Authors: Peter Regier1, Kyongho Son2, Xingyuan Chen2, Yilin Fang2, Peishi Jiang2, Micah Taylor2, Wilfred M Wollheim3, James Stegen2Affiliations 1Marine and Coastal Research Laboratory, Pacific Northwest National Laboratory, Sequim, WA, United States2Pacific Northwest National Laboratory, Richland, WA, United States3University of New Hampshire, Durham, NH, United StatesAbstract: Hyporheic zones regulate biogeochemical processes in streams and rivers, but high spatiotemporal heterogeneity makes it difficult to predict how these processes scale from individual reaches to river basins. Recent work applying allometric scaling (i.e., power-law relationships between size and function) to river networks provides a new paradigm to develop a scalable understanding of hyporheic biogeochemical processes. We used reach-scale hyporheic aerobic respiration estimates to explore allometric scaling patterns across two basins, and related these patterns to watershed characteristics. We found consistent scaling behaviors at lowest and highest exchange flux (HEF) quantiles, and consistent but HEF-dependent relationships to watershed elevation, precipitation, and land-cover. Our results also suggest variability of hyporheic respiration allometry for middle exchange flux quantiles, and in relation to land-cover. Our findings provide initial evidence that allometric scaling may be useful for predicting hyporheic biogeochemical dynamics across watersheds from reach to basin scales.Scientific Significance Statement: The hyporheic zone is a biogeochemical control point in streams and rivers, and processes like hyporheic respiration are important determinants of how watersheds move and process carbon and nutrients. However, the hyporheic zone is also characterized by high spatial heterogeneity, which makes it difficult to predict how hyporheic functions like respiration change across watersheds from reach to basin scales. This study applies allometric scaling theory, which suggests that function scales in a predictable way with size, to determine if hyporheic respiration scales with watershed area in two basins with contrasting watershed characteristics. We found some consistent patterns between basins that suggest allometric scaling of hyporheic respiration may be a tool for transferable knowledge of hyporheic function between basins, but also note some site-specific relationships may constrain the generalizability of this method to other regions and watersheds.

Denitrification in the hyporheic zone (HZ) of river corridors is crucial to removing excess nitrogen in rivers from anthropogenic activities. However, previous modeling studies of the effectiveness of river corridors in removing excess nitrogen via denitrification were often limited to the reach-scale and low-order stream watersheds. We developed a basin-scale river corridor model for the Columbia River Basin with random forest models to identify the dominant factors associated with the spatial variation of HZ denitrification. Our modeling results suggest that the combined effects of hydrologic variability in reaches and substrate availability influenced by land use are associated with the spatial variability of modeled HZ denitrification at the basin scale. Hyporheic exchange flux can explain most of spatial variation of denitrification amounts in reaches of different sizes, while among the reaches affected by different land uses, the combination of hyporheic exchange flux and stream dissolved organic carbon (DOC) concentration can explain the denitrification differences. Also, we can generalize that the most influential watershed and channel variables controlling denitrification variation are channel morphology parameters (median grain size (D50), stream slope), climate (annual precipitation and evapotranspiration), and stream DOC-related parameters (percent of shrub area). The modeling framework in our study can serve as a valuable tool to identify the limiting factors in removing excess nitrogen pollution in large river basins where direct measurement is often infeasible.

This work documents version two of the Department of Energy’s Energy Exascale Earth System Model (E3SM). E3SM version 2 (E3SMv2) is a significant evolution from its predecessor E3SMv1, resulting in a model that is nearly twice as fast and with a simulated climate that is improved in many metrics. We describe the physical climate model in its lower horizontal resolution configuration consisting of 110 km atmosphere, 165 km land, 0.5° river routing model, and an ocean and sea ice with mesh spacing varying between 60 km in the mid-latitudes and 30 km at the equator and poles. The model performance is evaluated by means of a standard set of Coupled Model Intercomparison Project Phase 6 (CMIP6) Diagnosis, Evaluation, and Characterization of Klima (DECK) simulations augmented with historical simulations as well as simulations to evaluate impacts of different forcing agents. The simulated climate is generally realistic, with notable improvements in clouds and precipitation compared to E3SMv1. E3SMv1 suffered from an excessively high equilibrium climate sensitivity (ECS) of 5.3 K. In E3SMv2, ECS is reduced to 4.0 K which is now within the plausible range based on a recent World Climate Research Programme (WCRP) assessment. However, E3SMv2 significantly underestimates the global mean surface temperature in the second half of the historical record. An analysis of single-forcing simulations indicates that correcting the historical temperature bias would require a substantial reduction in the magnitude of the aerosol-related forcing.

Quantifying the multiscale feedback between hydrodynamics and biogeochemistry is key to reliable modeling of river corridor systems. However, accurate and efficient hydrodynamics models over large spatiotemporal scales have not yet been established due to limited surveys of riverbed roughness and high computational costs. This work presents a semi-automated workflow that combines topographic and water stage surveys, computational fluid dynamics modeling, distributed wall resistance modeling, and high-performance computing to simulate flow in a 30-kilometer-long reach at the Columbia River during 2011-2019. The results show that this workflow enables a high accuracy in modeling water stage at all seven survey locations during calibration (1 month) and validation (65 months) periods. It also enables a high computational efficiency to model the streamflow during a 58-month solution-time within less than a 6-day wall-clock-time with mesh number, time step, and CPU hours of about 1.2 million, 3 seconds, and 1.1 million hours, respectively. Using the well-validated results, we show that riverbed dynamic pressure is randomly distributed over all spatiotemporal scales with its cross-sectional average values approximately quantified by a normal distribution with a mean and standard deviation of -0.353 m and 0.0352 m; bed shear stress is affected by flowrate and large- and small-scale topographic features with cross-sectional maximum values following a smooth but asymmetric distribution with 90% of its value falling between 5 Pa and 35 Pa; and hydrostatic pressure is influenced by flowrate and large-scale topographic features with cross-sectional maximum values quantified by a discontinuous and skewed distribution determined by streamwise topographic variations.

Quantifying hydrologic exchange fluxes (HEF) and subsurface water residence times (RT) are important for managing the water quality and ecosystem health in dynamic river corridor systems. Laboratory-scale experiments and models have shown that hydrodynamic pressure variations on the riverbed induced by dynamic river flows can strongly impact HEFs and RTs. In this study, the impacts of hydrodynamic pressure on HEFs and RT for a 30 km section of the Columbia River in Washington State over a three-year period were quantitatively evaluated using a coupled transient three-dimensional (3D) multi-phase surface and subsurface water flow transport approach. Based on comparisons between model simulations with and without considering hydrodynamic pressure, we found that hydrodynamic pressure increase the net HEFs by 7% of flowing into river from subsurface domain, and also leads to a reduction of the area with long RT, and increase of area with short RT.

Quantifying hydrologic exchange fluxes (HEF) and subsurface water residence times (RT) are important for managing the water quality and ecosystem health in dynamic river corridor systems. Because of limited field accessibility, numerical models typically have been used to quantify HEFs and RTs. These models, however, usually are limited to local-scale bedforms and are based on the assumption that hydrostatic pressure drives surface and subsurface water exchanges. Previous laboratory-scale experiments and models have shown that hydrodynamic pressure variations on the riverbed induced by dynamic river flows can strongly impact HEFs and RTs. In this study, the impacts of hydrodynamic pressure on HEFs and RT for a 30 km section of the Columbia River in Washington State over a three-year period were quantitatively evaluated using a coupled transient three-dimensional (3D) multi-phase surface and subsurface water flow transport approach. Based on comparisons of the model simulations with a reference case that considered only hydrostatic pressure, we found that hydrodynamic pressure significantly enhances upwelling and downwelling HEFs by 2 and 1.7 times, respectively, in the investigated river section and also leads to a reduction of the most likely RT by one order of magnitude (i.e., from hundreds of days to tens of days).