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NEW YORK STATE MUSEUM 



Division of the year into storage, growing and replenishing 

 periods. In reference to taking as a water year, the period 

 extending from December-November, inclusive, instead of from 

 January-December, inclusive, it may be stated that such or a 

 similar division has been customary with advanced hydrologists 

 for many years, although in the United States the advantages 

 of this division have not thus far generally appeared obvious 

 to writers on this subject.' In England a water year beginning 

 with September and ending with August is quite common. The 

 same thing has been done by the Philadelphia Water Department 

 in tabulating the data of Neshaminy, Perkiomen, Tohichon and 

 Wissahickon creeks and Schuylkill river. However, no hard and 

 fast rule can be laid dOAvn for the beginning and ending of these 

 periods. In some years the storage period will end soon after 

 May 1, while in other years it may be extended into the first or 

 second week in June. After some consideration a division of 

 the storage period from December-May, inclusive, has been taken 

 as, on the whole, best representing average conditions. 



The storage period is the period when evaporation is low r est 

 and the largest amount of water may be stored. On reference 

 to table No. 67, runoff data of Croton river for the water years 

 1877-1899, inclusive, it appears that the mean runoff for the 

 storage period is 16.83 inches ; the mean evaporation, 6.85 inches ; 

 and the mean rainfall, 23.68 inches. The mean runoff, therefore, 

 is 71 per cent, of the mean rainfall. In the growing period on 

 Croton river the mean runoff is 2.57 inches, with 13.58 inches 

 mean rainfall, or the runoff is 19 per cent, of the rainfall. In 

 the replenishing period the mean runoff is 3.42 inches and the 

 mean rainfall 12.08 inches, or the runoff is 27 per cent, of the 

 rainfall. It is obvious, therefore, that it is only during the 

 months of the storage period that any large amount of water 

 can be stored. A similar condition is shown by tables Nos, 43 

 and 61. 



During the storage period, vegetation is inactive and evapora- 

 tion takes place chiefly through wind action. It seems clear, 



