Impact of lateral flow on the transition from connected to disconnected stream-aquifer systems

Understanding the mechanisms by which stream water infiltrates through streambeds to recharge groundwater systems is essential to exploit sustainably the scare water resource in arid and semi-arid areas. An inverted water table (IWT) develops under stream in response to the desaturation between stream and aquifer as system changes from connected to disconnected status. But previous studies suggested that the IWT can only occur at the bottom of clogging layer in which only the vertical flow between stream and groundwater table was considered. HYDRUS -2D simulations show that the lateral flow induced by capillarity or heterogeneity also plays an important role on interaction between stream and aquifer. Three pathways of transition from connected to disconnected systems are possible in homogenous systems where the lowest point of IWT can develop not only at the bottom of clogging layer, but also in clogging layer or aquifer, in response to the first occurrence of desaturation at, above or below the interface between clogging layer and aquifer (ICLA), respectively. The sensitivity analysis shows that under wide streams the lowest point of IWT can only occur at the bottom of clogging layer but under narrow streams the lowest point can also develop in clogging layer if the clogging layer is thick and stream is shallow enough or in aquifer if the clogging layer is thin but relatively high permeable and stream is sufficiently deep. In the system with a heterogeneous streambed, two or three pathways can simultaneously occur where the different parts of IWT occur at different position relative to the ICLA. These challenge the commonly held assumption that the clogging layer under disconnected stream is always fully saturated and limit the methods which introduce a negative or atmospheric pressure value at the ICLA to calculate seepage rate or assess stream-aquifer connectivity.