A conceptual framework for incorporating surface–groundwater interactions into a river operation–planning model

Abstract

Groundwater discharge constitutes a significant proportion of the total flow volume in most rivers. The exchange flux between surface and groundwater greatly impacts the surface as well as the groundwater balance with serious implications on ecosystem health especially during low flow conditions. There is a move towards conjunctive river–aquifer management with the integration of surface–groundwater exchange fluxes into surface and groundwater models to manage water as a single resource. Groundwater–Surface water (GW–SW) exchange fluxes are seldom integrated into river operation and planning models. The time lags associated with the impacts of groundwater processes on nearby rivers can greatly compromise the forecasting capacity of river models especially during low flow conditions. This paper presents a conceptual framework for integrating GW–SW exchange fluxes into the new generation river operation–planning model ‘Source Integrated Modelling System’. The proposed GW–SW Link Module adopts a simple pragmatic approach for estimating the exchange fluxes between a river reach and the underlying aquifer using explicit analytical solutions. This flux becomes an inflow/outflow to that river reach and forms part of the routing and calibration processes. The exchange flux comprises four components: (1) natural exchange flux resulting from river stage fluctuations during low flow conditions, within bank and overbank fluctuations; (2) flux due to groundwater extraction; (3) flux due to changes in aquifer recharge; and (4) flux due to changes in evapotranspiration. The sum of those components during every time step dictates whether the river loses water to or gains water from the aquifer. The proposed analytical solutions were found to provide flux predictions that agree favourably with those derived from a numerical groundwater model. Recognising that the simplifying assumptions that underpin the explicit analytical solution are likely to be violated in the natural world, a suite of criteria was recommended for their use under many violating conditions related to boundary conditions, head gradients, and aquifer heterogeneity. Low flow indices were adopted to demonstrate the critical role of GW–SW exchange flux when predicting river low flows. Explicit accounting of GW–SW interactions into river operation and planning models greatly enhances their forecasting capacity during low flow conditions.