Modeling the hydrology and hydrochemistry of the Boulder Creek Watershed
PhD: University of Colorado Boulder, 2018.
Mountains in the western United States act as significant water storage towers, but mountain streamflow systems are poorly understood due to the harsh environment, variable climatic conditions, and difficult accessibility. As a result, this study seeks to constrain streamflow recharge sources and future streamflow conditions using a combination of hydrochemical analyses and hydrologic models in the 264 km2 Boulder Creek Watershed. The results of end-member mixing analysis (EMMA) demonstrated that on average, snowmelt and rain water from the subalpine zone contributed 54% and 22% of streamflow, while groundwater from the upper montane zone contributed the remaining 24% of the annual streamflow, respectively. Snowmelt from the subalpine zone was thus the dominant contributor to streamflow. A linear aggregation of EMMA results from the headwater catchments showed that streamflow source waters at the watershed outlet can be predicted from representative headwater catchments within the larger watershed
Assessing streamflow conditions under future climate scenarios is essential for ascertaining the long-term supply of water to the public and evaluating the aquatic health of watershed streams. The application of the Variable Infiltration Capacity hydrologic model shows that streamflow will increase in spring but decrease in summer in the Boulder Creek Watershed under a projected climate warming scenario. However, analyses at two different spatial resolutions suggested different trends of changes in annual streamflow. In the sub-basins of the Upper Colorado River Basin (UCRB), seasonal changes showed a similar trend but different magnitudes, which may be a result of topographical, climatic, and land cover heterogeneity. In the more humid basins, such as the Colorado headwaters and Gunnison basins in the eastern part of UCRB, annual streamflow is predicted to increase. In the more arid basins, such as the San Juan basin in the southern part of UCRB, annual streamflow is predicted to decrease. Overall, climatic inputs and model initializations had a greater influence on model results than parameter uncertainty. The results advance our understanding of streamflow source waters in complex mountain terrain, which necessitate water resources allocation and management under future climate scenarios.