4 BULLETIN 242, TJ. S. DEPAKTMENT OP AGRICULTURE. 



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 relative^ 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 st;tt ions i'<>r the years during which the 

 experiments here reported have been conducted. 



