The hydrology of headwater catchments from the Plains to the Continental Divide, Boulder Creek Watershed, Colorado
MA: University of Colorado Boulder, 2010.
Isotopic (δ18O, δD, and 3H (tritium)), geochemical (Na+, Si,) and nutrient (DOC) tracers were used to investigate residence times, source waters, and flow paths in four headwater catchments along a 1,500-m elevational gradient within the Boulder Creek Watershed in the Front Range of Colorado. Precipitation totals from 2009 ranged from 563 mm at 1800 m to a high of 1214 mm at 3528 m. The precipitation was 85% snow at the highest elevation and only 32% snow at the lowest elevation. Application of a convolution algorithm to the δ18O values in precipitation and stream waters produced relatively short mean residence times ranging from 1.12 years in the alpine to 2.08 years in the lower montane ecosystem. Tritium analysis indicated relatively young surface water ages and supported the results from the residence time calculations. Two-component mixing models were conducted using δ18O to identify new and old waters and reactive silicate (Si) to identify reacted and un-reacted waters. All streams consisted of greater then 50% old and greater than 50% reacted waters with the peaks in new and un-reacted water generally occurring during the receding limb of the hydrographs. Additionally, runoff efficiency decreases with elevation from a high of 75.9% in the alpine to a low of just 12.2% at the lowest elevation catchment. These results indicate that headwater catchments within Boulder Creek Watershed have relatively short groundwater residence times, yet groundwater plays an important role in streamflow generation. An overall improvement in understanding the spatial-temporal variations of streamflow generation under predicted changes in climactic conditions will become increasingly important as the ratio of rain to snow in precipitation increases over time.