Terrestrial and aquatic controls on watershed biogeochemistry respond differently to climate change: Walker Branch, TN

Background/Question/Methods

Most long-term watershed monitoring sites focus on catchments with shallow soils, consolidated bedrock, and small groundwater volumes such that hydrologic budgets can be accurately quantified. These characteristics make it difficult to disentangle the terrestrial and aquatic processes that govern biogeochemical cycles by limiting the pathways that water can be routed through watersheds. We analyzed 20 years of weekly streamwater chemistry data for the Walker Branch watershed (TN, USA), a site with exceptionally deep soils, complex soil flowpath structure, and large groundwater volumes that feed perennial springs. 

Results/Conclusions

We show that long-term trends in climate patterns have altered catchment flowpath structure, such that vegetation and surface soils are becoming increasingly hydrologically isolated from the stream channel and that deep groundwater comprises a greater proportion of streamflow. Groundwater is chemically distinct from water that passes through shallow soils and, thus, changing flowpath contributions have resulted in long-term trends in streamwater solute concentrations and fluxes. Groundwater also exhibits little variation in solute concentrations across seasons and years, providing an ideal backdrop against which aquatic processes can be assessed. Aquatic processes in Walker Branch can have important effects on streamwater biogeochemistry during baseflow, but are sensitive to hydrologic disturbance and storm frequency. In addition to the long-term restructuring of soil flowpaths affecting terrestrial controls on biogeochemical patterns, changes in the frequency and intensity of storms have had distinct effects on aquatic controls governing solute concentrations and fluxes. The characteristics of Walker Branch that are generally considered undesirable for constructing water budgets and which set it apart from most other long-term watershed monitoring sites—deep soils with complex flowpath structuring and large groundwater volumes—allow us to disentangle the effects of upland vs. in-stream processes.