Hydrologic-geologic threshold effects on groundwater-surface water interaction

Heterogeneity in the hyporheic zone or near-field geology can impart a threshold effect on groundwater-surface water (GW-SW) exchange. Variations in the texture of streambed sediments and lithologic variations in underlying geology are examples of common heterogeneities. Interfaces between textural zones tend to be sharp given the nature of fluvial environments, whereby sediment of widely contrasting sizes can be deposited by relatively rapid changes in surface water velocity during normal versus flood stages. Hydrologic interaction with these heterogeneities leads to distinct types of "behavior" that "switch" when surface-water or groundwater levels rise above or fall below the interface. This threshold effect, which causes different rates and volumes of GW-SW exchange, is often manifest as different rates of rise or recession in groundwater levels. A simple dynamic water-balance (linear reservoir) model was developed to investigate threshold effects at a location on the Sunflower River in the lower Mississippi River valley. Several conceptual models, each of which consists of a perched aquifer that receives recharge from the riverbank and loses water to an underlying regional aquifer, were tested: homogenous riverbank properties, different upper and lower riverbank properties, and different upper- and lower-layer aquifer properties. Models were run using a 382-day period of hourly stream-gauge measurements and calibrated to corresponding measurements in a nearby well. Threshold effects in observed data were apparent, with periods of lower rates of groundwater rise and recession during generally low river stage switching to periods of higher rates of groundwater rise and recession during high river-stage and groundwater-level events. All models matched observed groundwater levels reasonably well, with a maximum root-mean-square error of 0.006 m. However, the heterogeneous models matched high-stage events substantially better than the homogeneous model, with the best performance by the model incorporating threshold effects controlled by both riverbank and aquifer properties. These results illustrate the importance of considering threshold effects on GW-SW interactions and the efficacy of relatively simple models that incorporate salient hydrologic processes.