Moisture sources and pathways of annual maximum precipitation in the Lancang-Mekong River Basin

Extremely heavy precipitation leads to increasingly frequent floods, landslides, debris flow, storm surges, and other natural hazards in the Lancang-Mekong River basin (LMRB) that causes large amounts of economic loss and affected millions of residences. This study analyzed the spatial-temporal characteristics of the annual maximum precipitation (R1X) of the LMRB and identified the moisture sources and pathways conducive to the occurrences of these extreme precipitation events during 1965-2021. Results show that the R1X of the upstream region concentrated in July, while that of the downstream region mainly occurred from August to September. The regional mean R1X shows an increasing trend, especially after 2010. The moisture pathways of the historical R1X were identified through a Lagrangian back trajectory model and were classified into three clusters by the Self-Organize Map: West Pacific Ocean (WP), local evapotranspiration, and Bay of Bengal (BOB). BOB provided the main moisture source to the R1X of the LMRB which contributes 68.3% of the trajectories, while the local evapotranspiration and WP account for 20.4% and 11.3%, respectively. For most areas downstream of LMRB, the moisture from the BOB transported through the cross-equator flow is the main moisture pathways patterns. For the upstream of LMRB, the evapotranspiration from the local and neighboring terrestrial and oceanic surfaces provides the main moisture sources. For the east area of the downstream, R1Xs are high and mainly resulted from tropical cyclones bringing large amounts of moisture from the WP to the LMRB. As tropical cyclones moved northward under climate change, more extreme precipitation over the LMRB was fed by the moisture from WP, while those from the BOB is decreasing with the slowdown of cross-tropical flows.