During the week of September 10-17, 2013, about 44 cm of rain fell across Boulder County, Colorado, USA, representing a 1000-year precipitation event. The resultant stream flows corresponded to an extreme event not seen since the historical flood of 1894. For the Boulder Creek Critical Zone Observatory (BcCZO), this event provided an opportunity to study the effect of extreme rainfall on solute concentration-discharge relationships and biogeochemical catchment processes. We observed weathering-derived solutes and dissolved organic carbon (DOC) concentrations at two sites on Boulder Creek following the recession of peak flow. We also isolated four distinct fractions of dissolved organic matter (DOM) for chemical characterization. At the upper site, which represented the forested mountain catchment, the concentrations of lithologic solutes (Ca, Mg, Na, and Si) were greatest at the peak flood and decreased only slightly, in contrast with DOC and K concentrations, which substantially decreased after the first week of flow recession. At the lower site within the urban corridor, all solutes decreased abruptly during this period, with concentrations of lithologic solutes stabilizing or increasing while DOC and K continued to decrease. Additionally, we found significant spatiotemporal trends in the chemical quality of DOM exported, with consistent decreases in DOC over time highlighting the importance of the landscape in terms of depleting a limited reservoir of soluble organic matter. Our results further suggest such extreme events mobilize solutes through waters infiltrating deeper layers of the critical zone, and that this flushing regulates the hysteretic response as well as terrestrial-aquatic biogeochemical linkages.

Concentration-discharge relationships during the recession of an extreme flood in the Boulder Creek Watershed: Patterns of lithologic solute concentrations contrast with decreases in concentration and changes in chemical quality of dissolved organic material

253rd National Meeting (2017)

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