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 maximum 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 foregomg case, the time element was obtained by estimating 

 the distance at 30 miles and the velocity at IJ 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 River 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 run-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 rainf aU 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 (fig. 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, 

 wliich is equivalent to 31.5 second-feet per square mile for the area 

 of 1,200 square miles. 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 rainfall 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. 



