Impacts of Summer Monsoons on flood characteristics in the Lancang-Mekong River Basin

The impact of monsoon on rainfall in the Lancang-Mekong River Basin (LMRB) has been well understood, but its impact on flood characteristic across the basin is still unclear. To investigate this impact, the Variable Infiltration Capacity (VIC) hydrological model was used to generate the basin-wide discharge and extract flood characteristics. Indian Summer Monsoon (ISM), Western North Pacific Monsoon (WNPM), and their combined effect (ISWN) were considered and represented by monsoon index. The monsoon impact area was firstly obtained based on the monsoon impact on rainfall, followed by the anomaly analyses of flood characteristics within the impact area to quantify the monsoon impact on floods at local and spatial scales. The results show that the ISM and WNPM (or ISWN) can significantly modulate up to 20% of the rainfall interannual variability in the western and eastern parts of the basin, respectively. The monsoon impact on flood is regionally distributed with impact in tributary larger than mainstream. Over half of the monsoon impact areas show the flood start date averagely advances (delays) 8-12 days, flood volume averagely increases (decreases) by 9%-17.5% and Q10 averagely increases (decreases) by 7.4%-14.4% during the strong (weak) monsoon years. Also, the comparisons between monsoon local and spatial impacts reveal that the trade-off of water from different areas can disturb the monsoon impact on flood, suggesting that more stations should be used when using the observed data to analyze the monsoon impact. More importantly, the ISM tends to cause the severe flood in northern Thailand, while WNPM and ISWN mainly induce the severe flood in the southeastern part of the LMRB. This study could help to increase the knowledge of the impact of climate change on flood and help with the regional flood managements.