114: 



NEW YORK STATE MUSEUM 



what he has said in his report to the United States Board of 

 Engineers on Deep Waterways, viz : 



The data for estimating the water supply of a large canal, 

 specially when on a large scale, should be based, when such data 

 are available, upon actual gagings of streams, rather than on 

 general considerations derived from study of the rainfall alone. 

 An examination of a large number of estimates of canal water 

 supplies, based on the usual method, shows that rainfall data 

 alone are in close cases inadequate for solving a water-supply 

 problem of the magnitude of the one now under consideration. 

 When, however, actual gagings of the streams, extending over a 

 sufficient number of years, are available, there is no reason why a 

 water-supply problem on a large scale may not be worked out with 

 the precision of a proposition in mathematics. 



What is here said in regard to water supplies for canals is 

 equally true as regards all other water supplies, either municipal 

 or for water power, etc. Farther on in the same chapter it is 

 stated : 



It is not intended to say, however, that rainfall data are not of 

 use in a hydrologic discussion. When, as in the present case, in 

 addition to stream gagings an extended series of such data are 

 available, the argument is made doubly good and the demonstra- 

 tion strengthened. 



When records of gagings are available the computation becomes 

 very simple. It is merely a matter of simple addition and sub- 

 traction. 



The complete data required in order to compute the safe pos- 

 sible yield of a stream are as follows : 



1) The catchment area. 



2) The rainfall of the minimum year, as well as for a series of 

 years. 



3) A ground- water diagram of the stream or, lacking such, a 

 diagram for a neighboring stream lying in the same or a similar 

 geologic formation, and, so far as possible, with similar condi- 

 tions of forestation. 



4) The available storage capacity of the stream. 



5) The loss by water surface evaporation from the reservoirs, 

 together with an estimate of the loss by percolation. 



