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Impacts of climate change on stream hydrologic and geochemical fluxes
Warix, Sara
Warix, Sara
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2024
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Abstract
In the western U.S. climate change is causing more precipitation to fall as rain rather than snow. Precipitation shifts are decreasing total groundwater recharge, ultimately changing the way that groundwater is stored and transported before it contributes to streams. However, hydrologists don’t currently have the data needed to constrain or model how subsurface flowpaths are changing, thus making it difficult to predict future streamflow and chemistry. Characterizing changing flow and chemistry in headwater streams is critical due to their direct impact on downstream municipal, industrial, and agricultural water supplies. In this dissertation, I combine stream discharge and chemistry observations from to make predictions about how climate change is going to impact future stream drying patterns and stream chemistry. From my field-based work, I show that 1) stream drying is most probable in steep areas with high hydraulic conductivity, and 2) stream chemistry is relatively constant despite variable hydrology as it is primarily sourced by old groundwater at geochemical equilibrium. Conclusions from this catchment-scale work highlight the importance of groundwater storage in controlling streamflow, as well as a need for improved quantification of stream drying impacts on larger downstream water resources. I then build upon conclusions about how climate is altering stream hydrology and chemistry at the catchment scale by evaluating historic trends in stream discharge and chemistry across 35 watersheds throughout the United States over 20+ years. While historic trends in stream discharge are correlated with changes in precipitation, trends in stream chemistry do not exhibit a clear relation with climate. I show that stream chemistry is not changing uniformly through space, or for a single solute. Rather, changes in stream chemistry over the past 20+ years vary through space and for different solutes and are controlled by a combination of 1) climatic and hydrologic change; 2) changes in anthropogenic solute fluxes; and 3) biologic solute buffering. Conclusions from this work highlight the challenges in predicting how climate change will alter stream chemistry because of the compounding nature of geochemical reaction networks.
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