16 DRY LAND FARMING 



relatively in the late autumn and in the winter months, 

 which so far is less favorable to production except in 

 the case of a limited number of varieties of cereals and 

 grasses, but the loss is counterbalanced in a considerable 

 degree by the relatively large proportion of the precipi- 

 tation that falls in winter which may enter the soil at 

 that season, because of the limited extent to which it re- 

 mains frozen. Precipitation that is most timely, on the 

 whole, is that which falls but lightly in the winter season 

 and freely in the season of growth. 



Prominent among the influences that affect the evap- 

 oration of moisture are: (1) winds; (2) abundant and 

 hot sunshine, and (3) low humidity. The more dry and 

 warm the winds that blow over a given area, the more 

 forceful that they are, and the more constantly that 

 they blow, the greater is the amount of the moisture that 

 they remove from the soil. 



In the arid and semi-arid areas, the air is much less 

 moisture-laden than in humid areas; hence it takes 

 moisture more readily from the soil. In much of the 

 bench land and prairie areas, the winds blow with con- 

 siderable force and with no little constancy, especially 

 during the spring months. Unless it is prevented, they 

 will carry with them much of the moisture that is in the 

 soil. In the absence of preventive measures, so much 

 will have been lost that before the summer is well 

 under way the vegetation will languish. The hotter the 

 winds the more rapidly do they draw on soil moisture, 

 as transpiration from the plants is relatively more rapid. 

 The almost constant sunshine tends to draw heavily 

 on the moisture in the unprotected soil, and the heat of 

 the same proportionately intensifies this condition. The 

 heat of the summer sunshine is greatest in the valleys, 

 hence the drain on moisture in these is relatively great- 

 est; on the higher elevations the degree of the heat is 



