Ancient and Modern Engineering and the Isthmian Canal

Table VIII

Average Yield of Sudbury Watershed, 1875-1899, Inclusive, Variously Expressed

(Area of watershed, 75.2 square miles.)

Month.	Per Square Mile.		Rainfall.
	Cubic Feet per Second.	Million Gallons per Day.	Collected, Inches.	Per Cent Collected.	Total, Inches.
January	1.937	1.252	2.233	51.6	4.33
February	2.904	1.877	3.050	71.7	4.26
March	4.489	2.901	5.175	117.4	4.41
April	3.124	2.019	3.485	107.5	3.24
May	1.680	1.086	1.936	58.1	3.33
June	.735	.475	.821	28.0	2.93
July	.305	.197	.352	9.3	3.77
August	.478	.309	.551	13.3	4.16
September	.376	.243	.419	13.0	3.23
October	.829	.536	.956	21.9	4.37
November	1.474	.953	1.645	39.0	4.22
December	1.612	1.042	1.859	51.9	3.58
Year	1.655	1.070	22.482	49.1	45.8

168. Run-off of Sudbury Watershed.—Table VIII has been given by Mr. Charles W. Sherman, as representing the average yield of the Sudbury watershed for the period 1875 to 1899, inclusive, expressed in several different ways. The average rainfall was 45.83 inches, and the percentage which represents the run-off is 49.1 per cent of the total. The average monthly run-off varies from.305 cubic foot (for July) to 4.489 cubic feet (for March) per second per square mile. As a general rule it may be stated that the average run-off from the drainage areas of New England streams amounts very closely to 1,000,000 gallons per square mile per day. The area of the Sudbury watershed is 75.2 square miles, with 6.5 per cent of that total area occupied by the surface of lakes or reservoirs. As will presently be seen, the amount of exposed water surface in any watershed has an appreciable influence upon its run-off.

169. Run-off of Croton Watershed.—The total area of the Croton watershed, from which New York City draws its supply, i.e., the area up-stream from the new Croton Dam, is 360.4 square miles, of which 16.1 square miles, or 4.47 per cent, of its total area is water surface. Mr. John R. Freeman found in the investigations covered by his report to the comptroller of the city of New York in 1900 that the average annual rainfall on that area for the thirty-two years beginning 1868 and ending 1899 was 48.07 inches, and that the average run-off for the same period was 47.7 per cent of the total average rainfall, equivalent to a depth of 22.93 inches.

Aqueducts near Jerome Park Reservoir,
New York City.

Table IX gives the main elements of the rainfall and run-off for the Croton watershed during the thirty-two year period, for the averages just given.