Thursday, April 12, 2018, 2:00PM - 4:00PM
Mountains in the western United States are critically important to water resources, but mountain streamflow systems remain poorly understood due to complex terrain, variable climatic conditions, and difficult accessibility. As a result, this study seeks to constrain streamflow recharge sources and future streamflow conditions in the 264 km2 Boulder Creek Watershed using a combination of hydrochemical analyses and hydrologic models.
End-Member Mixing Analysis (EMMA) couples hydrochemical tracers and Principal Component Analysis to understand hydrological source waters and flow paths. EMMA results showed that snowmelt from the subalpine zone near treeline contributed approximately half of the annual streamflow in Boulder Creek, while rain water sampled from the subalpine zone and groundwater from the upper montane zone contributed the other half. A scaling exercise indicated that streamflow source waters at the watershed outlet can be predicted from linear aggregation of EMMA results from representative headwater catchments within the larger watershed, which highlights the disproportionate importance of nutrient cycling in headwater systems.
Application of the Variable Infiltration Capacity hydrologic model demonstrated that streamflow in Boulder Creek is likely to increase in spring but decrease in summer under projected climate warming scenarios for the 2050s and 2080s. However, analyses at 1/8 versus 1/16 degree spatial resolutions suggested opposite trends in the magnitude of total annual streamflow. Overall, climatic inputs and model initializations had a greater influence on model results than parameter uncertainty. These results advance our understanding of streamflow source waters in complex mountain terrain, which can be used to determine water resources allocation and management in the future.