Abstract: To investigate the influence of upstream rivers on the water body in Pingzhai Reservoir, water samples were collected from the inlet river and the reservoir area of Pingzhai Reservoir, as well as in the Sancha River Basin, in January and July 2020. The hydrochemical parameters and stable isotopes of the water samples were analyzed by using mathematical statistics, Durov plots, correlation analysis, and spatial interpolation. The spatiotemporal distribution characteristics and their causes were preliminarily explored. The study show that: (1) The hydrochemical characteristics of the reservoir water are highly influenced by the supply rivers, and the hydrochemical pattern of reservoir water is consistent with that in the basin, mainly HCO^-₃-Ca²⁺, with the ion concentration being higher during dry season than wet season. (2) The δD and δ¹⁸O of the water were linearly related, and the isotopes exhibit high values in summer and low values in winter. The reservoir water is supplied by precipitation on one hand, and is affected by karst groundwater on the other, thus displaying different seasonal variation characteristics from those of precipitation. (3) The hydrogen and oxygen isotopes of the water in both the reservoir and the basin are mostly distributed along the meteoric water line; however, the slope and interception values are smaller than global meteoric water line and local meteoric water line, indicating strong evaporation fractionation despite dominant supply from precipitation. (4)During wet season, the hydrogen and oxygen isotopes of the basin and the reservoir water are significantly correlated with ion concentration, while in dry season, the water bodies in the lower reaches and the reservoir are notably influenced by precipitation, and the upper reaches by evaporation.

Key words: hydrogen and oxygen isotopes, water chemistry, evaporative fractionation, meteoric precipitation line, Pingzhai Reservoir