REPORT OF INVESTIGATIONS NO. 48


to a rise of about 1 foot. Some fluctuation of the lake level would
result, of course, because inflow does not occur at a uniform rate.
Most of the net rise of the lake for December 1964, for example,
was due to a rise of 1.1 feet within a 24-hour period (figure 8,
graph (e)).
 The volume of water equivalent to a 1-foot rise of the lake
level depends on the surface area of the lake which, in turn, varies
with the lake level. The higher the lake level, the greater the
surface area and the greater the volume of water corresponding
to a given rise.
 The level of the lake prior to the outstanding rise beginning
in December 1964 was slightly below the altitude of 90 feet. At this
level the surface area of the lake is about 6,300 acres. The volume
of water corresponding to a 1-foot rise at this level is about 6,300
acre-feet. Removal of this volume of water within a period of
30 days would require an average outflow of 106 cfs (cubic feet
per second) or 48,000 gpm (gallons per minute).
 Flow by gravity past the controlling feature of the outflow
system would vary with the height of the water above the level
of the controlling feature. Thus, flow would not occur at an average
rate but, rather, would be greater than average part of the time
and less than average part of the time. To accomplish an average
flow of 106 cfs, therefore, the controlling feature of the outflow
system must be designed to pass a maximum flow substantially
greater than the average. A lower rate of outflow could be used
if more time were allowed to lower the lake to the desired level.
In this case, however, the cumulative lake rise would be greater,
thus requiring a greater allowance for the zone subject to flooding.

 DECLINING LAKE LEVEL

 During years having less-than-average rainfall the level of
 Lake Jackson usually declines. If the decline could be decreased
 by adding water to the lake during dry years, the utility of the
 lake for some purposes would be increased.
 The decline of the lake level averaged 2.2 feet per year for
 the 9 years of net decline since 1950 (table 2). The required
 inflow to offset such a decline with the lake level at an altitude
 of 90 feet, for example, would be equivalent to a continuous inflow
 of about 20 cfs or 9,000 gpm. As a matter of comparison, this
 amount is about 50 per cent greater than the pumpage for the
 City of Tallahassee, which during 1965 averaged about 6,200 gpm.