Search

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. [ABSTRACT FROM AUTHOR]

While continuous monitoring of streamflow and temperature has been common for some time, there is great potential to expand continuous monitoring to include water quality parameters such as nutrients, turbidity, oxygen, and dissolved organic material. In many systems, distinguishing between watershed and stream ecosystem controls can be challenging. The usefulness of such monitoring can be enhanced by the application of quantitative models to interpret observed patterns in real time. Examples are discussed primarily from the glacial meltwater streams of the McMurdo Dry Valleys, Antarctica. Although the Dry Valley landscape is barren of plants, many streams harbor thriving cyanobacterial mats. Whereas a daily cycle of streamflow is controlled by the surface energy balance on the glaciers and the temporal pattern of solar exposure, the daily signal for biogeochemical processes controlling water quality is generated along the stream. These features result in an excellent outdoor laboratory for investigating fundamental ecosystem process and the development and validation of process-based models. As part of the McMurdo Dry Valleys Long-Term Ecological Research project, we have conducted field experiments and developed coupled biogeochemical transport models for the role of hyporheic exchange in controlling weathering reactions, microbial nitrogen cycling, and stream temperature regulation. We have adapted modeling approaches from sediment transport to understand mobilization of stream biomass with increasing flows. These models help to elucidate the role of in-stream processes in systems where watershed processes also contribute to observed patterns, and may serve as a test case for applying real-time stream ecosystem models. [ABSTRACT FROM AUTHOR]

In natural waters, a variety of solutes would be appropriate for use as tracers in different situations. Comparisons were made of natural tracers at the confluence of an acidic stream (Snake River) with a pristine stream (Deer Creek) near Montezuma, Colorado. Three comparisons were made during late summer at low-flow conditions to identify the naturally occurring conservative solutes in the stream system: (1) a steady injection of chloride and sodium was used as an artificial, conservative baseline tracer to compare the behavior of silica, sulfate, and fluoride, (2) within the mixing zone of the confluence, comparisons were made of the lateral concentration gradients of sodium, calcium, magnesium, silica, sulfate, and fluoride, and (3) the conservative behavior of naturally occurring calcium, magnesium, silica, sulfate, fluoride, and manganese was evaluated from stream samples collected on four different days. On the basis of the results of these comparisons, manganese and sulfate were used as conservative solutes to demonstrate computation of: (1) the reactive losses of other solutes within the confluence and (2) the ratio of chemical inflows from Deer Creek and the Snake River. The approach used in this evaluation can be applied in the study of chemically similar acidic systems. [ABSTRACT FROM AUTHOR]