WHERE AND WHEN TRACER DATA SHOULD BE COLLECTED TO DIAGNOSE SUBSURFACE HYDROLOGIC FLOWPATHS THAT CONTROL TRANSPORT PROCESSES AT THE GROUNDWATER-SURFACE WATER INTERFACE (Invited Presentation)

Uncertainty about subsurface hydrologic flowpaths and residence times creates uncertainties about transport processes at groundwater-surface water interface, which escalates into uncertainties in predicting how stream water quality is influenced by climate or land use change. The observation of streamflow alone is unable to sufficiently constrain key features of catchment hydrologic transport. Different types of complementary hydrometric and tracer-based data have been suggested to constrain internal heterogeneity in flowpaths and residence times of hydrologic transport models. However, the extent to which these measurements and measurement-based transport metrics can resolve equifinality in the parameterization of flow and transport models are still not known.

Here we couple extensive hydrometric and tracer measurements, at a well-studied hillslope in Northern Sweden, with a new physically-based flow and transport model to explicitly test the degree to which different types of hydrometric and tracer measurements can reduce the risk of equifinality in defining water quality sensitive catchment internal heterogeneity in flowpaths and residence times. Specifically, we will explore:

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

Results show that groundwater level measurements (complementary hydrometric data) are hardly more effective than daily streamflow data in avoiding equifinality in the choice of catchment internal vertical heterogeneity. Years of stream ¹⁸O data can help resolve the equifinality but a much smaller number of ¹⁸O 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 heterogeneity in flowpaths and residence times. The findings of this research will guide experimentalists for an efficient design of tracer data networks in both gauged and ungauged catchments, and will guide modelers on compiling available tracer data and improving the structure of existing hydrologic transport models, to sufficiently constrain water quality predictions.