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NEW YORK STATE MUSEUM 



therefore, very largely on whether or not the rainfall of the stor- 

 age period — December to May, inclusive — is large or small. If 

 the winter rainfall is relatively large, the runoff will also be rela- 

 tively large, even though the total rainfall for the year is small. 

 This fact must be taken into account in estimating the value of 

 streams. Whether any given stream is low during the summer 

 months or has then a well-sustained flow will depend very largely 

 on the rainfall of the month of May. When the May rainfall 

 is heavy enough to produce full ground water, the flow is likely 

 to be well sustained, even though the rainfall is comparatively 

 low during the summer months following. If, on the contrary, 

 the Ma}' rainfall is so low as to leave a deficiency in ground water 

 for that month, the flow will be low during the summer, even 

 though the rainfall is large. 



The foregoing explains why for certain years the runoff of 

 a stream may be relatively small, even with rainfall considerably 

 above the average. 



To more particularly illustrate this, assume a stream with, say, 

 6 inches of ground-water flow and further assume that on any 

 convenient date the ground water is practically depleted. Under 

 these circumstances, the 6* inches of ground water must fill up 

 before any very large flow can occur. On the other hand, we may 

 consider the sequence of the rainfall such as to leave full ground 

 water, whence it results that there will be a much larger runoff, 

 even though rainfall and other conditions are the same. 



What is wanted in a stream, therefore, is as large a ground flow 

 as possible, with small evaporation. That there are very great 

 differences in streams in this respect may be easily seen by exam- 

 ining a series of tables of stream flow. It may be remarked that 

 these two conditions are obtained only on a forested area, for 

 proof of which see Bulletin No. 7, Department of Agriculture, 

 Forest Influences. 



Effect of loir </roini<l water. Moreover, when rainfall is below 

 the mean for several months, the ground water may be expected 

 to become continuously lower. This is a subject about which 

 comparatively little is known, although the data are very import- 



