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.