Conversion of agricultural lands to urban uses affects regional and global climate not only through the release of greenhouse gases but also through altering land surface physical processes such as energy and water balances. Most existing studies on the meteorological impacts of urbanization focus on urban heat island effects with little attention on its impacts of atmospheric humidity, a key variable in hydrometeorology and climate science. We define the influences of urbanization on reducing atmospheric humidity and elevating vapor pressure deficit as urban dry island (UDI) effects. We conduct a case study in the Yangtze River Delta, a typical humid area in southern China that is under rapid urbanization. We examine spatiotemporal characteristics of UDI and identify potential drivers during 2001–2014. Relationships and interactions between variations of air temperature, atmospheric humidity, evapotranspiration, and leaf area index of different land cover were determined using correlation and attribution analyses at both station and regional levels. We show that atmospheric humidity decreased dramatically and vapor pressure deficit increased sharply in the urban core, resulting in enhanced UDI. In addition to global warming and localized urban heat island, UDI is closely related to the loss of vegetation cover (i.e., natural wetlands and paddies). Reduction of evapotranspiration or latent heat is another important factor contributing to UDI effects. We conclude that the role of vegetated land cover and associated ecohydrological processes in moderating UDI and maintaining a stable climate and environment should be considered in massive urban planning and global change impact assessment in southern China.

‘Urban Dry (Wet) Islands’ (UDI/UWI) represents microclimate change that impacts ecosystems and human well-being. However, causes of the UDI (UWI) phenomena are not fully understood due to the lack of empirical data. Here, we quantify UDI (UWI) using global observations of atmospheric humidity, evapotranspiration (ET), and land surface characteristics across 25 large urban agglomerations. We show that UDI (17) and UWI (8) are closely tied to local ET, global warming, and ‘Urban Heat Islands’ through intertwined linkages with water and energy balances. UDI is most pronounced in humid vegetated regions where mean urban-rural annual ET differences are as high as 215 mm, whereas UWI is found in arid regions or in climates with dry summers. We conclude that ET can be used as a single variable to explain emerging urban environmental changes. Our study supports a concerted strategy of restoring nature’s ET power as effective ‘Nature-based Solutions’ to mitigate the negative environmental effects of urbanization.

Rapid global vegetation greening has been observed for the past two decades, but its implications to the hydrological cycle are not well understood in many regions, including the Yangtze River Basin (YRB). This study used a remote sensing‐driven ecosystem model, the Coupled Carbon and Water model, to fully examine the individual and combined hydrological effects of vegetation and climate changes through a series of modeling experiments. During the study period (2001–2018), the vegetation showed a significant greening trend with the mean annual normalized difference vegetation index increasing at a rate of 0.4% per year (p < 0.001). In contrast, climate exhibited a marginal wetting trend with annual precipitation increasing at a rate of 6.7 mm/yr (p = 0.08). Annual evapotranspiration (ET) in the YRB significantly increased (3.1 mm/yr,p = 0.01) primarily due to enhanced ecosystem productivity associated with vegetation greening, rather than climatic factors. However, the enhancement in ET did not lead to a significant decline in total water yield at the YRB scale. The large inter‐annual variability of precipitation masked the effects of vegetation greening on water yield. Overall, our study indicated that the recent land “greening up” has accelerated the regional hydrological cycle through increasing ET and resulted in enhanced risks of water resource shortage. Our findings highlighted the close connection between land cover dynamics and hydrological cycle under climate variability in one of the world’s largest river systems. Effective basin water resource management must consider hydrological response to vegetation greening and climate change.