Where and when tracer data diagnoses the architecture of catchment permeability that controls flow-paths and transit times?

Abstract:

Uncertainty about the architecture of catchment permeability creates uncertainty about hydrologic flow-paths and transit times which escalate into uncertainties in predicting how stream water quality is influenced by climate or land use change. The observation of a streamflow alone is unable to sufficiently constrain key features of catchment structure. Different types of complementary hydrometric and tracer-based data have been suggested to constrain internal heterogeneity in flow paths and transit times of hydrologic transport models. However, the extent to which these measurements and measurement-based transport metrics are able to resolve equifinality in the parameterization of flow and transport models are still not known.

Here we couple extensive hydrometric and tracer measurements with a new physically-based flow and transport model to explicitly test the degree to which different types of hydrometric and tracer measurements are able to reduce the risk of equifinality in defining catchment permeability architecture as represented by structures in hillslope hydraulic conductivity and the associated hydrologic flowpaths. Specifically, we will explore:

1. To what extent groundwater level, and hillslope and stream tracer data discriminate between distinct hillslope vertical heterogeneities (and flow-path patterns)?
2. Where and when to collect minimum samples of hillslope and stream tracer data to discriminate between distinct hillslope vertical heterogeneities and flow path patterns?

Modeling results show that groundwater level measurements (complementary hydrometric data) are hardly more effective than daily streamflow data in avoiding equifinality in the choice of permeability architectures. Years of stream O18 data can help resolve the equifinality but a much smaller number of O18 measurements from the hillslope were even more effective. Dissimilarity analyses were able to discern the areas within the hillslope and seasons that tracer data was most powerful in resolving flow paths and residence times.