REPORT OF INVESTIGATIONS No. 40


erage runoff at Barrineau Park is 26.0 inches per year. That is, the
average flow of 756 cfs for 1 year would cover the drainage area of 394
square miles to a depth of 26.0 inches. This is in comparison with the
State average runoff estimated to be 14 inches per year (Patterson,
1955). The high yield of the Perdido River basin can be attributed to
two factors: (1) a high annual rainfall-this area receives about 63
inches per year; and (2) the coarse sand and gravel surficial covering
that releases water to the streams as seepage from the water table or as
artesian flow from local aquifers.


200,000


100,000


50,000


10,000


5000


cn 2000 1-I 1 1 1-
50 o o00 500 10000 50o 10,000
 DRAINAGE AREA, IN SQUARE MILES
Figure 20. Regional flood-frequency curves for the Perdido, Escambia, Blackwater,
 and Yellow rivers.

 The pattern of flow with respect to time is similar to that of rainfall.
 March and April are by far the months of highest runoff, and October is
 the month of lowest runoff. The bar graphs in figure 22 show the aver-
 age, maximum, and minimum monthly discharges for the Perdido River
 at Barrineau Park for the 20-year period 1941-61.
 The flow-duration curve for Perdido River is given in figure 17. The
 slope of this curve indicates the variability of flow. This stream has high