The Science

The contribution of groundwater to streams emanating from high-elevation, snow-dominated basins of large topographic relief has not been well studied or understood. Now, a research team has examined the coupling of topography, vegetation, and snow accumulation on a seasonal basis in an upper Colorado River basin to develop seasonal stream concentration-discharge (C-Q) relationships. The method is applied across scale and within topographically complex, snow-dominated basins. First-order controls on seasonal streamflow generation are isolated and hydrochemical conceptual model development is initiated.

The Impact

The role of groundwater contribution to snow-dominated, low-order streams residing in basins of large topographic relief was found to be significant; with recharge increasing in the upper subalpine where maximum snow accumulation was coincident with reduced conifer cover and lower canopy densities. Error in estimated stream concentrations was attributed to differences in water partitioning, source rock, seasonal shifts in flow paths, and sulfate reduction in floodplain sediments.

Summary

To isolate first-order controls on seasonal streamflow generation within highly heterogeneous, snow-dominated basins of the Colorado River, a team of researchers from the Desert Research Institute, the Rocky Mountain Biological Laboratory, and the Lawrence Berkeley National Laboratory developed a multivariate statistical approach, called end-member mixing analysis (EMMA), that uses a suite of daily chemical and isotopic observations. Models of the mixing of groundwater and surface water were developed across 11 nested basins (0.4 km² to 85 km²) spanning a gradient of climatological, physical, and geological characteristics. Hydrograph separation using rain, snow, and groundwater as end-members indicated that seasonal contributions of groundwater to streams was significant. Mean annual groundwater flux ranged from 12% to 33%, while maximum groundwater contributions of 17% to 50% occurred during baseflow. Groundwater recharge was found to increase in basins of high relief and within the upper subalpine, where maximum snow accumulation was coincident with reduced conifer cover and lower canopy densities. The mixing model developed for the furthest downstream site was not transferable to upstream basins. When attempted, the resulting error in predicted stream concentrations in the upstream basins pointed toward weathering reactions as a function of source rock and seasonal shifts in flow path as the most likely cause. Additionally, the potential for microbial sulfate reduction in floodplain sediments along a low-gradient, meandering portion of the river was found to be sufficient to modify hillslope contributions and alter mixing ratios in the analysis. Soil flushing in response to snowmelt was not included as an end-member but was identified as an important mechanism for release of solutes from these mountainous watersheds.

Principal Investigator

Rosemary Carroll
Desert Research Institute
rosemary.carroll@dri.edu

Program Manager

Paul Bayer
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
paul.bayer@science.doe.gov

Funding

Work was supported by the Lawrence Berkeley National Laboratory’s (LBNL) Watershed Function Scientific Focus Area (WFSFA) through the Office of Biological and Environmental Research within the U.S. Department of Energy Office of Science under contract DE-AC02-05CH11231.

Related Links

• End-Member Mixing Analysis Data Package for the East River Watershed

References