Persistent Yellow River Influence on Soil Water Across the Soil Profile in the Delta: Evidence from Stable Isotopes and Bayesian Mixing Models

In the Yellow River Delta, changes in the Yellow River’s main stream flow, groundwater exchange, and seasonal precipitation jointly regulate soil water and influence local hydrological processes. Understanding the sources of soil water is crucial for water resource management in this region. A total of 210 soil water samples were collected between August and October 2022 and analyzed for δ2H and δ18O. Subsequently, the contribution of each water source in the 10–100 cm soil profile was quantified using a five-source Bayesian mixture model. The results showed that the isotopic characteristics of the six types of water bodies were different, with surface water and tidal water having a higher degree of evaporation enrichment than precipitation and Yellow River water. The isotopic changes in soil water during the flood season and the post-flood season are mainly concentrated in the shallow and middle layers of the soil profile, while the changes in the deeper layers are relatively small. The Bayesian mixture model analysis results show that the Yellow River water is the main water source of the entire soil profile, with a contribution rate of 39.7–57.1%, followed by precipitation (23.5–37.2%), while the contribution rates of groundwater, local surface water and tides are relatively low. The probability of the Yellow River as the main water source ranges from 55.9% to 80.8% at different depths. These findings indicate that the Yellow River water source has a continuous impact on soil water at different depths in the Yellow River Delta, deepening understanding of soil water recharge processes and water source connections in the coastal delta.