RAINFALL AS A DETERMINANT OF SOIL MOISTURE 



FORREST SHREVE 

 The Desert Laboratory. Tucson, Arizona 



The influence of rainfall upon the distribution and seasonal 

 activities of plants is obviously exerted chiefly through its power 

 to replenish the moisture of the soil. In physiological plant geog- 

 raphy much good use has been made of annual totals of rain, the 

 seasonal distribution of rain, and the correlation of rainfall and 

 evaporation, as criteria for explaining the distribution of various 

 types of vegetation. In more intensive investigations it is de- 

 sirable to replace the consideration of rainfall — which is mediate 

 in its relation to plants — by a considera;tion of soil moisture, 

 which is immediate in this relation. The general parallelism of 

 rainfall and soil moisture conditions over large areas, and the 

 ready accessibility of rainfall records, contrasted with the scar- 

 city of soil moisture data, will long continue to be ample justifi- 

 cation for gi\'ing rainfall a prominent place as a so-called factor 

 in determining plant distribution. 



It is the purpose of the present paper to present a digest of a 

 short record of desert rainfall, interpreted in terms of its possible 

 effect upon soil moisture; to give data showing the annual march 

 of water content at three depths in a retentive clay soil; to indi- 

 cate the relative potency which different falls of rain were found 

 to have in renewing the store of soil water; and to estimate the 

 relative efficiencies of various percentages of soil water for the 

 maintenance of plant activitj^ by correlating them with the con- 

 current rates of aerial evaporation. 



The desert regions of North America are characterised by a 

 low rainfall which is chiefly made up of a large immber of light 

 rains and a small number of torrential rains. The soil of such 

 regions may \'ery well be thought of as a gigantic reservoir which 

 is replenished at infrequent intervals, and is drained continually 



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