Abstract
A groundwater condition metric is presented and used to evaluate hydrologic changes in a regional population of wetlands in and around municipal well fields with large groundwater withdrawals. The approach compares a 26-year, monthly time series of groundwater potentiometric surfaces to light detection and ranging (LiDAR) land-surface elevations at 10,516 wetlands in a 1505-km2 area. Elevation differences between the potentiometric surface and the wetland land surface provide flow direction (upward or downward) and a proxy for vertical hydraulic-head difference in Darcy’s groundwater flow equation. The resulting metric quantifies the groundwater condition at a wetland through time as the potential for groundwater to discharge upward into a wetland or for water in a wetland to leak downward to recharge the underlying aquifer. The potential for wetland leakage across the regional wetland population decreased by 33% in the 13 years after cutbacks in groundwater withdrawals (2003 – 2015) compared to years before cutbacks (1990 – 2002). Inside well-field properties, wetland leakage potential decreased by 24%. In the wet season month of September, wetlands with the potential to receive groundwater discharge increased to 21.6% of the regional population after cutbacks compared to 13.3% before cutbacks. When mapped across regional drainage basins, discharging wetlands formed spatial connections, suggesting they play a critical role in generating streamflow.
Highlights
In Florida, protecting the ecology and hydrology of wetlands is a priority of water managers, and hundreds of freshwater wetlands are directly monitored in cooperation with federal, state, and local governmental agencies
In the wet season month of September, wetlands with the potential to receive groundwater discharge increased to 21.6% of the regional population after cutbacks compared to 13.3% before cutbacks
Monthly-average potentiometric surfaces were compared to wetland land-surface elevations derived from a gridded light detection and ranging (LiDAR) product provided by the Southwest Florida Water Management District
Summary
In Florida, protecting the ecology and hydrology of wetlands is a priority of water managers (see Florida Statute 373.414), and hundreds of freshwater wetlands are directly monitored in cooperation with federal, state, and local governmental agencies. A hundred thousand geographically isolated palustrine wetlands occur in the mid-peninsula of Florida alone (USFWS 2017). Most of these wetlands are small yet combined their area is comparable in size to the Everglades of South Florida (Haag and Lee 2010). Their scattered geographic distribution is key to another aspect of their importance: palustrine wetlands are the headwaters to most streams in central Florida (Ewel and Odum 1984; Haag and Lee 2010). The practical limits to monitoring a population of this magnitude requires that spatially-distributed hydrologic data, based on empirical measurements attributable to monitored and unmonitored wetlands alike, be used to assess the effects of groundwater withdrawals, climate, and land use on wetlands and streams in a region
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