Half-century urban drying in Shijiazhuang City**Submitted to Environmental Research Letters for possible publication.

Abstract

Climatic and environmental change in urban areas attracts more and more attention with rapid urbanization worldwide. Change in key surface climatic variables in urban areas is a core component of urban climatic change. Here we report an analysis of relative humidity (RH) change in Shijiazhuang City, China, over the period 1963–2012, based on the RH data of a typical urban station and the surrounding rural stations. We found that the annual mean RH at the urban station underwent a more significant downward trend (−0.74%/10 yr) than those at the four rural stations (−0.16%/10 yr on average), and the most significant decrease occurred after 1990s; urbanization effect on the decrease of annual mean RH at the urban station reached −0.59%/10 yr, which was highly significant, with the urbanization contribution to the overall RH decrease being 78.7%, indicating that the trend of urban drying was caused dominantly by the urbanization; urbanization effects on seasonal mean RH decrease at the urban station were −0.60%/10 yr, −0.61%/10 yr, −0.67%/10 yr and −0.46%/10 yr respectively for spring, summer, autumn and winter, which were all significant statistically, with a 100% urbanization contribution for winter; rising temperature, dropping evaporation and decreasing precipitation days relative to rural areas were found to be the main factors affecting the urban RH decrease. The conclusions are of practical significance to further understand the characteristics and causes of RH change at a typical urban meteorological station in North China, and to detect the large-scale climatic and environmental change for assessing the impact of climate change on agricultural production and water cycle.