HGS RESEARCH HIGHLIGHT – Understanding the vulnerability of surface–groundwater interactions to climate change: insights from a Bavarian Forest headwater catchment
Munir, M. U., Blaurock, K., & Frei, S. (2023). Understanding the vulnerability of surface–groundwater interactions to climate change: insights from a Bavarian Forest headwater catchment. In Environmental Earth Sciences (Vol. 83, Issue 1). Springer Science and Business Media LLC. https://doi.org/10.1007/s12665-023-11314-2
“This study uses an integrated hydrological model (HydroGeoSphere) in combination with 23 downscaled ensemble members from representative concentration pathways (RCPs) 2.6, 4.5 and 8.5 to examine how climate change affects water availability in a headwater catchment under baseflow conditions.”
Fig. 1&2: Topographical map and location of the Grosse Ohe catchment and its sub-catchment Hinterer Schachtenbach, along with the downstream river network (above). Model discretization of the (A) Grosse Ohe catchment and (B) the sub-catchment Hinterer Schachtenbach (below).
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A new article in the journal of Environmental Earth Sciences used HydroGeoSphere to evaluate the impact of climate change on streamflow and water availability within a small (19.2 sqkm) forested catchment in South-East Germany. Climate forecasts in the region predict a significant decrease in precipitation over the coming decades (simulations were run to the year 2099). Based on integrated hydrologic modelling of the catchment, this forecasted decline in precipitation combined with a relatively steady rate of evapotranspiration (compared to the historical period) will result in prolonged drought conditions, which in turn will result in declining groundwater levels, decreased baseflow to the upstream reaches of the stream network. In fact, baseflow discharge in the upper reaches of the stream network are expected to decline by such an extent that the overall flow length of the stream network is expected to decrease by up to 200 m.
This type of flow length analysis is a perfect use-case for the physics-based and fully-integrated groundwater-surface water approach at the heart of HydroGeoSphere. Traditional groundwater models typically ignore dynamics of overland/stream/wetland flow processes; conversely, traditional surface water models commonly lack rigor in the treatment of 3D subsurface variably-saturated flow. Interactions between ground and surface water systems are typically represented via sources/sinks without feedback. This means that things like your streamflow network are typically hard-coded into the framework of a surface water model, which makes it impossible for the model to simulate how dynamic hydrology can impact the distribution of the stream network itself. But with a fully integrated modelling engine like HydroGeoSphere the stream network itself is a product/output of the model. This means that we can simply apply climatological forecasts (i.e. precipitation & temperature) to the model, and see how future conditions will impact stream flow rates, and in fact the flow length of the stream network itself. In conjunction with declining groundwater baseflow to the stream network (and lower flow rates), expected increases in air temperature are predicted to result in increased water temperatures with negative impacts on ecosystem health.
This study also used an interesting approach to calibrating sub-catchment scale models, by taking a nested modelling approach. A larger regional model of the entire Grosse Ohe catchment was originally built for calibration/validation, but to improve computational efficiency the climate change impact analysis was run only in the Hinterer Schachtenbach sub-catchment. Conceptualization of boundary conditions in both models were quite simple, again demonstrating the stengths of a physics-based approach to integrated hydrologic modelling. Boundary nodes for the model were all set as no-flow conditions, with the exception of a critical depth boundary at the location of the catchment outlets. Finally, spatially homogeneous precipitation and evapotranspiration rates were applied to the surface domain.
“To achieve the objective of this study, we employed a nested modelling approach using HGS. This involved developing an HGS model for the entire Grosse Ohe catchment, as well as a separate model for one of its sub-catchments, Hinterer Schachtenbach (Fig. 2). As long-term discharge data were only available for the entire Grosse Ohe catchment, the larger model was used for calibration and validation, while the model for the sub-catchment, was used for investigating the impact of climate change on surface/groundwater interactions. Despite maintaining an accurate representation of surface/groundwater interactions, the sub-catchment model demonstrated greater computational efficiency than the larger catchment model. Due to its significantly lower runtimes, we utilized the sub-catchment model to simulate various long-term climate change scenarios.”
Abstract:
Fig. 7: Projected surface water depths for the stream during baseflow conditions in late summer. Water depths were estimated for September 15th for all RCP ensembles and the reference period, and the blue line represents the mean water depth for the reference period (1992–2018). The grey shaded area indicates the range of variation in water depths for the different RCP scenarios
Headwaters play a crucial role in maintaining forest biodiversity by providing unique habitats and are important for the regulation of water temperature and oxygen levels for downstream river networks. Approximately 90% of the total length of streams globally originate from headwaters and these systems are discussed to be especially vulnerable to impacts of climate change. This study uses an integrated hydrological model (HydroGeoSphere) in combination with 23 downscaled ensemble members from representative concentration pathways (RCPs) 2.6, 4.5 and 8.5 to examine how climate change affects water availability in a headwater catchment under baseflow conditions. The simulations consistently predict increasing water deficits in summer and autumn for both the near (2021–2050) and far future (2071–2099). Annual mean water deficits were estimated to be 4 to 7 times higher than historical levels. This is mainly due to a projected reduction in precipitation inputs of up to – 22%, while AET rates remain similar to those observed during the historical reference period (1992–2018). The declining groundwater storage reserves within the catchment are expected to result in a significant decline in surface water availability during summer and autumn, with a reduction in mean annual stream discharge by up to 34% compared to the reference period. Due to declining groundwater levels, upstream reaches are predicted to become intermittent in summer leading to a reduction of the total stream flow length by up to 200 m. Findings from this study will enhance our understanding of future water availability in headwater systems and may aid in the development of effective management strategies for mitigating local impacts of climate change and preserving these vulnerable ecosystems.