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Surface Water Chapter 5 SURFACE WATER Surface water originates mostly from rainfall and is a mixture of surface run-off and ground water. It includes larges rivers, ponds and lakes, and the small upland streams which may originate from springs and collect the run-off from the watersheds. The quantity of run-off depends upon a large number of factors, the most important of which are the amount and intensity of rainfall, the climate and vegetation and, also, the geological, geographi- cal, and topographical features of the area under consideration. It varies widely, from about 20 % in arid and sandy areas where the rainfall is scarce to more than 50% in rocky regions in which the annual rainfall is heavy. Of the remaining portion of the rainfall. some of the water percolates into the ground (see "Ground water", page 57), and the rest is lost by evaporation, transpiration and absorption. The quality of surface water is governed by its content of living organisms and by the amounts of mineral and organic matter which it may have picked up in the course of its formation. As rain falls through the atmo- sphere, it collects dust and absorbs oxygen and carbon dioxide from the air. While flowing over the ground, surface water collects silt and particles of organic matter, some of which will ultimately go into solution. It also picks up more carbon dioxide from the vegetation and micro-organisms and bacteria from the topsoil and from decaying matter. On inhabited watersheds, pollution may include faecal material and pathogenic organisms, as well as other human and industrial wastes which have not been properly disposed of. In rural areas, water from small streams draining isolated or uninhabited watersheds may possess adequate bacteriological and che- mical quality for human consumption in its natural state. However, in most instances surface water is subject to pollution and contamination by pathogenic organisms and cannot be considered safe without treatment. It should be remembered that clear water is not necessarily fit for human consumption and that one cannot depend wholly on self-purification to produce potable water. Collection of Rain-water in Cisterns This system is used by individual families and farms in areas where there is no ground water or where the quality of such water is objectionable for domestic use. Rain-water may also be collected to provide a supply of soft water, for laundry purposes, for example, in places where the avail- 162 WATER SUPPLY FOR RURAL AREAS able water is hard, i.e., possessing a high content of mineral salts. The collection of rain-water requires clean roofs. an even roof overhang, col- lection troughs, and storage tanks. Paved surface may also be used. In Bermuda and the Middle West of the USA, cisterns are commonly employed as a source of domestic water-supply. The quality of rain-water is affected by the nature and the degree of maintenance of the collecting surfaces. Certain wood or paint materials used on roofs, and dead leaves which u~uallyaccumulate in the troughs, are capable of imparting taste and colour to cistern water; rough roof surfaces, such as those made of thatch, are likely to retain wind-blown dust which is later collected by rain-water. Galvanized-iron roofs, which are extensively used in the tropics, provide excellent and smooth surfaces for the collection of rain-water. To strain out suspended matter, sand filters may be built at the entrance to storage tanks or cisterns. Such filters cannot be depended upon, how- ever, to remove bacteria and to produce safe water. Consequently, cistern water should always be boiled or otherwise disinfected before human consumption. In the storage tank itself, the chemical as well as bacteriological quality of the rain-water may deteriorate. For example, the mortar employed for the construction of unlined masonry walls may yield substantial amounts of mineral substances to the rain-water. Also, the number of bacteria may considerably increase because of the decomposition of organic matter held in suspension or in solution in the water stored for long periods, or by passage through sand filters which are not properly maintained. Most of these difficulties can be avoided, however, by suitably construct- ing and locating the cisterns and by providing for their thorough cleansing at regular intervals. Cisterns should be located in such a manner as to prevent any possibility of pollution of their contents by surface water or by sewage. For this reason they should preferably be sited on higher ground than the surrounding area and higher than, and at least 3 m (10 ft) away from, sewage-disposal installations. For convenience and economy, how- ever, they are frequently built underground. This has the further ad- vantage of keeping the stored water cool in warm climates and of protecting it against freezing in cold regions. Cisterns may be built of brick or stone masonry, or of reinforced concrete. The latter material is to be preferred, especially for under- ground cisterns, since it provides watertight walls in addition to necessary structural strength. A concrete mixture of 1 part cement to 2 parts of sand and 3 of gravel, and containing 23 litres (6 US gal., approximately) of water per bag of Portland cement will be satisfactory in most cases. A well-constructed concrete tank needs no surface treatment to make it suitable for water storage. However, in most cases a coating of silicate of soda or bituminous paint is recommended. Masonry walls should INSTALLATION OF WATER-SUPPLY SYSTEMS 163 be carefully built with strong cement mortar joints, and the interior surfaces should receive two 1.3-cm ( v2-in.) plaster coats of 1 : 3 (cement : sand) mortar to provide for waterproofing. Manhole openings should have edges which project about 10 cm (4 in.) above the level of the surround- ing surface, and their covers should be tight to prevent the entrance of light, dust, and other pollutional material. An overflow or air-vent provided with screen should be installed. The outlet should be located slightly above the bottom of the tank, and a clean-out pipe flush with the bottom and provided with a drain valve should be installed. Fig. 64 shows typical designs of filters and cisterns. The amount of rain-water which can be collected in cisterns depends upon the average annual rainfall and the available collecting area (either roofs or watersheds). Theoretically, 2.5 cm (I in.) of rainfall over 9.3 m2 (100 sq. ft) of horizontal surface will yield 236 litres (62 US gal.) of water. Allowing for losses due to evaporation, it may be safely estimated that 190 litres (50 US gal.) will reach the cistern. If, for example, the available roof area is 41.7 m2 (450 sq. ft) and the average annual rainfall is 71 cm (28 in.), the amount of rain-water which can be collected is equal to 450 50 x 28 x - - = 8550 US gal. (32 362 litres) per year. loo The size of cistern to be built will depend on the amount of water needed, the amount and distribution of the rainfall, and the size of the collecting surface. Capacity in excess of that required for normal rainfall conditions must be provided as a factor of safety for the abnormally dry year. The collection of rain-water in cisterns is not as widely practised as its advantages warrant, because the expense involved often makes this system unattractive as a solution for individual homes in rural communities with a relatively low standard of living. In the construction of such systems, pre- cautions must be taken to prevent water from standing in the collection troughs (or roof gutters) and serving as a breeding-place for a variety of nuisance mosquitos and for Aedes aegypti, the mosquito vector of yellow fever in many areas of the world. Such conditions may be avoided by keeping the drains in good repair and by giving them diagonal slope towards the cistern. Frequent inspections will also be required to clear them of accumulating leaves which may block their outlets. Drainage Basins and Catchment-Areas Rock and soil catchment-areas Rock and soil catchment-areas are sometimes used like roofs to collect rain-water which is led into storage tanks by means of suitable placed trenches or drainage pipes. In some regions where rainfall is extremely scarce, such as are found in southern and western Australia, bare granite WATER SUPPLY FOR RURAL AREAS Fig. 64. CISTERN WITH SAND FILTER (PUMP INSTALLATION OPTIONAL) CLOSED TOP WM 1" a nore above SECTION A-A Reproduced from US Public Health Service, Joint Committee on Rural Sanitation (1950) Indivklual u3ntpr rupply svstem5, Washington, p. 32 INSTALLATION OF WATER-SUPPLY SYSTEMS 165 outcrops are efficiently used for this purpose. Water gathered in this manner serves mainly for irrigation and stock-watering; but, if given adequate treatment and disinfection, it may also be used for domestic consumption. In the latter case, the quality of the water should be ade- quately protected by taking certain measures such as the provision of a fence around the area, designed to prevent pollution of the watershed by both humans and animals. In the case of soil catchment-surfaces, measures should also be taken against erosion, by the use of vegetative covering, for example, to reduce the turbidity of the water. Ponds and reservoirs Ponds and reservoirs may provide an abundant supply of water for use in rural as well as urban areas. Surface water from a small upland stream or spring can sometimes be accumulated in a location suitable from the topographical and geological standpoints. Such an area should possess a saucer shape, and the soil should be able to hold water.
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