DISTRIBUTION OF RAIN THROUGH THE YEAR.

In a general way the limit of agriculture without irrigation, or “dry farming”, is indicated by the curve of 20 inches rainfall, and where the rainfall is equally distributed through the year this limitation is without exception. But in certain districts the rainfall is concentrated in certain months so as to produce a “rainy season”, and wherever the temperature of the rainy season is adapted to the raising of crops it is found that “dry farming” can be carried on with less than 20 inches of annual rain. There are two such districts upon the borders of the Arid Region, and within its limits there may be a third.

First District.—Along the eastern border of the Arid Region a contrast has been observed between the results obtained at the north and at the south. In Texas 20 inches of rain are not sufficient for agriculture, while in Dakota and Minnesota a less amount is sufficient. The explanation is clearly developed by a comparison of the tables of rainfall with reference to the distribution of rain in different seasons.

Table V.—Precipitation of Texas.

Station.Latitude.Longitude.Height.Mean precipitation, in inches.Extent of record.
Spring.Summer.Autumn.Winter.Year.
° ´° ´Feet.Y. M.
Austin 30 17 97 44 650 8.61 7.9410.74 6.23 33.52 18 8
Camp Verde 30 00 99 10 1,400 6.11 9.81 8.30 5.05 29.27 5 9
Fort Belknap 33 08 98 46 1,600 6.41 9.44 8.34 3.86 28.05 5 10
Fort Bliss (El Paso) 31 47 106 30 3,830 0.43 3.49 3.38 1.23 8.53 14 3
Fort Brown 25 50 97 37 50 3.18 7.6413.02 4.04 27.88 15 0
Fort Chadbourne 31 58 100 15 2,020 5.77 6.53 7.06 3.52 22.88 8 7
Fort Clark 29 17 100 25 1,000 4.14 7.57 6.55 4.35 22.61 12 5
Fort Davis 30 40 104 07 4,700 1.84 8.76 4.72 1.80 17.12 8 11
Fort Duncan 28 39 100 30 1,460 3.56 8.60 6.54 2.63 21.33 11 7
Fort Griffin 32 54 99 14 — 4.95 6.25 6.14 4.17 21.51 5 3
Fort Inge 29 10 99 50 845 5.38 9.67 6.88 3.53 25.46 7 4
Fort Mason 30 40 99 15 1,200 6.3610.44 8.22 3.96 28.98 5 1
Fort McIntosh 27 35 99 48 806 3.22 6.56 5.38 2.35 17.51 14 7
Fort McKavett 30 48 100 08 2,060 5.21 6.71 7.81 4.22 23.95 9 7
Fort Stockton 30 20 102 30 4,950 1.24 5.66 3.31 1.29 11.50 5 8
Galveston 29 18 94 47 30 13.1514.9016.83 12.19 57.07 6 1
Gilmer (near) 32 40 94 59 950 13.36 9.9311.77 10.93 45.99 7 9
New Braunfels 29 42 98 15 720 7.60 6.90 8.83 4.25 27.58 5 1
Ringgold Barracks 26 33 99 00 521 3.71 7.00 6.31 2.58 19.60 14 2
San Antonio 29 25 98 25 600 6.77 8.91 9.30 6.32 31.30 10 2
Means 4.62 6.78 6.64 3.69 21.73 — —

Table VI.—Precipitation of Dakota.

Station.Latitude.Longitude.Height.Mean precipitation, in inches.Extent of record.
Spring.Summer.Autumn.Winter.Year.
° ´° ´Feet.Y. M.
Fort Abercrombie 46 27 96 21 — 4.80 8.67 3.46 1.85 18.78 13 6
Fort Buford 48 01 103 58 1,900 3.76 4.06 2.01 2.01 11.84 7 10
Fort Randall 43 01 98 37 1,245 4.72 6.22 3.40 1.18 15.52 15 6
Fort Rice 46 32 100 33 — 3.63 4.87 1.54 1.35 11.39 6 1
Fort Stevenson 47 36 101 10 — 3.41 4.97 2.15 1.31 11.84 6 2
Fort Sully 44 50 100 35 1,672 6.52 7.18 1.70 1.14 16.54 7 8
Fort Totten 47 56 99 16 1,480 5.18 7.17 2.50 1.59 16.44 5 5
Fort Wadsworth 45 43 97 10 1,650 7.0010.25 3.98 2.92 24.15 6 5
Pembina 48 57 97 03 768 4.02 7.24 2.71 1.53 15.50 4 8
Means 4.78 6.74 2.61 1.65 15.78 —

[Table V] includes every station in Texas that has a record of five years or more, in all twenty stations. If the means of rainfall for the state be compared with the means for single stations, it will be seen that there is a general correspondence in the ratios pertaining to the different seasons, so that the former can fairly be considered to represent for the state the distribution through the year. [Table VI] presents the data for Dakota in the same way, and the correspondence between the general mean and the station mean is here exceedingly close. At each of the nine stations, the greatest rainfall is recorded in summer, the next greatest in spring, and the least in winter. Placing the two series of results in the form of percentages, they show a decided contrast:

Spring.Summer.Autumn.Winter.Year.
Dakota30 431710100
Texas 21313117100

In Dakota a rainy season is well marked, and 73 per cent. of the rain falls in spring and summer, or at the time when it is most needed by the farmer. In Texas only 52 per cent. of the rain falls in the season of agriculture. The availability of rain in the two regions is therefore in the ratio of 73 to 52, and for agricultural purposes 20 inches of rainfall in Texas is equivalent to about 15 inches in Dakota.

For the further exhibition of the subject, [Table VII] has been prepared, comprising stations in the Region of the Plains all the way from our northern to our southern boundary. By way of restricting attention to the practical problem of the limit of “dry farming”, only those stations are admitted which exhibit a mean annual rainfall of more than 15 and less than 25 inches. The order of arrangement is by latitudes, and in the columns at the right the seasonal rainfalls are expressed in percentages of the yearly. The column at the extreme right gives the sum of the spring and summer quotas, and is taken to express the availability of the rainfall.

Table VII.—Seasonal precipitation in the Region of the Plains.

Station.Latitude.Extent of Record.Mean yearly rainfall.Percentage of annual rainfall.
Spring.Summer.Autumn.Winter.Spring and Summer.
° ´Y. M.Inches.
Pembina, Dak. 48 57 4 8 15.50 26 47 17 10 73
Fort Totten, Dak. 47 56 5 5 16.44 31 44 15 10 75
Fort Abercrombie, Dak. 46 2713 6 18.78 26 46 18 10 72
Fort Wadsworth, Dak. 45 43 6 5 24.15 29 42 17 12 71
Fort Sully, Dak. 44 50 7 8 16.54 39 44 10 7 83
Sibley, Minn. 44 30 7 11 24.74 21 40 29 10 61
Fort Randall, Dak. 43 0115 6 15.52 30 40 22 8 70
Fort McPherson, Nebr. 41 00 6 9 18.96 36 40 17 7 76
Fort Riley, Kans. 39 0320 10 24.52 22 43 24 11 65
Fort Hays, Kans. 38 59 6 11 22.70 31 27 25 17 58
Fort Larned, Kans. 38 1010 9 21.42 24 45 23 8 69
Fort Griffin, Tex. 32 54 5 3 21.51 23 29 29 19 52
Fort Chadbourne, Tex. 31 58 8 7 22.88 25 29 31 15 54
Fort McKavett, Tex. 30 48 9 7 23.95 22 28 32 18 50
Fort Davis, Tex. 30 40 8 11 17.12 11 51 28 10 62
Fort Clark, Tex. 29 1712 5 22.61 18 34 29 19 52
Fort Duncan, Tex. 28 3911 7 21.33 17 40 31 12 57
Fort McIntosh, Tex. 27 3514 7 17.51 18 38 31 13 56
Ringgold Barracks, Tex. 26 2314 2 19.60 19 36 32 13 55

The graduation of the ratios from north to south is apparent to inspection, but is somewhat irregular. The irregularity, however, is not greater than should be anticipated from the shortness of the terms of observation at the several stations, and it disappears when the stations are combined in natural groups. Dividing the whole series into three groups, as indicated by the cross lines in [Table VII], and computing weighted means of the seasonal ratios, we have—

Table VII (a).[2]

Groups of stations.Mean latitude of group.Total years of record.Percentage of annual rainfall.
° ´Spring.Summer.Autumn.Winter.Spring and Summer.
Eight stations in Dakota, Minnesota, and Nebraska 45 20 67 29 43 19 9 72
Three stations in Kansas 38 45 38 24 41 24 11 65
Eight stations in Texas 29 45 85 19 36 31 14 55

[2] In computing the several means of [Table VII] (a) from the seasonal means of [Table VII], the latter were weighted according to the lengths of the records by which they had been obtained.

A moment’s inspection will show that the middle group is intermediate between the northern and southern in all its characters. The spring quota of rainfall progressively diminishes from north to south, and so does the summer, while the fall and winter quotas increase. What is lost in summer is gained in winter, and thereby the inequality of rainfall from season to season is diminished, so that a rainy season is not so well defined in Texas as in Dakota. What is lost in spring is gained in autumn, and thereby the place of the rainy season in the year is shifted. In Dakota the maximum of rain is earlier than in Texas, and corresponds more nearly with the maximum of temperature.

Table VIII.—Seasonal precipitation in the San Francisco Region.

Station.Extent of record.Mean annual rainfall.Percentage of annual rainfall.
Y. M.Inches.Spring.Summer.Autumn.Winter.Winter and spring.
Alcatraz Island 9 5 16.49 16 0 11 73 89
Angel Island 5 11 18.58 19 0 15 66 85
Benicia Barracks 18 3 14.90 28 1 15 56 84
Fort Miller 6 9 19.00 38 0 16 46 84
Fort Point 14 11 17.36 21 0 13 66 87
Monterey 12 3 15.71 28 2 14 56 84
Sacramento 18 3 19.24 29 1 14 56 85
San Francisco; Presidio 20 2 20.29 24 2 13 61 85
San Francisco 24 4 21.49 24 1 14 61 85
Weighted means — — 25 1 14 60 85

Total extent of record = 130 years.
Mean of yearly rainfalls = 15.90.

Second District.—In the San Francisco Region a rainy season is still more definitely marked, but occurs at a different time of year. It will be seen by [Tables III] and [VIII] that no rain falls in summer, while the winter months receive 60 per cent. of the annual precipitation, and the spring 25 per cent. The general yearly rainfall of the district is only about 16 inches, but by this remarkable concentration a period of five months is made to receive 13 inches. The winter temperature of the district is no less remarkable, and supplies the remaining condition essential to agriculture. Frosts are rare, and in the valleys all the precipitation has the form of rain. The nine stations which afford the rainfall records given above show a mean spring temperature of 57° (see [Table IX]). Thirteen inches of rain coming in a frostless winter and spring have been found sufficient for remunerative agriculture.

Table IX.—Mean temperatures, by seasons, for the San Francisco Region.

Station.Extent of record.Mean temperatures, in degrees Fahr.
Y. M.Spring.Summer.Autumn.Winter.Year.
Alcatraz Island 8 6 55 57 60 54 57
Angel Island 3 1 58 63 61 52 58
Benicia Barracks 15 7 58 67 62 49 59
Fort Miller 7 6 64 86 67 49 67
Fort Point 10 11 55 59 58 52 56
Monterey 12 5 55 60 57 50 55
Sacramento 14 0 59 71 62 48 60
San Francisco; Presidio 19 0 54 57 57 50 55
San Francisco 11 2 55 58 58 50 55
Means — 57 64 60 50 58

The same winter maximum of rainfall is characteristic of the whole Pacific coast. The Region of the Lower Columbia, with an average rainfall of 46 inches, receives 47 per cent. of it in winter and 24 per cent. in spring. Southward on the coast, Drum Barracks (near Los Angeles) and San Diego receive more than half their rain in winter, but as the whole amount is only 9 inches agriculture is not benefited. The eastern bases of the Sierra Nevada and Cascade Range exhibit the winter maximum of rainfall, and this feature can be traced eastward in Idaho and Nevada, but in these districts it is accompanied by no amelioration of winter temperature. (See [Table X.])

Table X.—Seasonal precipitation and temperatures on the Pacific coast, etc.

Station.Mean annual rainfall.Percentage of rainfall.Mean temperature.
Inches.Spring.Summer.Autumn.Winter.Spring.Winter.
San Francisco Region 15.90 25 1 14 60 57 50
Region of Lower Columbia 46.45 24 6 23 47 51 40
Drum Barracks, Cal 8.74 26 3 4 67 60 56
San Diego, Cal 9.31 20 4 20 56 60 54
Camp Independence, Cal 6.60 17 5 9 69 57 39
Fort Bidwell, Cal 20.23 24 8 15 53 48 32
Camp Warner, Oreg 14.41 30 8 17 45 42 29
Camp Harney, Oreg 8.76 26 13 18 43 47 27
Fort Colville, Wash 14.06 26 22 18 34 45 24
Fort Walla Walla, Wash 19.36 24 11 26 39 52 34
Camp McDermitt, Nev 8.53 35 9 13 43 46 29
Camp Halleck, Nev 10.98 33 11 21 35 45 28
Fort Lapwai, Idaho 14.89 28 16 23 33 53 33
Fort Boisé, Idaho 15.48 33 8 16 43 52 30

Third District.—In Arizona and New Mexico there is a general maximum of rainfall in summer, and a restricted maximum in winter. The principal minimum is in spring. In [Table XI] the stations are arranged according to longitudes, a disposition well suited to exhibit their relations. In eastern New Mexico the distribution of rainfall has the same character as in adjacent Texas, but with a more decided maximum. Half of the total rainfall is in summer and half of the remainder in autumn. Westward the maximum diminishes slightly, but it appears in every station of the two territories. In western Arizona the winter maximum of the Pacific coast asserts itself, and it can be traced eastward as far as Fort Wingate, New Mexico. Except at Camp Mohave, on the western border of Arizona, it is inferior in amount to the summer maximum.

Table XI.—Seasonal precipitation in Arizona and New Mexico.

Station.Longitude.Mean annual rainfall.Percentage of annual rainfall.
° ´Inches.Spring.Summer.Autumn.Winter.
Western Texas — — 19 36 31 14
Fort Union, N. Mex 104 57 19.15 11 62 20 7
Cantonment Burgwin, N. Mex 105 30 8.65 18 34 28 20
Fort Stanton, N. Mex 105 38 20.94 14 51 23 12
Santa Fé, N. Mex 106 02 14.91 14 46 23 17
Albuquerque, N. Mex 106 38 8.11 10 54 25 11
Fort Fillmore, N. Mex 106 42 8.42 5 50 36 9
Fort Selden, N. Mex 106 55 8.49 7 57 22 14
Fort Craig, N. Mex 107 00 11.06 6 53 31 10
Fort McRae, N. Mex 107 03 11.59 21 53 20 6
Fort Wingate, N. Mex 107 45 17.32 11 38 20 31
Fort Bayard, N. Mex 108 30 14.32 11 50 16 23
Fort Defiance, Ariz 109 10 14.21 14 42 26 18
Camp Bowie, Ariz 109 30 15.26 9 48 13 30
Camp Grant, Ariz 110 40 15.08 14 41 22 23
Camp McDowell, Ariz 111 36 11.45 10 42 15 33
Camp Verde, Ariz 111 54 10.85 12 43 22 23
Camp Whipple, Ariz 112 20 19.28 20 42 11 27
Camp Mohave, Ariz 114 36 4.65 18 27 20 35
San Francisco Region — — 25 1 14 60

In all this region the daily range of temperature is great, and frosts occur so early in autumn that no use can be made of the autumnal rainfall. The yearly precipitation is very small, and the summer quota rarely exceeds seven or eight inches. Nevertheless the Pueblo Indians have succeeded, in a few localities, and by a unique method, in raising maize without irrigation. The yield is too meagre to tempt the white man to follow their example, and for his use the region is agricultural only where it can be watered artificially.

CHAPTER IV.
WATER SUPPLY.

By G. K. Gilbert.

The following discussion is based upon a special study of the drainage-basin of Great Salt Lake.