Spring floods and their major influential factors in the upper reaches of Jinsha River basin during 2001-2020

Study region: The upper reaches of Jinsha River basin (UJSB) in the northeastern Tibetan Plateau. Study focus: Spring floods in the UJSB have posed increasingly severe challenges to reservoirs operation and water resources management due to increased climatic variability under global warming. In this study, spring floods in the region were investigated for 2001-2020 based on station data and multisource remote sensing products. We seek to understand: 1) the characteristics including the peaks, the frequency, and duration of spring floods, 2) how snowfall and rainfall affect spring floods. New hydrological insights for the region: Late May has seen most of the highest spring flood discharge, while some springs have experienced multiple peaks. Extreme spring floods were identified in the years 2012, 2013, 2019, and 2020, with the highest peak discharge (1365.83 m3/ s) and longest flood duration (47 days) in 2019. Spring snowmelt dominated the extreme spring floods in most years while the floods in 2012 and 2020 were results of combined snowmelt and rainfall. We defined Snow Water Volume (SWV) as an indicator of the precondition for high spring flood. The key regions contributing to spring floods in the UJSB were concentrated around the Tongtianhe sub-basin and the northern parts of Batang-Zhimenda sub-basin where the SWV was large. The enhanced Westerly jets in winters brought about large snowfall and extended snow cover in the UJSB which can be released as floods triggered by rapid increase in air temperature in the coming spring.