Research

Photo: Bryn Stewart

Catchments are an ideal spatial scale for studying the hydrological and biogeochemical processes that drive stream chemistry dynamics, as they provide a drainage area and specific set of environmental conditions-- from climate, land cover, and land use to topography, soils, and geology.

Relevant publications:

• Adler et al. (2021). "Drivers of dissolved organic carbon mobilization from forested headwater catchments: A multi scaled approach."

• Stewart et al. (2022). "Streams as mirrors: reading subsurface water chemistry from stream chemistry."

• Ruckhaus et al. (2023). "Disentangling the responses of dissolved organic carbon and nitrogen concentrations to overlapping drivers in a northeastern United States forested watershed."

• Stewart et al. (2024). "Illuminating the “Invisible”: Substantial Deep Respiration and Lateral Export of Dissolved Carbon From Beneath Soil."

Figure: Stewart et al. (2022). "Soil CO2 Controls Short‐Term Variation but Climate Regulates Long‐Term Mean of Riverine Inorganic Carbon."

Because we cannot collect field measurements or model every catchment on Earth, we need ways of generalizing our understanding of catchment processes across a diverse range of landscapes. One approach to doing this is to identify common patterns in stream behavior, such as with concentration-discharge relationships.

Relevant publications:

• Li et al. (2022). "Climate Controls on River Chemistry." 

• Kincaid et al. (2024). "Solute export patterns across the contiguous USA."

• Stewart et al. (2025). "Mechanisms Underlying Near-Universal Export Patterns of Dissolved Carbon From Land to Rivers."

Figure: Stewart et al. (2024). "Illuminating the “Invisible”: Substantial Deep Respiration and Lateral Export of Dissolved Carbon From Beneath Soil."

Reactive transport models are computational models that couple physical, chemical, and biological processes through mathematical equations. For example, the movement of water from precipitation to infiltration to groundwater flow, and reactions in the subsurface like mineral weathering and microbial decomposition.

In my work, I have used a novel, watershed-scale reactive transport model, BioRT-HBV, to simulate key hydrological and biogeochemical processes that control dissolved carbon transformation and transport from the catchment subsurface to streams and rivers.

Relevant publications:

• Zhi et al. (2022). "BioRT-Flux-PIHM v1.0: a biogeochemical reactive transport model at the watershed scale."

• Sadayappan et al. (2024). "BioRT‐HBV 1.0: A biogeochemical reactive transport model at the watershed scale."

• Stewart et al. (2025). "Mechanisms Underlying Near-Universal Export Patterns of Dissolved Carbon From Land to Rivers."

• Wang et al. (2025). "Forest recovery reduces production and rising aridity diminishes export of dissolved inorganic carbon."

Photo: Andrew Cassel

Fieldwork and data compilation are the backbone of Critical Zone science! We need good, foundational data to support analysis and modeling efforts in research.

Importantly, we often lack data at the spatial and temporal scales that enable us to study impacts of long-term change as well as sudden, extreme events on catchment processes and stream flow and chemistry dynamics across a broad range of landscapes. Thus, it is imperative to continue supporting long-term watershed monitoring efforts and expand our collection of data across different spatial and temporal scales.

Relevant publications:

• Adler et al. (2021). "Drivers of dissolved organic carbon mobilization from forested headwater catchments: A multi scaled approach."

• Stewart et al. (2022). "Streams as mirrors: reading subsurface water chemistry from stream chemistry."

• Ruckhaus et al. (2023). "Disentangling the responses of dissolved organic carbon and nitrogen concentrations to overlapping drivers in a northeastern United States forested watershed."

• Kerins et al. (2025): "Controls From Above and Below: Snow, Soil, and Steepness Drive Diverging Trends of Subsurface Water and Streamflow Dynamics"