Snowmelt-dominated systems are a significant source of water supply for the Western United States. Changes in timing and duration of snowmelt are predicted to continue under climate change; however, the impact this change will have on water resources is not well understood. The ability to compare hydrologic processes across space and time is critical to accurately assess the physical and chemical response of headwater systems to climate change. This dissertation builds upon previous work by using long-term data from two snowmelt dominated catchments to investigate the response of hydrologic processes at different temporal and spatial scales. First, results from an hourly spatially-distributed energy balance snowmelt model were spatially and temporally aggregated to provide daily, catchment-wide snowmelt estimates, which, along with measured discharge and hydrochemical data were used to assess and compare hydrologic processes which occur on an annual scale in two headwater catchments for an eleven year study period. Second, the magnitude and timing of snowmelt, discharge fluxes and hydrochemical data were used to assess and compare inter-annual catchment response in two headwater catchments for an eleven year study period. Third, a pseudoinverse method was developed to compare mineral weathering fluxes in a series of nested sub-catchments over an eleven year study period. Advances from this work include the use of an independently-created energy balance snowmelt model for spatially-distributed hydrologic input for catchment-scale water balance, application of a quantifiable measure of catchment-scale hydrologic flux hysteresis and the development of a method to quantify and compare mineral weathering reactions between source waters across space and time. These methods were utilized to quantify and assess its role of dynamic storage in mitigating climate change response.

Determining where, when, and how subsurface flow affects soil processes and the resulting arrangement of soil development along flow paths is challenging. While hydrologic regime and soil solution acidity are known to influence weathering rates and soil transformation processes, an integrated understanding of these factors together is still lacking. This dissertation explores the effects of subsurface flow on the mobility and distribution of dissolved organic carbon (DOC) and base cations to explain spatial patterns in chemical weathering in a forested headwater catchment. In the first chapter, relationships between hydrologic behavior, fluxes of weathered elements, and the extent of soil elemental loss across landscape positions are established. The second chapter investigates what specific groundwater behavior best explains spatial patterns in solution DOC concentrations during storm events. Lastly, in the third chapter, near surface saturation dynamics are examined to determine when and where DOC mobilization might be enhanced by subsurface flow. Results show that weathering extent was greatest in the upper reaches of the catchment, where O horizon saturation frequency and DOC concentrations are highest. Annual base cation fluxes, which were also greatest in these positions, could indicate where weathering is likely still enhanced. Additionally, while O horizon saturation occurred across the catchment, spatial differences in DOC concentrations suggest there are other sources of acidity to groundwater solutions other than just leaching from O horizons. Shallow organic soils, near bedrock outcrops at the top of the catchment is likely this additional C source, in which drainage water is transported downslope to nearby mineral soils when water tables are high and hydrologic connectivity between soils is increased. Spring and fall storm events were identified as times when groundwater most frequently reached O horizons during the snow-free year, providing insight into the timing of these processes throughout the year. This dissertation highlights how catchment structure mediates DOC flushing events, which in turn, influences the spatial architecture of soil development and chemical weathering processes across the landscape. / Doctor of Philosophy / This dissertation explores how the movement and chemistry of groundwater influences chemical weathering in forest soils. Chemical weathering is an important process in which rocks and soils are broken down into soil nutrients and water-soluble elements. The control of weathering processes by spatial and temporal differences in water behavior across landscapes is not well understood. To address these knowledge gaps, this dissertation measured groundwater fluctuations, solution chemistry, and nutrient fluxes across a mountainous forested landscape. Results from this work found that areas with more frequent flushing of organic matter-rich soil horizons increases groundwater acidity, which can enhance weathering processes. Flushing frequency of organic horizons and soil nutrient fluxes were greatest in the highest elevation portions of the landscape, where soils were most weathered (greatest loss of soil nutrients). This study revealed that flushing events occurred most frequently in spring and fall storm events during the snow-free year, shedding light on the when weathering might be most enhanced. Overall, this research demonstrates that topographic graphic position described differences in catchment groundwater behavior and solution acidity, which contributes to predictable patterns of weathering and soil development across the landscape.