Groundwater discharge is a major contributor to stream baseflow. Quantifyingthis flux is difficult, despite its considerable importance to water resourcemanagement and evaluation of the effects of groundwater extraction onstreamflow. It is important to be able to differentiate between contributionsto streamflow from regional groundwater discharge (more susceptible togroundwater extraction) compared to interflow processes (arguably lesssusceptible to groundwater extraction). Here we explore the use ofgroundwater surface mapping as an independent data set to constrain estimatesof groundwater discharge to streamflow using traditional digital filter andtracer techniques. We developed groundwater surfaces from 88 monitoring boresusing Kriging with external drift and for a subset of 33 bores with shallowscreen depths. Baseflow estimates at the catchment outlet were made using theEckhardt digital filter approach and tracer data mixing analysis using majorion signatures. Our groundwater mapping approach yielded two measures(percentage area intersecting the land surface and monthly change insaturated volume) that indicated that digital filter-derived baseflowsignificantly exceeded probable groundwater discharge during most months.Tracer analysis was not able to resolve contributions from ungauged tributaryflows (sourced from either shallow flow paths, i.e. interflow and perchedaquifer discharge, or regional groundwater discharge) and regionalgroundwater. Groundwater mapping was able to identify ungauged sub-catchmentswhere regional groundwater discharge was too deep to contribute to tributaryflow and thus where shallow flow paths dominated the tributary flow. Ourresults suggest that kriged groundwater surfaces provide a useful, empiricaland independent data set for investigating sources of fluxes contributing tobaseflow and identifying periods where baseflow analysis may overestimategroundwater discharge to streamflow.
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