88 DRY LAND FARMING 



ment of air. The degree of the soil moisture thus re- 

 moved is increased with increase in the dryness and 

 warmth of the air. Warm air will hold several times 

 more vapor than cold air. As the air is usually more dry 

 in dry areas than in humid ones, the loss from this source 

 will be much more in the former, and, because of the in- 

 creased heat of summer, it will be much greater at that 

 season than in winter. 



Wind is a strong factor in removing moisture from 

 soils, especially moisture on or near the surface. The 

 influence of wind in thus taking up moisture may be 

 clearly seen in the rapidity with which water is removed 

 from the highway by strong wind blowing, after rain. 

 Winds are usually more prevalent in dry than in humid 

 areas, because of the comparatively treeless condition of 

 the former, hence the relative loss of the moisture from 

 this source is greater. These influences frequently act 

 in conjunction, and when they do the loss of moisture 

 from the soil will be very rapid in the absence of meas- 

 ures to prevent it. 



The extent to which soil moisture is lost through 

 evaporation will be proportionate: (1) to the extent to 

 which the agencies of air and wind are operative in re- 

 moving it; (2) to the extent to which other influences 

 are present that facilitate such loss, and (3) to the ex- 

 tent to which soil conditions are absent that would tend 

 to lessen and prevent the same. 



From what has been said, it will be apparent that 

 evaporation will be much greater in southern than in 

 northern areas of the dry belt. In the former the sun- 

 shine is hotter, and the humidity of the air is less. The 

 winds may not be any stronger but they are warmer- 

 The annual average evaporation of surface water in 

 dry areas is usually several times greater than the an- 

 nual precipitation. In the Panhandle of Texas, the annu- 

 al evaporation has been placed at about 54 inches, where- 



