3.2. Isotopic compositions in precipitation, plant water, soil water, and groundwater
The δ18O and δ2H values in the two sites varied greatly between water sources and months (Figure 3). Generally, the average values of δ2H were -53.86‰, -52.25‰, -45.95‰, and -38.48‰ for groundwater, precipitation, soil water, and plant water, respectively. The respective average values of δ18O were -8.83‰, -8.65‰, -6.93‰, and -2.96‰. Considering groundwater, precipitation, soil water, and plant water, a progressive enrichment in δ18O or δ2H existed from groundwater and precipitation to soil water to plant water for each month. The δ18O and δ2H in precipitation water enriched initially and then depleted significantly from May to October (p < 0.05). Moreover, the trends of δ18O and δ2H in plant water responded well to the trends of precipitation from May to July, but not well from August to October. Likewise, significant differences were observed for δ18O in plant water between shrub and grassland sites (p < 0.05), but non-significant differences were observed for δ2H (p = 0.06>0.05). There were no significant differences in the δ18O and δ2H of soil water and across months for the two sites (p>0.05). Groundwater isotopes remained relatively stable during the sampling period.