Streamflow generation in a large, alpine watershed in the southern Rocky Mountains of Colorado: Is streamflow generation simply the aggregation of hillslope runoff responses?

Abstract

[1] Spatial and temporal trends in stream chemistry were investigated in a large (1600 km 2 ) alpine watershed in the southern Rocky Mountains of Colorado to help understand mechanisms of streamflow generation. We observed linear increases of concentrations of chemical constituents in streamflow as accumulated drainage area increased along the main channel of Saguache Creek. We tested two conceptual models of streamflow generation against our stream chemistry observations. One model is essentially two-dimensional and treats streamflow generation at the large watershed scale as the aggregation of runoff responses from individual hillslopes, primarily surface and shallow subsurface flow paths. Alternatively, a fully three-dimensional conceptual model treats streamflow generation as being controlled by a distribution of large-scale groundwater flow paths as well as surface and shallow subsurface flow paths. The structure and magnitude of groundwater contributions in streamflow as a function of increasing scale provided a key distinction between these two conceptual models. End-member mixing analysis and measurements of hydraulic head gradients in streambeds were used to quantify basin-scale groundwater contributions to streamflow with increasing spatial scale in the Saguache Creek watershed. Our data show that groundwater contributions are important in streamflow generation at all scales and, more importantly, that groundwater contributions to streamflow do increase with increasing watershed scale. These results favor the three-dimensional conceptual model in which long groundwater flow paths provide a streamflow generation process at large scales that is not operative at smaller scales. This finding indicates that large watersheds may be more than simply the aggregation of hillslopes and small catchments.

Are we missing the tail (and the tale) of residence time distributions in watersheds?

“…During the deconvolution process, the temporal resolution of the response function is limited by the temporal span of the output time series [Karl, 1989]. In fact, it is common for published CIM residence times to be approximately equal to the temporal span of the period of observation (Table 1) [Frisbee et al, 2011]. This is consistent with these a Fractional uncertainties in the weathering rates, shown in parentheses, were estimated using standard error propagation techniques.…”

“…This homogenization of the stable isotopic signature appears to be caused primarily by mixing processes. In comparison, increases in the geochemical composition of spring waters and increases in the geochemical composition of stream water with increasing drainage area are largely controlled by geochemical kinetics [Frisbee et al, 2011]. Thus, we infer that the CIM is providing information on the time scale of mixing processes, while the radiocarbon and geochemical chronometer methods are providing information on the weighted RTD including the much longer time scales representative of the tail of the RTD (Figure 1b).…”

“…However, it is unlikely that longer input and output time series of stable isotope data will provide any additional information on the long tail of the RTD. At longer time steps, the stable isotopic composition of the output time series often show little, if any, temporal variability and little spatial variability with increased distance from recharge area and/or with increasing accumulated drainage area despite seasonal fluctuations in the input time series [Frisbee et al, 2011]. This homogenization of the stable isotopic signature appears to be caused primarily by mixing processes.…”

“…The notion that groundwater plays an important role in streamflow generation is not new [Pinder and Jones, 1969;Sklash and Farvolden, 1979]. However, the role of old groundwater in streamflow processes has largely been ignored until very recently [Frisbee et al, 2011;Gardner et al, 2011;Smerdon et al, 2012]. Are the watershed processes in some of these studies properly characterized, while others are missing important process information because their residence times are biased short?…”

“…In general, these studies suggest that residence times are short (days to years). However, there is concern that these approaches are not capturing the tail of the RTD representing contributions from long groundwater flow paths and are therefore not properly characterizing flow processes in watersheds [Stewart et al, 2010;Frisbee et al, 2011]. The notion that groundwater plays an important role in streamflow generation is not new [Pinder and Jones, 1969;Sklash and Farvolden, 1979].…”