To explore the temporal and spatial intrusion process of runoffs and the response of water quality during the flood season in the Jinpen Reservoir (JPR) in Xi'an. Continuous in-situ monitoring was carried out on the water quality indexes (WQI) from the upstream river channel to the reservoir of two runoffs in early August and mid-September 2019. The single factor WQI and comprehensive WQI were used to assess the water quality vertically. Different inflow conditions of rain storm runoffs evolved into different intrusions. The initial inflow of the two runoffs was small, the runoff experienced a full-section intrusion, bottom intrusion, and mid-intrusion process along the way; the position of mid-intrusion in reservoir changed from 545-565 m at the beginning of the runoff to 535-580 m at the end in early August, and developed from 540-575 m of mid-intrusion to 575 m below the bottom of the intrusion in mid-September. The continuous inflow weakened the thermal stratification structure and replenished the DO in the reservoir. Meanwhile, mass particulate pollutants sank into the reservoir, and vertically, the nutrients of middle and bottom parts were higher than at the surface. The single factor WQI showed that the TP and permanganate index values of underflow location increased to some extent, and both exceeded the class Ⅲ water quality standard of surface water at the end. The comprehensive WQI showed that the middle layer of runoff was moderately polluted in early August, while the bottom layer was heavily polluted due to the dual effects of anaerobic and particle deposition, and reached the peak after one week of runoff, while the bottom intrusion of below 575 m directly caused heavy pollution in the middle layer, and bottom layer was medium polluted due to the supplement of dissolved oxygen in mid-September. The discharge of the spillway tunnel and the intake of stratified water could effectively guarantee the safety of the water supply during the flood season.
Keywords: Jinpen Reservoir; intrusion process; nutrients; rainstorm runoff; water quality indexes (WQI) assessment.