Identifying soil water movement and water sources of subsurface flow at a hillslope using stable isotope technique

Knowledge concerning soil water movement and water sources of subsurface flow is crucial for understanding runoff generation mechanisms and nutrient-pollution migration. Although the spatiotemporal characteristics of soil water have been confirmed, water sources of subsurface flow during rainfall remain poorly understood at the hillslope scale. We measured the δ18O and δ2H in soil water before and after storm events to explore the mixing process of new water (rainwater) and old water (pre-event soil water) at the hillslopes with different farming practices (bare land, no-tillage and straw mulching). Water source partitioning of subsurface flow during three storms (April 11, April 24, June 10) were quantified using the MixSIAR model. The results showed that both preferential flow and piston flow existed in the process of soil water movement which were detected by the variation in the isotopic values of soil water after storms. The proportional contributions of new water to subsurface flow from 0 − 30 cm in no-tillage plot was the highest (46.60%) among experimental plots. The fraction of new water to subsurface flow for no-tillage, bare land, and straw mulching plot was 27.36%, 22.39%, and 16.28%, respectively, indicating that farming practices affected water source partitioning of subsurface flow. The average proportion of old water to subsurface flow of 0 − 30 cm, 30 − 60 cm and 60 − 90 cm during three storm events were 71.77%, 76.72%, and 87.48%, respectively, suggesting that subsurface flow was mainly derived from old water with soil depths specific responses. In addition, the proportional contributions of new water to subsurface flow varied dynamically with the progress of rainfall, implying that the mixing between new and old water affected water sources of subsurface flow. These findings have crucial implications for understanding the generation mechanisms of subsurface flow and the sustainable management of agro-environmental systems.