HGS RESEARCH HIGHLIGHT – Assessing the impact of surface water and groundwater interactions for regional-scale simulations of water table elevation
Delottier, H., Schilling, O. S., & Therrien, R. (2024). Assessing the impact of surface water and groundwater interactions for regional-scale simulations of water table elevation. In Journal of Hydrology (Vol. 639, p. 131641). Elsevier BV. https://doi.org/10.1016/j.jhydrol.2024.131641
“In HGS, variably saturated groundwater flow is simulated, which is the most realistic and theoretically rigorous way to simulate the water table.”
Fig. 1. Map of Southern Quebec. The locations of the virtual wells used for the sensitivity analysis are shown. Eight major river catchments are in the region and surface water divides are indicated by thick black lines. Streams and lakes are also shown. Simplified schematic representation of the surface water mass balance module computing potential infiltration fluxes available for the HydroGeoSphere model. η is the melting constant, also known as the day degree factor. DCRT, CRT and FN are the minimum, the mean soil water capacity and the maximum soil infiltration capacity, respectively.
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In this research highlight, researchers Hugo Delottier, Oliver S. Schilling, and René Therrien, conducted an in-depth exploration of how the interaction between surface water (SW) and groundwater (GW) affects the accuracy of regional-scale simulations of water table elevations in Southern Quebec. This investigation was conducted over a vast 36,900 km² regional aquifer system, which is marked by a complex hydrogeological setup. The area of study includes a regional bedrock aquifer that is overlain by discontinuous Quaternary sediments, presenting a challenging environment for accurate hydrological modelling.
The study centered around a comparative analysis of two distinct modelling approaches. The first approach utilized a fully integrated, two-way dynamic feedback system between surface water and groundwater, capturing the intricate and continuous exchange of water between these two domains. The second, more simplified approach, modeled surface water as a boundary condition that does not provide feedback to the groundwater system, representing a more traditional, one-way interaction.
Central to this study was the use of the HydroGeoSphere (HGS) model, which enabled researchers to conduct fully-coupled, spatially distributed, and physics-based simulations of both surface and groundwater flow. By utilizing HGS, the researchers were able to more accurately capture the complex interactions within the aquifer system, including variably saturated groundwater flow. This capability was essential for generating precise simulations of water table elevations across the study area.
The findings of the study were significant. The model that incorporated two-way SW-GW feedback produced simulations of water table elevations that were not only more accurate but also more stable over time. In contrast, the one-way interaction model, which lacked dynamic feedback, tended to predict deeper water tables, particularly in regions with shallow water tables and during periods of low flow. This model's failure to account for the feedback loop between surface water and groundwater led to less accurate predictions, especially under varying hydrological conditions.
Moreover, the inclusion of two-way feedback within the model demonstrated a reduced sensitivity of groundwater levels to changes in hydraulic conductivity. This suggests that models lacking this feedback mechanism might overestimate or underestimate groundwater levels depending on local conditions, leading to potential errors in regional water management strategies.
The study highlights the critical importance of incorporating detailed, two-way SW-GW interactions in regional-scale hydrological models. By neglecting these interactions, modelers risk significant inaccuracies in simulating water table elevations, which could have serious implications for water resource management and decision-making in the region. These findings highlight the need for comprehensive data collection and advanced modelling techniques that can adequately represent the complex interactions within aquifer systems, ensuring more reliable predictions and better-informed management practices.
Fig. 3. Model comparison based on (a-b) regional simulation of water table elevations in February and April (c) Volumetric exchange flux (infiltration/exfiltration) and (d) Mean sensitivity of water table elevations to bedrock hydraulic conductivity (K) for the pilot points distributed over the whole model domain. In a, b and d, the model comparison is presented in a combined fashion by subtracting the results (i.e., simulated water table elevation or sensitivities) of the one-way groundwater model from the results of the two-way integrated model. In c, the model comparison is presented by plotting water balance results for the entire model domain.
Abstract:
Groundwater flow models are increasingly considered for the regional scale simulation of hydraulic heads and water table elevation. In the most complete configuration, models explicitly simulate two-way interactions between surface water (SW) and groundwater (GW) to reproduce and forecast both SW and GW water levels. In most regional scale groundwater models, however, SW-GW interactions are represented by simplified boundary conditions that only allow one-way interaction from SW to GW, neglecting most of the dynamic exchange fluxes between SW and GW. To evaluate the potential consequences of such simplifications on the simulation of regional GW levels, we compare two models on a 36,900 km2 regional aquifer system in Southern Quebec. One model explicitly simulates both SW and GW flow with two-way SW-GW feedback and the other model only simulates GW flow with a surface boundary flux to represent a one-way interaction with the land surface. Both models are developed with the same numerical code to ensure that the only differences are the representation of SW water flow and SW-GW feedback. The one-way model simulates overall deeper water tables because it removes all exfiltrated groundwater from the system once it exits the subsurface, therefore not allowing exfiltrating groundwater to re-infiltrate. This effect is most pronounced in areas where the water table is close to the surface and for low-flow periods. The inclusion of two-way feedback also reduces the sensitivity of simulated GW levels to the magnitude of the hydraulic conductivity. This result highlights the need for additional data on other system states to improve the calibration of regional scale models that explicitly simulate two-way SW-GW interactions.
“HGS can simulate a wide range of different hydrogeological systems, from local to regional scales.”