Land use/cover change (LUCC) affects regional climate not only through its direct changes of land surface properties, but also through its further modifications of land-atmosphere interactions. Urban land expansion, as a typical case of LUCC in highly populated areas, would inevitably have a strong influence on atmospheric humidity, a key variable in hydrometeorology and climate. We employed observation data to examine atmospheric humidity changes by trend analyses of humidity indicators in the midstream urban agglomeration along the Yangtze River during 1965~2017, and found the evident urban dry island (UDI) effects which are characterized by significant relative humidity decrease 1.977% and vapor pressure deficit increase 0.588 hPa per decade, respectively. Furthermore, combining observation and remote sensing data, significant positive correlations between humidity and evapotranspiration, and between evapotranspiration and leaf area were detected during 2001~2017 when cities entered the accelerated stage of land expansion, indicating that LUCC affects regional atmospheric humidity through an eco-hydrological way. Besides, the obvious rise of air temperature was captured much earlier than the decline of atmospheric humidity, and air temperature has not shown marked changes in recent years. Thus, we speculated that the UDI effect will not appear until urban land expands to a certain scale and urbanization-induced LUCC may exert a larger influence on atmospheric humidity than on air temperature in the current later period of urban expansion.
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