Three-dimensional Copula-based Assessment of Meteorological Drought Characteristics and Driving Mechanisms in Haihe River Basin, China

Owing to the complexity of droughts, detailed assessments of drought events have become a key issue in water resource management and planning. In this study, three-dimensional copula models at Standard Precipitation Evapotranspiration Index (SPEI)-1, SPEI-3, SPEI-6, and SPEI-12 were used to assess drought risks in the Haihe River Basin (HRB) of China from 1961–2020. Drought duration, severity, and peak, as indicated by SPEI, were extracted based on run theory and fitted with suitable marginal distributions. The difference between the joint return period (T_or) and the co-occurrence return period (T_and) could explain the intrinsic correlation between drought characteristics. The smaller the difference, the stronger the correlation. The results showed that droughts in the northwestern region of the HRB were characterized by high peak, intense severity, and long duration. In contrast, the eastern region exhibited a higher frequency of drought occurrence. Furthermore, the decreasing trend in precipitation dominated droughts, and topography of the northwest region creates the features of low annual precipitation with more days of precipitation. The drought events in the HRB were influenced by the phase shift between El Niño and La Niña. There was a strong negative phase coupling between SPEI-12 and Niño3.4 (R² ≥ 0.77). The transition from La Niña to El Niño was responsible for severe droughts in the HRB. The El Niño-Southern Oscillation could predict droughts with lag times of 0.15–4.35 mon in mountainous areas.