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Flash drought (FD) is an onset and intensify rapidly type of drought that can harm the terrestrial ecosystem, and cause economic and agricultural losses. The North China Plain (NCP) is an important agricultural region in China where sustainable development is restricted by the frequent droughts and insufficient water resources. Coping with FD requires an understanding of the FD onset and identification in the NCP. Based on root zone soil moisture (RZSM), standardized evaporative stress ratio (SESR) and multiples of mean evaporative stress ratio (MESR), this study identified the FD events in the NCP from 1981 to 2022, revealed the FD characteristics such as frequency, duration, severity and intensity, explored the temporal and spatial trend, determined the FD hotspots, and demonstrated the impact of FD identification thresholds on the FD identification. The frequency distributions of FD events identified by RZSM, SESR, and MESR are all high in the central and northern NCP and low in the southern, whereas the total duration is high in the southern and eastern NCP and low in the northern. As the FD intensity increases, the onset stage lengthens, the recovery stage shortens, the total duration reduces, and the severity declines. The FD affected areas from various FD identification methods exhibit significant and similar seasonal variations, primarily occurring from May to August. Besides, NCP is prone to extreme and exceptional FDs. The NCP has a decreasing tendency of the FD characteristics, and three hotspots with frequent and serious FD events are identified in the northwestern, eastern and southwestern NCP. The FD frequency is also significantly influenced by the thresholds in the identification methods. This study provides insights into the FD characteristics in the NCP, and clarifies its trend and hotspots, which may be valuable for FD understanding and adaptation. [ABSTRACT FROM AUTHOR]

Coastal cities frequently face various types of flooding triggered by heavy rainfall and storm surges, such as fluvial flooding and pluvial flooding. Currently, Currently, there is a lack of comprehensive methods to analyse the sources of severe compound flooding. This study, using the Shahe River Basin in Guangzhou, China as an example, establishes and validates a coupled 1D and 2D hydrodynamic model. Based on historical data, it constructs joint probability distributions of rainfall and tidal levels with different return periods and durations. Using the results from the coupled model under various design scenarios, it proposes an impact index to quantify the contributions of rainfall and tides to flooding. Furthermore, it quantifies the interactions between fluvial flooding and pluvial flooding. Flood-prone areas are delineated, and the causes of flooding are analyzed. The results show that when the return periods of rainfall and tide level are both 10 years, the Kendall return period for the combined event of rainfall and tide level is 36.35 years, greater than the "Or" return period (5.40 years) and less than the "And" return period (66.88 years). The impact degree index of rainfall on flooding varies between 0.5 and 1, with the minimum at 24-hour duration, indicating that the study area is primarily affected by rainfall and the influence of tide level is most significant at 24-hour duration. The pluvial flooding caused by the influence of river water level on the drainage outlet accounts for 19.08 % of the total volume at most. This shows that fluvial flooding affects the seriousness of pluvial flooding by influencing the water levels of outlets. The flood-prone area is divided into different regions based on the main natural factors (rainfall and tidal level) and social factors (pipeline network, drainage outlets, and riverbank defenses) to help decision-makers identify the causes of flooding in each drainage unit and better formulate targeted disaster reduction strategies to improve flood control capabilities. [ABSTRACT FROM AUTHOR]