Study of meteorological-hydrological drought propagation under reservoir regulation using a Copula-Bayesian network in the Hanjiang River Basin

Abstract

Reservoir operations play a pivotal role in modifying drought propagation processes, particularly by influencing the transition from meteorological to hydrological drought. This study investigates the drought propagation characteristics in the middle reaches of the Hanjiang River Basin, China, under both natural and observed (reservoir-influenced) conditions. The Standardized Precipitation Evapotranspiration Index and Standardized Streamflow Index were utilized to characterize meteorological and hydrological drought, respectively. The Soil and Water Assessment Tool was employed to reconstruct natural streamflow, providing a baseline for comparison. A nonlinear copula function was applied to model the dependence between meteorological and hydrological drought characteristics, and a Copula-Bayesian network was developed to quantify propagation probabilities. Under the regulation of the Danjiangkou Reservoir, drought propagation characteristics for 1–12-month timescales have shifted markedly: the average propagation time downstream was prolonged from 0.25–0.70 months to 0.94–2.36 months, while the propagation rate declined from 0.83–0.89 to 0.48–0.65, and the sensitivity decreased from 0.83–0.96 to 0.68–0.79. In the natural scenario, the optimal propagation model was based on the Gumbel copula, whereas the observed scenario was best fitted by the Frank copula. The likelihood of hydrological drought increased with the intensity and duration of meteorological drought. However, compared to natural conditions, reservoir regulation significantly delayed the onset and reduced the probability of hydrological drought occurrence. These findings elucidate the nonlinear dynamics of drought propagation and underscore the regulating effect of large-scale reservoirs on downstream hydrological responses.