24 BULLETIN 181; U. S. DEPARTMENT OF AGEICULTUEE. 
follow, a rough estimate of the distance would be 20 miles, and assum- 
ing a velocity of 1^ miles per hour, the time required for the water 
to flow this distance would be about 16 hours. Hence the total 
time required for the water to flow from the upper edge of the water- 
shed to the lower end of the area under consideration would be 3 days 
and 4 hours. According to factor (2) a rain of 3 days' duration will 
produce a fiiaximum rate of flow from the total area. 
In the consideration of drainage areas of about 100 square miles 
the probable maximum rate of run-off from Apookta Creek was inves- 
tigated in conjunction with the rainfall records at Kosciusko, this 
rain-gauge station being in the neighborhood of Apookta Creek. The 
drainage area for this creek is 102.5 square miles and is approximately 
10 miles long and 10 miles wide. Employing the same method as 
in the foregoing case, the time element was obtained by estimating 
the distance at 30 miles and the velocity at 1^ miles per hour, which 
gives 24 hours as the time required for the water to traverse the 
watershed. 
As previously explained, the run-off from the Pearl Kiver water- 
shed for the two maximum storms was 55.6 and 56.1 per cent. Actual 
gaugings of the flow in Twenty-Mile Creek, near Baldwyn, Miss., 
were made by C. E. Ramser, who determined the run-off from the 
drainage area of 80 square miles to have been 1.17 inches from a 
storm of 1.88 inches in April, 1913. In that instance the rim-off was 
62.3 per cent. These data justify to a certain extent the assump- 
tion here made that approximately 60 per cent of the total rainfall 
will run off. Then, assuming as before that for any flood the rising 
and falling stages will be of equal duration and at a uniform rate, 
it can be shown that the maximum daily rate of run-off will be 60 
per cent of the average daily rainfall for the maximum storm of dura- 
tion equal to the period of rising flood. 
The rainfall records (fig. 3 a to 3 1) show the greatest three-day 
rain since 1903 on the 1,200 square miles at the upper end of the 
Big Black River watershed to have occurred in May, 1909 {G.g. 3h), 
the average total precipitation for the two stations having been 
5.85 inches, or 1.95 inches per 24 hours. If 60 per cent of the rain- 
fall be assumed to flow off, then the probable maximum rate of run- 
off would be 60 per cent of 1.95 inches, or 1.17 inches per 24 hours, 
which is equivalent to 31.5 second-feet per square mile for the area 
of 1,200 square mfles. The maximum rainfall of one days' duration 
for Apookta Creek, as taken from the records at Kosciusko (fig. 3b), 
was 5.8 inches, this rain havuig occurred February 6, 1903. Assum- 
ing 60 per cent of the rainfaU to flow off, the probable maximum 
run-off for 24 hours will be 3.48 inches, which is equivalent to 93.7 
second-feet per square mile. 
