Stable isotopes of soil water: Implications for soil water and shallow groundwater recharge in hill and gully regions of the Loess Plateau, China

There is relatively abundant groundwater in some hill and gully regions of the Loess Plateau aquifer. The region has an arid to semiarid climate with low precipitation and high potential evaporation, and thus, groundwater recharge processes remain unclear. This study aimed to investigate recharge mechanisms and soil water movement in the Loess Plateau aquifer and to assess the recharge to shallow loess aquifers using soil profiles. In general, we found that the δ18O and δ2H values of soil water varied both with season and with soil profile depth. The seasonal stable isotopic values in the soil profiles also differed between wet and dry years. In particular, soil water and groundwater isotopic values responded to precipitation. This response suggests that seasonal precipitation is the main source of recharge to soil water and groundwater and that soil water is involved in groundwater recharge. The precipitation infiltration, soil water replenishment, and groundwater recharge are complicated processes. Two main modes of infiltration allow water to move downward through the unsaturated zone and recharge groundwater can be concluded. The primary mode is preferential infiltration through microtopographic features. This channel-like infiltration can rapidly move precipitation downward to deep soil layers, greatly reducing evaporation even under the arid to semiarid conditions of the study area. The secondary mode is a combination of matrix flow, which includes inside preferential infiltration though finger-like flow in the upper layer and piston-like flow in the deeper layer. The upper finger-like flow can cause rapid infiltration to below the root zone during less intense rain or snowmelt. The annual precipitation through preferential flow is the dominant recharge mode for the shallow groundwater in the loess aquifers. These two modes of recharge maintain perennial groundwater in the arid to semiarid hill and gully regions of the Loess Plateau.