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Kendall's tau and linear-recession techniques "were used to determine if statistically significant (a = 0.10) long-term trends for period of record exist for Lake Parker stage data or for four other area lakes, four ground-water sites, four rainfall sites, Lakeland publicsupply pumpage, and pan evaporation. Findings were consistent between the two analytical methods. There were no statistically significant long-term trends indicated for seasonal or annual mean daily stage data at Lake Parker. Statistically significant Kendall tau slope estimators were detected for pan evaporation ( + 0.45 inch per year), Lakeland well-field annual average pumpage ( + 0.56 million gallons per day per year), and one ground-water site (+0.48 foot per year). Statistically significant negative slope estimators were indicated for three other lakes (-0.03 to -0.27 foot per year) and one ground-water site (-0.25 foot per year). Rainfall indicated no long-term trends. The seasonal change in Lake Parker stage from November to May was related by multiple linear regression to changes in ground-water levels, rainfall, and pumpage for the same time period. The regression coefficient of determination was 0.90, and the standard error was 0.24 foot. Monthly changes in average lake stage for November through May were related to evaporation, rainfall, and ground-water levels with a regression coefficient of determination of 0.67 and standard error of 0.14 foot. INTRODUCTION Lake Parker is a 2,291-acre natural lake within the cily limits of Lakeland in Polk County, Fla. (fig. 1). In addition to its esthetic value, the lake is used for recreation and for cooling water for two city-owned electrical generating plants. The mean lake stage for 1949-86 was 129.88 feet above sea level. Approximately 2 miles west of the lake is the city's Northwest well field, permitted to pump an average of 28.3 Mgal/d from the underlying Upper Floridan aquifer. Polk County's two largest industries, phosphate mining and related chemical processing, used about 117 Mgal/d of water from the same aquifer system in 1985 (Stieglitz, 1986). The confining layer between the surficial aquifer and the Upper Floridan aquifer varies from 50 to 100 feet thick in the Lakeland area (Buono and others, 1979). Water levels in the Upper Floridan aquifer wells in the Lakeland area have fluctuated between 30 to 60 feet below water levels in the lakes; hence, there is the possibility for downward leakage. In July 1985, the mean daily stage of Lake Parker fell to 127.62 feet above sea level, the lowest elevation since stage records began in 1949 (fig. 2). On June 8, 1986, the mean daily stage declined to 127.02 feet above sea level and threatened to drop below the cooling-water intakes for the power-generating plants. A sinkhole formed near the western shore of Lake Parker on May 17, 1986, and was reported to have collapsed, forming a natural plug by June 21, 1986 (Dyer, Riddle, Mills and Precourt, Inc., 1986). The city utilities department and local citizens, however, became concerned that Lake Parker might be undergoing a long-term declining trend in stage, possibly related to ground-water pumpage and resultant lowered ground-water levels. The U.S. Geological Survey has collected weekly stage readings for Lake Parker from May 1949 to June 1954 and has collected continuous stage recordings from 1954 through the present (1988). In November 1986, the U.S. Geological Survey, in cooperation with the Lakeland Department of Electric and Water Utilities, began a study of analyzing existing hydrologic data to determine the relation of lake stages to other hydrologic variables and to determine if lake stages were in a long-term declining trend. This report describes and presents results of those analyses based on data from May 1949 through the 1986 calendar year. Purpose and Scope The purpose of this report is to analyze existing data on lake stage, ground-water levels, public-supply pumpage, and climatic data to determine: 1. If Lake Parker stage is in a long-term declining trend; 2. If any detected trends are occurring only at Lake Parker or are typical of regional trends in lake stage, rainfall, ground-water levels, and evaporation; and 8 2 °1 0 ' 8 2 °0 0 ' 8 1 °5 0 ' 8 1 °4 0 '

Trend analysis of Lake Parker stage and relation to various hydrologic factors, 1950-86, Lakeland, Florida