Wetter or Drier? Spatiotemporal Evolution of Hydro-climatic Extremes in Hong Kong via High-Resolution Data Fusion and the Budyko Framework

Under global warming, high-density coastal cities face the dual challenge of intensifying precipitation extremes and increasing atmospheric evaporative demand. While rainfall trends in Hong Kong have been widely monitored, determining whether the region is becoming wetter or drier requires a comprehensive assessment of the water-energy balance beyond simple precipitation totals. This study investigates the spatiotemporal characteristics of hydro-climatic changes in Hong Kong over the past 40 years (1985–2024), utilizing a hybrid data fusion approach. We integrate hourly precipitation records from 84 Geotechnical Engineering Office (GEO) stations with temperature data from the Hong Kong Observatory (HKO) and reanalysis products from ERA5-Land. To address the coarse resolution of reanalysis data in complex terrains, a topography-based bias-correction and downscaling scheme is applied to generate high-precision, 1-km resolution fields of both Evaporation (E) and Potential Evapotranspiration (PET).The analysis evaluates hydro-climatic indices across wet (April to September) and dry (October to March) seasons to capture the changing patterns of the urban water cycle. Precipitation metrics include accumulated rainfall, total wet/dry days, and Consecutive Dry Days (CDD), while thermal stress is assessed through daily maximum temperatures, the aggregate count of hot days (>30°C), and the duration of consecutive hot days. Beyond statistical trend analysis, the study adopts the Budyko framework to physically characterize the shift in hydro-climatic regimes. We analyze the joint trajectories of the Aridity Index (PET/P) and the Evaporative Index (E/P) within the Budyko space. This framework is applied spatially across four distinct subregions—Hong Kong Island, Kowloon, New Territories, and Lantau—to reveal how varying degrees of urbanization and vegetation cover alter the partitioning of available water and energy.By exploring these metrics, this study elucidates the potential decoupling between water supply and atmospheric demand. The research aims to identify transitions towards compound extremes, such as the alternation between intense rainfall pulses and prolonged, hotter dry spells. These insights provide a physical basis for understanding the changing flashiness of the local climate, offering critical guidance for adaptive water resource management in the Guangdong-Hong Kong-Macao Greater Bay Area.