The decision taken to reconstruct, develop, renovate, repair, dredge and rehabilitate one thousand old tanks, which do not hold their capacity of water, due to siltage, seepage and leakage, is most welcome, provided it is done in good faith.
In the ancient past, the life of the people, whether rich or poor, depended mostly on agriculture to produce the staple food (rice). The numerous tanks built by our kings provided the necessary water to irrigate the paddy fields, during the dry season when water was scarce.
Rice was not imported and the farmers maintained their paddy fields well supplied with water, mostly by means of irrigation, to receive a bumper harvest.
Therefore, among other utility services, tanks received preferential treatment, and the kings did their best to maintain a uniform supply by impounding water in tanks and reservoirs, without allowing it to go waste.
People need water to drink and for many other domestic purposes, but by far the largest amount of water goes to agriculture, e.g., leaving for soil evaporation and plant transpiration from liquid to vapour and to grow paddy it requires about 4,000 tonnes of water per tonne of crop. A billion acre-feet of water per year, less than 4% of the river flow, is used to irrigate about 1% of the total land area.
In Sri Lanka, the first tank to be built was the Abhayavapi, by King Pandukabhaya (BC 437-407) in Anuradhapura, as mentioned in the Mahavamsa (Ch. 10:88). Today, it is known as Basavakkulama, with a surface level of 174 hectares of water. The following principal tanks were built, from time to time, by our kings, to harness the water potential going waste.
Senanayaka Samudra is not really a tank but a reservoir, covering the largest surface area of water, for purpose of irrigation, combined to produce hydro-electric power from the Inginiyagala Power House to feed the national grid.
Where perennial rivers were few and far between, our farmers from remote antiquity, felt the absolute necessity of water, in plenty, to irrigate their paddy fields to produce rice, and also for dry-land crops under 'chena' cultivation.
The kings supported in the exploitation of water resources, and tanks, big and small, were built, where the need arose, depending on the location, by raising embankments to hold water to form a reservoir.
The system first adopted was to impound the water in tanks with low embankments. The water collected in these was gradually passed out, either directly on to the paddy fields, or through excavated channels, to those fields far away.
However, when people acquired a better knowledge to harness rain water, they built tanks raising embankments to hold greater depths of water with more volume. The constructive genius of the tank builders well asserted itself and more comprehensive schemes were attempted. Great earthen embankments were raised intercepting the flow of water along streams, and thus storing up for use during the dry season.
The main motive was to avoid any impending danger of a famine. With the passage of time, and when the demand for irrigation facilities was felt badly, the kings took the initiative to build more and more tanks to irrigate the paddy fields.
From the primitive small tanks, large ones were planned with high bunds and miles in length, to cover a large area under inundation. The water meandered through these channels feeding tanks on its way regulating a smooth supply.
King Mahanaga, who established his kingdom in Rohana, during the reign of Devanampiyatissa (BC 307-267), built the Tissawewa at Mahagama, in the shallow valley about a mile away from the Kirindi-oya.
Today, this tank covers 576 acre-feet of water, and it was the first to have been built with a permanent stone dam. King Vasabha (65-109) did much for the development of agriculture by building many tanks, some of them being Magamwewa, Maharametiwewa, Kalivasawewa, Vadunnawewa etc. King Jettha Tissa (261-275) built the Alambagamawewa, Pisannawewa, Mahatombuwawewa etc., to provide irrigation facilities.
King Mahasena (275-302) built 19 tanks of which his greatest achievement was the Minneriya tank, and for this work, he came to be known as the 'Minneriya Deviyo'. The Giant's tank in Mannar is one of the largest tanks in the island.
It was built by King Dhatusena (460-478) by daming the Malwatu-oya, later known as Manawatuwewa. His greatest work was the Kalawewa, which irrigated 6,900 acres of lowland fields. The Parakkrama Samudra in Polonnaruwa was the work of King Parakramabahu (1153-1186), to feed 18,200 acres of paddy lands, with a bund 8 1/2 miles in length and rising to a height of 40 feet from the level ground.
The Kantalawewa was built by King Aggabodhi I (568-601), which provided water to 10,000 acres of paddy fields. In 1793, a report has been submitted by the engineer Tornbauer to the Dutch Governor Jacob Willem van de Graff, stating about the two aqueducts of hewn stone made to precision to his wonder.
According to Belidor's Hydraulic Architecture, in one second it would discharge 570 cubic feet of water, calculating to 49 million in a day. The engineer was genuinely impressed of the genius of the builders in such architectural design, when the West knew nothing about hydro engineering when the lake was built.
Today, most of these tanks have lost their capacity and are in ruin, because no Government, since Independence in 1948, ever tried to bring the tanks to their original condition. Some, weakened by the passage of time, broke their bunds allowing the water to go waste.
The Mahaveli Development Project is one of the recent engineering feats to provide water, by means of irrigation, to lowland areas deprived of water and the farmers depended on rain.
Although man has abandoned these tanks for centuries, the beasts of the jungle have invaded them, without any challenge from man, and have made them their fertile breeding grounds abound with luxuriant foliage and undergrowth and with an adequate supply of water to quench their thirst.
In times of yore, man had toiled and moiled their fields for the staple food. Anyone touring the North-Central province can see old tanks submerged with tree trunks telling the tale of neglect with sorrow.
Irrigation agriculture is the most productive kind of farming devised by man to overcome drought and shortfall of rain. It is also most expensive, and it cannot be achieve by individual level without state aid.
The potential evapotranspiration during the growing season, the permeability of the soil, the quality and quantity of water available and the salt toleration of crops, determine the amount of irrigation water that must be applied for maximum crop yields under specific situations.
Evapotranspiration depends largely on the length of the growing season and the amount of sunlight available.
The water requirement of different crops vary over a wide range. In sandy soil, much of the water will be lost by downward percolation, and hence for any particular crop, the quantum of water required will be far above the evapotranspiration requirement. To grow a tonne of sugarcane under irrigation, about 1,000 tonnes of water must be consumed to meet soil evaporation and plant transpiration from liquid to vapour.
With the outbreak of drought for the last few months, most tanks have gone dry, including wells as the sole means of water for domestic use.
This natural phenomenon has made many to suffer without adequate water, and some have to walk several miles to find a place to collect water full of mud caused by animals in search of water. Let us salute the Government's move to rehabilitate old tanks.
