Antagonism or synergy: Divergent surface water dynamics at the southern margin of the Eurasian permafrost

Abstract. Intensifying climate change and human activities are substantially altering frozen ground conditions, disrupting both surface water regimes and groundwater connectivity. The specific driving mechanisms behind these surface water shifts at the southern margin of the Eurasian permafrost, however, remain poorly quantified due to overlooked spatial heterogeneity. This study analyzed surface water dynamics in the Songhua River Zone (SHRZ) from 1988 to 2024 by integrating an improved water detection method with an interpretable geographical extreme gradient boosting framework coupled with shapley additive explanations. The results show a marked hydrological reversal from shrinkage to expansion around 2012. Expansion in the seasonal frozen ground region (24.77 %) significantly outpaced that in the permafrost region (9.38 %). Spatially explicit attribution identified a structural divergence in regulation mechanisms: the permafrost region is dominated by human activities (76.4 %), forming an "antagonistic" pattern where reservoir-driven expansion is constrained by environmental barriers. In contrast, the seasonal frozen ground region is governed by natural factors (72.4 %), exhibiting a "synergistic" pattern where climate and terrain jointly promote water expansion. Across distinct water types, natural factors control 93.6 % of lake dynamics, whereas human activities dominate river systems (71.0 %) and reservoirs (56.0 %). Furthermore, this surface water expansion occurred alongside accelerated groundwater depletion, suggesting that the surface recovery was achieved at the expense of subsurface storage. These findings demonstrate that surface water expansion does not equate to water security, highlighting the need for targeted surface-groundwater management strategies and prospective research integrating dynamic permafrost degradation processes to further elucidate these ecohydrological trade-offs.