Non‐Darcy Flow Through a Natural Streambed in a Disconnected Stream

Seepage flux from disconnected streams can be a main source of groundwater recharge in arid and semiarid regions. The seepage into the streambed will respond to a change in stream stage; however, the relationship between seepage flux and stream stage is uncertain due to a lack of in situ observations. This study investigated the change in seepage flow regime with stream stage in a natural streambed through field measurements along a 50.1 km long disconnected stream. Seepage fluxes were quantified using a seepage meter method involving the measurement and simulation of the variable stream stages in a seasonal stream. Flow regimes in the streambed were then evaluated using a developed model. Two seepage flow regimes were observed, the Darcy flow regime and the non‐Darcy flow regime, with the change of the stream stage in the streambed. Our analysis of the mechanisms of seepage flow indicates that drainable porosity and particle size of streambed media are the key factors controlling non‐Darcy seepage flow behavior. We demonstrated that the Reynolds number based on the particle diameter d30 instead of d50 can provide a more accurate criterion for non‐Darcy flow in natural heterogeneous streambeds. Model evaluation showed that predicted streambed seepage regimes and fluxes were consistent with field observations. Together, these results provide new insights into the seepage mechanisms, which have significant implications for the establishment of a better seepage model and the joint development of regional surface water and groundwater. Key Points Seepage flow in a streambed transfers from the Darcy flow regime to the non‐Darcy flow regime as the stream stages increase The Reynolds number based on the d30 diameter is shown to be a critical criterion for the onset of non‐Darcy flow in a natural streambed An analytical expression of the non‐Darcy streambed seepage has been developed