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Environmental Significace of the Groundwater-Surface Water Interaction Zone
System Models

System Models

Objectives and Scope:

  • Quantify the cumulative effects of mechanistic river corridor HEFs and biogeochemistry (RC-B and RC-C) on large-scale nutrient cycling, water quality (including temperature), contaminant mobility, and land surface fluxes.
  • Identify the critical scale-dependent process couplings that govern the translation of hydromorphic-scale mechanistic processes to system-scale outcomes. Quantify those process couplings in terms of reduced-order models applicable at system scales.
  • Advance the scientific basis for river-network-scale river corridor modeling by replacing empirical formulations in current state-of-art models with mechanistically-based reduced-order models and parameterizations.
  • Incorporate the new river-network-scale river corridor model into a prototype watershed-scale modeling framework that will have broad applicability to both scientific investigations of the cumulative effects of river corridor hydrobiogeochemistry on watershed function and applied predictions of watershed system response to environmental perturbations and human controls.

The System Models Campaign will reveal the cumulative effects of relatively small-scale governing processes and their appropriate representation in larger-scale models. Numerical experimentation using linked models at multiple scales and levels of process fidelity, integrated with experiments and observations, can shed light on the manner in which emergent system functions arise from interactions of complex hydrobiogeochemical processes. This campaign aims to use this approach to assess the importance of river corridor processes in hydrologic exchange flow, heat transport, contaminant mobilization and transport, and C and N transformations and their gaseous emissions at the land surface within a river reach or watershed. To enable this, we will develop a first-of-kind river-network model of hydrologic exchange flows (HEFs) and associated biogeochemical processes guided by high-resolution mechanistic model simulations at the hydromorphic unit scale (Mechanistic Models Campaign). This new river corridor model will be coupled with other existing watershed model components to create a prototype watershed-scale modeling framework that directly accounts for river corridor hydrobiogeochemistry in predictions of watershed-scale nutrient processing, water quality, and river ecology. A critical aspect of the coupling is implementation and testing of alternative reduced-order modeling approaches to determine the appropriate level of complexity for simulating watershed-scale complex system behavior.

Our new river corridor model will incorporate elements of the NEXSS modeling framework developed by our university collaborator (Prof. Jesus Gomez-Velez of New Mexico Tech) and of transient storage models (TSMs) commonly used to model effects of HEFs. However, by developing linkages between these phenomenological model approaches and mechanistic models at smaller scales (Mechanistic Models Campaign), we aim to create a framework that represents both small-scale processes and large-scale cumulative effects. These linkages will be enabled through implementation of models at multiple scales and iterative numerical experimentation validated against field observations. Comparison of model results among these scales will lead to identification of critical scale-dependent process couplings and guide development of reduced-order models and parameterizations. The four model domains to be considered are shown in the table below:

Model Domain Primary Emphasis Scientific Contribution
Hydromorphic Unit Mechanistic Understanding Understand influence of hydromorphic structure and variable discharge on HEF and biogeochemistry
Reach Resolution, Boundary Conditions, Coupling to Land Surface Process-resolved reach-scale model to formulate and evaluate reduced-order models; understand influence of HEF on terrestrial processes
River Network Reduced-Order Model of River Corridor Processes Identify and quantify critical scale-dependent process couplings
Watershed Integrate Surface-Subsurface Interactions with River Corridor Quantify cumulative effects of river corridor HEF and biogeochemistry on watershed function

The new river corridor model will be coupled with other existing watershed component models, modified as necessary to accommodate the coupling. Candidate models are the Soil and Water Assessment Tool (SWAT) river routing module (referred to here as SWATR) for river routing and modeling effects of impoundments, PFLOTRAN for groundwater flow and reactive transport, and CLM for land surface and vegetation modeling. Selected linkages among these codes are being implemented and tested, laying the foundation for a fully integrated groundwater-surface water-land surface watershed model to achieve the long-term project vision.

System Models efforts are not only closely linked with the high-resolution Mechanistic Models Campaign, but will also leverage reach-scale data collected by the Process Studies Campaign (i.e., spatially distributed carbon stocks to drive model simulations and HEFs quantified at select locations for model validation). Our research team will collaborate with the DOE IM3 SFA and ACME projects to leverage their developments in watershed-scale modeling with reservoir impoundments and maintain interoperability of software components. The design of the new river corridor module and methods for coupling with other model components will be developed in collaboration with the Interoperable Design of Extreme-scale Application Software (IDEAS) project to promote interoperable software design that will maximize its transferability. To ensure robustness of the model development and application process, we will also leverage datasets and modeling tools generated by efforts funded by other agencies, such as the United States Geological Survey (USGS) National Water-Quality Assessment Program (NAWQA) and NOAA National Water Model (NWM), to augment existing and ongoing data collection along the Hanford Reach.

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