HYDROLOGY OF NEW YORK 



115 



The data required for ordinary computations may be frequently 

 limited to the totals of the storage, growing, and replenishing 

 periods, although when ground water is to be taken into account 

 the monthly data should be given. 



Formulas for runoff. At the risk of being considered some- 

 what elementary the writer will give the more important of the 

 formulas for runoff, expressed in terms of inches on the catch- 

 ment area: 



tixQx 86400x12 

 lm— Ax 640x43560 ^ 



Whence we deduce, 



Also, 



and 



J m= X ^ 



Iy = QxC, (7) 



To change gallons per day into inches per month we have : 



I m = nxGxC 3 (9) 



Also, 



In the reports of the United States Geological Survey the dis- 

 charge of streams is sometimes given in acre-feet per month. To 

 reduce such to inches per month, we have, when total acre-feet 

 are given : 



T _BxC 4 . 



(11) 



In these formulas, 



A=area of catchment in square miles. 

 B=total acre-feet per month. 

 D=cubic feet per second per square mile. 

 O=gallons per day. 



I m z=inches in depth per month on the catchment area. 

 I y =inches'in depth per year on the catchment area. 

 n=number of days per month. 



