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Potential Climate-Induced Runoff Changes and Associated Uncertainty in Four Pacific Northwest Estuaries
Citation:
Steele, M., H. Chang, D. Reusser, C. Brown, AND I. Jung. Potential Climate-Induced Runoff Changes and Associated Uncertainty in Four Pacific Northwest Estuaries. U.S. Geological Survey, Reston, VA, EPA/600/R/12/634, 2012.
Impact/Purpose:
As part of a larger investigation into potential impacts of climate change on estuarine habitats in the Pacific Northwest (PNW), we estimated changes in freshwater inputs into four estuaries. These were the Coquille River estuary, the South Slough of Coos Bay, and the Yaquina Bay in Oregon, as well as the Willapa Bay in Washington. We used the U.S. Geological Survey’s Precipitation Runoff Modeling System (PRMS) to model watershed hydrological processes under current and future climatic conditions. The trends with the most agreement among climate models and among basins were an increase in autumn average monthly flows, especially in October and November, decreases in summer monthly average flow, and an increase in the top 5% of flow. Our results improve our understanding of how climate change may affect the saltwater/freshwater balance in PNW estuaries, with implications for their sensitive ecosystems.
Description:
As part of a larger investigation into potential impacts of climate change on estuarine habitats in the Pacific Northwest (PNW), we estimated changes in freshwater inputs into four estuaries. These were the Coquille River estuary, the South Slough of Coos Bay, and the Yaquina Bay in Oregon, as well as the Willapa Bay in Washington. We used the U.S. Geological Survey’s Precipitation Runoff Modeling System (PRMS) to model watershed hydrological processes under current and future climatic conditions. All modeled basins are located in rainfall-dominated coastal areas with relatively insignificant baseflow inputs, and their sizes vary from 74.3 km2 to 2,747.6 km2. This allowed us to explore possible shifts in coastal hydrologic regimes at a range of spatial scales. The basins also vary in mean elevation, ranging from 147 m in the Willapa to 1,179 m in the Coquille. The latitudes of basin centroids range from 43.037 degrees in the Coquille to 46.629 degrees in the Willapa. We calibrated model parameters using historical climate grid data downscaled at 1/16 degrees by the Climate Impacts Group, and historic runoff from sub-watersheds or neighboring watersheds. Nash Sutcliffe efficiency values for daily flows in calibration basins ranged from 0.71 to 0.89. After calibration, we forced the PRMS models with four North American Regional Climate Change Assessment Program (NARCCAP) climate models: CRCM-CCSM, CRCM-CGM3, HRM3-HADCM3, and RCM3-CGCM. These are global climate models (GCMs) downscaled with regional climate models that are embedded within the GCMs, and all use the A2 carbon emission scenario developed by the Intergovernmental Panel on Climate Change. With these climate forcing outputs, we derived the change in flow from the period around the 1980s (1971-1995) to that around the 2050s (2041 to 2065). Specifically, we calculated percent change in average monthly flow rate, coefficient of variation, top 5% of flow, and 7-day low flow. The trends with the most agreement among climate models and among basins were an increase in autumn average monthly flows, especially in October and November, decreases in summer monthly average flow, and an increase in the top 5% of flow. As an additional analysis, we estimated variance in PRMS outputs due to parameter uncertainty and the choice of climate model using Latin hypercube sampling. This showed that PRMS low flow simulations are more uncertain than medium or high flows, and that variation among climate models was a much larger source of uncertainty than the hydrological model parameters. Our results, presented in full in this report, improve our understanding of how climate change may affect the saltwater/freshwater balance in PNW estuaries, with implications for their sensitive ecosystems.
