HYDROLOGY OF NEW YORK 



97 



Francisco, + 9 per cent and — 10 per cent. For a 25 year period, 

 it was found that the extreme variation was 10 per cent, both 

 at St Louis and New Bedford. Mr Henry reached the conclusion 

 that at least 35 to 40 years' observations are required to obtain 

 a result that will not depart more than ± 5 per cent from the 

 true normal. The average variation of a 35 year period Avas found 

 to be ± 5 per cent, and for a total 40 year period ± 3 per cent. 



This preliminary study indicates slightly more range than was 

 found by Mr Binnie, although it may be remembered that the 

 observations of the latter are far more extensive than Mr Henry's. 



Again, since the Tunoff is a function of the rainfall, it follows 

 that it must be affected in some degree in a similar manner. As 

 to just the relation, so far as known, very few computations have 

 been made. Indeed, very few runoff tabulations are extant which 

 are long enough to settle this question. It is clearly, therefore, 

 very difficult to solve definitely so abstruse a problem as that of 

 the extent to which forests affect rainfall. All solutions are 

 necessarily tentative in their character and will be for some time 

 to come. 



Minimum precipitation in New York. Let us now examine as 

 to what the records of precipitation in New York indicate in 

 regard to the probabilities of extreme low-water periods. The 

 following records of minimum precipitation are herewith in- 

 cluded : At Albany the water years 1895 and 1896 represented a 

 period of very low precipitation. The following are the totals 

 of the several periods : 



1895 ' 1896 



Storage 12.58 14.79 



Growing 8.88 8.31 



Replenishing 8.93 6.64 



Total 30.39 29.74 



The total precipitation of the previous year, 1894, was 34.45 

 inches. It is probable that for the water year 1896 streams in 

 the vicinity of Albany did not run to exceed 9 inches on the catch- 

 ment area. 



