Abstract: As the most important transnational water system in Asia, the Lancang River plays a crucial role as part of the “Asian Water Tower”. Analyzing the evolutionary patterns and trends of upstream river runoff in the Lancang River has significant strategic implications for water resource utilization and regional green energy development. The present study focuses on the runoff characteristics and their evolution in the upstream of Lancang River over the past 30 years based on data from the Wunonglong section, as well as air temperature and precipitation records. Methodologies including linear regression analysis, the Mann-Kendall trend test, and Pearson correlation coefficient were employed for data analysis. The findings reveal that, over the decades since the availability of discharge data, the upstream runoff in the Lancang River has exhibited little change at the decadal scale. However, on a seasonal basis, there is a tendency of increased runoff in winter and decreased runoff in summer. Notably, a distinct shift occurred in August in the early 1970s, with a prominent decreasing trend of 4.8% per decade, approximately equivalent to a total volume reduction of 200 million cubic meters per decade. Moreover, this decreasing trend has been further accelerating in the past 30 years, consistent with the decreasing trend of precipitation in the middle region of the upstream area. The study also reveals that, over the past three decades, as spring temperatures have risen, the response of upstream runoff to temperature in May has weakened, while the relationship between temperature in April and runoff has become more pronounced. Given the background of global warming, the advancing snowmelt period in the upstream area of the Lancang River requires high attention and proactive measures.

Key words: water resources, runoff evolution, trend regression, response analysis, upstream of Lancang River