WATER SUPPLIES FOR IRRIGATION. 



73 



TABLE No. 1. 



Volume of precipitation per second of 



time for a given depth in feet, 



per twenty- four hours. 



Vol. of rainfall per 

 Rainfall in 24 hrs. second on 1 sq. mi. 



FEET. CUBIC FEET. 



0.25 

 0.50 

 0.75 

 1.00 



80.6683 

 161.3312 

 242.1635 

 322.8264 



From the above table the volume of 

 rainfall can be found for any of the given 

 depths, observed in twenty-four hours, by 

 multiplying the figures in the second col- 

 umn by the area of the watershed in square 

 miles. For depths of rainfall other than 

 those given, the cubic feet per second for 

 one square mile can be found by simple 

 proportion, and then for the whole water- 

 shed by multiplication, as before. 



Recorded measurements show that the 

 flood volume of streams varies inversely 

 as the area of the tributary watershed or 

 basin. Usually the larger the basin the 

 less in proportion to its area of watershed 

 will be the rate of flow of flood water at 

 its outlet. This is due to the fact that the 

 proportion of pervious soils, open and por- 

 ous ledges, intercepting stratified forma- 

 tions, easy slopes and still water basins is 

 so much greater in large drainage basins 

 than in smaller ones, that a much larger 

 proportion of the water falling reaches 

 underground courses and receptacles. In 

 small tributary basins the opportunity for 

 loss is much less and the rate of flow con- 

 sequently greater. 



The time when the flood flow takes place 

 at the outlet of a basin, like the rate of 

 flood flow, varies inversely as the area of 

 the basin. In small watersheds and the 

 basins of tributary streams the period of 

 maximum flow follows immediately after 

 the maximum rainfall, while in the main 

 outlet channel of a large basin the flow 

 may not begin for from one to four days, 

 and sometimes even more. 



The length of time which the flood dis- 

 charge from a stream lasts usually corre- 

 sponds very closely to the length of the 

 storm, although in the case of large basins 

 the discharge sometimes begins after the 

 storm is entirely over, this time being con- 

 sumed for the first water to reach the out- 

 let. 



In the case of elevated watersheds, where 



the larger portion of the precipitation comes 

 in the form of snow, the discharge of the 

 flood water coming from the subsequent 

 melting of the snow may continue for a 

 long time, and the effect of a storm may 

 continue for weeks or even months. 



DETEKMINATION OF FLOOD VOLUMES. 



A large number of different formulae 

 have been invented in different countries 

 for calculating the flood discharges from 

 river watersheds. Most of the experimetits 

 made for the purpose of devising satisfac- 

 tory formulae have been by Indian engi- 

 neers. Mr. Dredge proposes the following, 

 which has hitherto been very popular 

 among hydraulic engineers: 



in which 



Q = Volume of discharge in cubic feet 

 per second. 



M = Area of watershed in square miles. 



L = Length of watershed in miles. 



The record of maximum flood discharges 

 of American streams are few in number, 

 and for this reason it has been impossible 

 to construct a formula epecially for this 

 country which would give as close results 

 as might be desirable. In addition to the 

 difficulty due to the insufficiency of re- 

 corded observations, another serious diffi- 

 culty in the way of constructing a general 

 formula for the whole of the United States 

 arises from the fact that there is quite a 

 considerable difference, meteorologically, 

 between extremes of the country. In the 

 New England and Middle States, and most 

 of the United States lying east of the Miss- 

 issippi river and north of the Mason and 

 Dixou line, hydraulic engineers use the 

 following formula, which has been obtained 

 by comparing the data furnished from 

 measurements taken in that region: 



Q = 200 (Hi), in which 



Q = Volume of discharge from the 

 whole area measured in cubic feet per 

 second. 



M = Area of watershed in square miles. 



To find the discharge per square mile, 

 the fornmla may be expressed as follows: 



Q = 



200 (Mi) 

 M ' 



in which the 



letters have the same value as given above. 



In the southern and western portions of 



the United States the measurements made 



