Understanding Hydrological and Biogeochemical Processes in the HJ Andrews Watershed
Open Access
- Author:
- Ramesh, Shreya
- Millennium Scholars Program:
- Environmental Systems Engineering (ENVSE_BS)
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisor:
- Li Li, Thesis Supervisor
- Keywords:
- water quality
modeling - Abstract:
- It has been well-established that mountains are seeing exacerbated rates of warming compared to their lower-elevation counterparts, posing a threat to the billions of people that rely on them as a source of water. However, while shifting hydrology and biodiversity in these regions in response to increasing temperatures have been widely understood, little is known about the implications of Earth’s changing climate on water chemistry, specifically freshwater carbon levels. Here we examine data from a fully harvested catchment (WS01) in the HJ Andrews Experimental Forest, a long-term ecological research (LTER) site located in the Oregon Cascades, in order to determine how fluxes in dissolved organic carbon (DOC) and inorganic carbon (DIC) respond to climactic, hydrologic, and land-use modulations. The watershed-scale reactive transport model HBV-BioRT was used to simulate hydrology as well as the biogeochemical reactions that produce DOC and DIC over a seventeen-year period from 2004 to 2017. Results show that lateral flow was the most dominant flowpath in the catchment in the wet springs and winters, accounting for an inter-annual average of 50.3% of total annual discharge. Groundwater flow was also a large contributor to the streamflow, responsible for 40.3% of annual flow and becoming most prominent during the arid summers as a sole source to the river. The watershed also exhibited concentration-discharge flushing behavior of DOC, or increasing concentrations with higher discharge, and dilution of DIC, or decreasing concentrations with higher discharge. The model revealed that DOC was most reflective of seasonal upper zone hydrological highs and lows, while DIC peaked during the dry summer when baseflows prevailed. These results suggest that due to increased reliance on older water from deeper groundwater supplies as the climate warms and higher surface-level transpiration from younger plants, clear-cut mountainous catchments can expect to see lowered export of DOC from these regions to marine environments, an important carbon sinking process and source to aquatic life. Consequently, higher fluxes of DIC can be expected in the stream which can subsequently be evaded to the atmosphere as CO2. We suggest similar future study on controlled old-growth watersheds in the mountainous Western United States to further characterize the possibly differentiating role of land-use on watershed hydrology and biogeochemistry in response to warming.