Changes in runoff in a typical temperate continental-monsoon transitional zone in the last four centuries

摘要

Temperate continental-monsoon transitional zones are sensitive to variations in the global hydrological climate, and the long-term patterns of hydrological variability in this zone should be explored urgently. In this study, based on the tree-ring width of Platycladus orientalis, we reconstructed the annual runoff sequence during 1666-2016 (351 years) on Wula Mountain, which lies within the low-latitude temperate continental-monsoon transitional zone. Our reconstruction model is stable and reliable because our reconstruction results overlap well with the arid and flood years and periods found in local historical records. Our analysis results show that over the last four centuries, the annual runoff in the low-latitude transitional zone is characterized by an overall decrease, with heightened fluctuations. Specifically, in the 18th century, wet and extremely wet years appeared frequently, while continuous wet periods grew progressively shorter. Later, this area experienced a mean-flow period of approximately 100 years ending in the 1930 s, when progressively longer dry periods began to occur. In the last 30 years, this area has suffered from its longest dry period in 351 years. Regarding runoff variability, over two centuries in the temperate continental-monsoon transitional zone, high-latitude areas had more wet periods with significant variation in large-scale periodicities; the majority of the zone had simultaneous variation in wetting and drying and a significant drying trend in the first half of the 20th century. High- and low-latitude areas had long and slowly changing arid periods, while mid-latitude areas had short and quickly changing arid periods. This study enriches the understanding of the dendrohydrology of the low and middle latitudes of the temperate continental-monsoon transition zone, analyzes the runoff variations throughout the transition zone area, and helps reconstruct the long-term historical runoff sequence over four centuries in the transitional zone.