8 BULLETIN 181^ IJ. S. DEPAETMEI^T OF AGEICULTITEE. 
DETERMINATION OF RATE OF RUN-OFF. 
By the establishment of a sufficient number of measuring stations 
over the watershed, the amount of rain falling during any period of 
time may be determined Y\'ith comparative accuracy. However, the 
rate of run-off is influenced not only by the total amount of rain 
failing, 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 rehable method of ascertaining the maximum rate of run- 
off for any district consists in making accm'ate 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 River watershed have 
been made, but such measurements have been taken on the Pearl 
River watershed wliich 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 would result, since the water f alhng would immediately be 
