Interaction of soil water and shallow groundwater in a hillslope in the Ordos Basin: Insights from transient 2D flow system analysis

Study region A semi-arid region with shallow groundwater and distinct topographic relief in the Ordos Plateau, China. Study focus This study aimed to investigate the interaction between soil water with groundwater and the transient two-dimensional unsaturated–saturated water flow systems. Particular attention was given to the effects of rapid infiltration and strong evapotranspiration in the lowland and their role in driving lateral groundwater flow (LGF) between the upland and lowland positions. New hydrogeological insights from the region The results reveals the occurrence of transient 2D unsaturated-saturated water flow systems induced by rapid infiltration and strong evapotranspiration in the lowlands. The former causes lateral groundwater flow (LGF) from the lowland to the upland, whereas the latter generates LGF with an opposite direction. In unsaturated zone of the upland, there are convergent or divergent zero flux planes separating upward or downward soil water flow. In drought period, the proportion of water loss caused by downward flux in the profile in the upland ranges from 33% to 72%, demonstrating that a large proportion of soil water loss is contributed by ex situ evapotranspiration (water from topographic highs is consumed by evapotranspiration in topographic lows) via LGF, not by in situ evapotranspiration. Therefore, in regions with shallow groundwater, analyzing transient 2D unsaturated-saturated water flow systems can be useful in interpreting soil water flow as long as there is topographic relief.