Groundwater abstraction impacts on spring flow and base flow in the Hillsborough River Basin, Florida, USA

Groundwater abstraction has resulted in spring flow and groundwater base-flow declines in the Hillsborough River system of central Florida, USA. These declines have resulted in reduction of inflows to the Tampa city reservoir as well as likely adverse environmental effects on riverine and estuarine biota. Causes evaluated for the declines include effects of groundwater development, reduced rainfall, and land alterations. The karstic, heterogeneic nature of the area renders groundwater flow modeling an ineffective method for overall evaluation. Therefore, the evaluation of these declines is accomplished through the systematic use of parametric and nonparametric statistical techniques. These techniques include contingency table analysis, linear regression, Kendall-Theil and Mann-Kendall trend analysis, locally weighted regression, Pearson correlation, Kendall-tau correlation, Spearman correlation, runs test, Student’s t test, and the Kruskall-Wallis test. Data evaluated include groundwater withdrawals, rainfall, base flow, streamflow, stream stage, spring flow, and groundwater levels. Additional methods used include double mass analysis, base flow separation, a low-stage trend analysis, data visualization techniques, and water level change maps. The methodical application of these analyses and techniques to the hydrologic and climatic data yields the conclusion that the primary factor causing the spring flow and base-flow declines is lowered groundwater levels caused by over-abstraction.