Characteristics and their influencing factors of water recharge under different groundwater levels in the Mu Us Sandy Land of China

Abstract: Water and soil conservation have been highly critical to the vegetation restoration and ecological reconstruction in the arid and semi-arid regions, particularly in southeastern China. Thus, it is necessary to characterize the soil water recharge and movement under different Groundwater Levels (GWL) for water use efficiency and sustainable agriculture. In this study, the hydrogen and oxygen isotopic compositions (δ2H and δ18O) of rain water, soil water and groundwater were measured in three different woodlands (i.e., Salix psammophila woodland with GWL range of 253-260 cm, Pinus sylvestris L. var. mongholica Litv. woodland with GWL range of 87-93 cm and Amygdalus pedunculata Pall. woodland with GWL range of 172-176 cm) in the Gechougou watershed in the southeastern edge of Mu Us Sandy Land. The characteristics and influencing factors of soil water recharge were clarified in three types of woodland. Results showed that the δ2H and δ18O of soil water were plotted below the local meteoric water line, in Salix psammophila, Pinus sylvestris L. var. mongholica Litv., and Amygdalus pedunculata Pall. Woodland, where the slope of soil water line in the three types of woodlands (5.69-7.13) was less than that of the local meteoric water line (7.79), indicating that the soil water in each woodland was replenished by the local precipitation. The δ2H and δ18O of soil water in Salix psammophila (0-20 cm), Pinus sylvestris L. var. mongholica Litv. (0-40 cm), and Amygdalus pedunculata Pall. Woodland (0-40 cm) exhibited the depletion of heavy isotopes and enrichment of light isotopes in monitoring period, while significantly positive correlated (P < 0.05) with the hydrogen and oxygen isotopic compositions of precipitation, indicating that the shallow soil water (< 40 cm) under different woodlands was more easily replenished by the precipitation. The mean values of δ2H and δ18O of deep soil water in different types of woodlands (Salix psammophila (> 180 cm), Pinus sylvestris L. var. mongholica Litv. (> 60 cm), and Amygdalus pedunculata Pall. Woodland (> 120 cm)) were close to those of the groundwater, where the δ2H and δ18O of soil water tended to be more stable (CV < 10%) with the increase of depth. The depth ranges of soil water recharged by groundwater were about 73-80, 27-33, and 52-56 cm, respectively, during the monitored period in the three types of woodlands (Salix psammophila, Pinus sylvestris L. var., and Amygdalus pedunculata Pall. woodland), according to the correlations of δ18O between soil water and groundwater (i.e., in term of how easily the δ18O of soil water were mixed and displaced with the fluctuating groundwater). Consequently, the shallow soil water (<40 cm) in the three woodlands was mainly recharged by precipitation, especially in the rainy season, whereas, the deep soil water (27-80 cm above the GWL) was dominantly recharged by groundwater. Therefore, the deep soil water recharge by shallow groundwater can be expected to alleviate the deficit of soil water in the dry season, in turn providing potential water sources for vegetation growth in the study area. In any way, the shallow and deep sources of soil water were inevitable prerequisites for the construction of sand-fixing plants under the arid climate in the southeastern edge of Mu Us Sandy Land.