16 FORREST SHREVE 



showed that readings may be expected to vary from 2% to 4% 

 at the same depth. 



The entire series of readings has been brought together in fig- 

 ures 3 and 4 in such a way as to present a graphic picture of the 

 annual march of soil moisture at the three depths. Each vertical 

 series of three figures shows the moisture conditions on the date 

 given at the top of the vertical line, the topmost figure giving the 

 percentage of moisture at 3 cm., the middle one at 15 cm., and 

 the lowest at 30 cm. Each horizontal series of figures shows the 

 percentages of moisture that were found, week by week, at the 

 same depth. In order to make clearer the march of the moisture 

 conditions a series of curves has been drawn between the mois- 

 ture figures in such a way as to separate the latter by gradations 

 of 2 %. Over the soil curves are given two graphs which show 

 the weekly rate of evaporation (solid line) and the weekly ratio 

 of evaporation to the moisture of the soil at 30 cm. (dotted line), 

 and also vertical lines which indicate the dates and amounts of 

 rainfall. It is thus possible to examine together the march of 

 the soil moisture conditions, and the relation which rainfall and 

 evaporation sustain to this march. 



RELATION OF RAINFALL TO SOIL MOISTURE 



A few instances may be given of the manner in which the rain- 

 fall affects the soil moisture conditions: On November 3 there 

 was a shower of 0.04 in. and on the following day a rain of 0.34 

 in. When the soil moisture was determined on November 7 it 

 was found to have risen from 3 % (on the preceding week) to 

 17 % at 3 cm. and from 7 % to 9 % at 15 cm., but to have fallen 

 from 12 % to 10% at 30 cm. The moisture at 3 cm. was, of 

 course, much above 17 % on the days immediately following the 

 rain; the moisture had had time to penetrate to 15 cm., but not 

 sufficient time to reach the 30 cm. level. On November 13 there 

 was a rain of 0.70 in. and on November 14 a rain of 0.03 in. As a 

 result the moisture at 3 cm. was 24 % on the 14th, from which it 

 fell to 14 % on the succeeding week. At 15 cm. the soil had 

 risen from 9 % on November 7 to 11 %, but as a result of the 

 rain of November 4 and not of the rain of the 13th, for it" was 



