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Inferring snowpack contributions and the mean elevation of source water to streamflow in the Willamette River, Oregon using water stable isotopes.
Citation:
Brooks, J. Renee, H. Johnson, K. Johnson, R. Comeleo, S. Cline, AND W. Rugh. Inferring snowpack contributions and the mean elevation of source water to streamflow in the Willamette River, Oregon using water stable isotopes. AGU Fall Meeting, Chicago and virtual, IL, December 12 - 16, 2022.
Impact/Purpose:
Snow is an important water resource to rivers and streams in the western USA, and is predicted to decline with climate change. Declines in mountain snowpacks can cause water shortages during the dry western summers when water demand is at its highest. Understanding the degree that snowpack water supplies interact with other climate variables such as temperature and total precipitation will be important to managing streamflow in western rivers. Here, we are partnering with the USGS to identify factors driving water supply within the Willamette River over the past decade.
Description:
Much of the water that people in Western Oregon rely on is derived from the Cascade Range snowpack, and this snowpack is predicted to decrease in coming years with climate change. Snowpack has varied dramatically in the past decade, ranging from a high of 170% of normal in 2010-11 to a low of 11% of normal for 2014-15, the lowest on record. During this timeframe, we have monitored the stable isotopes of water at two locations in the Willamette River twice monthly and mapped the spatial and temporal variation of water isotopes across the basin using streams draining small catchments across a range of elevations. Within the Willamette Basin, stable isotopes of water in precipitation vary strongly with elevation and provide a marker for determining the mean elevation from which water in the Willamette River is derived. In the winter when snow accumulates in the mountains, low-elevation precipitation (primarily rain falling below 1200 m) contributes the largest proportion of streamflow to the Willamette River. During summer when rainfall is scarce and demand for water is the greatest, streamflow in the Willamette River is mainly derived from the snow zone above 1200 m (less than 12% of the basin area). Our data indicate that during summer low-flow, the proportion of streamflow in the Willamette River derived from the snow zone (above 1200 m) in the watershed declined with decreasing snowpack. We use this information to estimate the volumetric reduction in streamflow related to snowpack reduction, both annually and during the dry summer. Over the last decade, average annual streamflow in the Willamette River has decreased, and we explore the role of increased temperature, decreased snowpack, and precipitation trends to understand flow vulnerabilities within the Willamette River. Reconciling human demands with biological instream requirements during summer will be challenging under climatic conditions in which winter snowpack is reduced compared to historical amounts and summer temperatures are reaching record highs.