Origin of Supply. Rainfall.— All supplies of fresh water come primarily from the clouds, though portions may eventually be drawn from the bowels of the earth. Water when distilled is obtained in its purest form; and the heat of the sun is continually drawing up large quantities of moisture from sea and land, forming clouds which return it as rain to the earth. Some of the rain is quickly evaporated from the surface of the earth, and returned to the clouds; some sinks into the ground to feed springs and underground stores of water; and some passes into streams and rivers, whence it flows into the sea, from which the greater portion of the rain is derived. The available water-supply of any district, accordingly, principally depends on the rainfall of the locality and the extent of the gathering ground. The rainfall varies greatly in different places, and at different periods of the year (see Meteorology); and, in England alone, the average annual rainfall at the Stye, the wettest part of the lake district in Cumberland, is about eight times the rainfall at Hunstanton, the driest locality in Norfolk. The rainfall, moreover, varies from year to year; and the driest years must be taken into consideration in estimating the available water-supply.

Evaporation.— The proportion of the rainfall which is actually serviceable for water-supply depends greatly upon the season of the year in which the rain falls; for evaporation is very active in the hot season, whereas in the cold season its influence is slight. Accordingly, for hydrological purposes, the year may be divided into two seasons in the latitudes of the British Isles,—the warm season, extending from the beginning of May to the end of October, and the cold season, embracing the other half of the year. Comparatively little effect is produced by variations in the amount 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 replenishing 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 evaporation. Rain descending in a heavy continuous fall, by sinking into the ground or escaping into the watercourses, 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 permeable 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 introduced 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 proportion of the rain falling on the sloping roofs can be stored in underground tanks. In large towns, the rainwater 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 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 determined by the dip AS of the underlying impermeable stratum and the line of least resistance to the underground 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 oxidized and eliminated in the passage of the water through the ground. The water, however, collects any soluble gases and salts which exist in the strata through which it flows; and most springs contain some inorganic compounds in solution, depending upon the nature of the strata and the distance traversed. Occasionally springs are so strongly impregnated with certain substances as to receive specific names, such as sulphuretted, chalybeate, and saline springs; but in such cases they are of more value for medicinal purposes than as sources of water-supply. The abundant springs derived from the chalk, though containing considerable quantities of carbonate of lime, are quite suitable for domestic purposes. Springs from large underground supplies possess the advantage of a fairly constant temperature, as their sources are protected from atmospheric changes; but underground waters are subject to the rise of temperature, experienced in descending below the surface of the earth, of 1° F., on the average, for each 52 feet of depth.

Small springs frequently supply little hamlets; and a shallow tank, formed at the spring, into which the water trickles, serves as a reservoir, which is gradually filled, and from which water is readily drawn. Large springs may afford adequate supplies for towns; but before relying upon such a source it is essential to gauge their discharge at the close of the autumn, in a dry year, so as to ascertain their sufficiency under unfavourable conditions. When springs have been selected, it only remains for the engineer to design suitable conduits for conveying the water to the place to be supplied. The fine chalk-water springs of Amwell and Chadwell, in Hertfordshire, have been thus utilized by the New River Company for supplying London, by means of a conduit 40 miles long, completed in 1613; and the springs issuing from the Malvern Hills furnish Malvern with a plentiful supply of the purest water.

Springs were naturally much prized in ancient times, when the simplest means of procuring water had to be resorted to; and ignorance of their real origin led to their being the subject of mythological legends. They were used for the supply of public fountains by the Greeks and Romans (see Fountain), and provided water for some of the Roman aqueducts.

Streams and Rivers.— In olden times, the only other obvious sources of water-supply, besides rain and springs, were the watercourses which carried off the surplus rainfall. Streams and rivers afford the most ample supply, but they become turbid in flood-time; and when they have a rapid fall, and drain an impermeable basin, they fail in times of drought. These objections have, however, been overcome by settling-tanks, filter-beds, and storagereservoirs,—so that now the principal supplies are drawn from these sources. The increase, indeed, of population, and especially the introduction of the system of discharging the sewage of towns into the adjacent streams, have polluted many rivers. Fortunately, in process of time, some of the organic impurities are removed by aquatic animals and plants, and some become oxidized and thus rendered innocuous (see Water), —so that, after a sufficient length of unpolluted flow, the river again becomes suitable for supply. The enactments against river-pollution have attacked this evil at its source; but some rivers, passing through large manufacturing centres, are hopelessly contaminated with refuse products. The best sources of water are found in streams draining uncultivated mountainous districts, where a plentiful rainfall on steep impervious strata affords a very pure though somewhat intermittent supply. The freedom from habitations, the rapid flow of rain off the surface, the absence of organic impurities and of soluble salts, prevent any chance of contamination beyond occasional discoloration by peat. Distance, however, from hilly regions and a considerable population render it frequently necessary to resort to larger rivers, which, passing towns and villages in their course, become more or less contaminated, and, being fed by springs from permeable strata, contain the salts dissolved by those springs. London, for instance, draws its principal supplies from the Thames and the Lea; and, though microscopical investigations and the reports of the water-examiners are not always reassuring about the qualities of the water thus supplied and the recuperative powers of nature, waters containing some organic matters and certain kinds of micro-organisms do not appear to be injurious, as is evidenced by the general absence of specific diseases in the inhabitants supplied from these sources.

Wells.— There are two distinct kinds of wells, namely, shallow wells, sunk into a superficial permeable stratum; and deep wells, sunk through an impermeable stratum into an underlying permeable stratum. Both kinds of wells tap the underground waters which are the sources of springs, and furnish artificial outlets for waters which would either find a natural outlet in springs at the outcrop, or which, owing to a depression of the strata, may not possess a natural outlet at a low enough level ever to drain the lower part of the underground reservoir.

Shallow wells, sunk in the ordinary manner, have long been used for collecting moderate supplies of water, where a permeable stratum, such as the Bagshot sands, or the gravel covering parts of the London basin, overlies a watertight stratum such as the London Clay, especially where a slight depression in the impervious stratum towards the centre, or a considerable expanse of the surface stratum, prevents a ready outflow from the permeable beds. Many parts of London were supplied for a long time in this manner; for the rain percolating the bed of gravel flowed into the wells sunk in it, from whence the water could be drawn up. Indeed, as pointed out by Professor Prestwich, the growth of London was restricted, till the regular establishment of waterworks, to those districts possessing a gravel subsoil, in which water could be readily procured, as no such facilities existed where the clay rose to the surface. The sites also of many of the older towns and villages were doubtless determined by similar considerations. Shallow wells are still very useful in supplying scattered populations, but they are exposed to the worst forms of contamination when the houses are near together. Any surface impurities are washed in with the rain; frequently cesspools are given an outlet into the permeable stratum from which the water-supply is derived; and pumping in the well, to increase the supply, creates a flow from the contaminated areas to the well. Such utter neglect of sanitary precautions has led to serious outbreaks of illness; and the sparkling water from some of the old wells in the City of London has proved very deleterious, from the infiltration into them of the decaying matter from graveyards and elsewhere. Shallow wells, in fact, must be resorted to with great caution, and only when an absence of habitations, or a thorough inspection of the district drained by the well, affords assurance of freedom from organic pollution.

Deep wells, passing generally through impervious beds into a permeable water-bearing stratum to a depth at which an adequate supply of water is obtained, are mostly free from organic impurities, partly owing to the protection of the superincumbent impervious stratum, and partly to the filtration any impurities must undergo before reaching the well. The wells are usually formed by sinking a shaft lined with brick for the upper portion, and then carrying down a boring below to the requisite depth. The level of the water in the well depends upon the water-level in the stratum: and generally the water has to be raised by pumping to the surface. Occasionally, owing to a depression of the strata, the top of the well is below the level of the lowest part of the outcrop of the permeable stratum into which the well is sunk, and the water rises in the well directly this stratum is reached, and flows over if the hydrostatic pressure is sufficient to overcome the friction. These latter wells, known as Artesian wells, have been already described (see Artesian Wells); and the methods of boring other deep wells are precisely similar. The most favourable strata for deep wells in England are the Chalk, Oolites, New Red Sandstone, and Lower Greensand. The yield of these wells depends upon the extent of the portion of the underground reservoir which they can drain; and the reservoir depends for its supply, as in the case of springs, on the extent of the stratum exposed at the surface, the drainage it may receive from adjoining impermeable strata, and the amount of rainfall over these areas. As these points can only be roughly estimated, it is impossible to judge of the yield beforehand; and much depends on the fissures the well may happen to pierce, as the main flow in many rocks takes place along their fissures. It is disadvantageous to sink a well where the superincumbent impervious stratum is very thick, not merely because of the depth that has to be sunk before reaching water, but also on account of the slow rate of the underground flow at a long distance from the outcrop, and owing to the compression of the porous stratum by the mass above it. For instance, the deep well on Southampton Common, sunk through 465 feet of impervious beds, has only yielded a small supply of water, though carried 852 feet into the chalk. A well may also prove a failure owing to a fault or an impervious barrier interrupting the underground flow, if it is sunk on the side of the fault or barrier away from which the dip of the stratum inclines. Thus a well sunk at A (fig. 2) will receive the water flowing along the permeable stratum; whilst an adjacent deeper well at B will yield no water, on account of the interruption of the flow. As there is a limit to the underground waters, only a limited number of wells can be advantageously sunk within a certain area; and a multiplication of wells, such as has occurred in the London basin, permanently lowers the water-level of the underground reservoir, and involves an increased lift in pumping to maintain the supply. Wells drawing their supplies from the same sources as springs reduce the yield of the springs issuing from the strata which they pierce; and when these springs feed rivers the volume of these rivers is thereby somewhat diminished. Wells, however, sunk into strata draining to the sea-coast merely intercept water which otherwise would be absolutely lost. A useful well for small supplies is a tube well, which consists of a series of strong wrought-iron pipes, between 1 and 2 inches in diameter, the bottom length being terminated in a point, and perforated with little holes for a short distance up. The point is driven into the ground by a falling weight, as in pile-driving; and, as the tube descends, fresh lengths of pipe are screwed on the top. When the...