Toward catchment hydro‐biogeochemical theories

Abstract

Headwater catchments are the fundamental units that connect the land to the ocean. Hydrological flow and biogeochemical processes are intricately coupled, yet their respective sciences have progressed without much integration. Reaction kinetic theories that prescribe rate dependence on environmental variables (e.g., temperature and water content) have advanced substantially, mostly in well‐mixed reactors, columns, and warming experiments without considering the characteristics of hydrological flow at the catchment scale. These theories have shown significant divergence from observations in natural systems. On the other hand, hydrological theories, including transit time theory, have progressed substantially yet have not been incorporated into understanding reactions at the catchment scale. Here we advocate for the development of integrated hydro‐biogeochemical theories across gradients of climate, vegetation, and geology conditions. The lack of such theories presents barriers for understanding mechanisms and forecasting the future of the Critical Zone under human‐ and climate‐induced perturbations. Although integration has started and co‐located measurements are well under way, tremendous challenges remain. In particular, even in this era of “big data,” we are still limited by data and will need to (1) intensify measurements beyond river channels and characterize the vertical connectivity and broadly the shallow and deep subsurface; (2) expand to older water dating beyond the time scales reflected in stable water isotopes; (3) combine the use of reactive solutes, nonreactive tracers, and isotopes; and (4) augment measurements in environments that are undergoing rapid changes. To develop integrated theories, it is essential to (1) engage models at all stages to develop model‐informed data collection strategies and to maximize data usage; (2) adopt a “simple but not simplistic,” or fit‐for‐purpose approach to include essential processes in process‐based models; (3) blend the use of process‐based and data‐driven models in the framework of “theory‐guided data science.” Within the framework of hypothesis testing, model‐data fusion can advance integrated theories that mechanistically link catchments' internal structures and external drivers to their functioning. It can not only advance the field of hydro‐biogeochemistry, but also enable hind‐ and fore‐casting and serve the society at large. Broadly, future education will need to cultivate thinkers at the intersections of traditional disciplines with hollistic approaches for understanding interacting processes in complex earth systems. This article is categorized under: Engineering Water > Methods

A parsimonious model of longitudinal stream DOC patterns based on groundwater inputs and in-stream uptake

“…As such, the identification and characterization of groundwater chemistry of DRIPs can be used to represent major lateral groundwater inputs to stream channels, and to highlight reactive reaches within stream networks. This study contributes to the greater goal of integrating hydrological and biogeochemical models (Li et al, 2020), explicit consideration of groundwater discharges in quantitative frameworks (Briggs 430 and Hare, 2018), and the spatial assessment of removal mechanisms of DOC in stream networks (Mineau et al, 2016).…”

“…To integrate the supply of terrestrial DOC fluxes and removal in aquatic ecosystems, we need to combine source-transport hydrochemical and in-stream C spiraling frameworks (Li et al, 2020). While there are hydrochemical frameworks that integrate spatial heterogeneity of terrestrial DOC fluxes to streams (Seibert et al, 2009), they are mostly suitable during high flow conditions, when stream DOC dynamics are dominated by source-transport mechanisms, and in-stream uptake processes are less significant (Raymond et al, 2016).…”

Drivers of Dissolved Organic Carbon Mobilization From Forested Headwater Catchments: A Multi Scaled Approach

“…These spatial and temporal gaps currently limit statistical power and the development of a larger consensus on processes, suggesting a need for more long-term monitoring and data fusion/integration efforts as illustrated here. Overall, consistent, extensive, and multiple-perspective monitoring systems are urgently needed to record long-term alteration of water quantity and quality response to climate change and human perturbation efforts (Lovett et al, 2007;Magner and Brooks, 2008;Li et al, 2021;Zhi et al, 2021). In the meantime, we believe that such integrated datasets, novel data science tools, and process investigations will allow the catchment science community to make progress addressing the problem of scale.…”

Comment on hess-2021-16

“…Nevertheless, nitrate concentration and load variability can be predicted at catchment scales when relying on detailed process understanding regarding transport and biogeochemical processing (Alexander et al, 2009;Schlesinger et al, 2006;Wollheim et al, 2008). Moving beyond small scale variability and characterizing nitrate processing at the catchment scale however remains a challenge (McDonnell et al, 2007;Li et al, 2020).Within river reaches and streams, reactive solutes like NO 3 − are affected by complex interactions of physical, biological and chemical processes. Physical transport is driven by local discharge and channel geomorphology and dictates the NO 3 − residence time in a reach, thus influencing the timescales at which biogeochemical processing can take place (Kirchner et al, 2000;Runkel and Bencala, 1995;Zarnetske et al, 2011).…”

“…Nevertheless, nitrate concentration and load variability can be predicted at catchment scales when relying on detailed process understanding regarding transport and biogeochemical processing (Alexander et al, 2009;Schlesinger et al, 2006;Wollheim et al, 2008). Moving beyond small scale variability and characterizing nitrate processing at the catchment scale however remains a challenge (McDonnell et al, 2007;Li et al, 2020).…”