Page:Graphic methods for presenting facts (1914).djvu/180

 John R. Freeman's Report on New York City's Water Supply, 1900

Fig. 139. Cumulative or Mass Curve, Showing the Relation of Drought to the Storage Capacity Required for Different Rates of Water Consumption from the Croton Watershed. The Rainfall in Million Gallons per Square Mile of Watershed Is Plotted from 1868 to 1899

Note the arrangement to get a long cumulative curve on a small page by showing the curve in three sections. In Fig. 138 the curve was plotted on a fluctuating consumption and the line joining humps on the curve showed the minimum rate of steady supply. Here the curve is plotted for a varying rainfall supply, and the straight lines joining different humps show the maximum possible steady rate of consumption with different sized reservoirs for storage purposes. The necessary reservoir capacity is determined by the greatest vertical distance between the rainfall curve and any sloping line drawn for rate of consumption or "draft"