Figure 2 Spatial and temporal distribution of surface water and
groundwater nitrate by season in the Huashan watershed (a winter
November 2021, b spring April 2022, c summer July 2022, d autumn October
2022)
3.2 Origins of surface water and
groundwater
The Isotopic ratios of hydrogen and oxygen can effectively indicate the
strength of evaporation, tracing water cycle processes and
distinguishing the source of water (Darling et al., 2016). The values of
δD-H2O in surface water ranged from -22.42‰ to -52.66‰,
with an average of -39.24‰; the values of
δ18O-H2O
ranged from -1.22‰ to -8.18‰, with an average of -5.36‰. The values of
δD-H2O in groundwater ranged from -28.41‰ to -55.05‰,
with an average of -43.05‰, and the values of
δ18O-H2O ranged from -2.34‰ to -8.45‰,
with an average of -6.42‰. As shown in Fig.3, the values of
δD-H2O and δ18O-H2O of
surface water and groundwater all fall near the meteoric water line,
indicating the precipitation origin of surface water and groundwater.
The slopes of the regression lines for both surface water and
groundwater are lower than LMWL (Wang et al., 2016), indicating that
both surface water and groundwater are affected by evaporation, and
evaporation is stronger for surface water. The wider distribution of
hydrogen and oxygen isotopes in surface water indicates a more diverse
supply source for surface water than for groundwater. Additionally,
there is overlap in the hydrogen and oxygen isotopes of surface water
and groundwater, suggesting that there may be hydraulic connections
between the two in the Huashan watershed. Results from on-site
monitoring and a coupled surface water-groundwater model also confirm
the presence of strong interaction between surface water and groundwater
in the study area. Groundwater is the primary source of recharge for
surface water in the Huashan watershed, according to the SWAT-MODFLOW
model (Zhang et al., 2022).