Nonuniform Groundwater Discharge across a Streambed: Heat as a Tracer

Time series analysis of continuous streambed temperature during a period of 47 d revealed that discharge to a stream is nonuniform, with strongly increasing vertical fluxes throughout the top 20 cm of the streambed-aquifer interface. An analytical solution to the transient heat transport equation was used to analyze several pairs of observed amplitude damping with depth. A nonuniform pattern in discharge across the stream width was also observed, which could have been caused by lateral or horizontal flow. Head measurements in a meadow area and below the stream showed strong convergence of flow near the streambed. Seepage meter measurements in the middle of the stream often resulted in highly variable flux estimates, which could have been caused by hyporheic flow due to the presence of a gravel layer. Discharge and recharge to the stream at the bank near the meadow was relatively steady throughout the period. On the other hand, discharge to the stream at the opposite bank near a steep hillslope decreased significantly toward the end of the period (early June), which was attributed to a drop in the water table on this side of the stream. The results from the time series analysis were compared with seepage meter measurements and the results from a steady-state analytical solution to the heat transport equation. The different methods agreed on the pattern of discharge across the stream width, and the mean values during the studied period generally agreed well but with different ranges.