Letters from Sri Lanka – The World’s Largest Water Harvesting Earthworks Project


A mahout and his elephant (the elephant is the one on the left)
Photos Copyright © Craig Mackintosh

Preamble: Okay, the elephant has nothing to do with the story below (except that it’s also large in scale), but it is an appropriate way to share the geographic context for this article — Sri Lanka. You can read more articles from this visit here.

Permaculture earthworks projects for supplying water and rehydrating the landscape are, generally, supposed to be small scale by nature. And, also in the spirit of Permaculture, they’re (ideally) done with voluntary, community/collective labour where possible.

What I’m going to describe below fits into neither category.

Let not even a drop of rain water go to the sea without benefiting man – Parakrama Bahu the Great, King of Sri Lanka, 1153–1186 AD

The quote above – a highly valid argument for Permaculture engineers by the way – sets the scene for this story. King Parakrama Bahu is credited with being the greatest water harvesting earthworks engineer of all time. Is it believed that during the 12th century he constructed or restored: 165 dam walls, 3910 canals, 163 major and 2376 minor reservoirs (or ‘tanks’, as they’re known here) and 328 stone sluices. He also repaired 1,969 embankment breaches. This is all no mean feat considering his reign only lasted 33 years – and considering the size of the dam walls themselves.


An ancient dam wall/embankment at Anuradhapura forms a reservoir behind it.
This is a medium sized dam, with a catchment area of 32.5 square miles (84 square kms)

Of course, he didn’t do this single-handedly…. History books indicate that forced labour was his earthmoving equipment of choice. Putting the ethics of this behind us though, these earthworks transformed the country into the granary of the Orient – enabling the early Sinhalese to grow rice (and other crops) across the large flatlands of the island state.

I say early Sengalese, because our ambitious king, although the most famous hydrologist, was actually only one of a long line of Sri Lankan dam and canal builders – the earliest of who dates back as far as the fourth century BC.

Originally devoid of natural lakes, century upon century of work in Sri Lanka has seen hundreds of large and thousands of smaller dams pepper the landscape. The work was meticulous in its detail. For example – the thousands of miles of canals were designed so precisely as to descend at a steady rate of less than 20 centimeters per kilometer (or less than a foot per mile), with individual canals reaching up to 80 kilometers (or 50 miles) in length. And some of the reservoirs, like the one I’ve photographed for this post, also had an overflow spillway as a feature, ensuring that if a particularly strong wet season did its worst, the water would outflow passively without breaching the restraining wall.

I’m told that originally all the dams were filled through rainwater harvesting only, although over the course of time stream and river diversions played a considerable role in feeding the system as well.


A sluice at the Anuradhapura reservoir

As fascinating as all this is, it gets even more so since much of the ancient system is still in use right now – helping make today’s Sri Lanka the largely food-self-sufficient country it is, on land that would otherwise be predominantly bone dry. The sluice above and below is ancient in placement, but modern in its restoration – with concrete walls and an iron sluice gate.

The shot below progresses this photo series to show the other side of the dam wall – where the open sluice allows water to flow into the adjacent region.


Water exits the reservoir, or ‘tank’, via a sluice through the dam wall (right)
and begins its life-giving journey across the countryside (left)

From here the canal winds through the countryside, with the man-made tributaries branching off into smaller and smaller lines along the way, some of which are controlled by minor sluice gates, with the precious cargo finally reaching rice farmers across the area.

While today’s legislation demands that water diverted to your property only be used for rice paddies, in real terms the water ultimately gets used for a lot more. Let me explain: today’s canals are fully concrete lined, but only initially. As the water enters smaller offshoots, they become concrete sided only (i.e. no seal on the bottom). This means water seeps into the ground to lift the water table – recharging the many thousands of wells across the countryside.


An end-of-the-line category channel, showing an officially controlled, slightly ajar
mini-sluice at right. This will divert waterflow into an individual
farmer’s paddy field – when it’s his turn at least….

Given the extreme nature of water flows in this region – monsoon rains in the wet season, followed by extended dry periods – this massive water harvesting network was a highly appropriate investment for Sri Lanka. And, as it turned out, a very long term one at that! The many thousands of streams flowing out of these vast reservoirs not only supply a large population with their staple, but also allow countless trees of every kind imaginable to get to work in shading and feeding people, slowing evaporation, and providing habitat for Sri Lanka’s rich biodiversity.


Rice harvested as egret supervises

Not all was perfect with the ancient system though. The large scale, centralised nature of the system did backfire at one point – when a major but natural change in the course of the Mahaweli Ganga river in the thirteenth century completely undermined a large part of the network.

The fall of the ancient hydraulic civilisation of Sri Lanka in the 13th century was due to sudden Natural Cataclysmic change of the river course of the Mahaveli Ganga and was not due to foreign invasions as historians would want us to believe. The scientific evidence is clearly seen in the aerial photographs of the old course of the Mahaveli Ganga and its new river course…. This sudden geological catyclysm that changed the river course that sustained our ancient hydraulic civilization, led to disease and famine. This resulted in the major part of the population to abandon these areas and move to the Wet and Intermediate Zones…. – A. Denis N. Fernando,
Fellow National Academy of Sciences

The moral of the story? Keep it small, keep it ethical, but do make sure not a drop reaches the ocean without benefiting the land, and the men and creatures on it.

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