Research Highlight - Model simplification to simulate groundwater recharge from a perched gravel-bed river
Di Ciacca, A., Wilson, S., Durney, P., Stecca, G., & Wöhling, T. (2024). Model simplification to simulate groundwater recharge from a perched gravel-bed river. Journal of Hydrology, 643, 132016. https://doi.org/10.1016/j.jhydrol.2024.132016
“The HydroGeoSphere modelling code used to develop the 3D integrated surface and subsurface model (HGS3D) simulates water flow in both the surface and subsurface domain in a fully coupled fashion. In the surface domain, it solves a 2D depth-averaged diffusion-wave approximation of the Saint Venant equations, while in the subsurface it solves a 3D modified formulation of the Richards’ equation, allowing the simulation of variably saturated flows. The coupling between the surface and subsurface is achieved using the so-called dual node approach.”
Figure 4. Water content fields simulated using the 2D (a) and the 3D (b) models for 18 August 2020.
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This publication co-authored by Antoine Di Ciacca, Scott Wilson, Patrick Durney, Guglielmo Stecca, and Thomas Wöhling, investigates model simplification strategies to simulate groundwater recharge from perched gravel-bed rivers. This study leverages HydroGeoSphere (HGS) as a fully integrated 3D surface–subsurface model, alongside 2D cross-sectional and 1D analytical models, to address long-standing challenges in representing river–aquifer interactions while reducing computational demands.
Traditional groundwater models often represent river recharge using simple conductance functions that assume the riverbed acts as an impeding layer. While efficient, these approaches fail to capture the dynamics of gravel-bed rivers, where permeable sediments act as storage reservoirs and drive complex hyporheic exchanges. By starting with detailed HGS simulations of the Selwyn River in New Zealand, the researchers developed a multi-model framework to progressively simplify the system while preserving realistic groundwater recharge dynamics.
The study showed that HGS captured the full 3D variability of river–aquifer exchanges, including seasonal fluctuations and hyporheic storage. These results were then used to test whether simpler models could reproduce recharge patterns. Findings revealed that a properly parameterized 1D analytical model could replicate groundwater recharge time series with high accuracy when driven by groundwater levels in the braidplain aquifer. By contrast, models relying only on river stage significantly overestimated recharge, underscoring the importance of using aquifer heads rather than surface water levels to represent pressure gradients.
Key findings demonstrated that although HGS remains the most physically complete tool, simplified models validated against HGS outputs can provide efficient and transferable solutions for regional groundwater management. This approach shows how HydroGeoSphere can underpin the development of reduced-complexity models, ensuring that computationally light methods still maintain physical realism.
HydroGeoSphere proved essential in enabling this work by reproducing the coupled surface and subsurface processes that control groundwater recharge from gravel-bed rivers. By serving as the benchmark for simplification, HGS allowed the researchers to demonstrate that efficient analytical formulations can be confidently implemented in regional MODFLOW applications. This research highlights the value of using HGS both as a primary modelling tool and as a foundation for building scalable methods that improve the representation of river recharge in water resource planning.
This work provides critical insights for hydrogeology and groundwater management, showing how advanced, physics-based modelling with HGS can bridge the gap between detailed process representation and practical implementation at regional scales. By combining physical realism with computational efficiency, the study offers a pathway for more sustainable management of groundwater resources in gravel-bed river systems.
Abstract:
Gravel-bed rivers are an important source of groundwater recharge in some regions of the world. Their interactions with groundwater are complex and highly variable in space and time, with considerable water storage in the riverbed sediments. In losing river sections, where most of the groundwater recharge occurs, the river can be separated from the regional groundwater system by an unsaturated zone (i.e., perched). The complexity of groundwater–surface water interactions in these environments calls for the use of 3D fully integrated hydrological models to represent them, but their computational intensity limits their practicality for parameter inference, uncertainty quantification and regional scale problems. On the other hand, the simple groundwater–surface water exchange functions currently implemented in regional scale groundwater models are not suited to represent complex gravel-bed river systems such as braided rivers. There is therefore a need for developing groundwater–surface water exchange functions tailored to gravel-bed rivers that can be used in regional scale models.
To address this issue, we developed a model simplification framework that combines a 3D integrated surface and subsurface hydrological model, a 2D cross-sectional river-aquifer model and a 1D conductance-based analytical model. We aim at broadly simplifying the 3D model while ensuring the appropriate simulation of groundwater recharge. We demonstrate our modelling approach on the Selwyn River (New Zealand) using piezometric data and groundwater recharge estimates derived from field observations and satellite imagery.
Our results indicates that groundwater recharge from this river can be simulated using a simple 1D analytical model, which can easily be implemented in regional groundwater models (e.g., MODFLOW models). However, to represent properly the time variability of groundwater recharge, it is essential to use the groundwater level in the shallow aquifer associated with the river as input to the regional groundwater model. Our approach is generally transferable to other gravel-bed rivers but requires some observations of river losses for proper calibration.