Reservoir regulation alters meteorological–hydrological drought propagation in the Lancang–Mekong River Basin

Understanding how cascade reservoirs alter the propagation from meteorological to hydrological drought is critical for transboundary water management. Focusing on the Lancang-Mekong River Basin (1960–2016), we integrated Convergent Cross Mapping, Copula-Bayesian frameworks, and dominance analysis to quantify propagation dynamics, dynamic thresholds, and underlying driving mechanisms. Results reveal that reservoir regulation spatially reconfigures hydrological drought risk. During dry seasons, midstream and downstream sub-basins experience significant drought attenuation (dry-season SRI increasing by up to 0.85), whereas upstream droughts paradoxically intensify (by up to 63%). Furthermore, the time required for meteorological drought to propagate into hydrological drought lengthened basin-wide, suggesting a weakened dependence of hydrological responses on concurrent meteorological forcing, alongside distinct seasonal asymmetry downstream. Crucially, reservoir operations correspond to lower local meteorological drought thresholds required to trigger downstream hydrological droughts (by up to 0.6 SPI units in midstream sub-basins), associated with a reduced probability of this cross-type propagation under moderate conditions. According to the dominance analysis, the presence of reservoirs alters the established runoff-generation regime, significantly enhancing the relative contribution of local precipitation in driving downstream runoff (from 13.5% to 32.4%). These findings highlight how hydraulic infrastructure alters the spatial patterns of drought risk, providing a robust scientific basis for adaptive reservoir operation.