4 BULLETIN 242, L T . S. DEPARTMENT OP AGEICULTUEE. 
It will be noted that at each of the stations, from 40 to 45 per cent 
of the total precipitation falls during the growing season of the 
crops studied ; that is, in the months of June, July, and August. 
WIND VELOCITY. 
Considered as a whole, the southern portion of the Great Plains 
has a high wind velocity. It is not, however, the high average ve- 
locity as much as the occasional high winds, which usually last only 
a short time, that must be considered as the injurious factor in crop 
production. The damage to crops in this region by wind may be 
accomplished either by soil blowing, by excessive transpiration from 
the leaf surface, or by direct loss of soil moisture by drying. Trans- 
piration from the leaf surface is an uncontrollable factor in crop 
production and will not be further discussed. The wind reaches its 
maximum velocity during the months of March, April, and May, 
although there are occasional days throughout the entire year when 
the velocity is high. Wind velocity, in so far as its soil-blowing effect 
is concerned, does not readily lend" itself to any form of scientific 
measurement, and data other than those gathered by general observa- 
tions can not be given. The extent to which wind velocity may affect- 
soil movement also depends largely upon the condition of the surface 
soil. 
EVAPORATION. 
The amount of evaporation from a free water surface during the 
growing season is very high in the southern portion of the Great 
Plains. This is due to a combination of high altitude, dry air, 
excessive wind, high temperatures, and long periods of drought. 
The seasonal evaporation for this area is about 55 inches, as com- 
pared with about 30 inches for the northern portion of the Great 
Plains. This relatively high evaporation doubtless accounts for some 
of the differences in crop yields, It is also one of the determining 
factors in the crop variety which can be successfully produced in this 
region. 
The amount of evaporation from a free water surface should not 
be confused with the evaporation from the soil except that it may 
offer rather a close relation to the water lost from the first few 
inches of soil when the soil is thoroughly saturated. Considerable 
work is now being done at all of these stations to determine the 
rate and amount of water lost from the soil by evaporation, but this 
subject will not be considered here. 
Table II gives the monthly and seasonal evaporation in inches 
from a free water surface at the level of the ground at the Garden 
City. Dalhart, and Amarillo stations for the years during which the 
experiments here reported have been conducted. 
