Influence of evaporation, ground water, and uncertainty in the hydrologic budget of Lake Lucerne, a seepage lake in Polk County, Florida

Abstract

Evaporation losses and the interaction of ground water with Lake Lucerne were studied to determine the influence of these two processes on the hydrologic budget of a seepage lake. Lake Lucerne is representative of the numerous seepage lakes of sinkhole origin in the karst terrain of central Florida. Because of permeable surficial deposits, ground-water inflow is the only significant contribution from the surrounding watershed. The lake recharges the underlying Upper Floridan aquifer and, as a result, is susceptible to increased leakage induced by pumping from this aquifer. Ground-water fluxes determined in the study were analyzed to define the proportion of the total lake leakage induced by pumping from the Upper Floridan aquifer. A hydrologic budget is analyzed for the 1-year period from October 1985 to September 1986. Ground-water inflow and leakage are significant components of the hydrologic budget. Changes in the quantity of either of these fluxes can substantially alter lake stage. Ground-water inflow contributed from 20 to 37 percent of the total annual inflow to the lake. Leakage from the lake accounted for 18 to 23 percent of the total annual outflow. Water withdrawals from the Upper Floridan aquifer increased annual lake leakage by 22 percent over nonpumping conditions. Most of the increase (92 percent) in leakage occurred during April, May, and June 1986, when local citrus irrigation was highest. For the study year, ground-water inflow and leakage volumes were calculated by flow-net analysis to be equal to 10.5 and 12.6 inches, respectively, of water depth above (or below) the lake surface. These estimates were revised upward on the basis of an analysis of the error in the hydrologic-budget equation. Revised ground water inflow exceeded annual leakage from the lake. Ground-water inflow rates were increased by 120 percent to 23.6 inches, and leakage was increased by 40 percent to 17.5 inches. Differences between the two estimates probably reflect the uncertainty in the hydraulic conductivity estimates of the porous media around the lake and the unaccounted effect of transient ground-water inflow. The geometry of the sinkhole complex beneath Lake Lucerne and pumping in the Upper Floridan aquifer are primary controls on ground water interactions with the lake and, in particular, lake leakage. A numerical ground-water model was used to test the effects of these two factors on ground-water interactions with the lake. Results indicate that the intermediate confining unit below Lake Lucerne has been breached and replaced by materials about two orders of magnitude more conductive. Anisotropy in the surficial aquifer is approximately 100 and controls the depth of the ground-water flow intercepted by the lake. Lake sediments having low permeability may control the distribution of leakage through the lakebed but did not appreciably reduce total leakage rates in these simulations. Evaporation loss was the major outflow component of the hydrologic budget. Annual lake evaporation determined by the energy-budget method was 57.9 inches, about 8 inches greater than long-term estimates for the region. The greater rate was attributed to drier than normal conditions: rainfall totaled 40.9 inches during the study year, about 10 inches less than the long-term average. Similar annual evaporation rates were determined by the energy-budget method, the simpler mass-transfer method, and by corrected pan evaporation from an onsite pan; however, the probable errors associated with these other two methods were greater than for the energy budget. Weekly energy-budget evaporation rates ranged from 0.04 inch per day in early January 1986 to 0.26 inch per day in early May 1986. The largest monthly energy-budget evaporation rates occurred in April and May 1986, 7.16 and 7.12 inches per month, respectively. Monthly evaporation estimated from corrected pan evaporation generally was within 10 percent of the energy-budget estimate but differed by as much as 35 percent. Daily energy-budget evaporation also was computed for 321 days during the year.