Concentration-discharge relationships during an extreme event: Contrasting behavior of solutes and changes to chemical quality of dissolved organic material in the Boulder Creek Watershed during the September 2013 flood

During the week of 9-15 September 2013, about 44 cm of rain fell across Boulder County, Colorado, USA, representing a very rare precipitation event. The resultant streamflows 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 processes. We measured weathering-derived lithologic solutes (Ca, Mg, Na, K, and Si) and dissolved organic carbon (DOC) concentrations at two sites on Boulder Creek during the recession of peak flow. We also isolated four distinct fractions of dissolved organic matter (DOM) for chemical characterization. At the upper and lower sites, all solutes had their highest concentration at peak flow. At the upper site, which represented a mostly forested catchment, the concentrations of lithologic solutes decreased slightly during flood recession. In contrast, DOC and K concentrations decreased by a factor of three. At the lower site within the urban corridor, concentration of lithologic solutes decreased substantially for a few days before rebounding, whereas the DOC and K concentrations continued to decrease. Additionally, we found spatiotemporal trends in the chemical quality of DOM that were consistent with a limited reservoir of soluble organic matter in surficial soils becoming depleted and deeper layers of the Critical Zone contributing DOM during the flood recession. Overall, these results suggest that despite the extreme flood event, concentration-discharge relationships were similar to typical snowmelt periods in this Rocky Mountain region.