Groundwater behavior response to rainfall and reservoir infilling: Insights from long-term in-situ monitoring

Jinlongshan slope in the Ertan Reservoir. The study focuses on exploring the interplay between rainfall, reservoir level, and groundwater dynamics (encompassing groundwater levels, temperatures, and flow characteristics) within a reservoir slope. Specifically, it examines three key aspects: the characteristics of groundwater in different periods; the hysteretic behavior of groundwater levels in response to variations in rainfall and reservoir level; and the behavior of groundwater temperature and its influencing factors, with an emphasis on analyzing the lag effects and their underlying causes (e.g., time lag in slope deformation). Rainfall and reservoir level impacts on groundwater levels, exhibiting notable lag effects. Pre-impoundment, rainfall induced a two-month lag in middle and rear slope sections. During initial rapid impoundment (2 m/day), the front edge responded swiftly, while middle and rear sections lagged by 14 and 35 days, respectively. These lags decreased during normal operations. Groundwater temperatures fluctuated significantly (21°C-26°C) during rapid impoundment, stabilizing around 22°C during normal operations. Additionally, a notable lag effect was observed between changes in groundwater temperature and reservoir water level, primarily attributed to the time lag in slope deformation. This analysis provides insights into the complex hydrogeological responses of reservoir slopes to environmental and operational changes. • A 20-year dataset assesses how rainfall and reservoir levels impact slope groundwater across all impoundment stages. • Results show rainfall and reservoir levels significantly drive groundwater fluctuations, often with notable lag effects. • Groundwater temperature lags behind reservoir levels, primarily driven by the timing of slope deformation.