DRAINAGE OF JEFFERSON COUNTY, TEXAS. 11 
FACTORS AFFECTING RUN-OFF. 
The amount of nin-off depends upon the rate and amount of rain- 
fall; modified by the size and form of the drainage area, the slopes 
of the ground, the aiTangement of tho watercourses, the nature of 
the soil, tho transpiration of plants and evaporation, the natural 
storage reservoirs, and tho drainage improvements within the water- 
shed area. In the following discussion the rate of run-off is expressed 
in inches depth over the entire drainage area removed in 24 hours, 
or in cubic feet per second, commonly abbreviated to ''second-feet." 
On the proper determination of the maximum rate of run-off that 
the drainage works should remove will depend largely the economy 
and efliciency of those improvements. Since the run-oif is dependent 
upon several factors of variable values, the determination of the 
proper rate requires wide experience and mature judgment. 
RAINFALL. 
It is usual to determine first the intensity of the most severe storm 
against which protection should be provided. The rainfall records 
secured by the United States Weather Bureau in Jefferson County 
and the vicinity show a very uneven distribution of the precipitation, 
with storms of great intensity. More rainfalls and more heavy 
storms occur from March to September than during the rest of the 
year, usually; therefore overflow is the more likely to occur during 
the growing season. The average annual precipitation at Beaumont 
dming the past 15 years is about 42.6 inches, at Galveston 46.3 inches, 
and at Lake Charles 53.9 inches. 
Some of the most notable storms recorded at Beaumont during 
those years are as follows: October 14, 1902, 9.26 inches; November 4, 
1902, 6.25 inches; and November 31, 1902, 6.87 inches. Lesser storms 
during and immediately following these several precipitations must 
have kept the ground saturated. June 25, 1905, 6.51 inches fell, 
preceded and followed by heavy rains, and October 14, 1906, 5.12 
inches of rain occurred. During May, 1907, 19.40 inches of rain 
were recorded, 13.3 inches of which fell in two storms on the 23d 
and 30th. April 14, 1908, 5.80 inches rainfall occurred, preceded 
and followed by lesser storms, and during the first 10 days of July of 
tlie same year 9 inches fell. It will be noted that these excessive 
storms are of infrequent occurrence, but the records from aU the 
stations show that from 1892 to 1912, between March and October 
each year there occurred one or more storms of 3 to 4 inches at times 
when the ground must have been well saturated, and that heavier 
storms were rather local and not general over the county. 
In view of the facts stated above, it seems wise to design the drain- 
age improvements for this county to care for the run-off from a rain- 
fall of 4 inches in 24 hours. At very infrequent intervals the ditches 
