Attribution analysis of groundwater level fluctuation in the Xinxiang section of the Lower Yellow River’s suspended river after the construction of the Xiaolangdi Reservoir

The clear identification and quantification of the factors affecting groundwater systems is crucial for protecting groundwater resources and ensuring safety in agricultural production. The Lower Yellow River (LYR) is a suspended river that replenishes groundwater continuously due to clear differences in the water head, especially in the Xinxiang section. Since its construction, the Xiaolangdi Reservoir has reversed the LYR’s deposition. To accurately determine the factors influencing the groundwater level (GWL), the study area was divided into five subzones based on hydrogeology. A dynamic factor model (DFM), variational mode decomposition (VMD), and a multiple linear regression model were used to identify and quantify the factors influencing the GWL. The impact of the suspended river on the ground-water before and after the construction of the Xiaolangdi Reservoir was examined. The results show that: (1) The rate of decrease in the GWL was 8.53 × 10⁻⁴ m/month, and the rate of decrease in the Yellow River water level (RWL) was 4.63 × 10⁻⁴ m/month. (2) Mountain front recharge (MFR) (scale = 3 months) and precipitation (scale = 9 months) were the dominant factors in subzones I and II, accounting for more than 40% of the fluctuation in the GWL. Subzone III was dominated by exploitation (scale = 7 months) and precipitation (scale = 12 months), accounting for 28.43%, and 23.44% of changes in the GWL, respectively. In subzone IV, agricultural irrigation (scale = 12 months) was the major factor, accounting for 32.47% of GWL changes, while in subzone V, the RWL (scale = 12 months) accounted for 52.52% of these changes. (3) The Xiaolangdi Reservoir has increased the lateral seepage of the suspended river and altered the inter-annual distribution. The results of this study can provide a valuable reference for controlling groundwater overexploitation and ensuring water supply security.