CLIMATIC CYCLES. 251 



low that almost any deficit is equivalent to a drought of some degree. How- 

 ever, while a dought year involves inconvenience and loss, it rarely causes 

 disaster and the general abandonment of recently settled regions. Hence, in 

 the discussion of climatic cycles and drought, the latter is understood to be a 

 drought period several years in length and extending over all or nearly all of 

 the West. 



Recurrence of drought periods.— The assumption that the 11-year cycle 

 could be traced in the present and future as well as in the past (Clements 

 1916: 330; 1917: 304; 1918: 295) led to a study of the coincidence of drought 

 periods and sun-spot maxima from 1860 to 1915. The sun-spot maxima for 

 this interval of 55 years occurred in 1860, 1870-72, 1883, 1893, and 1907. 

 The maxima of 1870-72 and 1893 were known to coincide with times of 

 general and critical drought in the West, and it was found that similar con- 

 ditions had prevailed in 1859-60. In the case of the maximum for 1907, the 

 deficit fell in 1908-€9 for most regions and was less marked, while for the 

 maximum of 1883 the record showed an excess of rainfall quite as often as a 

 deficit. The close correspondence of sun-spot maxima and drought in 1870- 

 72 and 1893-95, and the decrease or absence of agreement in 1883 and 1907, 

 suggested that the maximum effects occurred in multiples of the 11-year 

 period (Plant Succession, 336) . The period involved in the two maj or droughts 

 was 21 to 23 years, and this appeared to warrant the suggestion that a similar 

 critical drought would recur in connection with the sun-spot maximum of 

 1917. The year 1915 proved to be exceptionally rainy, perhaps due to a lag 

 of the effects expected at the sun-spot minimum in 1913, with the result that 

 the ensuing drought period of 1916-18 appears to have been the most general 

 and severe ever known in the West. 



Drought periods not only bear a relation to the maximum of the sun-spot 

 cycle, but also to periods of increased rainfall, with which they show a definite 

 alternation. This alternation of dry and wet phases constitutes the climatic 

 cycle which corresponds with the 11-year sun-spot cycle. As Douglass has 

 found in the case of tree growth (1919), the drought period is much more 

 marked, at least in its effects, and its rings are consequently used as basing 

 points. The wet phase is related to the dry one in a cause-and-effect sequence, 

 in accordance with which a deficit is followed within a year or two by an excess, 

 or an excess by a deficit. While a preliminary investigation of this point 

 indicates that it is the general rule, it is often obscured by local variations in 

 the spatial distribution of rainfall. The wet phase likewise shows a correla- 

 tion with the minimum of the sun-spot cycle, but it is usually less definite and 

 striking than that of the dry phase. However, in a large number of localities 

 representing different regions of the West, the rainfall at the sun-spot minimum 

 is usually above the normal. It seems more or less probable that periods of 

 excessive rainfall for 1 to 3 years occur on the second or third multiple of the 

 11-year cycle, and that they precede or follow a drought period as a rule. 



The evidence that drought has occurred at frequent intervals during the 

 past 300 years is conclusive. It is equally certain that drought periods have 

 regularly alternated with wet ones, though these are naturally less frequently 

 noted in the human record. Moreover, it must be recognized that the alter- 

 nation of dry and wet phases will be seen most clearly in the grassland climax 

 of the prairies and plains, where the rainfall ranges between 15 and 30 inches. 



