10 FOREST BELTS OF WESTERN KANSAS AND NEBRASKA. 
remarkable for their steady flow of pure, cold water. The sand-bar 
willow is a native shrub of the sand hills in both Kansas and Ne- 
braska, and evidences the good moisture condition of the soil. , 
A striking example of correlation between soil and forest growth is 
shown by the closeness with which the rock pine follows the sand- 
stone and sand of the Arikaree formation in Nebraska. 
CLIMATE. 
The climate of nearly all this region is essentially semiarid. It is 
characterized by light and unevenly distributed precipitation, high 
winds, excessive evaporation, and great fluctuations of temperature. 
AJ] these conditions become more marked from east to west, and are 
clearly reflected in the character of the forest growth and the gradual 
disappearance of tree species. This is fully shown in the discussion 
of the valley type and in the notes on the ranges of the various native 
species. (See p. 25.) 
The average annual precipitation at Hays, Kans., is 20.9 inches, 
while that at Wallace, which is due west of Havs, near the Colorado 
line, is but 16.1 inches. In Nebraska the average precipitation at 
Kearney is 25.8 inches annually, at North Platte 17.7 inches, and at 
Gering 14.8 inches. These three places are all on the Platte River, 
and strikingly illustrate the imecreasing aridity toward the west. 
Fort Robinson, Nebr.. in the Pine Ridge region, has an annual precip- 
itation of 16.6 inches. 
Of even more moment to forest growth than the low average rain- 
fall are the fluctu: tions from year to year and the irregular distribu- 
tion of rains throughout the vear. The Weather Bureau records 
have been kept at Dodge, Kans., for thirty years, and during that 
period the precipitation of the driest year was only 10.1 inches, while 
the wettest year had 33.7 inches. The average precipitation at Dodge 
is nearly 20 inches, which, if well distributed, is sufficient for the 
erowth of many tree species. The species actually found at that 
place, however, are necessarily those which are hardy enough to 
endure a year in which the precipitation is only 10 inches. This 
same fluctuation in precipitation prevails throughout the plains 
region. 
One fact of great importance in connection with the rainfall is that 
the greater part of it comes at the time it is needed by vegetation of 
every kind. Three-fourths of the annual amount commonly falls 
between April 1 and October 1. 
High wind velocities increase the evaporation, and experiments 
show that a wind which has a velocity of 10 miles per hour causes 3.8 
times the evaporation from a water surface that there is in a calm. 
Since the average velocity on the plains is about 12 miles per hour, 
it may be safely said that the wind causes at least four times the evap- 
