A study of soil water movement combining soil water potential with stable isotopes at two sites of shallow groundwater areas in the North China Plain

In the shallow groundwater areas of the North China Plain (NCP), precipitation infiltration and evapotranspiration in the vertical direction are the main processes of the water cycle, in which the unsaturated zone plays an important role in the transformation process between precipitation and groundwater. In this paper, two typical sites in Cangzhou (CZ) and Hengshui (HS) of Hebei province with shallow water tables were selected to analyse the relationship among precipitation, soil water and groundwater. At each site, precipitation, soil water at depths 10, 20, 30, 50, 70, 100, 150, 200, 300 cm, and groundwater were sampled to analyse the stable isotope compositions of hydrogen and oxygen. The soil water potentials at the corresponding depths were observed. Although the climates at the two sites are similar, there are some differences in the infiltration process, soil water movement and groundwater recharge sources. Evaporation occurred at the upper depths, which led to the decrease of soil potential and the enrichment of heavy isotopes. At the CZ site, precipitation infiltrated with piston mode, and an obvious mixture effect existed during the infiltration process. Preferential flow may exist in the soil above 100 cm depth. However, at the HS site soil water moved in piston mode, and groundwater was mainly recharged by precipitation. When precipitation recharged the groundwater it experienced a strong evaporation effect. The results of the soil water movement mechanism provides the transformation relationship among precipitation, soil water and groundwater in the middle and eastern NCP. Copyright (C) 2009 John Wiley & Sons, Ltd.