Beyond Data Assimilation: A Hypothesis-Driven Model-Experiment (ModEx) Approach to Predictive Modeling of Sediment Respiration Across the Continental U.S.
Authors
Jianqiu Zheng* (jianqiu.zheng@pnnl.gov), Kenton Rod, Vanessa Garayburu-Caruso, Amy Goldman, James Stegen, Tim Scheibe
Institutions
Pacific Northwest National Laboratory, Richland, WA
URLs
Abstract
Hyporheic zones play a dominant role in the metabolism and biogeochemistry of river corridor systems. Hyporheic sediment respiration exhibits high spatial variability within and across river basins. The variable permeability and diverse exchange mechanisms of hyporheic zones result in heterogeneous conditions, posing a significant challenge for modeling coupled physical and biogeochemical processes. Studies often focus on individual aspects (e.g., physical, chemical, biological) of hyporheic zone dynamics due to data constraints, limiting comprehensive understanding and scaling efforts. The team took a hypothesis-driven approach that integrates theoretical knowledge generation, model development and experimentation to address this challenge. Starting with conceptual models built from known theory and data patterns, researchers identified key hypotheses that were testable with available data. Using a continental-scale dataset from the Worldwide Hydrobiogeochemistry Observation Network for Dynamic River Systems (WHONDRS) consortium, researchers first tested the prediction that sediment respiration rates would decrease as the number of unique organic molecules (i.e., OM molecular richness) increases. The analyses rejected the hypothesis of a direct limitation of respiration by OM molecular richness. Rather, the team found that OM concentration and thermodynamics impose primary constraints on sediment respiration. Researchers further conducted numerical experiments to explore the potential controls using a simple thermodynamic-based kinetic model, where OM thermodynamics, microbial biomass and substrate accessibility were included as key drivers.
Integrating numerical experiments and observational data, it was determined that the OM molecules fueling respiration comprised less than 1% of the measured OM. These results motivate deeper evaluation of the physical characteristics of river sediments that modulate the physical accessibility of OM, such as particle size, specific surface area and density of surface binding sites. The hypothesis-driven approach is a powerful way of integrating models and observations. The continental-scale WHONDRS dataset allowed researchers to challenge and refine existing hypotheses with numerical experiments, and to build geographically transferable predictive models and deeper, process-informed understanding.