Pingzhai Reservoir dam was built in a deep canyon of the southern tributary of the Wu River and is the main water resource used in the Hydro-junction Project to satisfy the water requirements of Central Guizhou Province in southwestern China. The interval basin (i.e., subbasin, encompassing lower reaches between the dam and the backwater end of the main stream contributing to the dam reservoir) of Pingzhai Reservoir is characterized by thick karstified carbonate strata. Hydrological and hydrogeochemical data, including the inflow of the main stream to the interval basin, the reservoir water level, rainfall, reservoir discharge, and concentrations of major ions and stable isotopes of oxygen and hydrogen in water, were collected and analyzed. A conceptual model was constructed using the back-propagation artificial neural network approach and the water balance equation was used to calculate the total storage capacity of the karst underground reservoir associated with the Pingzhai surface reservoir. The results showed that the intensively karstified Lower Triassic Yongningzhen Formation carbonate strata resulted in the creation of a karst groundwater reservoir with a macroscopic volumetric karst rate of ~5.0%. The total storage capacity of the karst underground reservoir is (311 ± 108) × 10⁶ m³, and it accounts for 29% of the total capacity of the Pingzhai surface reservoir. The reliability of the total storage capacity value of the karst underground reservoir was assessed by hydrochemical and stable isotopic methods. These results will aid in the joint management of surface reservoirs and associated underground reservoirs in karst canyon areas.