TIME SERIES OF STREAMBED SEEPAGE IN A STRONGLY-LOSING STREAM FROM THERMAL RECORDS: RESOLVING SEEPAGE DYNAMICS AND CHANGES IN STREAMBED HYDRAULIC CONDUCTIVITY

We evaluate seepage dynamics in the Pajaro River, a losing stream in central coastal California, using differential gauging and time-series analysis of long-duration, streambed temperature records. Data were collected throughout the water year at multiple locations along an 11.42-km experimental reach. Differential discharge measurements were made directly, at various locations along the reach, and using continuous gauges installed at fixed stations. The analytical method applied to thermal data uses changes in the phase and amplitude of subsurface diurnal temperature variations to infer the direction and rate of seepage once per day, without direct application of Darcy's law. Differential discharge values indicate that the Pajaro River loses 0.2 to 0.4 m³/s of discharge during the second half of the water year, when discharge into the reach is less than 4.5 m³/s, with most of the loss occurring in the lower part of the reach. Along the entire reach, downward seepage rates increase in magnitude with distance downstream, and hydraulic gradients are increasingly negative. Streambed seepage rates inferred from thermal data are 0 to -0.3 m/day near the top of the experimental reach and -0.3 to -1.7 m/day near the bottom of the reach (negative values indicate flow into the streambed). When combined with measurements of the hydraulic gradient within the streambed, observed seepage rates indicate hydraulic conductivities of K = 10^-6 to K = 10^-4 m/s, with lower conductivities near the bottom of the experimental reach. Hydraulic conductivities decrease significantly with time during the water year, particularly within the lower part of the reach, when there is lower discharge in the channel, probably as a result of deposition of fine-grained material.