The Qinling-Bashan Mountains (QBMs) serve as an important boundary between southern and northern China and are dubbed China's Central Water Tower (CCWT). However, the spatiotemporal structures and dynamics of summer hydroclimate, as well as the water vapor sources 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. This knowledge gap stems from the short length of existing observational records, which span only a few decades.

To address this gap, researchers from the Institute of Earth Environment of the Chinese Academy of Sciences have presented a two-century-long reconstruction of summer relative humidity (RH_JJA) for the southern CCWT. The reconstruction relies on high-resolution tree-ring δ¹⁸O records.

Notably, this reconstruction explains 43.60% of the variance in instrumental RH_JJA data. Moreover, using proxy data for the first time, it highlights that two notorious mega-droughts—the "Dingwu drought" and the "1940–1943 drought"—also had a significant impact on the study area.

The reconstruction identifies three dry periods (1850–1859 CE, 1920–1943 CE, 1966–1982 CE) and three wet periods (1861–1875 CE, 1885–1898 CE, 2009–2013 CE). These periods largely align with broader hydroclimatic oscillations across the CCWT, indicating decadal-scale synchronicity in the region's climate patterns.

However, a key finding reveals annual RH_JJA discrepancies between the northern and southern CCWT during 1943–1953 CE. Specifically, years that were comparatively wetter in the southern CCWT were drier in the north, and vice versa. Furthermore, the study identifies a "dry-warm/wet-cold" pattern in the CCWT, suggesting that future warming may exacerbate dry conditions in the region.

The study also clarifies the drivers of summer hydroclimatic variations in the CCWT: these changes are primarily driven by the Asian Summer Monsoon (ASM), with water vapor transported by the Indian Summer Monsoon (ISM) playing a dominant role. Additionally, the El Niño-Southern Oscillation (ENSO) further modulates these hydroclimatic shifts.

The findings are pivotal for comprehending the impacts of climate change, managing water resources sustainably, and safeguarding ecological systems—not only within the CCWT but also in other monsoon-influenced regions globally.

This work, recently published in Journal of Hydrology, was support by the National Natural Science Foundation of China, and the Natural Science Basic Research Program of Shaanxi Province, among other sources.