REPORT OF INVESTIGATIONS No. 40


 SURFACE WATER
 Escambia and Santa Rosa counties have an abundant supply of sur-
face water of excellent quality flowing in the streams and additional
supplies are found in small natural ponds and a few man-made ponds.
Streams are the main source of fresh surface water, discharging an aver-
age of 8.5 bgd into the bays along the southern boundary of the counties.
Small reservoirs created by dams are few in number at present. How-
ever, much of the terrain lends itself well to the development of small
reservoirs, and more will probably be built as the economy of the area
expands. The bays along the coast cover more than 230 square miles and
provide excellent facilities for boating, fishing, swimming, and shipping.
 The streams that flow into Escambia and Santa Rosa counties or
along their boundaries drain about 6,000 square miles before reaching
the counties. An average of slightly more than 10,000 cfs (cubic feet per
second), or 6.5 bgd, is brought into the counties by the surface streams.
Streams within the two counties pick up an average flow of 3,100 cfs, or
2.0 bgd, from the 1,700 square miles of land of the area.
 The flow of 2.0 bgd that is derived from within the two counties is
equivalent to 25 inches, or 40 percent, of the 63-inch annual rainfall of
the area. The combined losses by evaporation, transpiration, and under-
ground flow averages about 38 inches per year.
 Average unit runoff varies from basin to basin from 14 inches to 50
inches. The map in figure 12 shows approximate average annual runoff
in inches from stream basins within the two counties.
 Runoff during an extremely wet year is about 2% times that for a
 dry year. Figure 13 shows runoff in inches for 1956, a year of low runoff,
 and for 1961, a year of high runoff. The 1956 rainfall was near normal
 but the low runoff for that year reflected the rainfall conditions during
 the two previous years, which were well below normal. The cumulative
 deficiency of rainfall for the 3-year period 1954-56 was about 40 inches.
 This 3-year deficiency in rainfall reduced the amount of direct surface
 runoff and caused a decline in ground-water levels which in turn caused
 a decline in the base flow of streams. Streams in this area have a high
 rate of base flow that comes as seepage from the ground.
 The surface waters of Escambia and Santa Rosa counties are of ex-
 cellent quality, except in the coastal reaches where tides bring salt wa-
 ter up the streams. The Escambia River coming out of Alabama brings
 water of higher mineral content (about 100 ppm, parts per million);
 however, this mineralization is diluted somewhat by the lower mineral-
 content waters of the Florida tributaries.