The vast majority of theses in this collection are open access and freely available. There are a small number of theses that have access restricted to the WWU campus. For off-campus access to a thesis labeled "Campus Only Access," please log in here with your WWU universal ID, or talk to your librarian about requesting the restricted thesis through interlibrary loan.

Date of Award

Spring 2024

Document Type

Masters Thesis

Department or Program Affiliation


Degree Name

Master of Science (MS)



First Advisor

Hooper, David U., 1961-

Second Advisor

Olson, M. Brady (Michael Brady)

Third Advisor

Compton, Jana E.

Fourth Advisor

Behling, Shawn


The eutrophication of fresh and coastal waters is a growing global concern. Agricultural best management practices (BMPs) mitigate nutrient pollution, but their effectiveness at the watershed scale is often untested, creating uncertainty around which practices should be prioritized. In the Salish Sea, on the Pacific coast of Washington State and British Columbia, seasonal hypoxia threatens food webs, already-impacted salmon populations, and tribal fishing rights. To what extent can different BMPs alleviate N export in watersheds with heavy agricultural influence? We used the InVEST® Nutrient Delivery Ratio (NDR) model to estimate reductions in N export under BMP scenarios in the transboundary (US and Canada) Nooksack River watershed. In the process, we evaluated several aspects of NDR relevant to assessing watershed nutrient retention, including the addition of a groundwater component. We then calibrated and validated NDR against measured annual fluxes of total N from the whole Nooksack River watershed and multiple subwatersheds. We used the validated model to explore reductions in N export possible from factorial combinations of BMPs: riparian restoration on all waterways, 20% lower agricultural N input, and improving crop nitrogen use efficiency (NUE) to 75%. NDR estimated that lowering agricultural N input was the most effective BMP, followed by riparian restoration and improved NUE, with reductions in anthropogenic export by 22%, 17%, and 14%, respectively. Combining all BMPs yielded a 44% reduction in anthropogenic export from 2,458 to 1,725 (Mg N yr-1), though total N export was still over 2-fold greater than pre-industrial fluxes. Our results suggest that combining management practices is needed to effectively reduce N export from watersheds with heavy agricultural influence. When using default parameters, NDR chronically underestimated measured export, and we found that including subsurface N flux was essential for model estimates to match measured values of export. NDR did not allow detailed analysis of N retention in different land use or vegetation types. After extensive calibration, however, NDR was reliable in estimating the reduction of N export at the watershed level achievable from multiple BMPs. A 20% reduction in N inputs is estimated to be more effective than restoration of 100% of riparian buffers on waterways to reduce anthropogenic N export from the Nooksack River watershed. Future efforts should examine the cost effectiveness of these different practices and incentives available to implement such reductions.




nitrogen, watershed, transboundary, biogeochemistry, riparian buffers, ecosystem service model, agriculture, best management practices, eutrophication, hypoxia


Western Washington University

OCLC Number


Subject – LCSH

Biogeochemistry--Washington (State)--Nooksack River Watershed; Best management practices (Pollution prevention)--Washington (State)--Nooksack River Watershed; Riparian areas--Management; Eutrophication--Washington (State)--Nooksack River Watershed; Hypoxia (Water)--Washington (State)--Nooksack River Watershed

Geographic Coverage

Nooksack River Watershed (Wash.)




masters theses




Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.

Included in

Biology Commons