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Quantitative Assessment of Temperature Sensitivity of the South Fork Nooksack River under Future Climates using QUAL2Kw
Butcher, J. B., M. Faizullabhoy, H. Nicholas, P. Cada, AND J. T. Kennedy. Quantitative Assessment of Temperature Sensitivity of the South Fork Nooksack River under Future Climates using QUAL2Kw. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-14/233, 2016.
The purpose of this report is to provide documentation on the climate change modeling that was initiated as a research pilot for the draft South Fork Nooksack River (SFNR) temperature Total Maximum Daily Load (TMDL) developed by Washington’s Department of Ecology (Ecology). Further, this report discusses and considers the relevant Clean Water Act (CWA) water quality standards developed to protect beneficial uses, including cold water fisheries.
The Total Maximum Daily Load (TMDL) program, established by the Clean Water Act, is used to establish limits on loading of pollutants from point and nonpoint sources necessary to achieve water quality standards. One important use of a temperature TMDL is to allocate thermal loads to achieve water temperature criteria established for the protection of cold water fisheries. The pollutant in this case is thermal load and allocations to reduce the load often involve restoration of stream shading, which reduces the solar input. While many temperature TMDLs have been established, the supporting analyses have generally assumed a stationary climate under which historical data on flow and air temperature can serve as an adequate guide to future conditions. Projected changes in climate over the 21st century contradict this assumption. Air temperature is expected to increase in most parts of the US, accompanied in many areas by seasonal shifts in the timing and amount of precipitation, which in turn will alter stream flow. This study evaluates the implications of climate change for the water temperature TMDL developed for the South Fork Nooksack River in northwest Washington by the Department of Ecology, where multiple water body segments exceed temperature criteria established for the protection of cold water salmonid populations (Ecology, 2016). The purpose of this report is to provide a “companion technical methods manual” as documentation for the draft SFNR temperature TMDL developed by Ecology. The TMDL analyses use Ecology’s QUAL2Kw stream simulation model in conjunction with an analysis of shading to predict the temperature profile during critical conditions of summer low flow and elevated air temperatures. The modeling shows that restoration of system potential vegetation shading would significantly mitigate increasing water temperature. We reran the QUAL2Kw model for future climate conditions (multiple climate models for the 2020s, 2040s, and 2080s) using gridded downscaled climate data and hydrologic model runoff predictions developed by the Climate Impacts Group at the University of Washington to modify the critical conditions inputs using a change factor approach. Results show that the risk of higher water temperature will accelerate over time. Projected increases in heat inputs and lower summer flows associated with a reduction in the storage of winter snowpack combine to exacerbate summer water temperature extremes and may begin to overwhelm the mitigating impact of increased shading by the 2040s, with a high probability of exceeding cited lethal temperature thresholds under low flow critical conditions. We note, however, that the TMDL focuses on extreme conditions (e.g., 7-day low flow with 10-year recurrence), and predictions are more favorable for less extreme flow conditions – for instance, at a 2-year low flow recurrence water temperatures through the 2080s are generally predicted to remain below lethal thresholds with enhanced shading in place. Indeed, the importance of system potential shading is of even greater importance under future climate conditions. Establishing a mature riparian forest canopy can take 100 years, so it is important to begin planting riparian buffers now to reduce the anticipated climate change impacts on water temperature. Protection and restoration of local cold water refuges is another important adaptation strategy to mitigate the effects of climate change on aquatic life during high temperature events.
URLs/Downloads:Charge to Reviewers (PDF, NA pp, 279 KB, about PDF)
Peer Reviewers (PDF, NA pp, 287 KB, about PDF)
Quantitative Assessment of Temperature Sensitivity (PDF, NA pp, 8852 KB, about PDF)
Response to Comments (PDF, NA pp, 491 KB, about PDF)
Peer Review Report (PDF, NA pp, 283 KB, about PDF)