HYDROLOGY OF NEW YORK 



123 



At this point Mr Vermeule was confronted by the difficulty of 

 ground storage. In regard to the effect of this it may be men- 

 tioned that, with rainfall above the average continuously for sev- 

 eral years, ground water may be expected to stand above its 

 average night, yielding to streams the maximum flow possible to 

 ground water. On the other hand, when the rainfall is below the 

 average for a number of years ground-water flow will be lower, 

 becoming less and less as the rainfall approaches the minimum. 

 It is very important that this fact be taken into account, because 

 without it one is certain to fall into error. The formulas for 

 average depletion may be given as follows : 



d^^+e+f— r; (24) 



d= 4 +d ^ T= T' (25) 



In which d x and cZ 2 =depletion at end of previous month and 

 for the month under consideration; d— average depletion; e and 

 r=monthly evaporation and monthly rainfall, respectively, and 

 /—computed monthly flow. 



The foregoing does not fully express the use of these formulas, 

 but as all that is wanted at this time is an illustration of methods, 

 this brief account may be deemed sufficient. 



Mr Vermeule gives a diagram showing ground flow for the sev- 

 eral different streams mentioned for a given depletion, which is 

 to be used in conjunction with the foregoing formulas. In his 

 opinion the diagrams present advantages over a ground-flow 

 formula with varying constants and coefficients for different 

 streams, being more readily compared and insuring greater accu- 

 racy. Later, in his Report on Forests, 1 Mr Vermeule modifies his 

 formula, as follows : 



E=(ll+0.29 R) M. (26) 

 In which E=evaporation, R=rainfall, and M is a factor de- 

 pending upon the mean temperature of the atmosphere. The 

 writer understands Mr Vermeule to say that this is also an 

 expression for annual evaporation. 



i Report on Forests, by C. C. Vermeule: Ann. Rept. State Geologist New 

 Jersey for year 1899. Trenton, 1900. 



