Assessing spatially-compounding drought probabilities in inter-basin water diversion: Insights from the Tao River water diversion project in China

Study regions The water-donating (Tao River Basin, TRB) and water-receiving (Zuli River Basin, ZRB, and Wei River Basin, WRB) regions of the Tao River Water Diversion Project (TRWDP) in Gansu Province, China. Study focus Inter-basin water diversion projects are critical for mitigating water shortages in arid and semi-arid regions. By addressing misestimations arising from solely precipitation-based analyses, this study integrates meteorological and hydrological perspectives to explore spatially-compounding drought probabilities between the water-donating and water-receiving regions. Moreover, the meteorological-hydrological propagation is explored using correlation analysis, runs theory and copula functions, while variance decomposition is applied to identify the dominant climatic drivers. New hydrological insights for the region Results show that: 1) Spatially-compounding hydrological droughts occur less frequently than meteorological droughts due to varying geomorphologies and human activities. From 1970–2020, meteorological droughts occurred in five years, while only one hydrological drought event (2016) was identified across the three sub-basins, which was not detected by meteorological indices. The probability of severe spatially-compounding droughts varies regionally, with a higher joint probability in TRB–WRB than in TRB–ZRB, highlighting regional differences in drought propagation. 2) Meteorological and hydrological droughts exhibit distinct seasonal propagation characteristics, with short PTs in summer and autumn due to rapid hydrological responses and longer PTs in winter and spring due to delayed runoff generation. Basins with short PTs require real-time monitoring and rapid response strategies, while those with prolonged PTs need adaptive measures. 3) Precipitation and temperature are the primary climatic drivers of drought, with extreme climatic indices—monthly maximum of daily minimum (TNx) and maximum (TXx) temperatures—contributing the most. The decline in Rx5day (maximum five-day precipitation) suggests a decreasing frequency of extreme rainfall events, further intensifying drought propagation. These findings improve understanding of concurrent droughts and provide practical insights for drought mitigation and sustainable operation of inter-basin water diversion project. • Spatially-compounding droughts show distinct characteristics from both meteorological and hydrological perspectives. • Spatially concurrent meteorological droughts occur more often due to similar large-scale climatic conditions across basins. • Spatially concurrent hydrological droughts are less frequent, shaped by terrain differences and human water interventions. • Meteorological-hydrological drought propagation shows strong seasonality based on three statistical methods.