26 FORREST SHREVE 



at 3 cm. falls as low as 1 % and that at 30 cm. rises to as much as 

 32 %. The average moisture of the soil from the surface to 30 

 cm. in the driest weeks of the year is 6.5 %, in the wettest is 

 29 %. At its minimum water content the clay soil contains 

 about one-fourteenth of the annual supplj^ of water furnished it 

 by the normal rainfall. 



The weekly rate of atmospheric evaporation ranges from a 

 minimum of 173 cc. to a maximiun of 1084 cc. — the annual 

 total being 31,447 cc, in terms of loss from a porous cup atmome- 

 ter. This is equivalent to a loss of 345 cc. per square centi- 

 metre from a free water surface. The ratio of evaporation to 

 rainfall is as 9.3 is to 1. 



The ratio of evaporation to soil moisture fluctuates from a 

 minimum amount to a maximum which are in the proportion 

 of 1 to 10. The ratio of evaporation to soil moisture at the foot 

 of the Santa Catalina Mountains is 9.7 times the same ratio for 

 their summit. The annual amplitude of moisture conditions at 

 the Desert Laboratory is as great, therefore, as that which exists 

 in the most arid portion of the year between localities which are 

 5000 vertical feet apart. 



