HYDROLOGICAL MIXING IN THE NEAR-STREAM ZONE

Quantifying the processes by which water and conservative tracers are transformed from rainfall into streamflow is a key to understanding biogeochemical cycling, pollutant movement, and aquatic ecosystem functioning in headwater catchments. The saturated region immediately surrounding the stream channel has been characterized as a zone of active mixing. Simple two-component mixing models, widely used in catchment hydrology, often neglect such mixing. An extension of these simple models to account for riparian-zone mixing can be used to explore the implications attending such mixing zones. If chemically distinct ‘‘hillslope'' and ‘‘groundwater'' runoff components are routed through a near-stream reservoir, representing the channel and surrounding alluvium, to produce streamflow, the resulting hydrological-chemical signatures differ from those produced by conservative mixing of two reservoirs. For physically plausible near-stream zones, groundwater contributions estimated from hydrograph separation on streamflow ranged from 45 to 80 percent larger than the true inflow values on the rising limb, and 19 to 57 percent larger at peak flow. This simple routing of hillslope and groundwater components through a mixing reservoir alters the travel-time distribution of a chemical constituent and illustrates how inferences that ignore near-stream mixing can be erroneous. Another observation from measured stream chemical compositions that confounds simple mixing models is power-law scaling -- the form of the decrease in the variance spectrum as frequency increases. We also have investigated a two-component hydrology model coupled with a mixed linear reservoir with the addition of a “dead storage” volume. Preliminary results indicate that the power spectrum of simulated data for a small catchment in the Blue Ridge Mountains of Virginia exhibits very similar scaling as do the measured data.