OUR WATER SUPPLY— MEINZER 295 



tated water upon or under the land surface while on its way to the 

 sea or to points of re-evaporation on the land. Both phases are very 

 complicated. 



FIRST PHASE OF THE HYDROLOGIC CYCLE 



Natural fluctuations in humidity and aridity. — Let us consider the 

 first of these two phases of the hydrologic cycle. The terms aridity 

 and humidity, as the opposite of aridity, are difficult to define in pre- 

 cise terms. In a widely accepted sense, however, the term aridity 

 relates to the deficiency of the precipitation in a given area for the 

 normal growth of mesophytic vegetation that is otherwise adapted 

 to the conditions of that area. In this sense the aridity of an area is 

 intensified by decrease in the amoimt of precipitation and also by 

 increase in the evaporativity of the area — that is, in the potential 

 rate of evaporation. Both precipitation and evaporativity vary radi- 

 cally from place to place and from time to time, largely because of 

 temperature variations, which are produced by a complex of different 

 causes. 



The geologic record, covering some hundreds of milHons of years, 

 seems to show that long ages of relatively warm and equable cHmate, 

 perhaps with a general tendency toward aridity, were at several times 

 interrupted by shorter periods of more variable cHmate including 

 some cold, humid stages. The latest of these variable periods began, 

 perhaps a million years ago, with the first of the Quaternary glacial 

 stages and is apparently still in progress. The geologic record shows 

 that the Quaternary, and perhaps also older periods of the same sort 

 consisted of several major glacial stages alternating with distinct 

 interglacial stages, and that the glacial stages, or at least the last one, 

 consisted of two or more substages involving considerable climatic 

 fluctuations. The greater humidity of the glacial stages was in large 

 part caused by decrease in evaporation. From biological evidence and 

 the evidence of marine terraces, it appears that we are at present in 

 an intermediate position, having receded only part of the way from 

 the last glacial stage. From intensive study of geologic, archeological, 

 and historical records it is, however, evident that recent time has not 

 consisted of a gradual change from glacial to interglacial conditions, 

 but rather of complicated fluctuations of climate, in part regional 

 rather than world-wide, between periods that were more humid and 

 periods that were more arid than the present. 



All about us we have impressive evidence of climatic change, such 

 as the great sheets of glacial drift and trains of outwash gravel, the 

 scores of desiccated or partly desiccated lakes, including the extensive 

 Lakes Bonneville and Lahontan, and the great mantle of loess or 

 wind-blown sUt that covers much of the interior of this country and 

 is largely responsible for its great fertility. Looking more closely at 



