Parameters in a new multiscale model for stream transport are estimated from stream tracer tests.
Estimated joint distribution of parameters representing transport in a stream channel and exchange with adjacent groundwater. Comparison between breakthrough curves observed and simulated at four measurement locations. [Reprinted by permission from Rathore, S. S., et al. 2021.]
The exchange of water between stream channels and adjacent groundwater is an important control on exports of carbon, nutrients, and contaminants from watersheds. That exchange is typically estimated by analyzing tracer tests using fluorescent dyes. Indirect estimates of exchange measured that way are sensitive to measurement and model uncertainties. Oak Ridge National Laboratory researchers reanalyzed previous tracer tests using uncertainty-aware inverse modeling and a new multiscale model for stream transport. The results demonstrate that the new model accurately represents transport in streams. The results also show that tracer tests can be used to estimate model parameters.
This study provides additional confidence in a recently developed multiscale model for transport in river corridors. The model is implemented in the Advanced Terrestrial Simulator (ATS) software and provides the foundation for new watershed modeling capabilities that require fewer restrictive assumptions about stream-groundwater interactions. The analysis framework provides estimates of model parameters and their uncertainties. The study shows how different measurement locations and test durations can affect uncertainty in estimated model parameters. It also shows how data from multiple locations can be leveraged for reliable parameter estimates. That information can inform experimental design of future tracer tests including those involving reactive tracers.
Hyporheic exchange, the bidirectional movement of water and solute between stream and river channels and adjacent groundwater, is difficult to measure directly. Tracer tests offer an indirect method for estimating hyporheic exchange and constraining transport parameters in river corridor models. However, uncertainties and detection limits in measured concentrations, the indirect and ill-posed nature of inverse modeling, and model structural uncertainties present challenges for tracer test interpretations.
This study used a recently developed multiscale stream and river corridor model that represents solute flux to and from the hyporheic zone without explicitly solving threedimensional subsurface transport. Implementation of the model in the ATS software combined with a Bayesian parameter estimation process and high-performance computing made it possible to estimate key model parameters and uncertainty in those parameters. Significantly, the hyporheic travel time distribution was estimated without assumptions on the distribution shape and simultaneously with channel transport parameters.
Analyses of nonreacting tracer data from a low-gradient stream suggested that short reach length, brief measurement duration, and a time-dispersed source may impede parameter identifiability and result in broad or multimodal parameter distributions. Joint-fitting of data at multiple locations with uniform hyporheic zone and non-uniform channel parameters improve parameter identifiability with well-constrained unimodal parameter distributions.
Rathore, S. S., A. Jan, E. T. Coon, and S. L. Painter. 2021. “On the reliability of parameter inferences in a multiscale model for transport in stream corridors.” Water Resoures Research. 57(5):e2020WR028908. DOI:10.1029/2020WR028908.
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