Spatiotemporal characteristics of urban dry/wet islands in China following rapid urbanization

Evapotranspiration links land surface hydrological processes with the atmosphere and has an important influence on regional water cycles and the water balance. Research has shown that urbanization can impact the hydrological balance, leading to urban wet and dry island effects; however, the mechanisms driving these phenomena remain uncertain. In this study, the humidity levels of urban areas in China were compared with the surrounding areas prior to (1960-1969), and following (2010-2019) rapid urbanization to determine the drivers and mechanisms of urban wet and dry island effects. Monthly observational datasets of temperature, water vapor pressure, and relative humidity from 613 meteorological stations were combined with the processes of urban artificial water dissipation to analyze the temporal and spatial characteristics of urban dry/wet islands. Results showed that cold and dry urban areas tended to have wet island effects, while warm and humid urban areas had dry island effects. The dry/wet island intensity was less than 3.5% in most cities. Furthermore, urban areas tended to be dry islands during summer and wet islands during winter. Coastal cities were an exception, being dry islands throughout the year. Comparisons of humidity prior to and following rapid urbanization revealed that absolute humidity had generally increased, during winter, all regions displayed increased absolute humidity, by as much as 8%-16% in northern China. This was despite an overall trend towards increased dry island intensity, and was attributed to intensifying urban heat island effects. These results provide valuable information to help understand the causes of dry/wet island effects in urban areas.

Automated summary

Evapotranspiration plays a crucial role in connecting land surface hydrological processes with the atmosphere, impacting regional water cycles and water balance. Urbanization has been shown to affect the hydrological balance, leading to urban wet and dry island effects. The study compared humidity levels in urban areas in China before and after rapid urbanization, and found that cold and dry urban areas tended to have wet island effects, while warm and humid urban areas had dry island effects. Coastal cities were exceptions, remaining dry islands throughout the year. Despite an overall trend towards increased dry island intensity, absolute humidity generally increased, especially during winter with some regions in northern China experiencing up to 8%-16% increases, attributed to intensifying urban heat island effects. These results provide valuable insights into understanding the drivers and mechanisms of dry/wet island effects in urban areas.