Regime shifts in South Florida groundwater: distinguishing upstream recharge from coastal sea level influences (1997–2022)

Groundwater dynamics in South Florida play an important role in shaping the hydrologic balance of the region. This study investigated long-term trends in groundwater elevation, surface water stages, and their interactions across Water Conservation Areas 3A and 3B, Everglades National Park, coastal watersheds, and Florida Bay. Time series decomposition of long-term groundwater elevation (1997 to 2022) was used to identify trends, seasonal patterns, and anomalies. Analysis of groundwater elevations revealed statistically significant regime shifts defining distinct phases of decrease (2005-2015) and increase (2016-2022) associated with changes in rainfall patterns and water management. Average groundwater elevation has increased in recent years, with upstream areas showing stronger associations with surface water flow volumes, while coastal areas showed stronger correlations with sea level changes. This distinction suggests separate hydrological mechanisms for the observed trends: upstream recharge likely dominates in freshwater marshes, whereas coastal areas are associated with sea level variability. Groundwater levels in coastal areas now remain above the ground surface even during dry periods, associated with external factors such as sea level rise. The relationships between surface water stages and groundwater elevations in most parts of the study area were linear, whereas coastal regions, parts of the Water Conservation Areas 3A and 3B and northern parts of the Taylor Slough were nonlinear. In Florida Bay, sea level variability was a dominant driver of surface water variability (88.3% relative importance), while freshwater delivery through creek discharge (2.2%) and rainfall (2.6%) showed small but measurable contributions, along with groundwater elevation (2.4%) reflecting regional hydrologic conditions.