Transient analysis of fluctuations of electrical conductivity as tracer in the stream bed

Spatial patterns of water flux in the stream bed are controlled by the distribution of hydraulic conductivity, bedform-induced head gradients and the connectivity to the adjoining groundwater system. The water fluxes vary over time driven by short-term flood events or seasonal variations in stream flow and groundwater level. Variations of electrical conductivity (EC) are used as a natural tracer to detect transient travel times and flow velocities in an in-stream gravel bar. We present a method to estimate travel times between the stream and measuring locations in the gravel bar by non-linearly matching the EC signals in the time domain. The amount of temporal distortion required to obtain the optimal matching is related to the travel time of the signal. Our analysis revealed that the travel times increase at higher stream flows because lateral head gradients across the gravel bar become significantly smaller at the time.