8 - BTJLT.ETIlSr 181^ U. S. DEPARTMENT OP AGEICULTTTBE. 



DETERMDJATION OF RATE OF RUN-OFF. 



By the establishment of a sufRcient number of measuring stations 

 over the watershed, the amount of rain falling during any period of 

 time may be determined with comparative accuracy. However, the 

 rate of run-ofF is mfluenced not only by the total amount of rain 

 falling, but also by the duration, intensity, frequency, and distribu- 

 tion of storms, it being the composite effect of the rainfall occurring 

 during the overflow together with that of other recent storms. Thus 

 it may be seen that the determination of the maximum rate of run-off 

 becomes a complex problem. 



The most reliable method of ascertaining the maximum rate of run- 

 off for any district consists in making accurate measurements of the 

 amount of water flowing from the district during its highest flood. 

 Since in most cases it is impossible to obtain this information for the 

 stream under consideration, recourse must be had to other methods. 

 Fairly rehable data may be obtained by investigating some stream in 

 the same locality with the one in question whose channel and water- 

 shed are of similar size, shape, and slope, where the soil and vegeta- 

 tion are similar, where rainfall records are available, and run-off 

 measurements have been made. 



No run-off measurements for the Big Black Eiver watershed have 

 been made, but such measurements have been taken on the Pearl 

 River watershed which adjoins that of the Big Black on the east, and 

 which is quite similar in size, shape, topography, character of soil, 

 and vegetation. The rainfall data collected at the Weather Bureau 

 stations near the divide between these rivers are applicable to both 

 watersheds. It was therefore decided to investigate the run-off of the 

 Pearl River and to apply the results obtained to the Big Black 

 watershed. As the size, shape, and topography of the watersheds, 

 character of soil, and vegetation are quite similar, it was assumed that 

 the effect of these factors would be the same on both watersheds. 



RUN-OFF FROM PEARL RIVER WATERSHED. 



A gauging station was established by the United States Geological 

 Survey on the Pearl River at the county highway bridge near Jackson, 

 Miss., June 24, 1901. From that date until the present time continu- 

 ous daily gauge readings have been recorded and numerous discharge 

 measurements have been made for river stages ranging from that of 

 minimum flow to within a few feet of the maximum recorded by the 

 gauge. From these data a discharge curve was constructed, and by 

 extending this the corresponding discharges for higher gauge heights 

 were estimated. The maximum discharge obtained in this manner is 

 the probable maximum discharge that will occur under existing drain- 

 age conditions. If drainage improvements were made, a greater rate 

 of run-off woultl result, since the water falling would immediately be 



