The ancient Sri Lankan 'tank cascades' tackling drought

 

A 2,000-year-old Sri Lankan hydraulic system uses natural features to help harvest and store rainwater. In a rapidly warming world, it is providing a lifeline for rural communities.

Each April, in the village of Maeliya in northwest Sri Lanka, Pinchal Weldurelage Siriwardene gathers his community under the shade of a large banyan tree. The tree overlooks a human-made body of water called a wewa – meaning reservoir or "tank" in Sinhala. The new stretches out beside the village's rice paddies for 175 acres (708,200 sq m) and is filled with the rainwater of preceding months.    

Siriwardene, the 76-year-old secretary of the village's agrarian committee, has a tightly guarded ritual to perform. By boiling coconut milk on an open hearth beside the tank, he will seek blessings for a prosperous harvest from the deities residing in the tree. "It's only after that we open the sluice gate to water the rice fields," he told me when I visited on a scorching mid-April afternoon.

By releasing water into irrigation canals below, the tank supports the rice crop during the dry months before the rains arrive. For nearly two millennia, lake-like water bodies such as this have helped generations of farmers cultivate their fields. An old Sinhala phrase, "wewai dagabai gamai pansalai", even reflects the technology's centrality to village life; meaning "tank, pagoda, village and temple".

But the village's tank does not work alone. It is part of an ancient hydraulic network called an engawa, or "tank cascade system". As such, the artificial lake at Maeliya links up with smaller, man-made reservoirs upstream in the watershed. Together with their carefully managed natural surroundings, these interconnecting storage structures allow rainwater to be harvested, shared, and re-used across the local area.

Constructed from the 4th Century BC up to the 1200s, these cascade systems have long helped Sri Lankan communities cope with prolonged periods of dry weather. "As most of the country is made up of crystalline hard rock with poor permeability, it induces runoff, " says Christina Shanthi De Silva, senior professor in agricultural and plantation engineering at The Open University of Sri Lanka. "Our forefathers built tank cascades to capture this surface runoff," she explains, preventing it from being washed away into rivers and, ultimately, the sea.

Such knowledge has since been passed down the generations. In a laminated box file, Siriwardene carefully safeguards a map his father, the village head, drew of Maeliya's cascade. There are nine tanks in this particular cascade, his father writes. A copy of another handwritten booklet documents the tanks' history and the folk poems that villagers sang in gratitude for its continuous water resource.

A lifeline in a heating climate    

Today, although some of the tanks in Maeliya's system have been abandoned, the main tank still provides its harvested rainwater to 202 farmers, irrigating 155 acres, says Siriwardene. At a time when climate change is projected to increase both Sri Lanka's drought and flood risk, tank cascades are receiving new attention.      

In the north-central plains of Sri Lanka, rehabilitation of village tank cascades under a World Bank-supported project has helped farmers grow rice and cultivate vegetables year round. Similarly, in the northwestern dry zone, during 2017's prolonged drought when many farmers had to abandon their crops, a United Nations project to rehabilitate a cascade system of 27 tanks helped farmers in Kurunegala to continue growing their rice.

One of the ways that cascade systems reduce drought risk is by connecting different tanks together, researchers have found. According to a study that measured the changing density of vegetation, tanks that were part of cascades retained more water during the dry season than small isolated reservoirs that operate alone.

The structures were formed by constructing embankments around natural depressions in the landscape, explains Punchi Bandage Dharmasena, an independent soil and water management researcher, and a national consultant at the Food and Agriculture Organization of the United Nations in Sri Lanka. To control the release of water, the tank builders constructed a sluice gate and used a natural stone gauge near the gate for measuring the water levels of the tank. Water from one tank passes to another through small streams in paddy fields, Dharmasena says. "We can say that it's a recycling of the water resource."

 

Our forefathers built tank cascades to capture this surface runoff - Christina Shanthi De Silva

 

The ecological features of tank systems also help prevent drought by regulating groundwater levels, says Nalaka Geekiyanage, senior lecturer in forestry at the Rajarata University in Sri Lanka. "Cascades have large tree covers, providing high capacity to cool the ecosystem. Because [such tank system features] maintain the groundwater table, restoring them can also help maintain water in rivers that otherwise run dry during hot months."

Tree belts planted alongside the tank act as a wind barrier and reduce evapotranspiration, while community-owned forest cover in the catchment area supports the groundwater table and slowly releases water to the tank during dry periods. Furthermore, the trees help protect communities from the flash floods that can follow periods of drought by intercepting rain, reducing water's velocity, and controlling soil erosion, Geekiyanage adds.  

    

In addition to providing irrigation and helping reduce drought and flood risk, the cascade systems have supported villages in less direct ways too.

About 30km northwest of Maeliya, in the village of Ullalapola, 72-year-old Tikiri Kumari remembers a small tank situated outside the village: "This one was so small it didn't have a bund [embankment]. We didn’t take water from it. We always saw cattle swimming in it," she says. These small forest tanks, as well as trapping silt and regulating the release of excess rainwater, were constructed to provide water for wildlife and so discourage animals from coming to the village.

Water purification is another advantage. Tank builders constructed a soil ridge to prevent sediment from ending up in the tank, and grew different grass-like plants in this area. "It was like a filter that retained sediments and purified water before slowly releasing to the main water body, " says Geekiyanage. The roots of the large trees growing near the tank also created water cages for fish to breed.  A joint initiative by the United Nations Development Programme and Reforest Sri Lanka has thus recently helped farmers grow native trees and plant species around their tanks, and so restore the original tree-belts.

Animal husbandry, beekeeping, agroforestry, and food cultivation beyond rice can all also be supported by a local tank system. Domestic gardens in the cascade landscape bear a high diversity of foods like neglected fruit species, edible medicinal plants, indigenous vegetables, tubers, and spices, says Shiromi Dissanayaka, associate professor at the department of Agricultural Engineering and soil science at Rajarata University.  These gardens can, in turn, mitigate the socioeconomic impact of climate change, she notes. "Tank cascades help conserve the soil moisture in these home gardens for an extended period, so villagers can grow crops and ornamental plants throughout the year."

A modern overhaul

Researchers suggest there were once 18,000-30,000 small tanks in Sri Lanka, with 90% organized into clusters or cascades. But today only 14,421 active tanks and 1,661 cascades are estimated to remain.

The causes of this decline are numerous, explains Geekiyanage. After the 12th Century, ancient kingdoms in the dry zone fell out of power while local settlements and kingdoms shifted to rainy, wet zones in central Sri Lanka, leading to the abandonment of many tank systems. Plus, over time, Sri Lanka experienced several South Indian invasions that destroyed tanks and other forms of traditional irrigation in the country.

 

The British Empire also played its part. Before British colonization in the late 18th Century, village tank cascade systems were owned by farmers who collectively managed and maintained them under the customary laws of the village. Under this process, it was mandatory for the community to take part voluntarily in maintaining the tank system. However the British regarded this system to be forced labor, abolishing it in 1832 and centralizing responsibility for maintenance. This was followed by decades of local government neglect.

After independence from the British in 1948, new government institutions like the Department of Agrarian Development and farmer organizations took control of tank system management. These institutional changes meant that tank systems today have no clear ownership and in many places are no longer managed and maintained by village communities. 

For those tank systems that do remain, more recent urban sprawl and expanding agriculture have also had various negative effects on their capacity to support local ecosystems and tackle drought. Aquatic invasive plants like water hyacinth and salvinia have blocked canals and streams in the irrigation network. Tree cover has been reduced and heavy reliance on chemical fertilizer has impacted the soil and biodiversity that the tank systems rely on.

 

But restoration efforts are now underway. In 2017, Unesco and FAO recognized tank cascades as a Globally Important Agricultural Heritage System. Following this, the restoration of small tank systems and cascade management programs have been included in the country's National Adaptation Plan for Climate Change Impacts in Sri Lanka.

Geekiyanage says that restoration of tank cascades – although a big challenge – has been a top agenda of development and conservation practitioners for several years now. One climate resilience project has restored 325 tanks in 30 cascades, and according to Dharmsena, another has addressed 1700 tanks in 280 cascade systems.

Such efforts have already had positive impacts. According to a 2013 study, rehabilitation of the Thumbulla tank system in northwestern Sri Lanka improved yields and allowed villagers to cultivate 30 acres of non-rice crops, such as corn and vegetables, during the rainless period from May to September. Another study in 2016 explained that restoration of the Kapiriggama cascade system in north central Sri Lanka – which contains 22 tanks and supplies water to 800 acres of rice fields – has helped farmers in 11 villages to cultivate rice during the drought months.   

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There are some limits, however, to what restoration alone can achieve. Both Geekiyange and Dharmasena stress the need for proper coordination between different government and non-government entities involved in the preservation, such as the irrigation and forestry departments over the management of the surrounding tree cover. Plus Geekiyanage notes that many villages' tank systems "cannot sustain" the larger cultivation areas needed to meet the demands of a growing population.

But integrating new techniques like precision agriculture could also help increase land productivity, Geekiyange adds. "Sri Lanka should test these out for the benefit of our agriculture, so it will reduce the pressure on traditional systems," he says. "That would help promote the sustainability of our agriculture in the dry zone."

And while Dharmasena says that the cascade systems can only be replicated in regions with the requisite geological conditions, Geekiyanage explains that some of the accompanying ancient water-management technologies, such as conservation of the surrounding forest, could be applied in modern irrigation projects in Sri Lanka and beyond.

For instance, soil ridges can be used to control sediment flow and prevent blockages, Geekiyanage says. Unlike tank cascades, modern irrigation projects in Sri Lanka, such as the Mahaveli Development Project, have little concern for the environment, Geekiyangae says, "Every year, the soil is washed out from vegetable farms in the upland and ends up in the reservoirs."

Ancient knowledge, new uses

Back in Maeliya, Siriwardene remembers his childhood years; swimming with friends to pluck water lilies and white lotus to offer to the Buddha image in the temple. And still today, the tank is integral to the community.         

Sri Lanka's recent economic crisis has doubled the country's poverty rates, but villagers in Maeliya are thankful for the tank that enables them to survive.

"We live because of this tank," Siriwardene says. Every morning, fishers from the village hop on small wooden boats to catch freshwater fish like tilapia and snakeheads. They sell them fresh and dried on small makeshift stalls along the main road. People grow rice because we get water from the tank. We get fish from the tank. Everything has become very expensive in the country, but in our village people can sustain themselves thanks to this water."

Traditional farming practices tied to tank cascade systems also have modern relevance, Geekyange says. During drought years, farmers with land close to the tank grew rice in a smaller area and shared the remaining land with others. When there was extreme drought, farmers grew rice in the tank bed itself, so they could preserve seed paddy for the next season. "It allows people to share available water resources for farming. It's helped civilizations to survive."

Everything has become very expensive, but in our village, people can sustain themselves thanks to this water - Pinchal Weldurelage Siriwardene

Geekiyanage says that tank cascade systems represent "traditional ecological knowledge" that Sri Lanka can promote as "a tourism product". These could include guided tours for both local and foreign tourists. But farmers need to be integrated into these tourism initiatives to benefit economically, he adds. "They are the original owners of these systems; they are the ones preserving them."

Siriwardene is hoping that younger generations in Maeliya will look after the tank system like he, his father, and his grandfather have done for many decades. "Our life depends on it," he says. "Without this tank, we can't survive."