Table V

General Rainfall Statistics for the United States

LEGEND:

(A) = Mean Yearly Rainfall.

(B) = Per Cent of Summer and Autumn Rain to Mean Yearly.

(D) = Per Cent Two Driest Years.

(E) = Per Cent Three Driest Years.

(F) = Number of Years’ Record.

Station.	(A)	(B)	(C)	(D)	(E)	(F)
North Atlantic:
Boston	45.4	50	60	70	80	79
New York	44.7	52	62	77	80	61
Philadelphia	42.3	52	70	75	80	72
Washington	42.9	51	69	71	74	45
South Atlantic:
Wilmington	53.7	61	75	80	81	26
Charleston	49.1	61	48	55	62	89
Augusta	48.0	50	81	88	87	27
Jacksonville	54.1	65	74	77	83	27
Key West	38.2	70	54	61	73	49
Gulf and Lower Mississippi:
Montgomery	52.5	42	76	80	83	24
Mobile	62.6	51	68	75	78	26
New Orleans	60.3	52	64	75	77	26
Galveston	47.7	58	50	65	72	26
Nashville	50.2	46	67	73	83	32
Ohio Valley:
Pittsburg	36.6	53	70	78	85	54
Cincinnati	42.1	50	60	72	71	62
Indianapolis	42.2	51	59	76	82	27
Cairo	42.6	47	62	75	81	25
Louisville	47.2	48	74	81	85	25
Lake Region:
Detroit	32.5	56	65	72	79	46
Cleveland	36.6	54	71	74	81	41
Duluth	30.7	63	65	81	88	26
Upper Mississippi Valley:
St. Louis	40.8	52	55	65	75	60
Chicago	34.0	54	66	80	86	30
Milwaukee	31.0	55	66	74	73	53
Madison	33.2	58	39	58	68	28
The Plains:
Omaha	31.4	63	57	63	70	27
North Platte	18.1	61	56	67	72	22
Denver	14.3	48	59	71	77	27
Cheyenne	12.7	55	39	62	75	27
The Plateau:
Tucson	11.7	65	44	79	80	19
Santa Fé	14.6	69	53	63	66	37
Salt Lake City	18.8	39	55	64	74	29
Walla Walla	15.4	38	46	81	86	27
Pacific Coast:
Astoria	77.0	33	64	68	77	34
Portland	46.2	31	67	76	79	27
Sacramento	19.9	16	42	67	84	47
San Francisco	23.4	17	51	73	78	47
Los Angeles	17.2	15	33	48	59	24
San Diego	9.7	18	30	54	61	47

The diagram Fig. 1 and Table V are constructed from data given in the bulletins of the Weather Bureau and exhibit some of the general features of the rainfall for different points throughout this country. The heavy lines of the diagram show the average monthly precipitation at the points indicated, for periods of a considerable number of years, as shown in the table. It will be observed that the rainfall is comparatively uniform in the North Atlantic States but quite variable on the Pacific coast, as well as in the Mississippi and Missouri valleys and west of those valleys.

164. Hourly or Less Rates of Rainfall.—Although not often of great importance in connection with public water-supply systems, it is sometimes necessary to possess data regarding maximum hourly (or less) rates of precipitation in connection with sewer or drainage work. The earlier records give exaggerated reports of maximum rates of rainfall, although that rate varies rapidly with the time. Throughout a rain-storm the rate of precipitation is constantly varying and the maximum rate seldom if ever extends over a period equal to a half-hour; usually it lasts but a few minutes only. In this country an average rate of 1 inch per hour, extending throughout one hour, is phenomenal, although that rare amount is sometimes exceeded. A maximum rate of about 4 inches per hour, lasting 15 to 30 minutes, is, roughly speaking, about as high as any precipitation of which we have reliable records. The waste ways or other provisions for the discharge of surplus or flood-waters of the Metropolitan Water-supply of Boston are designed to afford relief for a total precipitation of 6 inches in twenty-four hours. It is safe to state that an excess of that accommodation will probably never be required.

165. Extent of Heavy Rain-storms.—In all engineering questions necessitating the consideration of these great rain-storms it is necessary to remember that their extent is frequently much greater than the areas of watersheds usually contemplated in connection with water-supply work. The late Mr. James B. Francis found in the great storm of October, 1869, which had its maximum intensity in Connecticut, that the area over which 6 inches or more of rain fell exceeded 24,000 square miles, and that the area over which a depth of 10 inches or more fell was 519 miles. Again, in the New England storm of February, 1886, 6 inches or more of rain fell over an area of at least 3000 square miles. Storm records show that as much as 8 or 10 inches in depth have fallen over areas ranging from 1800 to 500 square miles, respectively, in a single storm.

166. Provision for Low Rainfall Years.—The capacity of any public water-supply must evidently be sufficient to meet not only the general exigencies of the year of lowest rainfall, but also the conditions resulting from the driest periods of that year. It is customary among civil engineers to consider months as the smaller units of a dry year. It is necessary, therefore, to examine not only the annual rainfalls but the monthly rates of precipitation during critical years, i.e., usually during dry years.

It is impossible to determine absolutely the year of least rainfall which may be expected, but evidently the longer the period over which observations have extended the nearer that end will be attained. It is sometimes assumed that the lowest annual rainfall likely to be expected in a long period of years is 80 per cent of the average annual rainfall for the same period. Or, it is sometimes assumed that the average rainfall for the lowest two or three consecutive years will be 80 per cent of the average for the entire period, and that the year of minimum rainfall may be expected to yield two thirds of the annual average precipitation. Such features will necessarily vary with the location of the district considered. Conclusions which may be true for the New England or northern Atlantic States probably will not hold for the south Atlantic and Gulf States. Data for such conclusions must be obtained from the rainfall of the locality considered. Table VI exhibits the comparative monthly rainfall which J. T. Fanning suggests may be used approximately for the average Atlantic coast districts.

TABLE VI.

	Mean Monthly Rainfall, Inches.	Respective Ratios.	Probable Depth in Inches of Actual Rainfall.
January	4	× 1.65	= 6.6
February	4	× 1.50	= 6.0
March	4	× 1.65	= 6.6
April	4	× 1.45	= 5.8
May	4	×.85	= 3.4
June	4	×.75	= 3.0
July	4	×.35	= 1.4
August	4	×.25	= 1.0
September	4	×.30	= 1.2
October	4	×.45	= 1.8
November	4	× 1.20	= 4.8
December	4	× 1.60	= 6.4

If the average monthly rainfall throughout the year were one inch, the values of the ratios would show the actual monthly precipitation. In general the table would be used by dividing the total yearly rainfall by 12, and then multiplying that monthly average by the proper ratio taken from the table opposite the month required. Such tables should only be used for approximate purposes and when actual rainfall records are not available for the district considered.

167. Available Portion of Rainfall or Run-off of Watersheds.—If the public water-supply is to be drawn from a stream where the desired rainfall records exist, it is necessary to know what portion of the rainfall, either in the driest or in other years, may be available. This is one of the departments of the hydraulics of streams for which much data yet remain to be secured. The watersheds or areas drained by some streams, like the Sudbury River of the Boston, and the Croton of the New York water-supply, have, however, been studied with sufficient care to give reliable data. The amount of water flowing in a stream from any watershed for a given period, as a year, is called the annual “run-off” of the watershed, and it is usually expressed as a certain percentage of the total rainfall on the area drained. For certain purposes it is sometimes more convenient to express the run-off from the watershed as the number of cubic feet of water per second per square mile of area. Table VII, taken from Turneaure and Russell, exhibits run-off data for a considerable number of streams in connection with both average and minimum rainfalls.

TABLE VII.

STATISTICS OF THE FLOW OF STREAMS.

Stream.	Area Drained, Square Miles.	Years.	Average Yearly.			Year of Minimum Flow.
Stream.	Area Drained, Square Miles.	Years.	Rain, Inches.	Flow, Inches.	Per Cent.	Rain, Inches.	Flow, Inches.	Per Cent.
Sudbury	75.2	1875-97	45.77	22.22	48.6	32.78	11.19	34.1
Cochituate	18.87	1863-96	47.08	20.33	43.2	31.20	9.76	31.3
Mystic	26.9	1878-96	43.79	19.96	45.6	31.22	9.32	29.8
Connecticut	10,234	1871-85	44.69	25.25	56.5	40.02	18.25	45.6
Croton	338	1870-94	48.38	24.57	50.8	38.52	14.54	37.8
Upper Hudson	4,500	1888-96	39.70	23.36	59.0	33.49	17.46	52.2
Genesee	1,060	1894-96	39.82	12.95	32.5	31.00	6.67	21.5
Passaic	822	1877-93	47.08	25.44	54.0	35.64	15.23	42.7
Upper Mississippi	3,265	1885-99	26.57	4.90	18.4	22.86	1.62	7.1
Stream.	Area Drained, Square Miles.	Years.	Average for December to May.			Average for June to November.
Stream.	Area Drained, Square Miles.	Years.	Rain, Inches.	Flow, Inches.	Per Cent.	Rain, Inches.	Flow, Inches.	Per Cent.
Sudbury	75.2	1875-97	22.98	17.52\|	76.0	22.61	4.70	20.8
Cochituate	18.87	1863-96	22.97	14.87	64.7	24.10	5.46	22.7
Mystic	26.9	1878-96	22.11	15.12	68.4	21.66	4.84	22.4
Connecticut	10,234	1871-85	20.13	17.95	89.1	24.56	7.30	29.7
Croton	338	1870-94	23.39	17.81	76.1	24.99	6.76	27.0
Upper Hudson	4,500	1888-96	18.20	16.23	89.0	21.50	7.13	33.0
Genesee	1,060	1894-96	19.58	10.20	52.2	20.24	2.75	13.6
Passaic	822	1877-93	22.47	18.22	81.1	24.39	7.19	29.5

The information to be drawn from this table is sufficient to give clear and general relations between the recorded precipitation and run-off. The percentage of run-off is seen to vary quite widely, but as a rule it is materially less for the year of minimum flow than for the average year. That feature of the table is an expression of the general law, other things being equal, that the smaller the precipitation the less will be the percentage of run-off. A number of influences act to produce that result. During a year of great precipitation the earth is more nearly saturated the greater part of the time, and hence when rain falls less of it will percolate into the ground and more of it will run off. Again, if the ground is absolutely dry, a certain amount of rain would have to fall before any run-off would take place. The area and shape of a watershed will also affect to some extent the flow of the stream which drains it. A larger run-off would reasonably be expected from a long narrow watershed than from one more nearly circular in outline. The greater the massing of the watershed, so to speak, the more opportunity there is for the water to be held by the ground and the less would be the run-off.

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