Linking river, floodplain, and vadose zone hydrology in a coastal wetland impacted by saltwater intrusion: The Loxahatchee River (Florida, USA)

Floodplain forests provide unique ecological structure and function, which are often degraded or lost when watershed hydrology is modified. Restoration of impacted ecosystems requires an understanding of surface water, groundwater, and vadose (unsaturated) zone hydrology in the floodplain. However, finding direct relationships between basic hydrological inputs and floodplain hydrology is hindered by complex interactions between surface water, groundwater, and atmospheric fluxes in a variably saturated matrix with heterogeneous soils, vegetation, and topography. Soil moisture and porewater salinity are of particular importance for seed germination and seedling survival in systems impacted by saltwater intrusion, but are difficult to monitor and often overlooked. This study contributes to the understanding of floodplain hydrology in one of the last bald cypress (Taxodium distichum [L.] Rich.) floodplain swamps in southeast Florida (USA) by investigating hydrology in the floodplain of the Loxahatchee River, where reduced freshwater flow has led to inadequate hydroperiod and saltwater intrusion into historically freshwater wetlands.;Twenty-four dielectric probes measuring soil moisture and porewater salinity every 30 minutes were installed along two transects---one in an upstream, freshwater location; the other in a downstream tidal area. Data collected over four years quantified the spatial variability and temporal dynamics of vadose zone hydrology, showed that soil moisture can be closely predicted based on river stage and topographic elevation, and helped to explain observed vegetation patterns. Groundwater elevation and salinity were measured in a network of twelve wells along a gradient from fresh to saline conditions. Dynamic Factor Analysis (DFA), a time series dimension reduction technique, was used to model temporal variation in groundwater and soil moisture datasets as linear combinations of common trends (which represent unexplained common variability) and appropriate explanatory variables. The resulting dynamic factor models (DFMs) were useful for assessing the effects of ecosystem restoration and management scenarios on floodplain hydrology. These relationships were coupled with ecological performance measures to predict floodplain vegetation dynamics using a spatially distributed ecohydrological model. This study offers a methodological and analytical framework for floodplain monitoring in locations where restoration success depends on vadose zone hydrology and provides relationships for evaluating proposed management scenarios for the Loxahatchee River.