The Qinling-Bashan Mountains (QBMs) serves as an important boundary between southern and northern China, and is dubbed China's Central Water Tower (CCWT). However, the spatiotemporal structures and dynamics of summer hydroclimate, as well as water vapor source and mechanisms in this CCWT during the peak and most concentrated precipitation period, which is crucial for forest growth, crop yield and water management remain unclear due to the short length of existing decades of observational records.

Tree-ring researchers from the Institute of Earth Environment, Chinese Academy of Sciences, have made significant strides in addressing this knowledge gap by presenting a two-century-long summer relative humidity (RH_JJA) reconstruction for the southern CCWT using high-resolution tree-ring δ¹⁸O records.

This reconstruction explains 43.60% of the instrumental RHJJA variance and, for the first time using proxy data, highlights that two notorious mega-droughts—"the Dingwu drought" and "the 1940–1943 drought"—also significantly impacted the study area. The identified three dry periods (1850–1859, 1920–1943, 1966–1982 CE) and three wet periods (1861–1875, 1885–1898 and 2009–2013 CE) in this reconstruction largely align with broader CCWT hydroclimatic oscillations, indicating decadal synchronicity. However, an annual RH_JJA discrepancies was found between the northern and southern CCWT during 1943–1953 CE, revealing a comparatively wetter (drier) year in southern CCWT versus a comparatively drier (wetter) year in the north. Furthermore, a dry-warm/wet-cold pattern in the CCWT suggests that future warming may exacerbate dry conditions.

The study demonstrates that summer hydroclimatic variations in the CCWT are primarily driven by the Asian summer monsoon (ASM), with water vapor transported by the Indian summer monsoon (ISM) playing a dominant role. These hydroclimatic changes are further modulated by the El Niño-Southern Oscillation (ENSO). 

The finding is pivotal for comprehending the impacts of climate change, managing water resources, and safeguarding ecological systems within the CCWT and other monsoon regions.

This work published in the Journal of Hydrology, was jointly supported by grants from the National Natural Science Foundation of China, the Natural Science Basic Research Program of Shaanxi, the Strategic Priority Research Program of Chinese Academy of Sciences, the Fund of Shandong Province and the National Natural Science Foundation of China.