Long-term Phase Identification and Persistence Characteristics of Salinization and Its Driving Mechanisms under Water‑Salt‑Vegetation Coupled Evolution Zoning in the Yellow River Delta

Based on the retrieval results, trend analysis was carried out, and eight evolutionary zones were delineated according to the trend combinations of the three types of indices. Cross‑wavelet analysis was performed for different zones to reveal periodic coupling characteristics, enabling fine identification of the phase relationships among elements within various evolutionary zones, which overcomes the previous limitation of determining the temporal sequence of coupled system evolution.

Furthermore, the Hurst exponent was used to identify the dominant mechanism and intensity of future trends, addressing the deficiency of previous studies that only performed trend analysis on a single salinity indicator without considering the differences among various elements. The generalized additive model (GAM) was introduced to reveal the nonlinear regulation mechanisms of the interaction between natural and human factors on salinization.

The results show that: (1) The optimal model M6 achieved the highest accuracy (R2 = 0.92, RMSE = 1.19 g/kg); (2) Moderate salinization covered the largest area (26%), mainly concentrated in Lijin County, the southern part of Dongying District, and the central‑western part of Kenli District, whereas severe salinization occupied the smallest area (11%); (3) Among the eight evolutionary scenarios, Scenario 3 (salinity decrease – vegetation increase – water decrease) was the most widespread (about 48%), and cross‑wavelet analysis revealed significant frequency‑band differentiation in the water‑salt coupling cycles under different scenarios; (4) Areas with a persistent decreasing trend in the future accounted for the largest proportion (about 28%), and changes in salinity dominated the future evolution (about 42%), concentrated in the northern part of Hekou District and the western part of Kenli District.

Overall, regional salinization showed an improving trend, but areas with anti‑persistent characteristics faced a risk of deterioration. The research results can provide a reference for regional salinization management.