Long-term reconstruction of flash floods in the Qilian Mountains, China, based on dendrogeomorphic methods

In China, flash floods are one of the main natural disasters causing loss of life and damage to infrastructure. The threat of flash floods is exacerbated with climate change and increased human activities, such that the number of disasters has shown a clear upward trend in recent years. However, due to the scarcity of instrumental data or overly short timeseries, we are still lacking critical data to understand spatio-temporal patterns and driving factors of extreme flash floods. This missing knowledge is however crucial for a proper management of these hazards, especially in remote mountain environments. In forested catchments, dendrogeomorphology allows the reconstruction of past process activity based on growth disturbances (GDs) in trees that have been affected by past flash floods. Therefore, in our study, for the first time, we reconstruct past flash floods in the Qilian Mountains, northeast Tibetan Plateau, over past centuries. To this end, we sampled 99 Qinghai spruce (Picea crassifolia) trees affected by flash floods, with a total of 194 increment cores, and identified 302 GDs induced by past flash floods. These GDs have been caused by at least 21 flash floods that we are able to reconstruct over the last 170 years. The position of GDs within tree rings and the intra-seasonal dating of past events also allowed discussion of the likely synoptic situations that may have led to the triggering of flash floods in the past. Logistic regression analysis confirms that significant correlation exists between cumulative maximum 5-day August–September precipitation and reconstructed flash floods, which is corresponding to the majority of scars and related tangential rows of traumatic resin ducts (TRDs) found in the latewood portion of growth rings. These results support the idea that abundant precipitation occurring at the end of the summer season and early fall is the key factor driving flash floods in our study area. Our research not only fills the gaps regarding historical flash flood histories in the Qilian Mountains, but also provides a scientific basis for the region’s response to climate change and flood prevention and reduction.
This work was supported by the National Natural Science Foundation of China [Grant No. 41977392], the National Key R&D Program of China [Grant No. 2019YFA0606602], the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA20090000), and Qilian Mountain National Park Qinghai Province management bureau. Jiazhi Qie was financed by the China Scholarship Council