The exact groundwater divide on water table between two rivers: A fundamental model investigation

The groundwater divide within the plane has long been delineated as the water table ridge composed of the local top points of the water table. This definition has not been examined well for river basins. We developed a fundamental model of two-dimensional unsaturated-saturated flow in a profile between two rivers. The exact groundwater divide can be identified from the boundary between two local flow systems and compared with the top of the water table. It is closer to the river of the higher water level than the top of the water table. The catchment area would be overestimated (up to ~50%) for the high river and underestimated (up to ~15%) for the low river by using the top of the water table. Furthermore, a pass-through flow from one river to another would be developed below two local flow systems when the groundwater divide is significantly close to the high river.