44 WINDBREAKS. 



IMPORTANCE OF EVAPORATION. 



Evaporation of moisture from the soil is considerable in amount, 

 but depends more upon capillary action, which brings moisture to the 

 surface, and upon the direct heat of the sun, than upon the rate of 

 wind movement. The chief part played by the wind is to carry off the 

 moisture as rapidly as it is evaporated from the surface. Only when 

 the surface is wet from a recent rain is the action of wind apparent in 

 drying out the uppermost crust of the soil, but this influence is so 

 slight that it is not appreciable in soil samples taken at a depth of 4 

 inches. The upward capillary movement of soil moisture may be 

 reduced by from 80 to 90 per cent by a covering of loose soil or other 

 mulch. In checking evaporation from the soil, therefore, protection 

 from wind will be an unimportant item, except in the effect upon the 

 evaporation of rain water, or irrigation water, before it has oppor- 

 tunity to soak into the ground. 



Evaporation or " transpiration " of moisture from the leaves of 

 plants, whether grasses, field crops, or trees, is much greater than 

 the direct evaporation from the soil, which, when the soil is covered 

 with vegetation, as in fields, becomes comparatively unimportant. 



Transpiration or loss of moisture from the leaves of plants is a 

 natural function and is going on constantly, but is only in a moderate 

 degree necessary to the vigorous growth of the plant. It is always 

 a source of great loss of moisture, is greatly increased 'by unfavorable 

 atmospheric conditions, and may easily become so excessive as to 

 be very injurious to the plant. Normally there is an unavoidable 

 loss of moisture through the pores or "stomata" and a slight loss 

 through the epidermis of the leaves. Usually when evaporation 

 exceeds the supply of moisture to the leaf the stomata are closed. 



Many cases have been observed in which the evaporative or desic- 

 cating power of the wind has had a most marked effect upon the 

 growth, or even the very existence of vegetation. The most extreme 

 cases were in the Middle West a few years ago, when, after long 

 periods during which the rainfall had been hardly sufficient for crop 

 growth, hot, dry winds blew across the prairies of Kansas and 

 Nebraska, bringing almost complete destruction to the field crops 

 of the region. Often in a day or two crops which had given fair 

 promise of success were parched to crispness. The same effect is 

 produced upon the half-dormant plants in a field of winter wheat, 

 when, their protective covering of snow removed, they are subjected 

 to a desiccating wind. In all such cases a windbreak is very bene- 

 ficial to vegetation, not only because it checks the mechanical force 

 of the wind, but because the stagnation of the air permits the forma- 

 tion of a more or less complete blanket of humid air or, in other words, 

 the rapid dissemination of the moisture is stopped. 



