RAINFALL AND SOIL MOISTURE 23 



69.3 sq. cm. area.-* The conversion of the total gi\-es an annual 

 rate of evaporation of 345 cc. per square centimetre. The aver- 

 age annnal rainfall for the Desert Laboratory— 14.60 in. — yields 

 an annual fall of 37.1 cc. per square centimetre of soil surface. 

 The ratio of the evaporation to the rainfall is as 9.3 : 1. The 

 average annual rainfall for the 34-year record of Tucson is 

 11.57 in., an amount which yields a fall of 29.4 cc. per square 

 centimetre. The ratio of the evaporation at the Desert Labor- 

 atory to the rainfall at Tucson is as 11.6 : 1. 



The ratio of evaporation to rainfall has been used as a crite- 

 rion of the moisture conditions of climates in relation to vegeta- 

 tion,^ and as a criterion of extremely general application it is 

 not without its usefulness. The fact, however, that it is not the 

 actual amount of rainfall which is of importance to plants, but 

 the amount of soil moisture, makes it worth while to determine 

 the ratio of evaporation to soil moisture. Such a ratio does not 

 contain the error due to the fact that the insignificant and the 

 superfluous rainfall forms a part of the annual total, and is an 

 actual expression of the relation of the moisture supply for plants 

 to the atmospheric moisture drain which is continually made upon 

 them. The fact that there is a certain residuum of soil moisture 

 at low percentages which is not available to plants would make 

 it desirable to use, in calculating this ratio, only the available 

 moisture content of the soil. Although such a procedure would 

 make more comparable the ratios determined for soils of differ- 

 ent textures, it has not been done in this case owing to the fact 

 that the lower limit of available soil moisture is different for 

 different species of plants, and that the limit for a given spe- 

 cies has been shown to vary with the atmospheric conditions 

 themselves.^ 



* The arbitrary character of all conversions of atmometer losses into terms of 

 evaporation from a water surface, and the dependence of the conversion on the 

 character of the water container, has been emphasized by: Livingston, B. E., 

 Operation of the Porous Cup Atmometer. The Plant World 13: 111-119. 1910. 



'" Notably by: Transeau, E. N., Forest Centers of Eastern North Amei'ica. 

 Am. Nat. 39: 87.5-889. 1905. 



'' Brown, W. H., The Relation of Evaporation to the Water Content of the Soil 

 at the Time of Wilting. The Plant World 15: 121-134. 1912. 



