Model developed and documented in: O’Reilly, A.M., Holt, R.M., Davidson, G.R., Patton, A., Rigby, J.R., 2020. A dynamic water-balance/nonlinear-reservoir model of a perched phreatic aquifer–river system with hydrogeologic threshold effects: Water Resources Research 56(6): e2019WR025382. https://doi.org/10.1029/2019WR025382

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 riverbed sediments and lithologic variations in adjacent and underlying geology are examples of common heterogeneities. 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 of the layers of differing lithology. A dynamic water-balance/nonlinear-reservoir model incorporating threshold effects was developed for a perched phreatic aquifer–river system. Four conceptualizations of the system were modeled, each of which simulates a perched aquifer as a dynamical system that receives recharge from the riverbank and loses water to an underlying regional aquifer, using combinations of zero, one, or two thresholds representing layered heterogeneity in riverbank and/or aquifer lithology. Application of the model code was demonstrated at a location in the Lower Mississippi River Valley, USA. Models were run using hourly river-gage measurements, calibrated to a 382-day period of corresponding measurements in a nearby well, and further assessed for a 3.5-year period representing varied hydrologic conditions. The best performance was demonstrated by the model incorporating threshold effects, which elucidated four modes of GW-SW system behavior controlled by both riverbank (riverbed hydraulic conductivity) and aquifer (hydraulic conductivity and storage coefficient) properties. The dynamical system modeling approach incorporates the salient hydrologic processes of a GW-SW system with layered heterogeneity. Based upon fundamental mass-conservation concepts, the simple dynamic water-balance/linear-reservoir model has broad applicability to many hydrogeologic settings.