Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/429

Rh WATER-SUPPLY 403 end of October, and the cold season, embracing the other half of the year. Comparatively little effect is produced by variations in the ampunt of rain in the warm season, except in extreme cases, owing to the large proportion drawn off by evaporation, whereas the rainfall during the cold season is of the utmost importance for replenish ing the sources of supply which have been drained during the summer. The period of the year, therefore, in which the rain falls is of more consequence than the total amount in the year ; and a drought is much more likely to result from a dry winter than from a dry summer. Any deficiency in the supply is generally felt towards the close of the warm season, when the reserves of water, furnished by the rains of the preceding cold season, have been reduced to their lowest level by the demands of the summer. Other circumstances also modify the influence of evapor ation. Rain descending in a heavy continuous fall, by sinking into the ground or escaping into the water courses, is less exposed to diminution by evaporation than several separate showers of rain, equivalent in total volume, but spread over a longer period. Forests and vegetation shelter the ground from the influence of evaporation ; and thereby, in spite of abstracting some of the moisture for their own requirements, they augment the proportion of available rainfall. The nature and slope of the surface stratum, moreover, notably affect the loss from evaporation. Rain falling on an impermeable stratum is almost wholly evaporated in hot weather, when the surface is flat ; but it flows off from a steep slope before evaporation can produce its full effect. Rain readily sinks into a porous stratum; and when a depth of three or four feet below the surface is reached it is to a great extent withdrawn from the influence of evaporation. Percolation. The percolation of rain through porous strata is the origin of springs and subterranean reservoirs of water, from whence so many supplies are derived. Sand, gravel, chalk, and sandstone are very absorbent strata ; whilst the oolites and other limestones are per meable to a smaller extent. The excess of rainfall over evaporation sinks into the ground till it reaches the level of saturation of the stratum by previous rainfalls, and adds to the underground supply. The water thus intro duced is prevented from sinking lower down into the earth by encountering an underlying impermeable stratum; and it is hindered in flowing away to the lowest point of the outcrop of the permeable stratum by friction, which causes the surface of saturation to slope towards its outlet, the inclination varying with the head of water and the resistance offered to its flow. The amount of percolation depends upon the rainfall, the porosity of the stratum, and the extent of its exposed surface ; and it varies inversely as the evaporation, being greatest in winter and during heavy long-continued rainfalls, and least in the summer and with short showers of rain. Watercourses. On impermeable strata, the whole of the rain not removed by evaporation finds its way into the watercourses. The streams, however, draining these strata have a very variable discharge, as they are rapidly swollen after a heavy fall of rain, and soon subside (see RIVER ENGINEERING), whilst in fine summer weather they are liable to be dried up. Accordingly, torrential streams, in their natural condition, are not suitable for water-supply, as they tend to fail when they are most wanted, and owing to their rapid flow they carry along a large quantity of matter in suspension. The flow of streams draining permeable strata is more regular, both on account of the smaller fall generally of the river-bed, and also owing to the rainfall being delayed by percolation in its passage to the river. Some of the rain sinking into a permeable stratum tends to find an outlet outside the valley in which it falls, but most of it reappears again in the form of springs which feed the river gradually at a lower part of its course ; and the loss which may occur is more than compensated for by the greater regularity of flow. In olden times, towns, villages, and monasteries were established on the banks of these streams, owing to the ready, ample, and never-failing water-supply thus ensured. SOURCES OF SUPPLY. Tanks. The simplest method of procuring pure water is to collect the rain as it falls from the clouds ; and this method is a necessity where, as in tropical countries, there is an excessive rainfall during one period of the year, followed by a long drought, unless the rain sinks into a permeable stratum whence it can subsequently be drawn. These open tanks, however, excavated in the ground, have to be numerous, and often large in extent, to collect sufficient rain to supply the wants of the surrounding population for several months ; and the water in them is subject to loss from evaporation during the dry season. The collection of rain-water is also advantageous in the rural districts of temperate regions, especially for large institutions and isolated mansions and farms, where, by a simple arrangement of gutters and pipes, a large pro portion of the rain falling on the sloping roofs can be stored in underground tanks. In large towns, the rain water is liable to contamination by smoke, dust, and other impurities, and is only serviceable for gardens and water- closets, or where softness is of more consequence than purity. Springs. A very valuable source of water-supply is provided by springs. These springs appear at the lowest point S of the outcrop of a permeable stratum, where it rests upon an impermeable stratum (fig. 1) ; and they constitute the outflow of the rain which has percolated c Fig. 1. through that stratum. A spring depends for its supply upon the extent of the underground reservoir furnished by the permeable stratum ; and its discharge is regulated by its level in relation to the line of saturation of the stratum and the resistance offered to its flow. The gathering ground of a spring consists of the portions AB, DS of the permeable stratum drained by it which are actually exposed at the surface, provided the surface slope is not very steep, and also of any impermeable surface strata BCD sloping from a higher level towards the permeable outcrop. The position of the spring is deter mined by the dip AS of the underlying impermeable stratum and the line of least resistance to the under ground flow. When the permeable stratum covers the surface, and is of small extent, as when it forms a thin cap to a hill, an outflow only occurs after a fall of rain. Where a permeable stratum, with a limited gathering ground, has a sufficient depression at some point to cause the line of saturation to sink occasionally below the level of the outcrop, the outflowing spring is intermittent ; and the time of the appearance of such springs (or bournes, as they are termed) can be accurately predicted by observing the rise of the water in the neighbouring wells sunk into these permeable strata. A spring is generally clear, and free from organic impurities, as particles in suspension are removed by the natural filtration, and organic matters are