equally contributes to 40% of the country’s hydropower potential. Moreover, the Mahaweli river has facilitated the development of adjacent river basins, addressing water scarcity for irrigation and domestic needs. Lastly, the Mahaweli River Basin plays a significant role in inland fisheries due to its large floodplains and reservoirs.
Challenges
Deforestation, agricultural expansion, and infrastructure development have degraded forests and wetlands, reducing the basin’s capacity for water infiltration and retention. What’s more, the dry zone area has doubled in the span of 40 years , increasing the potential of droughts in the area. Paddy cultivation in dry zonal areas mainly depends on irrigated water. Therefore, paddy cultivation is mainly based on the river basin-oriented complex water management systems. Nevertheless, poor water management has led to soil salinization in the lower catchment areas, causing declining water quality, habitat loss, and increased variability in water availability. Climate change is exacerbating these issues, intensifying both drought and flood risks.
Noteworthy
Before large-scale development, 50% of the area in the Mahaweli Basin was covered with mature tropical humid, sub-humid, or dry forests. The forests supported high biodiversity, including numerous endemic and rare species. Despite several large towns being located within this basin, it has one of the lower population densities in Sri Lanka.
Hydrology in Sri Lanka
Location
The Kalu River originates in Sri Lanka’s central hills and flows through Ratnapura and Horana before reaching the Indian Ocean at Kalutara. It stretches for 129 km, draining a catchment area of around 2,690 km². The river begins at an altitude of 2,250 meters in the wet zone. It then quickly drops to 14 meters above sea level within the first 36 km, resulting in a narrow bed and high banks. The entire basin is located in Sri Lanka’s wet zone. Rainfall varies from 6,000 mm in the mountains to 2,000 mm in the lower plains.
Significance
The Kalu Ganga River Basin is located in one of the country’s highest rainfall areas. Despite its size and rainfall, significant water resource development in the basin has been limited primarily to rural drinking water supply and minor irrigation schemes. One notable exception is the Kukule River hydropower project. Kukule is a tributary of the Kalu River, which operates an 80 MW power station. The basin’s high rainfall results in an annual flow of approximately 4,000 million cubic meters, making it a crucial water resource for the region.
Challenges
The Kaluganga Basin is characterized by steep gradients in its upper reaches and milder gradients in its lower plains. These topographical features contribute to frequent flooding, particularly during the Southwest monsoon. The lower floodplain, especially around Ratnapura, is highly susceptible to floods. Floods, in particular, have caused significant socio-economic damage due to the area’s dense population and agricultural importance. Serious floods have occurred in 1913, 1940, 1941, 1947, 2003, and more recently in 2008. The May 2003 flood alone caused an estimated Rs. 73 million in damage, highlighting the ongoing challenges of flood management in the basin.
Noteworthy
Despite being the second-largest river basin, the Kalu River discharges the highest volume of water into the sea among Sri Lankan rivers, totaling around 4,000 million cubic meters annually. This is partly due to the lack of large-scale water regulation projects like those in the Mahaweli River Basin. The Kalu Ganga Basin also boasts the highest annual per capita water availability in Sri Lanka. The water availability stands at about 7,750 cubic meters, significantly higher than the national average of 2,300 cubic meters. The consistent and substantial rainfall in the basin, averaging around 3,818 mm annually, ensures a steady water supply.
Hydrology in Sri Lanka
Location
The Kelani River originates from the western face of the central highlands within the Horton Plains National Park and Peak Wilderness Sanctuary in Sri Lanka. The river drains an area of approximately 2,292 square kilometers. Its middle and lower reaches flow through the Colombo and Gampaha districts with the highest population and industrial densities in Sri Lanka. The annual rainfall within the basin varies significantly. Hence, it can range from 500 mm to 5,000 mm, with an average annual rainfall of about 3,450 mm.
Significance
The Kelani River Basin passes through three distinct agroecological zones: the Up-Country Wet Zone, Mid-Country Wet Zone, and Low-Country Wet Zone. Within these zones, the basin is further divided into several sub-regions based on climate, topography, and vegetation, ranging from the hills to the coastal areas. The basin supports diverse agricultural activities, with each agro-ecological zone offering desired conditions for different crops. The river’s importance is also underscored by its role in supplying water to Colombo, the capital of Sri Lanka, and other urban areas, as well as supporting a variety of industrial activities.
Challenges
The basin is highly prone to flooding, especially downstream during the Southwest monsoon. A major flood in May 2016 brought 350 mm of rain over three days, causing the Kelani River at Hanwella to rise to dangerous levels. Flooding is a serious concern due to the dense population and concentration of industries in the lower basin, resulting in significant damage to property and livelihoods. Pollution is another major issue, particularly in urban and industrial areas. Municipal wastewater, industrial discharges, stormwater runoff, and household pollutants degrade water quality. This pollution endangers aquatic life, contaminates drinking water, and threatens the health of communities relying on the river.
Noteworthy
The Kelani River Basin boasts some of Sri Lanka’s most scenic landscapes, offering great potential for eco-tourism. The upper reaches, rich in biodiversity, are especially appealing for tourism. For this reason, the Kelani River landscapes can help diversify the local economy beyond its industrial focus. Besides, the basin’s lowland, submontane, and montane forests are home to many endemic and threatened species, making it a vital area for conservation. Because of this, sustainable management of the basin will require balancing urbanization and industrialization with the need to preserve its biodiversity and protect communities from flooding and pollution.
Hydrology in Sri Lanka
Location
The Malwathu Oya River Basin spans a total length of 162 km. It originates from the Ritigala and Inamaluwa Hills in the North Central Province, with elevations of 766 meters and 383 meters above mean sea level, respectively. The river flows through the Anuradhapura District, where 70% of its upper catchment is located. It then continues through the Vavuniya and Mannar districts before reaching the sea.
Significance
The Malwathu Oya Basin is one of Sri Lanka’s major agricultural regions, with rice farming as the predominant livelihood. This region, situated in the dry northern part of the country, has a rich history of rice cultivation. It dates back thousands of years, supported by ancient irrigation systems that have sustained local communities. Furthermore, two significant ancient reservoirs, Giant’s Tank and Akattimurippu Tank, are located in the lower reaches of the Malwathu Oya Basin. These tanks irrigate areas of 9,895 hectares and 2,521 hectares, respectively. Both tanks are fed by diversions from the Tekkam Anicut, an ancient structure located 36 km upstream from the river’s mouth.
Challenges
The Malwathu Oya Basin faces significant challenges due to its high vulnerability to climate change and the fluctuating availability of water. During the Yala (dry) season, the flow in the Malwathu Oya is insufficient to support cultivation under both Giant’s Tank and Akattimurippu Tank, leading to water shortages. Conversely, during the rainy season, excessive water flow over the Tekkam Anicut causes flooding in the lower reaches of the basin, particularly in the Musali, Nanattan, and Manthai West Divisional Secretary Divisions. This dual threat of drought during the dry season and flooding during the wet season severely impacts the livelihoods of communities dependent on agriculture. Moreover, the increasing water demand due to rapid population growth and expanded agricultural activities in the dry zone exacerbates water scarcity.
Noteworthy
The Malwathu Oya holds deep historical and cultural significance, as it was a vital water source for the ancient city of Anuradhapura, one of Sri Lanka’s most important archaeological sites. This river nourished the city’s intricate network of reservoirs and irrigation systems, which were engineering marvels of their time and crucial for the development of ancient Sri Lankan civilization. The river’s role in sustaining these ancient hydraulic structures reflects not only its environmental importance but also its contribution to the cultural and historical heritage of the region.
Hydrology in Sri Lanka
Location
The Deduru Oya River Basin encompasses an area of approximately 2,622 square kilometers. The river originates in the Matale hills, part of the central highlands’ wet zone, and flows westward through the North Western Province, passing through the Kurunegala and Puttalam Districts before emptying into the Indian Ocean near Chilaw. The basin also includes a small area in the North Central Province. The main river stream is 115 kilometers long, and the basin features 15 sub-watersheds.
Significance
The Deduru Oya River Basin is crucial for Sri Lanka’s agriculture, particularly for paddy cultivation. The fertile alluvial plains along the river support extensive rice farming, which is vital for the local economy. The basin is also home to seven major and medium-sized reservoirs, supplying water for irrigated agriculture. These reservoirs, along with approximately 1,560 small village tanks, irrigate about 18,320 hectares of farmland. The Deduru Oya Reservoir, completed in 2014 as part of a significant irrigation project, has a storage capacity of approximately 75 million cubic meters and plays a critical role in ensuring year-round water availability, flood control, and drinking water supply for 3,000 families.
Challenges
The Deduru Oya River Basin faces several challenges due to its dependence on rainfall, which is unevenly distributed both spatially and temporally. The region experiences significant variations in annual rainfall, ranging from 2,600 mm in the upper basin to 1,100 mm in the lower basin. This variability can lead to severe water management challenges, including droughts and floods. Flooding has become more frequent and severe, partly due to climate change, with recent years seeing sudden flash floods that were previously uncommon. Water pollution is another significant issue, with agricultural runoff containing pesticides and fertilizers degrading water quality and affecting aquatic life. Sedimentation from deforestation and poor land management practices further reduces the storage capacity of reservoirs and water channels, complicating water management efforts. Additionally, rapid population growth and unplanned urbanization have led to encroachment on riverbanks, exacerbating flood risks and disrupting natural river flows.
Noteworthy
The Deduru Oya Reservoir Project supports the irrigation of over 30,000 hectares of farmland during both the Yala and Maha cultivation seasons. It also contributes to flood control, though its effectiveness in preventing unprecedented floods downstream remains a topic of concern, as flooding incidents have spread to areas previously not susceptible. The Deduru Oya basin is also characterized by its diverse land use, with coconut plantations covering about 36% of the basin, paddy lands accounting for 17%, and home gardens making up 14%.
The river basins discussed in the BWI blog post are integral to hydrology in Sri Lanka, agriculture, and socio-ecological balance. While each river differs in size, flow, and climatic zone, they collectively sustain the island’s ecosystems, support agriculture, and shape the landscape. The challenges these basins face, from seasonal floods and droughts to deteriorating water quality, highlight the delicate equilibrium between Sri Lanka’s natural environment and human activity.
“Hydrology in Sri Lanka” A-S
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“Hydrology in Sri Lanka” U-Z
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