Concentration-discharge (CQ) relationship bears the convoluted signature of hydrological and biogeochemical processes at the watershed scale. Contrasting CQ behavior have been observed for different chemicals at different watersheds yet the underlying mechanism of such contrasts remains elusive. In this work, a coupled hydrological and biogeochemistry model (bioRT-Flux-PIHM) is developed to understand key controls of CQ behaviors in Coal Creek watershed (Colorado), where data have shown flushing behavior (i.e., concentration rises with increasing discharge) for dissolved organic carbon (DOC) and nutrients (e.g., nitrogen and phosphorus) and dilution behavior (i.e., concentration decreases with increasing discharge) for geogenic species (e.g., Na, Cl, Mg, and Ca). Coal Creek is seasonally snow-covered with the spring snowmelt dominating the annual water balance. The calibration of water model estimates an average of 37% of groundwater (in contrast to shallow soil water) contribution to the stream annually. The calibrated model shows that the relative composition and fluxes of groundwater versus shallow soil water is key to generate contrasting CQ behaviors. In particular, the predominance of organic-rich soil water in the stream during spring melt results in high DOC concentration at high discharge, whereas the organic-poor groundwater under dry conditions lead to low DOC concentration at low discharge. The opposite is true for Na that is originated from the dissolving Na-containing albite in the groundwater. A Monte-Carlo simulation of 500 runs that randomly sample across the uncertainty range of volume fractions of SOC and albite (0.1% to 10%) and across the range of groundwater DOC and Na concentration (0.1 to 10 mg/L). The analysis reveals that at the groundwater level for Coal Creek, flushing behaviors occur at the high concentration ratio of soil water to groundwater (1.8 to 200) while dilution behaviors occur with low concentration ratio range of 0.01 and 0.6. Chemostatic behaviors occurs in between these ranges. These results underscore the importance of flow paths in influencing stream chemistry, and conversely the usefulness of chemistry data in illuminating water flow paths.
Wei Zhi*, Li Li, Jason P Kaye, Wenming Dong, Wendy Brown, Carl I Steefel, Kenneth Hurst Williams (2018): Understanding Contrasting Concentration-discharge (CQ) Behaviors in a Seasonally Snow-covered Watershed. Abstract EP11C-2077 presented at 2018 AGU Fall Meeting, Washington, D.C., 10-14 Dec.
This Paper/Book acknowledges NSF CZO grant support.