Study on the structure and hydrological cycle of the Pingjiang underground river system

This study clarifies the hydrological cycle structure of the Pingjiang underground river system using hydrogeological mapping (1:50,000), tracer tests, automated groundwater monitoring, and RTK surveys. The obtained results indicate the following: (1) The Pingjiang underground river basin covers an area of 110.66 km² and consists of five tributary conduits, forming a dendritic-network composite structure dominated by unilateral recharge; (2) The threshold rainfall required to generate surface runoff is around 20 mm, which may exceed even 50 mm under prolonged drought conditions. When the daily rainfall exceeds 50 mm, the discharge at the underground river outlet increases significantly. The karst conduit system demonstrates a pronounced flood attenuation effect, capable of delaying the flood peak by up to 21 h. (3) The flow dynamics at the sinking stream inlet and underground river outlet exhibit strong consistency. The discharge at the outlet ranges from 0.35m3/s to 25.57m3/s, displaying flashy characteristics. The Bujing sinking stream inlet contributes 18.86% of the annual runoff to the underground river. (4) The underground river has a baseflow runoff modulus of 3.21 L/s·km2 during the dry season.