Groundwater dynamics and renewal responses in the plain section of the Yongding River, exemplified by extreme precipitation events

Abstract

The groundwater system, though invisible, is vital to the global water cycle, and it is essential to quantitatively assess its renewal capacity in light of climate change and anthropogenic activities. This study investigates the response of groundwater to extreme recharge events, focusing specifically on its renewal capacity and spatial-temporal variations. We analyze representative extreme precipitation events to quantify rebound amplitude, renewal rate, and the proportions of groundwater recharge. Additionally, we examine groundwater dynamics under varying precipitation intensities and initial conditions. The results indicate a strong correlation between groundwater levels (GWLs) and precipitation, characterized by rapid infiltration and mixing, indicating a highly open hydrogeological system. This conclusion is supported by the significant correlation between rainfall intensity and GWLs fluctuations, with correlation coefficients of 0.896, 0.871, and 0.801 from upstream to downstream. The similarities in δD and δ¹⁸O, along with the high young water fractions (0.27–0.35) recorded in 2023, further confirm rapid recharge through permeable media. Initial conditions influence lag time and GWLs changes, highlighting the importance of hydrogeological characteristics on groundwater dynamics. Furthermore, spatial variations in GWLs and stable isotopes reveal distinct recharge patterns: concentrated recharge zones upstream (aligned with fault zones) and broader, slower recharge areas downstream. The diminishing hydraulic gradient across the alluvial fan results in reduced flow velocities and renewal rates, while decreasing d-excess suggests evaporative enrichment or mixing with older water downstream. These results provide critical insights into groundwater system's response to extreme precipitation, particularly under climate variability and strong human activities.