Unraveling the Temporal Asymmetry of Drought-Wetness Abrupt Alternations: A Process-Aware Framework Based on Pentad Soil Moisture

Drought-wetness abrupt alternation (DWAA) represent a classic form of hydroclimatic whiplash, posing escalating threats to global water and food security. Yet, conventional identification frameworks often simplify these shifts as instantaneous binary events, neglecting the critical, evolving physical dynamics during the transition phase. This study challenges the prevailing “instantaneous-transition” assumption by establishing a process-aware identification framework based on pentad-scale soil moisture dynamics.

Applying this framework to multiple datasets (GLDAS-NOAH-2.1, ERA5, and SMCI) across China from 2000 to 2022, we reveal a fundamental temporal asymmetry in DWAA events. Our results show that drought-to-wetness (DTW) transitions typically occur as rapid, explosive shocks, completing within approximately 10 days with a mean transition rate of +30% per pentad. In contrast, wetness-to-drought (WTD) transitions unfold as prolonged depletions, taking roughly one month to conclude with a significantly slower transition rate of -15% per pentad.

This two-fold timescale difference is driven by distinct physical mechanisms: DTW is dominated by external, high-intensity atmospheric precipitation pulses, whereas WTD is constrained by the internal, nonlinear memory of terrestrial water storage and evapotranspiration-driven depletion. Spatially, these events exhibit segregated hotspots, with DTW clustering in the Huai River Basin and WTD concentrated across the broader Yangtze River Basin and southeastern coast. These findings offer a new physical basis for developing asymmetric, mechanism-based identification systems, shifting the paradigm from simple event cataloging to a dynamic understanding of compound hydroclimatic extremes.