204 Nebraska Agricultural Exp. Station, Research Bui. 6. 
CLIMATIC FACTORS. 
It has been sufficiently demonstrated that the water require- 
ments vary greatly with the climate. A low annual rainfall in a 
region with a cool, humid, "wind-still" climate would go as far 
as a much larger rainfall in a region where the air, at least during 
the growing season, is dry, hot, and windy. This is due to a 
lower value of the climatic water-dissipating influences. In re- 
gard to this, Briggs and Belz (1910) show in connection with 
studies of Shantz (1911), that the natural vegetation is a good 
index to crop production as related to climate. Short grass, con- 
sisting chiefly of buffalo grass and grama grass, is typical of semi- 
arid conditions, such as found in the Great Plains area of the 
United States. A strip of short grass extends from Montana to 
Texas, limited on the west by drouth and on the east by compe- 
tition with other grasses. In Montana the annual rainfall is 14 
inches, while in Texas it is 21 inches. The difference of 7 
inches in rainfall represents the additional amount of rainfall 
needed to offset the increased evaporation. Thus, we see that 
the necessary supply of available water for crop production 
cannot be measured by the rainfall without also knowing the 
evaporation rate, which Briggs and Belz (1910) have shown 
to vary from 24 to 72 inches annually from a free water surface 
in different parts of this country. Prospective settlers should 
keep in mind that in regions of low annual rainfall and high 
evaporation rate, the transpiration rate is also very high, 
working double hardships for crop production. In some sec- 
tions, the evaporation rate is subject to great temporary ac- 
celeration from strong hot winds. A few hours of hot winds, 
coming as they are apt to do in periods of drouth, seem to be 
more fatal to crops, particularly corn, than are as many days of 
drouth minus the hot winds. 
The evaporation from a shallow free water surface is affected by 
atmospheric conditions in a manner rather similar to the evapora- 
tion from the plant. Thus, knowing the free water evaporation 
during the growing season may be an index as to the relative 
amounts of water transpired in crop growth in any given locality. 
This explains why crops may be grown in the Northwest on lower 
rainfall than in the south central portion of the United States. 
A knowledge of the evaporation rate of a region is almost as 
important as to know the rainfall. The new meteorological term, 
"effective precipitation," covers this combination of factors. 
