Estimating seepage flux from ephemeral stream channels using surface water and groundwater level data

Seepage flux from ephemeral streams can be an important component of the water balance in arid and semiarid regions. An emerging technique for quantifying this flux involves the measurement and simulation of a flood wave as it moves along an initially dry channel. This study investigates the usefulness of including surface water and groundwater data to improve model calibration when using this technique. We trialed this approach using a controlled flow event along a 1387 m reach of artificial stream channel. Observations were then simulated using a numerical model that combines the diffusion-wave approximation of the Saint-Vénant equations for streamflow routing, with Philip's infiltration equation and the groundwater flow equation. Model estimates of seepage flux for the upstream segments of the study reach, where streambed hydraulic conductivities were approximately 10¹ m d^-1, were on the order of 10^-4 m³ d^-1 m ^-2. In the downstream segments, streambed hydraulic conductivities were generally much lower but highly variable (∼10^-3 to 10 ^-7 m d^-1). A Latin Hypercube Monte Carlo sensitivity analysis showed that the flood front timing, surface water stage, groundwater heads, and the predicted streamflow seepage were most influenced by specific yield. Furthermore, inclusion of groundwater data resulted in a higher estimate of total seepage estimates than if the flood front timing were used alone. Key Points Longitudinal streambed seepage flux are estimated on a reach scale Surface- and ground-water data contributed to the seepage flux calibration Longitudinal variability in streambed seepage flux are successfully estimated