8. On reaching the point of distribution an tintirely different set of conditions is encoun tered. The water heretofore concentrated in large masses has to be distributed over a wide area in a great ninnber of small pipes in which orifices are opened and shut at irregular inter vals of time; these pipes, moreover, are under a great head of water producing a pressure of 50 to 100 pounds per square inch, with conse quently a high velocity of efflux from any ori fice. The problem to be solved is so to arrange the connections and sizes of these pipes that, under the ordinary conditions of use, the pres sure in the pipes will not be materially altered at any time, and a constant supply may be kept up in the entire system. As illustrating the magnitude and complication of the distributing system in a large city, the conditions existing in the city of Chicago may be cited. There were there, in 1916, in an area of 128,000 acres, 2,600 miles of pipes for the delivery of water, with 275,000 taps or orifices from which water is drawn at irregular intervals 20,000 of these being controlled by meter. 'the development of such a system as this involves the exercise of not only theoretical knowledge of the prin ciples governing the flow of water under all conditions, but also a thorough acquaintance with materials of construction and the methods of using them to produce the best results at the least expenditure.
Irrigation and Drainage.— Next in im portance to municipal and domestic supply are the hydraulic engineering problems connected with the obtaining of an adequate quantity of water or the proper regulation of it to enable the production of food. Throughout the western two-fifths of the United States, on much of the best agricultural lands,. the rain fall in insufficient in quantity, or so irregularly distributed throughout the year, that valuable crops cannot be produced with certainty without an artificial application of water. In the Missis sippi Valley and to a certain extent in most of the States of the Union there are vast tracts of otherwise fertile lands which have an excess of water to a degree such that crops cannot be profitably raised. Here the hydraulic engi neer. is called upon to solve the problems of drainage. In many respects these are similar to those of irrigation and are intimately con nected with it as the object to be attained is the maintaining of the moisture in the soil within relatively narrow limits, for ordinary soils not dropping below 8 per cent of the volume nor rising above 15 per cent. There are also simi lar problems of providing adequate supply of water for cattle and other domestic animals used largely for food.
For the production of crops by irrigation or for relieving the lands of an excess of water by drainage, quantities of water must be han dled which are relatively very large when com pared with those needed for city supply. For
example, a 160-acre farm will require for its irrigation or may need for drainage the hand ling of a volume of water as large as would be needed for domestic or general supplies if the area were covered with dwelling-houses or factories. When it is considered that an ordi nary American city of say 100,000 persons cov ers an area of about 10,000 acres, while an irri gation or drainage project may include 100,000 acres or more, some conception may be had of the relative magnitude of the works needed for the two purposes. Although for irrigation or drainage there must be constructed works of large capacity, yet it is not practicable to pay for these works an amount comparable with the expenditures which may properly be incurred by a municipality. For farming purposes a cost of irrigation exceeding, say, $50 per acre or for drainage $20 per acre may be practically pro hibitive, but for municipal supply the cost of providing water for a similar densely populated area may properly run into thousands of dol lars. Thus the hydraulic engineer while en countering similar problems of quantity and quality of water, adequacy of supply and diffi culties of storage and distribution, must keep down the cost of these irrigation or drainage works to a small fraction of that which is feasible in considering questions of municipal supply.
Extensive studies must be made by the hy draulic engineer and detailed maps prepared to show the topography of the country from which water may be obtained for irrigation and to which it may be carried. This mapping should be accompanied by measurements not only of the rainfall, wind movement and other meteor ological phenomena, but especially of the flow of various streams at typical points on their course. Usually problems of flood conserva tion or water storage are involved, these being on a larger scale than those in connection with municipal supply. The result of these meas urements of rainfall and run-off should be available for a considerable period of time, as the fluctuations during five consecutive years, particularly in the arid region, may not fully reveal the ordinary conditions. Ten years are better, but it appears from study of data now available that the engineer cannot assume to have complete knowledge of the climatiefluctu ations from observations extending for a shorter period than half a century. Of course, it is impossible to wait that length of time be fore preparing plans for works, but when util izing data which extends over a short period, a large factor of safety, especially with refer ence to extreme drought and flood, should be employed.