4io 



REPORT OF THE COMMISSIONERS OF 



rainfall in relation to the run-off of streams may be considered a division of 

 applied meteorology. 



ANNUAL RAINFALL. 



In compiling rainfall records for purposes of comparison with run-offs of streams, 

 it is the author's custom to arrange them in a water year beginning with the month of 

 December, and ending with November. Such a year is again divided into three 

 periods, December to May, inclusive, constituting the storage period ; June to 

 August, inclusive, the growing period; and September to November, inclusive, the 

 replenishing period. During the storage period a large per cent, of the total rainfall 

 appears as run-off in the streams, while in the growing period the percentage of 

 rainfall appearing in the streams is very small, the bulk of the precipitation of that 

 period being used up by plant life and evaporation from the surface of the ground. 

 Usually the stored ground water is so drawn upon during this period as to produce a 

 low water table. The replenishing period is an intermediate stage during which 

 evaporation gradually decreases, and provided the rainfall is not deficient, ground 

 water rises to its usual height. Naturally these periods run more or less into one 

 another, depending upon whether seasons are advanced or retarded.* 



The English writers on hydrology make a water year beginning with September 

 and ending with August. This water year is again divided into a winter period 

 extending from October to March, inclusive, and a summer period from April to 

 September, inclusive, as best fitting the conditions of English climate. Mr. Beardmore, 

 in his Manual of Hydrology, proposes a water year of three periods of four months 

 each, the first including November to February, inclusive, which he calls the winter 

 division; the second, March to June, inclusive, the spring division; and July to 

 October, inclusive, the summer division. This plan, Mr. Beardmore considers, gives 

 better opportunity for comparison. As regards American climatic conditions, how- 

 ever, the author considers the division into storage, growing and replenishing periods, 

 as given in the foregoing, on the whole, the best arrangement.f 



* For examples of rainfall and stream run-off records written up with reference to a water year 

 extending from December to November, inclusive, and divided into storage, growing and replenishing 

 periods, see the author's several reports on Genesee and Hudson Rivers Storage Projects, in the 

 Annual Reports of the State Engineer and Surveyor, for 1895-96, inclusive. 



f See (1) Beardmore's Manual of Hydrology, pp. 2-81; (2) Harrison's paper On the Subterra- 

 nean Water in the Chalk Formation of the Upper Thames, and its Relation to the Supply of London, 

 in the Proc. Inst. C. £., Vol. CV (1891), pp. 35-46; (3) Evans' paper On the Percolation of the 

 Rainfall on Absorbent Soils, in Proc. Inst. C. E., Vol. XLV (1876), pp. 208-216; and (4) Graves' 

 paper On Evaporation and Percolation, also in Proc. Inst. C. E., Vol. XLV (1876). 



