4 Discussion

4.1 Spatial and temporal variation characteristics of hydrogen and oxygen isotopes in water

In this study, the hydrogen and oxygen isotopes of groundwater were the most depleted in the three water bodies, but the hydrogen and oxygen isotopes of the precipitation, which is one of the main water recharge sources for groundwater (Joshi et al.,2018; Maria et al.,2007), were the most enriched. This phenomenon might be related to the amount effect of the hydrogen and oxygen isotopes in rainfall (Yeh et al., 2011; Yamanaka et al.,2004; Dalai et al.,2002) (Fig. 9). The annual frequency of occurrence of short-duration light rains in the Loess Plateau could reach up to 86.70%, with relatively small rainfall and a large quantity of isotope. While the annual rainfall frequency of medium and heavy rains with large rainfall and little isotope was only 13.30%, which made the hydrogen and oxygen isotopes of the precipitation relatively abundant. Liu et al. (2009), Oiro et al. (2018) and Grismer et al. (2000) all found that groundwater recharge generally occurred after heavy rains and continuous rainfall, thereby implying that the probability of groundwater being recharged by heavy rain was greater. In this study, the δ D and δ 18O of the heavy precipitation were respectively -85.12‰ and -11.75‰, which was more depleted than the values for groundwater (-64.95‰, -9.04 ‰). The hydrogen and oxygen isotopes gradually approached those of the groundwater due to the combination of evaporation fractionation and the mixing of previous enriched soil water during the infiltration process of precipitation into the soil to recharge groundwater (Natalie et al.,2016). At the same time, Earman et al. (2006) found that the groundwater recharge ratio by snowmelt could reach 40% to 70% in the southwestern United States, while more than 50% of the groundwater recharge in Norway taken place during snowmelt (French et al.,2004), although snowfall only accounts for 25% to 50% of the annual precipitation. This indicates that snowfall in winter may also produce greater recharge to groundwater. In this study, the mean values ofδ D and δ 18O in snowfall were -95.08% and -14.11%, respectively, which were more depleted than those of groundwater. Groundwater isotopes had become the most depleted of the three water bodies under the combined recharge of heavy rains and snowfall with depleted isotopes.
At the same time, the hydrogen and oxygen isotopes of the rainfall have an elevation effect (Peng et al., 2016; Campani et al.,2012) (Fig.9). The precipitation isotopes in the upperstream with the higher elevation were more depleted, and the isotopes in the downstream area with the lower elevation were more enriched. In the process of surface water flowing from the gully head to the downstream, evaporation fractionation occurred and caused the hydrogen and oxygen isotopes to become gradually enriched (Lu et al., 2012). This finding is in agreement with the results reported by Song et al. (2009) in the Chabagou watershed. Under the combined recharge of precipitation and surface water, the hydrogen and oxygen isotopes of groundwater were also depleted in the upperstream and enriched in the downstream (Ho J C et al., 2004). However, the difference of the water hydrogen and oxygen isotopes was not significant among the different parts.