14 FORREST SHREVE 



In order to show graphically the duration of the rainless pe- 

 riods, figure 1 has been drawn, so as to indicate by lines the length 

 of periods of two weeks or more which received no rain or only 

 a trace or 0.01 in. (thin fines), and the periods which received no 

 rains above 0.15 in. (heavy lines). It wifi be shown presently 

 that few rains of less than 0.15 in. have an influence on soil 

 moisture at 15 cm. or below, and that most rains above that 

 amount have an appreciable wetting effect. The thin lines of 

 figure 2 indicate the rainless periods from a climatological stand- 

 point, the heavy lines indicate the rainless periods from the 

 standpoint of the influence of rain upon soil moisture. 



The lines of figure 2 show at a glance that the principal drought 

 periods of the year fall in April, May and June, and that July and 

 August are the months most regularly free of drought. The rain- 

 less intervals of October and November are less regular in their 

 occurrence and not so great in their duration as those of the arid 

 fore-summer, and the winter rainy period is also much more 

 irregular and uncertain than the summer rainy period. 



The longest abeyance of rain was in 1909, from March 23 to 

 June 26 — 94 days in which there was one day with 0.01 in. of 

 rain, and were three days with traces. The longest period with- 

 out significant rain extended from January 11 to June 1, 1911 — 

 a stretch of 140 days during which occurred one rain of 0.13 in., 

 one of 0.05 in. and six showers of less amount. 



The extent to which the soil water that is derived from the 

 heavy rains of restricted periods is conserved through the weeks 

 of drought will be brought out in the discussion of soil moisture. 

 It is not the absolute rainfall figures alone which furnish a cri- 

 terion of the climate, in this case and in the case of other desert 

 regions; the maximum duration of the drought periods consti- 

 tutes a limiting climatic factor of the foremost importance to 

 plants. 



