Snow-Dependence Modulates Warming-Induced Seasonal Extremization in Mountain Watersheds

Mountain watersheds are experiencing divergent hydrological responses to warming, yet the mechanisms controlling this divergence remain unclear. We introduce the Seasonal Extremization Index (SEI), the ratio of dry-season mean flow to annual median flow, to quantify changes in seasonal flow structure. Across 78 watersheds in the western United States, spring warming drives SEI decline (r = -0.277, p = 0.043), but this effect is strongly modulated by snow reliance (SRF). In snow-influenced watersheds (n = 54), the interaction between SRF and spring warming explains 18.0% additional variance in SEI trends (p < 0.001; stratified bootstrap 95% CI [-0.721, -0.092]). We validated the predictive power of this relationship using a temporal split: early-period SRF (1990-2005) significantly predicts late-period SEI decline (2006-2020) across 36 watersheds (r = -0.405, p = 0.014). High-SRF watersheds show significant SEI decline (r = -0.492, p = 0.009), while low-SRF watersheds show no significant response (r = +0.091, p = 0.652). We identify four anchor types with distinct response pathways. A cross-region application to 14,406 grid points in Chinese mountains reveals a latitudinal gradient: snow anchor failure in the north (>40°N) and monsoon-driven anchor strengthening in the south (<35°N). These results suggest that mountain hydrological futures are determined not by warming alone, but by the strength of snow reliance, which acts as an amplifier of warming effects.