10 BULLETIN 193^ U. S. DEPARTMENT OF AGEICULTUKE. 
RUN-OFF. 
Practically all of the water which falls upon any drainage area, 
except that which is returned to the air by evaporation, that which 
is taken up by vegetatioQ, and that which sinks far into the earth, 
passes over or through the ground to the main outlet w^atercourse, 
and is termed the run-off from the area. To insure thorough drain- 
age at all times, the ditches and the pumping machinery must be of 
sufhcient capacity to remove the water of the heaviest rainstorm 
within a reasonable time. Economy, however, dictates that the 
design should not contemplate handhng the run-off from the ex- 
tremely heavy storms that occur only at intervals of several years. 
The exact length of time that water may stand on the ground with- 
out doing any appreciable damage to crops or without interfering 
materially with field operations is variable and indeterminate, but 
it is assumed that the surplus water should be removed from the 
ground surface within 24 hours. 
RUN-OFF MEASUREMENTS. 
Of course the best basis for determining the proper capacity of 
drainage channels or pumping plants is a number of measurements 
of the maximum flood flow following storms of the greatest severity 
against wliich it seems practicable to provide. More often than not, 
drainage improvements must be designed with only the most meager 
data of this kind, or none at all. Few opportunities for actually 
gaging flood run-off in this locality were presented during tlie survey, 
but the following data were obtained, which serve at least as an im- 
portant check on the run-off computations discussed later. 
Gagings of Taylors Bayou were made April 18, 1912, at two places 
near Hamshire, and the following day a gaging was made of Hille- 
brant Bayou at the Iron Bridge. Flood conditions prevailed then 
due to a storm of 3 to 4 inches on April 16, covering the watershed 
area, preceded by general rains that had saturated the soil. A 
gaging of Pine Island Bayou at the liighway bridge near Voth was 
made May 12. Heavy rains two days previous caused the high 
water, but the crest of the flood occurred about 24 hours before the 
gaging was made; therefore the maximum run-off rate was sUghtly 
greater than that observed. 
A private firm of civil en^neers made gagings of Brays Bayou 
near Houston on December 16, 1911, during one of the wettest periods 
on record. The results may be used with those obtained in Jefferson 
County, as run-off conditions are similar. Run-off data gathered 
from pumping districts in southern Louisiana during several years 
of investigation are also available. 
