FLORIDA GEOLOGICAL SURVEY


to 34.5 inches. Surface runoff varied through a wide range from
wet to dry years, while ground-water outflow varied little. The
data show that the annual losses by evapotranspiration varied
little from wet to dry years. Evaporation losses from Lake Helene
amounted to 53.1 inches during 1962.
 Comparison of water-budget factors for the eastern and western
parts of the area shows that higher rates of ground-water recharge
to the Floridan aquifer occur in the eastern part.
 The amount of annual runoff from the total area has not
significantly changed in recent years. However, the distribution
of the runoff has been changed by drainage canals that divert
some of the flow from the upper Oklawaha River into the
Withlacoochee River.
 Impoundment of water in Green Swamp would provide some
flood protection for the lower Hillsborough River and the lower
Withlacoochee River basins. Impoundment of the total discharge
from Green Swamp to the Hillsborough River during the March
1960 flood would have reduced the flood crest at 22nd Street, Tampa,
by about 1 foot. Impoundment of the March 1960 flood discharge
in reservoirs proposed for the Green Swamp area (Corps of
Engineers, 1961) would have reduced the flood crest of the
Withlacoochee River at the Trilby gaging station by about 4 feet
and at the Croom gaging station by about 1.7 feet.
 Impoundment of water in Green Swamp Reservoir would have
 little effect on ground-water outflow from the total Green Swamp
 area because of increased seepage rates beneath the levee, increased
 evaporation losses, and because the aquifer under present conditions
 is essentially full. Impoundment of water in the Southeastern
 Conservation Area (Johnson, 1961) would increase the seepage
 rates during dry periods by about 60 percent. Impoundment of
 water will become more significant relative to ground-water
 recharge as pumpage from the Floridan aquifer increases.
 High piezometric levels in the southeastern part of the Green
 Swamp area are caused partly by a relatively slow rate of
 ground-water outflow due to sand-filled fractures, caverns, and
 sinkholes in the Floridan aquifer.
 Mineral content and calcium carbonate saturation studies show
 that generally the water in the Floridan aquifer in central Florida
 is low in mineral content and undersaturated.
 Interpretation of quantitative and qualitative data indicate
 that recharge to the Floridan aquifer in the Green Swamp area is
 about the same as that in other parts of central Florida.