266 REPORTS ON THE STATE OF SCIENCE.—1914. 
seasonal evaporation, and the depth and character of the soil appears 
to be indispensable. 
Reference has already been made to the striking difference in the 
monthly distribution of the rainfall in the Great Plains as compared 
with the Intermountain districts. This difference is illustrated in 
fig. 1, which shows the monthly distribution of rainfall. at repre- 
sentative stations in each area. Three Pacific Slope stations with 
a distinctly winter type of rainfall are also included. In this latter 
region, owing to the mildness of the climate, an annual crop of wheat 
is grown during the winter months either for grain or hay. 
Grain-farming under the alternating fallow and cropping system 
has been satisfactorily established in Utah, where the annual rainfall 
is 13 inches or more. In the southern part of the State of Washing- 
ton, where the conditions are unusually favourable, land with an 
annual rainfall as low as 10 inches is used for growing winter wheat 
by the summer-fallow method,* but the returns are uncertain. When 
the annual rainfall is reduced to 8°5 inches the crop will barely return 
the cost of production. 
The rainfall required when the rain comes chiefly in the summer 
is higher than for winter rainfall. This appears to be due to the 
greater evaporation-loss from the fallow when wet frequently by 
summer rains. In the Great Plains, where a summer rainfall prevails, 
dry-farming is not successfully conducted on an annual rainfall less 
than 14 inches, and this minimum is still higher in the southern part 
of the area, due, as we shall see, to the higher rate of evaporation. 
Evaporation. 
The evaporation-rate may fairly be considered as ranking next in 
importance to the annual rainfall in determining the dry-farming 
possibilities of a region. The evaporation from a free-water surface 
represents a summation of the intensity of solar radiation, temperature, 
saturation-deficit, and wind velocity, all of which enter also into the 
determination of the transpiration-rate of the growing crop, though 
not necessarily in the same proportion as in free evaporation. Evyapora- 
tion has been measured daily during the summer months at each of 
the experimental farms located in the dry-farming sections. Tanks 
6 or 8 feet in diameter and 2 feet deep are used, the tanks being 
sunk in the ground to within four inches at the top. The free-water 
surface is maintained at ground-level, i.e., about 4 inches from 
the top of the tank. Observations are now available for seven years 
at the stations first established. The observations are limited to 
the six months from April to September inclusive, since freezing 
weather is encountered at the stations during most of the remaining 
months. The average seasonal (April to September inclusive) evapora- 
tion in inches for each station, together with its location, is shown 
on the accompanying map (fig. 2). The evaporation increases rapidly 
as one proceeds southward in the Great Plains; the evaporation in 
Northern Texas, for example, is 54 inches, compared with 31 inches 
3 Briggs, L. J., and Belz, J. O., Dry Farming in relation to Rainfall and Hvapora- 
tion, U.S. Department of Agriculture, Bureau of Plant Industry, Bulletin 188, p. 25. 
