Page:The New International Encyclopædia 1st ed. v. 20.djvu/424

WATEK SUPPLY. 354 WATER SUPPLY. cardinal principle in estimating the quantitative fitness of a source of water supply is to compare the maximum demand with the minimum sup- ply. Averages are also of great importance, but the foregoiirg is the crucial test. There should be determined not only the actual or estimated yield of the source of supply during the driest month on record, but also the corresponding yield for the driest two or three years in succession. This will enable those concerned to calculate what storage will be necessary for use during droughts, what supplementai-y steam jilants may be required in connection with water supply de- velopments, and wliat delays to navigation may be expected in ease it is not feasible to provide all the storage indicated as necessary to main- tain the levels of navigable canals or rivers. Althougli the yields of both surface and under- ground supplies depend primarily u])on the rain- fall, other factors vary in the two cases. Streams and lakes are supplied partly from the water that flows into them directly from the surface of the ground, hence the name surface water, and partly from water that soaks into the ground, then percolates through the soil to brooks, rivers, and lakes. The underground supplies are re- plenished from time to time from that portion of the percolating water that does not find its way to the surface supplies, or else is intercepted by artificial means. The drainage areas of sur- face supplies are well defined and the flow of streams may be readily measured. The drainage areas of underground sources are not so easily determined, particularly deep- seated waters, and tlie determination of their volume is often extremely difficult and generally only an appi"oximation. Instead of a stream flowing in a well-defined channel, as is true of surface supplies, underground waters are hidden in the earth and follow a tortuous course through thousands of tiny cliannels in the voids between the separate grains that compose their water bed. The size of these channels, the general slope of the underground water surface, the depth of the water-bearing stratum, or absence of the pressure that gives rise to artesian con- ditions, all play their part in the possible yield of wells and springs. Some or all of these fac- tors are but poorly known and their ascertain- ment may be surrounded with grave dilUculties. The safest and often the only practicable means of determining the yield of an under- ground source of supply is to measure it, or a portion of it. In the case of springs this may 1)e done by means of ,a weir (q,v. ). Where wells are proposed one or more may be sunlv and tested experimentally, but unless the test is for a long period and the deductions are made in the light of the best engineering and geological knowledge the results are quite certain to be misleading and the future yield grossly exag- gerated. If a relatively small supply is all th.it is required one or more test wells may settle the question of capacity with sufficient definite- ness at the outset, with the understanding that when the supply, as finally developed, proves inadequate, the ])lant will be enlarged. If. how- ever, a quantity close to the probable safe yield of the underground basin be desired, the test wells may be so located as to determine approx- imately the slope of the water level and the porosity of the water-bearing material. From these facts, combined with other valuable data, an estimate of the available supply may be made. The efl'ect of a well is to lower the water level for a greater or less distance around it as a centre, so the new water level in this zone as- sumes the shape of a flat inverted cone, which has had its surface curved somewhat in the vicinity of the well. The more the water is low- ered the flatter becomes the cone and the greater the area of the zone influence, until the water practically fails through exhaustion, or through diminution due to increased friction. Obviously, if wells are placed too close together their zones of influence will overlap and one will rob the other. In determining the possible yield of surface supplies the first step is to ascertain what gaug- ings. if any, of the actual flow are available, and particularly whether such gaugings cover a series of years of minimum rainfall. Where no gaug- ings are to be had it is desirable that a gaug- ing station or stations should be established. If this cannot be done all the available rainfall records in and near the drainage area should be gathered and studied. These figures may be compared with the rainfall and corresponding yield, or run-oft', of other drainage areas, as nearly similar as possible. Deductions may then be drawn as to the average, minimum, and maxi- mum yields. The latter must be known to make possible the provision of adequate means for the passage of flood waters without damage to any of the structures connected with the supply works. Storage reservoirs may often be provided to make good the deficiency of run-ofi' or stream flow in dry periods. The extent to which this is feasible will depend partly upon the character of the available reservoir sites and largely upon the economic value of the water thus conserved. Absence of proper reseiwoir sites frequently turns the scales in favor of some other source of sup- ply. The yield of undergromid water supjilies may be supplemented by storage, also, but to a very small extent, as compared with the storage of surface su]iiilics, since most underground waters would have to be piimped as well .as stored, and since the conditions for water stor- age on a large scale are rarely favorable in the vicinity of underground sources. Moreover, undergroimd sources are chiefly used for public water supplies, and ground Mater in storage for more than brief periods is liable to .deteriorate unless protected from the light. The expense of covering large reservoirs would be prohibitive. Relatively small supplies of well water are some- times stored for irrigation (q.v,). The yield of drainage areas is expressed in a variety of ways, depending somewhat u)ion the uses to which the water is put. For preliminary studies the yield may be stated in inches of depth, or in percent;ige of the total rainfall. The former is readily converted into cubic feet per second, hour, or day, per mile of drainage area, or into millions of gallons per squai-e mile. The gallon is the most convenient unit where watei-- works are involved, and the cubic foot where water is to be applied for pover or navisration. In the case of irrigation either cubic feet or acre feet may be employed. (See Irrigation,) Stream gaugings are recorded, ])rimarily, in cubic feet ]ii'r >,i'c(ind.