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).