446 DRY LAND FARMING 



to the less degree of the humidity in the air; (2) to the 

 character of the winds at certain seasons of the year, 

 and (3) to the greater tendency in many soils to form 

 cracks than is usual in soils in humid areas. It is rea- 

 sonable to suppose that dry air moving over a soil will 

 evaporate water on the earth and for some distance 

 beneath it more quickly than air that is already sur- 

 charged with moisture. It is also reasonable to con- 

 clude that the more rapidly the currents move along, 

 the larger will be the amount of moisture removed. 

 Winds are strong relatively in the spring months in 

 the semi-arid country, the time when in many areas 

 moisture is present in most abundance. This means that 

 unless the escape of this moisture is prevented in some 

 way much of it will be lost to the soil. The contrac- 

 tion of the soil when drying forms cracks. The more 

 numerous and larger and deeper the cracks the greater 

 the amount of the moisture that will be lost. 



For the reasons given above, the shortage of water 

 in dry areas is intensified. It explains why ponds or 

 basins are so few that hold water all the year. It makes 

 it clear why living streams are so few save in proximity 

 to the mountains. It gives the reason also why lakes 

 are so generally absent and why springs are so rare. 

 Under these conditions it could not be otherwise than 

 that water from wells would be more difficult to obtain 

 in abundant supply than in humid areas. 



The sources of water in detail. The following are 

 the principal channels or sources from which water is 

 obtained in dry areas: (1) that caught from roofs and 

 in draws ; (2) that obtained from springs and streams ; 

 (3) that which comes from irrigating ditches ; (4) that 

 which is obtained from wells, and (5) that which comes 

 from artesian sources. 



The water obtained from roofs must always be 

 limited in supply for the reason, first, that the precipita- 



