RAINFALL AND SOIL MOISTURE 21 



as in the oa-se of the rain of November 26, with surface soil mois- 

 ture of 14 %. A rain of 0.18 in., with surface soil moisture of 

 5 %, was of some effect. The actual potency of a given amount 

 of rainfall in replenishing the store of soil moisture depends on 

 the moistness of the surface soil at the time of the fall, but to 

 some extent on the moistness of the lower soil as well. The at- 

 mospheric conditions immediately following a rain also help to 

 determine whether it sinks into the soil or is rapidly evaporated. 

 It would require a long series of observations to establish the 

 force of these and other conditions in determining what is the 

 minimum amount of significant rain. ' So far as the above data 

 indicate, a conservative approximation of the lower limit of sig- 

 nificant rainfalls may be placed at 0.15 in. The plotting of the 

 periods during 1907 to 1912 w^hich were without significant rain 

 (fig. 1) has been done on the basis of a lower limit of 0.15 in. 

 For soils of greater wettability than the Tumamoc clay the limit 

 of significant rainfall would be much lower, and the entire march 

 of soil moisture conditions, and the relation of rainfall and of 

 atmospheric and soil evaporation to the moisture of the soil, 

 would be altogether different. 



From a knowledge of the dr}^ weight of Tumamoc clay, and 

 of the percentage of water in it at a given time, it is possible to 

 calculate the amount of water contained in a unit of the soil at 

 the given percentage. The weight of the dry soil, free of all 

 rocks and stones, is 1.38 grams per cubic centimeter. In a unit 

 of soil with a surface area of 1 sq. m. and a depth of 30 cm. there 

 are 414 k. The average moisture down to a depth of 30 cm. may 

 be approximated by using the actual readings for 3, 15 and 30 

 cm., interpolating percentages for 10, 20 and 25 cm., and taking 

 the average of the six values. The actual amount of water in 

 an ideal, rock-free unit of soil can be determined by multiplying 

 the average percentage of soil moisture by 414. * 



During the driest weeks of June the average soil moisture was 

 6.5 %, and in the wettest week of August, 1911, it was 29 %. 

 These percentages would give a water content of 26.9 k. for June, 

 and of 120 k. for August, per soil unit. The actual amounts 

 of water contained in average units, as they vary from spot to 



