Our irrigation heritage in retrospect

M U A Tennakoon - PhD, DSc Former Executive Director, Central Bank of Sri Lanka

Sri Lanka has more than a 2,500 years old recorded history of irrigation, that is surpassed only by Egypt, Messopotamia, Indus valley in India and China dating back to 4,000 to 6,000 years. The uniqueness in Sri Lankan tank-based irrigation is that reservoirs are constructed on hard-rock bases across streams ephemeral or perennial in ‘etched’ plains to store water for irrigation and to meet all other human needs during the dry seasons. In all other countries mentioned above, the deep alluvial soil deposits in the river banks of Nile, Tigris, Euphrates, Indus, Yellow River basin etc., sometimes as thick as 30 metres, are irrigated by the natural river up-swellings and flooding.

Parker (1909) commenting on ancient irrigation in Sri Lanka states that “Pandae-waewa’, (presently known as Pandawewa) near Hettipola in the North Western Province, built across the Kolamune Oya, believed to have been constructed in the first half of the 4th century BC, would be the first great reservoir ever constructed, if we omit from consideration the great lakes of Egypt, since they were merely immense natural hollows into which the river water was turned”. It was also the Sri Lankan engineers who, for the first time at least by 300.B.C, invented those remarkable cistern (bisokotuwa) type sluices for our tanks, the device, for which the western scientists had to wait until the 18th century to invent. A large reservoir like ‘Pandae-waewa’ referred to by Parker (1909) would have required sophisticated bisokotuwa type sluices built using bricks and stone.

There were many other design and construction achievements of our ‘master engineers’ in that distant past. Human ingenuity and endurance discernible in perfected irrigation planning, designing and construction completed by our spirited engineers in that distant past, induce us to be proud of them, to learn more about them, and to let the world know about their ingenious irrigation task functions. But we should not be unnecessarily emotional, over-patriotic or too boastful in laying open the intricacies of our irrigation civilization before the world. We need to have a descent scientific methodological approach well balanced in this display of sense and substance of our irrigation heritage patiently, intelligently and scholarly well distanced from boasting. We need to be Parakrama Bhahus and not Nissanka Malalas.

Before we set upon a methodology to do a balanced display of our irrigation heritage before the world, let us understand what irrigation means and what are its functional components. Irrigation is any form of water supply to land to grow any type of crop or plant stress-free, in times of water shortage. Irrigation functions are : i. water collection; ii. water storing; iii. water release as and when necessary; iv. water conveyance to where it is necessary and v. water use for necessary purposes.

Water collection

It includes receiving rainwater from the intended storage’s catchment and water received from all other up-stream sources - tanks, springs, streams and diverted canal supplies from other large sources of water etc.

Water storage

Water is stored on the surface of land in ponds, villus, meadows, sink holes, small tanks (wewas) and in large reservoirs. These together contain the largest volume of fresh water collection on the earth’s land surface. Water is also made to accumulate in aquifers as groundwater, where geology favours their existence.

The active aquifers in Sri Lanka are: i. Shallow karstic aquifer of Jaffna Peninsula; ii. Deep confined aquifers; iii. Coastal sand aquifers; iv. Alluvial aquifers; v. Shallow regolith aquifers of the hard rock region; and vi. South western lateritic (cabook) aquifers. (Panabokke, 2005). In some areas, if not for the availability of this precious groundwater, human sustenance, both in terms of agriculture and other human needs, would not have been possible.

To be continued next week