48 



BTTLLETTX 1017, IT. s. DEPARTMENT OF AGRICULTURE. 



While figure 19 does not alone form sufficient basis for estimating' 

 maximum future run-off, yet the relations existing between volumes 

 and intensities of precipitation and run-off. time periods, and areas 

 are of importance in estimating future run-off which may result 

 from summer storms. 



ESTIMATES OF PROBABLE RUN-OFF. 



In Table 5 are shown the estimated rates of run-off for various 

 subdivisions of the Red River watershed. The basis for these esti- 

 mates is all available run-off records for annual, monthly, and daily 

 periods, and for flood periods during spring and summer seasons, 

 as well as data on precipitation and the results of a study of the 

 various other conditions that affect run-off. As may be noted from 

 Table 5, the rates of run-off shown are not expected to occur in such 

 relation with respect to time that the excessive flow from all sub- 

 divisions will concentrate in the outlets. 



Frequently, in connection with engineering reports on undertak- 

 ings similar to the Red River project, certain formulae or equations 

 are developed or equations previously evolved are adopted for use 

 in estimating run-off. In the present instance, however, each water- 

 shed is a problem of itself and any attempt to calculate run-off for 

 the larger watersheds (say those of more than 200 square miles) by 

 formula? is likely to give results which would be quite unreliable 

 owing to the wide variation of the influencing conditions and to the 

 difficulty of assigning definite values to them. Accordingly no 

 formula, equation, or curve has been developed and none is to be 

 recommended. 



ET7X-OFF FROM SMALL WATEKSHEDS. 



For small watersheds, containing less than about 200 square miles, 

 the size of watershed drained is usually the dominating feature in 

 determining the rate of run-off. For such areas a run-off formula 

 in which the area of watershed is the principal factor may give satis- 

 factory results if care and judgment are used in applying the 

 formula. 



Table 12. — Estimated run-off. per square mile, from small irate/shea's. 



Area (square miles). 



Minne- 

 sota. 1 



North 

 Dakota. 2 



Area' square mile-, . 

 °5 



Minne- 

 sota. 1 



North 

 Dakota 2 



1 



Cu.ft. 

 per sec. 

 12.0 

 11.2 

 10. 7 

 10.4 

 10.0 

 9.7 

 9.5 

 9.1 

 8.9 



Cu.ft. 

 per sec. 



Cu.ft. 



per sec. 

 8.7 

 8.5 

 8. 3 

 8.1 

 7.8 

 7.7 

 7.3 

 7.0 



Cu.ft. 

 per sec. 

 6.7 



2 



30 



6.6 



3 





40 



50 



75 



100 



6.4 



4 



13.4 

 11.8 

 11.2 

 10.0 



8.4 

 7.7 



6.2 







5.9 



8 



5.8 



10 



150 



5.3 



15 



200... 



5.1 



20 







1 By J. T. Fanning, 1886. 



2 By J. T. Stewart, 1907. 



