10 BULLETIN 198, U. S. DEPABTMENT OF AGRICULTURE. 



from the gaugings. The formula was changed and the investiga- 

 tions continued until a satisfactory curve was obtained. 



After an examination of the results of the gaugings heretofore 

 described, taking into consideration the probable effect that the 

 reclamation of the district will have on the rate of run-off, and after 

 a study of some of the larger drainage districts in the immediate 

 vicinity of the Cypress Creek drainage district, the following tenta- 

 tive assumptions were made as to run-off: 



From 5 square miles, 1 inch in 24 hours (26.88 second-feet per square 

 mile) ; from 25 square miles, f inch in 24 hours (20.20 second-feet per 

 square mile) ; from 400 square miles, | inch in 24 hours (13.44 second- 

 feet per square mile) . 



It was found that Fanning's formula could be converted into an 

 expression whose curve fulfilled these assumptions. The conversion 

 of this formula is as follows : 



Fanning's formula is: Q = 200 M^ 



Where Q = run-off from whole area, in second-feet, 

 and Jf = area of watershed, in square miles. 

 Substituting It for 200, and RM for Q (where i?= the run-off in 

 second-feet per square mile), we have: 



RM=KM^ovR=-j^ = -= (1) 



whence K=Ry'M 



Substituting the three assumed values of R and M, we have: 



For J? = 26.88, Z=35 



Fori? = 20.2, Z=34.5 



For 7? = 13.44, Z=36.4 

 Replacing the constant, K, by 35 in formula (1), we have: 



7?-^ 



This expression, which has been used for calculating run-off in this 

 project, is represented by the curve in figure 5. It was found to 

 agree fairly closely with what gaugings have been made, giving in 

 most cases values somewhat greater than the gaugings showed. 

 As has been pointed out, however, overflow and backwater affected 

 some of the gaugings and tended to give discharges less than the 

 actual ones. Allowance has also been made for increased run-off 

 to be expected after drainage. 



DRAINAGE PLANS CONSIDERED. 



Before the final plan, as hereafter stated, was decided upon, other 

 possible methods were carefully w^orked out and compared. 



