8 BULLETIN 114, U. S. DEPARTMENT OF AGRICULTURE. 
RUN-OFF. 
The " run-off " from a natural drainage basin is generally under- 
stood to be that water which flows from the area through the surface 
outlet channel. In designing drainage improvements, consideration 
must be given to the maximum rate of run-off for which provision 
should be made rather than to the total quantity to be removed. The 
principal factors affecting this rate in the Black and Boggy Swamps 
drainage district are rainfall, soil, and topography. 
RAINFALL. 
There are no stations of the United States Weather Bureau within 
this district, although it is well surrounded by stations at which con- 
tinuous precipitation records have been kept for years. From a study 
of the records secured at Allendale, Walterboro, and Yemassee, S. C, 
and Statesboro, Ga. (see fig. 1), the rainfall in the Black and Boggy 
Swamps district can be learned with sufficient accuracy. These 
daily records for the years 1903 to 1912, inclusive, have been carefully 
compared with each other and with the daily records secured at 
Whiteville, N. C., for comparing this district with the Chadbourn and 
Lyon Swamp districts as explained in the following paragraphs. 
PROBABLE RATE OF RUN-OFF. 
Drainage districts where conditions are similar to those in the 
Black and Boggy Swamps drainage district have been completed in 
southeastern North Carolina and have been in operation for sometime, 
but no measurements of flood run-off from these ditches have been 
made. These districts, however, afford the best guide in designing 
the ditches for this district. At Chadbourn, N. C, the rainfall, topog- 
raphy, evaporation and transpiration of plants, climate and seasons, 
the natural reservoir and storage capacities of the streams, and the na- 
ture of the vegetable growth are very nearly the same as in the district 
under discussion. Several ditches which drain areas varying in size 
from 400 to 2,150 acres have been constructed in that locality and 
have been in operation for about two years. The principal difference 
between conditions at Chadbourn and in Black and Boggy Swamps 
district is that the soil at Chadbourn is a sandy loam 3 to 8 inches deep 
underlain by a clay or clayey sand that is quite permeable and which 
will absorb water very readily; this, as compared with the soil in the 
district under discussion, will give a low run-off. The ditches at 
Chadbourn were designed to remove 1 inch, in depth of water from 
the entire watershed in 24 hours when running bank full. Where 
those ditches were constructed as designed they are working satis- 
factorily. 
In Bladen and Pender Counties, N. C, the Lyon Swamp Canal, 
which drains an area of about 18,000 acres, has been completed for a 
