HYDROLOGY OF NEW YORK 



191 



conclusions from data about which there is considerable doubt. 

 The rainfall differs by 4.33 inches and the evaporation by 4.89 

 inches, or from 20 per cent to 25 per cent. 



In preparing these tables the figures of table No. 26 of Mr. Free- 

 man's report have been used. This table is in million gallons per 

 24-hour day, and has been reduced to inches per month on the 

 catchment area of 338.8 square miles. The following gives the 

 water surfaces exposed to evaporation at different periods: 



It may at first thought be imagined that these large water sur- 

 faces exposed to evaporation have considerably increased the 

 ground evaporation over the entire catchment. When, however, 

 one considers that it is only the difference between what a water- 

 surface evaporation and what a ground-surface evaporation would 

 be, the difference is seen to be not very much. For instance, 

 assuming the water-surface evaporation at 36 inches per year and 

 the ground surface evaporation at 27 inches per year, the differ- 

 ence becomes 9 inches. With 12 square miles of water surface in 

 1900, giving 3.56 per cent of the whole, the excess of water-surface 

 evaporation over ground-surface evaporation is 0.32 of an inch, a 

 quantity which is so far within the limit of possible error in other 

 directions as to be negligible. At the most, taking the catchment 

 area at 338.8 square miles, it would only reduce the evaporation 

 from 26.5 inches to 26.2 inches. 



The minimum year in this table is seen to be 1880, when only 

 . 13.71 inches ran off. In 1883 the runoff was also very low, being 

 only 13.74 inches. 



Table 61 gives the rainfall, runoff, and evaporation of the stor- 

 age, growing, and replenishing periods for the Hudson river area 

 for a period of 14 years, from 1888-1901, inclusive. The minimum 



Per cent 



6.9 square miles, 1878-1891, 



8.4 square miles, 1891-1893, 



9.5 square miles, 1893-1895, 

 11.0 square miles, 1895-1897, 

 12.0 square miles, 1897-1900, 



5.8 square miles, 1868-1873, 



6.2 square miles, 1873-October, 1878, 



= 1.73 

 = 1.83 

 = 2.03 

 = 2.48 

 = 2.82 

 = 3.28 

 = 3.56 



