Twenty-Five Years Advance in Water Works

Abstract

PRESENTING a resume of what has been accomplished in water works during the past twenty-five years one must, logically, first consider the rainfall and the resultant water available as the source of supply. While there has been no advance in either the amount, distribution or chemical constituents of rainfall that can be properly claimed by water works engineers, the knowledge of the amount of rainfall and its fluctuations throughout the country has been advanced by the establishment, during this period, of thousands of rainfall gaging stations. Coupled with the great increase in rainfall recording has come the setting up of stream-flow gaging stations, where automatic records of stream levels have multiplied manyfold the previous known runoff of both large and small watersheds. The measurement of flood flows has resulted in an increase in the spillway capacity provided for water supply reservoirs and shown that for watersheds with an area of a few square miles, the flow may reach a peak of some 2,000 cu.ft. per sec. per sq.mi. of tributary area. Eastern dams have been constructed with spillway capacities designed for over 500 cu.ft. per sec. per sq.mi. of watershed. The severe drought of ten years ago in the eastern half of the United States demonstrated the inadequacy of many surface supplies and some well developments. From this experience have come increases in supply sources and scaling down of estimates of dependable yield. In the New York system for both the Catskill and Croton watersheds, the previously assumed dependable yields were reduced by some 10 to 15 per cent. Instead of estimating the yields on low stream flows and utilization of all the water held by storage reservoirs, in the New York system, it was assumed that 25 per cent of the total