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Major irrigation schemes of Lanka, as evident from the earliest written records in the Mahawansa, date to the fourth century B.C. The purpose and determination in the construction of the irrigation systems are depicted by the words of King Parakrama Bahu the Great, 1153–1186 AD: "Let not even a drop of rain water go to the sea without benefiting man". The final achievements were highlighted by "Sir Henry Ward", Governor of Sri Lanka : "It is possible, that in no other part of the world are there to be found within the same space, the remains of so many works of irrigation, which are, at the same time, of such great antiquity, and of such vast magnitude as Ceylon. Probably no other country can exhibit works so numerous, and at the same time so ancient and extensive, within the same limited area, as this Island."

(Reservoirs are also referred to as tanks in English and as Wewa in Sinhalese. The word wewa may be incorporated in to the name of the reservoir when it is not followed by either reservoir or tank. As such there are inconsistencies in the ways of referring to different reservoirs.)

[edit] Irrigation Systems

Two irrigation systems were adopted from ancient times:

The first was impoundment in reservoirs and the second was the diversion of rivers through excavated canals which conveyed the water into distant lands or reservoirs.

The works at "Pandu wewa", was the first great reservoir ever constructed in the world, if the great lakes of Egypt, which are immense natural hollows into which streams were turned, are not considered. This reservoir is thought to have been built by King Dappula II (807–812 AD). The earliest constructive work which can be identified with certainity in the Island is the "Abhaya wewa" in Anuradhapura, built by King Pandukabhaya in about 300 BC; this reservoir is now known as the "Basawakkulama wewa".

[edit] The Six Stages of Developement

Water conservation and utilization by means of a reservoir was mainly a Sinhala development, six stages in the development of the irrigation systems can be traced;

1. First Stage: collection of rain water in a pond or valley, lift irrigation using primitive implements to irrigate the surrounding paddies.
2. Second Stage: development of low artificial embankments or weirs built across the valleys of small ephemeral rivulets, such a tank would have a depth of 2.5 m, tank followed downstream by yet another tank and paddy fields.
3. Third Stage: improvement of the former type, the bunds strengthened; extent of irrigable land improved, but still not part of a complex network of tanks.
4. Fourth Stage: damming of the bed of comparatively large non-perennial rivers, e.g. "Kala Oya" ( King Dhatusena,459 AD), water distribution capabilities increased, special channels constructed to transmit water, catchments linked.
5. Fifth Stage: construction of reservoirs on large, ephemeral rivers and tributaries, and linking these to anicuts built on rivers having catchment areas of perennial water supply in the wet zone e.g. anicut at "Elahera" across the "Amban Ganga" (tributary of Mahaweli River, built by King Vasaba (65–109 AD), later enlarged and diversified by King Mahasena (276–303 AD), feeds the "Minneriya wewa". This stage was also characterised by building of spills and sluices.
6. Sixth Stage: trans-basin transfer of water from a perennial catchment to an ephemeral catchment based reservoir, e.g. "Amban Ganga" catchment linked with "Kala Oya" catchment, highest complexity by 8th century AD.

All historical evidence indicates that the systems reached their highest complexity well over 1000 years ago, and it will be seen later that modern engineering has had little effect on the planning and layout of the old systems. The ancient irrigation systems in three districts, viz. Polonnaruwa, Anuradhapura and Ruhuna, are thought to have acted as the focal points of the civilization in these areas. 44 ancient, major irrigation reservoirs approximating 39000 ha of surface area can be recognized. Almost all the major ancient irrigation works have now been rehabilitated, except perhaps for a reservoir or two which may have gone into disrepair and are covered by thick jungle.

[edit] Engineering/Hydrology

The ingenuity of the Sinhala irrigation engineers is best exemplified by the invention of the “biso-kotuwa” (which literally means queen - enclosure), the enclosure where the water level lowers. The “bisikotuwa” is the equivalent of the valve-pit, which functions in the regulation of the outward flow of water and is therefore essentially an invention made by the Sinhala irrigation engineers more than 2200 years ago. It has remained essentially unchanged since then. “It was this (=biso-kotuwa) invention alone which permitted the Sinhalese to proceed boldly with the construction of reservoirs that still rank among the finest and greatest work of its kind in the world” (Parker, 1981).

It is also recorded that in the design of dams built across rivers the early Sinhala engineers showed ingenuity. The dams were built at an oblique angle, exposing the masonry to a lesser degree of violent shocks caused by impact of large floating tree trunks and other debris.

No historical account on the reservoir systems in the island will be complete without reference to the works of King Parakrama Bahu (1153–1186 AD). This ruler is reputed to have been responsible for the construction or the restoration of 165 dams, 3910 canals, 163 major tanks (=reservoirs) and 2376 minor tanks, all in a reign of 33 years, perhaps reaching the zenith of development in irrigation and agriculture of the Sinhala race during its 2500 year history.