Page:EB1911 - Volume 28.djvu/408

COLLECTING AREAS]

Note.—The figures in the right-hand column at the ends of the curved lines are inches of mean annual rainfall over a period of 50 years.

The co-ordinates to any point upon any curved line give respectively the required reservoir capacity and daily yield in gallons per acre of drainage area, corresponding with the mean annual rainfall represented by that curved line.

The curves have been drawn for a mean annual evaporation of 14 in. For any increased rate of evaporation 1¼ in. are to be subtracted from the rainfall for each inch of evaporation above 14 in. For any decreased rate of evaporation 1¼ in. are to be added to the rainfall for each inch of evaporation below 14 in.

Any excess of evaporation from the water surface and allowance for bottom water are to be added to the storage so found.

The period over which the reservoir equalizes the flow is shown by the number of years marked on the straight radial lines.

Where the absolute minimum stream flow is known to be greater than the minimum of $1/5$ cubic foot per second per 1000 acres (108 gallons per acre per day) assumed in the diagram, the capacity of the reservoir as taken from the diagram may be reduced by the amount corresponding to that minimum flow for the particular rainfall and evaporation.