Effects Of Ecohydrological Changes On “Urban Dry Islands” Across China

Urbanization leads to differences in near-surface atmospheric humidity between cities and surrounding suburbs,creating an urban dry island/wet island effect.Changes in atmospheric humidity associated with urbanization have important implications for climate change prediction and impact assessment,such as cloud formation,rainfall intensity,human thermal comfort,and wildfires at the urban-forest interface.In this paper,we use various techniques and methods such as ground-based comparative observation,remote sensing monitoring technology,mesoscale numerical simulation,statistical analysis,and based energy balance contribution splitting to finely characterize the urban hydrothermal change process at multiple scales.Using vapor pressure deficit and specific humidity as atmospheric dry / humidity indicators,the spatial and temporal distribution patterns and change characteristics of urban dry islands in five typical urban agglomerations in four climate zones of China in the past four decades are examined,including the Pearl River Delta in the humid zone,the Yangtze River Delta in the semi-humid zone,the Chengdu-Chongqing in the semi-arid region,the Beijing-Tianjin-Hebei in the semiarid region,and the northern Tianshan in the arid region.Regional land cover changes(e.g.,leaf area index),hydrological changes(e.g.,reduced evapotranspiration due to urbanization)and urban climate change(temperature and humidity)are coupled to quantitatively characterize the hydroclimatic feedbacks of cover changes in urban agglomeration and to elucidate the chain reaction mechanism of ‘land cover pattern ? ecohydrological processes ? coupled hydrothermal balance ? urban dry island effect'.The specific findings are as follows.(1)The five large urban agglomerations in China have experienced aggravated atmospheric drying during the past 30 years.The UDI effects were significantly enhanced and more pronounced during daytime and growing seasons.Based on empirical data,this study revealed a plausible connection between land-use change and an urban atmospheric moisture drying phenomenon,or UDI effect across a large climatic gradient.We show that the five large urban agglomerations in China have experienced aggravated atmospheric drying during the past 30 years.During 1980-2018,the urban near-surface atmosphere became drier than the surrounding suburbs in the five major urban agglomerations with different climate gradients,and the urban dry island effect(?VPD > 0 or ?q < 0)appeared around 2000.The urban dry island effect was stronger and occurred more frequently during daytime;the intensity and frequency of urban dry island were more pronounced in the growing season than in other seasons.Although nighttime UWI partially offset the daily UDI effect and made daily UDI underestimated during 2000–2010,after 2010,it weakened or disappeared in humid and semihumid areas,resulting in a significant increase in daily UDI.(2)Urbanization exacerbates global warming and ‘Urban Heat Islands' effects on UDI through reducing evapotranspiration and water vapor availability.Attribution analysis found that the rise of air temperature alone was not sufficient to explain the observed increase in ?VPD after 2000.The loss of water vapor due to urbanization aggravated atmospheric drying(?VPD > 0)after 2000.Moreover,correlations between annual mean?VPD and ?q and change in common biophysical variables such as leaf area index(?LAI),evapotranspiration(?ET),impervious surface area(?ISAs),and near-surface air temperature(?Ta)across a climatic gradient as quantified by aridity index(PET/P)confirmed that ecohydrological processes such as landcover change-induced change in ET contributed to UDI.Due to differences in vegetation and local climate,the intensity and frequency of urban dry islands are more pronounced in humid areas than in arid areas.Urbanization is more likely to cause ‘Oasis Wet Islands'(?q > 0)effects in the arid zone(e.g.,the TSB area),while urbanization is more likely to trigger the ‘Urban Dry Island'(?q < 0)effects in the humid regions.The arid zone(TSB)became drier with increased VPD as those in other humid regions,but the drying trend was mostly caused by global climate warming and localized UHI effects.However,in humid ‘energy limited' regions where soil water does not limit ET,biophysical factors such as leaf biomass that directly affects ET becomes more important.In addition to global warming and localized UHI effects,the loss of vegetation cover(i.e.,forests,natural and man-made wetlands such as rice paddies)can result in loss of ET and water vapor sources,thus jointly caused the increases in VPD and decline in q in urban areas in humid regions.With the acceleration of worldwide urbanization and a drying trend of atmospheric humidity(i.e.,increased VPD)due to climate warming,the UDI effects may become more pronounced and common in the future.(3)Numerical simulations and contribution splitting further confirmed that the surface evaporation impedance term associated with the vegetation canopy is the main control factor influencing the difference in atmospheric humidity ?q between different subsurfaces.Our simulation and statistical analysis also found that strong UHI and UDI processes are often closely coupled through latent heat or ET processes,especially after 2010.Less ET in cities leads to less latent heat consumption,which is conducive to urban warming and causes a UHI effect.The occurrence of the UHI strengthens the vertical thermal turbulence,which is conducive to the transportation of near-surface water vapor to the upper layer,resulting in the reduction of the near-surface water vapor pressure and strengthening of the UDI effect.The results of the two different splitting methods based on energy balance show that in the intrinsic biophysical mechanism(IBM)splitting results indicate that the air and the vegetation canopy-related surface evaporation resistance term are the dominant factors influencing the difference in atmospheric humidity ?q between different land use and land cover.The research highlights the importance of vegetation in affecting local atmospheric moisture and temperature,and for developing mitigation and adaptation strategies in response to climate change.Maintaining forest vegetation and wetlands,thus the evapotranspiration power of nature,should be a core element of ‘Low-Impact Development' and ‘Nature-based Solutions'.These strategies represent modern integrated watershed management options to mitigate the negative environmental impacts of urbanization.Measures that mitigate impacts of urbanization on UHI and UDI likely benefit ecosystems as well as human health in an increasingly urbanized world.