Abstract:

This study investigated the floodplain that is approximately 10 m away from the Yellow River near the Jinniu Street Wharf in Anning District, Lanzhou City in Gansu Province, China. The hydrogen and oxygen stable isotopes of the floodplain soil water were analyzed. Moreover, together with the line-conditioned excess of different water bodies, the variation characteristics of the soil water hydrogen and oxygen stable isotopes and the soil water content at different depths of the floodplain soil profile were studied. The process of precipitation replenishing the soil water in the piston and preferential flow modes was assessed. The results are presented herein. First, the local meteoric water line (δD=7.00δ¹⁸O+3.81, R²=0.95, P<0.001) in Lanzhou was significantly different from the global meteoric water line, and the slope was affected by evaporation. Second, the δD of the shallow soil water showed a larger variation. The deeper it was, the smaller the variation, which gradually stabilized. The δD value first decreased with depth, then increased, and finally stabilized. Meanwhile, the shallow soil water content was smaller and gradually increased with the soil depth, indicating that the river water replenished the soil water in the deep layer (>80 cm). Lastly, in April, June, and October, precipitation was observed to replenish the soil water in a plug flow mode, while in May and September, precipitation was observed to replenish the soil water in a preferential flow mode (i.e., soil water recharging in the flood plain is performed under two modes). Clarifying the soil water characteristics is greatly significant for the accurate analysis of the amount of precipitation infiltration in the soil and leads to a deep understanding of its replenishment process and an accurate evaluation of both groundwater replenishment and pollution analysis.

Key words: soil water, precipitation, piston flow, preferential flow, floodplain