Model simplification to simulate groundwater recharge from perched gravel-bed braided rivers

Braided rivers are an important source of groundwater recharge in New Zealand. They consist of multiple temporary channels in a gravel environment and, as a consequence, their interactions with groundwater are complex and highly variable in space and time at different scales. Recently, the gravels of the contemporary braidplain of these rivers have been described and referred to as the ‘braidplain aquifer’. It is within this aquifer that hyporheic and parafluvial flows occur. In these systems, the groundwater recharge to the deeper regional aquifer is actually the water exchange between the braidplain and the regional aquifers. Some of these braided rivers are perched above the regional water level in their main losing section, which means that an unsaturated zone exists between the braidplain and regional aquifer. This complexity calls for the use of 3D fully integrated hydrological models to represent groundwater – surface water interactions in these environments. However, these complex models are very computationally intensive, which strongly limits their use in parameter inference and uncertainty quantification schemes as well as their applicability to regional scale problems.

We present a modelling framework that includes a 3D fully coupled HydroGeoSphere (HGS) model and several 2D cross-sectional HYDRUS-2D models (with 1, 2 and 3 layers). This framework aims at simplifying the model while ensuring the appropriate simulation of the groundwater recharge. We demonstrate our modelling approach on the relatively small Selwyn River. Piezometric data and groundwater recharge estimates derived from satellite photography were available for this river. First, stochastic simulations were performed using the 2D cross-sectional models and compared to observations in order to explore the validity of different subsurface conceptualizations and parameter values. Second, the selected conceptualization and parameter values were used to parameterize the 3D fully coupled HGS model. Third, the groundwater recharge simulated by the 3D and the 2D models were compared. Our results demonstrate that the observations can only be reproduced with a minimum of 3 distinct layers, with a lower permeability layer in the middle. Furthermore, this modelling exercise revealed the primary importance of the width and thickness of the braidplain aquifer as they determine the infiltration front width and the pressure head applied to the braidplain aquifer bottom, respectively. This shows that the properties, dimensions and water level in the subsurface are controlling the groundwater recharge from the perched braided river rather than the river characteristics. Moreover, we show that a 2D cross-sectional model can effectively replace the 3D fully coupled model to simulate groundwater recharge from the perched braided river and that this reduces the model run time by 3 orders of magnitude. Finally, some analytical equations, which can be easily implemented in regional groundwater models, were tested as a further simplification of the 2D model.