THE WATER SUPPLY IN DRY AREAS 445 



frozen; (2) of the too rapid melting of the snow, and 

 (3) because of evaporation under some conditions. 

 Should it come in the late spring and early summer 

 months, the loss from evaporation under good manage- 

 ment will be very considerable. Should it come in down- 

 pours much of it may run away over the surface and 

 into the gullies. Should much of it come in the form 

 of light showers, it is much liable to escape from the 

 surface soil through evaporation. Showers long and 

 moderate are much to be preferred. 



The soil while yet unbroken is hard. For long 

 centuries the soil has been trodden upon by the feet of 

 animals. In many places the rains have frequently fallen 

 upon it in dashes, which has tended to impact rather 

 than to open up the surface. There has been an almost 

 entire absence of the rains which sometimes fall gently 

 for successive days in humid climates, the water enter- 

 ing the earth rather than running over its surface. The 

 outcome has been that a large proportion of the pre- 

 cipitation that fell never entered the soil at all, and, as 

 a rule, the amount that fell was relatively small. Of 

 course, once in the streams it was forever lost to the 

 soil. 



In much of the semi-arid country warm winds come 

 occasionally, even in the winter. These winds are thought 

 to be influenced by the waters of the Pacific. They 

 are so warm that in some instances they are capable 

 of melting a foot of snow or more, in 24 hours. When 

 deep snow is melted thus quickly, the principal portion 

 thereof runs away over the impacted unbroken soil, 

 even though the soil should not be frozen. When it is 

 frozen, the condition is still further aggravated. In this 

 way but a fractional amount of the winter's precipitation 

 may enter the soil. 



In the semi-arid country evaporation is more rapid 

 than it is ordinarily in humid areas. This is owing: (1) 



