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by Aryadasa Ratnasinghe

Water is the commonest molecular compound (H2 O), and it covers about 70% of the earth's surface the largest being the Pacific Ocean. As a liquid water freezes to ice at 0 deg Celsius and builds up to steam at 100 deg. Celsius, the equivalent opposites under Fahrenheit being 32 deg. and 212 deg. respectively. Water dissolves everything to some extent, but is a poor solvent for substances which are found in solutions as molecules, e.g., oxygen and methane.

Water is essential to life and occurs in all living organisms. Usually, in solid form it is less dense than in liquid form, and this results in ice floating in water. Water containing substantial concentrations of calcium and magnesium ions is called `hard water'. It becomes `soft water' when the ions are placed with sodium and potassium. Soap is not easily miscible in hard water than in soft water.

Natural springs

Water supply in sparsely populated regions comes usually from natural springs, supplemented by pumps and wells. Other sources are deep artesian wells, rivers, lakes, ponds and reservoirs, from where water is conveyed by pipes, conduits and aqueducts to filter beds in order to make water safe for drinking purposes. By making water to seep through layers of shingle, gravel and sand, harmful organisms are removed to make it safe for drinking. Sometimes chlorine is added to purify water.

Man needs water to drink, wash, cook, bathe and for many other purposes which are indispensable. It is also used for industrial purposes, such as in cooling machinery, washing textile fibres, generating hydro power etc., but by far the largest quantity of water goes in the pursuit of agriculture mostly in dry regions, where the rainfall is either scanty or irregular.

Under such circumstances, irrigation water from lakes, tanks and reservoirs, is fed into the fields under a viable irrigation system, to be equally distributed, to enable farmers to cultivate their fields for bumper harvests.

Historically, among the works of public utility, with which our ancient kings stand accredited, tank building deserves more than a mere passing note. It was king Pandukabhaya (BC 437-407), who constructed the tank called Abhayavapi (now Basavakkulama) in Anuradhapura. It was the earliest constructive work which can be identified with certainty in Sri Lanka, featuring the ancient civilisation of the island.

Channels

These irrigation works were intended to store or convey the necessary water for paddy cultivation and other secondary crops, which the Sinhalese had begun to form at places wherever found suitable to grow the staple food.

The system they adopted, at first, was to impound the water in reservoirs or tanks with low embankments. From these, the water was gradually passed out, either directly on to the fields, or made to flow down by means of excavated channels to pass under gravity pressure.

Later, this system was changed to conform to better methods to store more water than what was collected. When the people had acquired a better knowledge and understanding in regard to the art of raising embankments to hold greater depths of water, the constructive genius of the Sinhalese asserted itself and schemes of a more comprehensive nature were attempted.

Great earthen embankments, often miles in length were raised across many suitable valleys, intercepting in this manner the flow of streams to store up water, during the rainy season to be made use of during the dry season. Immense sheets of water filled the tanks, providing irrigation facilities for cultivation purposes in the lower down areas.

Cultivation

Among the tanks built by our kings for irrigation cultivation, the most well known are Kantalai tank, Minneriya tank, Yodakandiya tank, Namaloya tank, Ampara tank, Lahugala tank etc., which still serve the purpose. The basic needs in the proper utilisation of water depends on how wisely and carefully it is used for cultivation purposes, without allowing to flow waste.

The quantities of water required for different uses vary over a wide range. On average, the amount of drinking water needed each year by man and animal, is in the order of 10 tons per ton of living tissue. The requirement of water in industry is much above the domestic needs, but even the largest quantities of water required for industry is too little when compared with water needed for agriculture since much of the water in the fields percolates into the ground.

To grow a ton of sugarcane, under irrigation facilities, about 1,000 tons of water is required by plants, with provision for soil evaporation and plant transpiration, from liquid to vapour.

Rice cultivation requires about 2,000 tons of water per ton per crop, and unless the plants are well fed at the proper time, the harvest will, obviously, be poor. Where there are no irrigation facilities for cultivation, due to the terrain of the fields or the topography of the land, farmers have to depend on rain-fed water which has no guarantee.

Severe drought

Irrigation agriculture is the most productive kind of farming devised by man for cultivation purposes when rains fail to come down or during severe drought. It is also the most expensive. The capital cost in providing water for irrigation agriculture is usually very high and, therefore, the expenditure cannot be fully met by the farmers alone, without aid from the government.

Four factors, viz: (i) the potential evapotranspiration during the growing season, (ii) the permeability of the soil, (iii) the amount of water requirement and (iv) the salt tolerance of the crops, determine the amount of irrigation water that must be applied for maximum crop yields, under different situations and the kind of crop. Evapotranspiration depends largely on the duration of the growing season and the amount of sunlight available.

The amount of water required by different crops varies over a wide range. In sandy soil, much of the water will be lost by downward percolation, and hence, for any particular corp, the amount of water necessary will be far greater than the evapotranspiration needs. In tight or silty or clayey soil, little water will be lost by percolation.

Even agriculture, man's principal consumer of water, takes a little of the available supply. It is said that a billion acre-feet per year (less than 4% of the total river flow) can be made to irrigate 310 million acres of land, or about 1% of the land area of the earth.

Most of the river waters flow to the sea almost unused by man, and more than half of the water evaporating plays little part in human life.

In the less developed countries, water development by itself does not produce much added value to the economy. Development of water resources must be accompanied by other measures to raise agricultural yields per acre-foot and per man-hour, and, in general, to increase the economic value of water. New mulching methods are already being applied in tropical countries to reduce evaporation from soil surfaces, thereby making more water available for transpiration by plants.

A very profitable venture is to generate hydro power for electricity, by harnessing the water resources. The engineer D. J. Wimalasurendra (1874-1953) is remembered as the pioneer in hydro-electricity in Sri Lanka. It was his brainchild and he estimated that 114.5 MW of power could be generated by harnessing the Kehelgamuwa Oya (now Laxapana).

His dream was fulfilled by the Hydro Electric Scheme (Stage I) in 1950, i.e., 3 years before his death.

National grid

Today there are numerous powerhouses generating electricity in Sri Lanka, which is fed to the national grid. As there is a demand over supply, the Ceylon Electricity Board is unable to provide adequate power evenly throughout the country, without interrupted powercuts.

Large scale water development projects, involving the construction of dams across water-courses, diversion structures and canal systems take many years and often involves bringing new lands under irrigation agriculture.

It is said that many irrigation projects fail due to lack of drainage systems. Excess water, instead of flowing off the irrigated fields, seeps into the ground and raises the water table, which may also be raised by canal leakage.

When the underground water has risen sufficiently it is sucked up by capillary action and evaporates at the surface, a residue of salt accumulates in the fields, and the productivity of the land gradually diminishes. Small scale water improvements can be accomplished by the farmers themselves. They include the construction of earthen tanks and small hill-side dams, the construction of water spreading structures to increase re-charge of the underground aquifer, digging and rigging shallow wells, the construction of water courses and wells as are found necessary.

Urban areas

Drinking pure water is a problem in most towns and urban areas. The Colombo city gets a supply of more than 30 million gallons of water daily from three sources and through five trunk mains. The water from Labugama reservoir was pumped to Colombo in 1886. This supply was augmented in 1938, by drawing water from the Kalatuwawa reservoir.

In 1949, water was pumped from the Kelani River at Ambatalen-pahala, on the Colombo-Avissawella old road, to meet the increasing demand of the city dwellers for more water.

The city's distribution system consists of over 275 miles of mains, from 3 in. to 30 in. in diameter. Water storage of irrigation is one of the principal justifications for such spectacular public works as the construction of dams and aqueducts.

These structures, though very popular with politicians in providing water to their electorates, is a very costly affair, but may be beneficial to politicians. Pipe-borne water is a luxury but the service is very limited due to the lack of storage tanks providing gravity pressure for distribution.

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