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

Kendall tau test and regression analysis were used to determine if statistically significant long-term trends exist for Lake Parker, Florida stage data or for four other area lakes, four groundwater sites, four rainfall sites, Lakeland public-supply pumpage, and pan evaporation. A 10% significance level was used for criterion of an existing trend. Findings were consistent between the two analytical methods. There were no long-term trends indicated for seasonal or annual stage data at Lake Parker. Statistically significant Kendall tau slope estimators were detected for pan evaporation (+0.45 in/yr), Lakeland well-field pumpage (+0.56 million gal/day/yr), and one groundwater site (+0.48 ft/year). Decreasing trends were indicated for three other lakes (-0.03 to -0.27 ft/year) and one groundwater site (-0.25 ft/year). Kendall tau tests of four annual rainfall records indicated no long-term trends. Change in Lake Parker stage from November to May was related by multiple linear regression to change in groundwater level, rainfall, and pumpage for the same time period. The regression coefficient of determination was 0.90, and the standard error was 0.24 ft. Monthly change in lake stage for November through May was related to evaporation, rainfall, and groundwater levels with a coefficient of determination of 0.67 and a standard error of 0.14 ft. (USGS)