Abstract: The Yibasan Reservoir, located in Maguan County, Wenshan City, is a natural seasonal reservoir consisting of 11 karst depression reservoirs. Transforming these reservoirs into functional water storage facilities not only enhances the local ecological environment but also addresses the water supply challenges faced by the local community.The study area exhibits significant karst development, with an intricate network of underground river channels connecting the karst depressions. To investigate the patterns and distribution of underground river channels within the reservoir area, tracer tests were conducted with sodium fluorescein and rhodamine as tracers. The hydrogeological parameters and underground river network scale were estimated with Qtracer2. The results reveal abundant water storage in the major underground channel YBS01 -YBS05 and the southern branch YBS03-YBS05, reaching 79 060 m³and 72 546 m³, respectively. YBS01 and YBS02 in the northern part of the Damagu Reservoir are hydraulically connected. On the other hand, the southern branch is obstructed by the northeast-oriented compression torsion fault F3, leading to no hydraulic connection with the main channel. These findings provide crucial support for the reservoir formation process, and also offer fundamental data for calculating the capacity of the Yibasan Reservoir.

Key words: tracer test, karst hydrogeology, tracer , Qtracer2, Yibasan reservoir