Examining the Utility of Continuously Quantified Darcy Fluxes through the Use of Periodic Temperature Time Series

Abstract Fluxes across the groundwater-surface water interface are spatially and temporally variable, difficult to observe and measure, and play a major role in regulating ecological habitat distributions, water quality, and water quantity. The long-term quantification of these fluxes is difficult as field conditions dictate when data can be collected. Ultimately there is a pressing necessity to quantify these long-term flow regimes, as impacts from a changing climate are altering the timing and extent of key groundwater-surface water interactions. The use of periodic temperature time series data is one method that can be utilized to capture these fluxes over long periods of time. Long term deployment of temperature sensors, continuously logging temperature time series data, can be leveraged into time-varying Darcy fluxes via time series analysis and the advection-dispersion equation. However, as hydrologic boundary conditions change, fluxes transition in both magnitude and direction and these temperature time series-based methods are less capable of accurately quantifying fluxes. Real world data taken from five existing United States Geological Survey paired stream gages and riparian groundwater wells sites were used as boundary conditions to inform a one-dimensional heat and mass transport model, with the simulated temperatures used to quantify Darcy fluxes through time. Comparing the Darcy fluxes found using Darcy's Law and the estimated Darcy fluxes from the use of heat as an environmental tracer, periodic temperature time series derived fluxes accurately matched those derived from Darcy's law at three of the sites. For the remaining two sites, hydrologic conditions resulted in erroneous flux estimates, allowing for the identification of specific conditions where temperature time series methods relying on a periodic signal cannot be applied.