Vertical surface water–groundwater exchange processes within a headwater floodplain induced by experimental floods

Abstract

Restoring hydrologic connectivity between channels and floodplains is common practice in stream and river restoration. Floodplain hydrology and hydrogeology impact stream hydraulics, ecology, biogeochemical processing, and pollutant removal, yet rigorous field evaluations of surface water–groundwater exchange within floodplains during overbank floods are rare. We conducted five sets of experimental floods to mimic floodplain reconnection by pumping stream water onto an existing floodplain swale. Floods were conducted throughout the year to capture seasonal variation and each involved two replicate floods on successive days to test the effect of varying antecedent moisture. Water levels and specific conductance were measured in surface water, soil, and groundwater within the floodplain, along with surface flow into and out of the floodplain. Vegetation density varied seasonally and controlled the volume of surface water storage on the floodplain. By contrast, antecedent moisture conditions controlled storage of water in floodplain soils, with drier antecedent moisture conditions leading to increased subsurface storage and slower flood wave propagation across the floodplain surface. The site experienced spatial heterogeneity in vertical connectivity between surface water and groundwater across the floodplain surface, where propagation of hydrostatic pressure, preferential flow, and bulk Darcy flow were all mechanisms that may have occurred during the five floods. Vertical connectivity also increased with time, suggesting higher frequency of floodplain inundation may increase surface water–groundwater exchange across the floodplain surface. Understanding the variability of floodplain impacts on water quality noted in the literature likely requires better accounting for seasonal variations in floodplain vegetation and antecedent moisture as well as heterogeneous exchange flow mechanisms. Copyright © 2016 John Wiley & Sons, Ltd.

Modeling Connectivity of Non‐floodplain Wetlands: Insights, Approaches, and Recommendations

“…However, the WHC is relatively simple and does not take into account local variation in hydrologic storage (e.g., variable upland soil depth and texture; Deemy and Rasmussen ) and fluxes (e.g., preferential flow, Hester et al. ). Thus, site‐specific application of this model would likely be unrepresentative without significant model improvements to allow for such parameterization.…”

“…However, the WHC is also limited because it does not simulate potentially dominant hydrologic processes like preferential flow paths (e.g., Hester et al. ) and spatially variable water storage. Therefore, while McLaughlin et al.…”

“…Finally, because simulations can be based on a simplified, synthetic landscape (i.e., a landscape based on modeler conceptualization), inference can be generally applied to many wetland-rich landscapes such as cypress domes in Florida, Carolina and Delmarva bays in the eastern U.S. coastal plain, and vernal pool systems in California and Texas. However, the WHC is relatively simple and does not take into account local variation in hydrologic storage (e.g., variable upland soil depth and texture; Deemy and Rasmussen 2017) and fluxes (e.g., preferential flow, Hester et al 2016). Thus, site-specific application of this model would likely be unrepresentative without significant model improvements to allow for such parameterization.…”

“…If this modeling exercise had been confined to a specific site and/or time period, the broad inference (i.e., the degree of WHC varies with wetland configuration, soil texture, and climatic regimes) would be lost. However, the WHC is also limited because it does not simulate potentially dominant hydrologic processes like preferential flow paths (e.g., Hester et al 2016) and spatially variable water storage. Therefore, while McLaughlin et al (2014) provide a compelling narrative of wetland functioning and are suggestive of several potentially fruitful areas for further inquiry, it has limited practical application for site-specific management.…”

Prediction and uncertainty analysis of a parsimonious floodplain surface water‐groundwater interaction model

“…Surface water can enter the floodplain laterally across the channel banks or vertically across the floodplain surface through Darcy flow. Consequently, the groundwater level of the floodplain increases in response to increased stream stage [ Hester et al ., ].…”