USE OF WEIGHTED REGRESSIONS ON TIME, DISCHARGE, AND SEASON TO ELUCIDATE THE ROLE OF DIFFUSION, PHENOLOGY, AND COLLOIDS IN THE WEATHERING OF A SUB-ALPINE WATERSHED

The goal of the Clean Water Act is to assure the protection and propagation of a balanced indigenous population of fish, shellfish, and wildlife in the nation’s waters. States are directed to determine total maximum daily load of constituents that should not be exceeded if this goal is to be attained. Existing regression models are limited in their ability to extrapolate relations of season, flow, and water quality into the future and existing mechanistic models assume constant weathering rates. The USGS Water, Energy, and Biogeochemical Model (WEBMOD), developed to overcome existing limitations, was used to simulate daily variations in discharge, temperature, pH, alkalinity, major ion concentrations, and stable isotopes for Andrews Creek that drains 1.74-km² of granitic terrain at the continental divide in Rocky Mountain National Park in Colorado. The creek was sampled almost 900 times for physical and chemical parameters from 1992 through 2012. As a measure of model performance, weighted regressions on time, discharge, and season were completed for both observed values and values simulated for the same days.

For both observations and simulations, flow-normalized concentrations and flow-normalized fluxes increased from 1991 through 2006 in response to rising temperatures and a significant drought that lasted from 1996 through 2002. We propose that weathering rates increased during the drought as desiccation of the soils enhanced the activity of available oxygen in the unsaturated zone.

Seasonal processes possibly related to phenology and transport of colloids were also revealed. The concentrations of base cations as a function of discharge for observations showed a transition from constant concentrations (unlimited source) in the spring, to decreasing concentration with discharge (source-limited) during peak summer flows, to increasing concentration with discharge (energy-limited) in the fall. We propose that the increased concentration with flow in the fall results from erosion and transport of colloids, and/or the release of ions as soil root tension decreases during senescence. These findings will be used to refine the fundamental processes already included in WEBMOD to produce reliable and robust simulations of the quantity and quality of water affecting the nation’s aquatic ecosystems.