Effects of alpine swamp wetland change on rainfall season runoff and flood characteristics in the headwater area of the Yangtze River

The effects of changes in alpine swamp wetland (ASW) on rainfall season runoff and flood characteristics have not been quantified adequately in the headwater area of the Yangtze River (HYR).To address this, we focus on detecting changes in ASW based on remote sensing data. In particular, we analyzed the effects of ASW on runoff (both runoff and runoff coefficient) and flood characteristics (i.e., peak flow, rising time, duration and flood runoff coefficient) during the rainfall season based on observed runoff and climate data. Our results indicate that the area of ASW decreased by 28.22% and 30.54% at Zhimenda (ZMD) and Tuotuohe (TTH), respectively, from 1975 to 2004. Moreover, 76% and 71% of the decreases at ZMD and TTH, respectively, occurred during the period from 1990 to 2004. Both runoff and the runoff coefficients decreased at ZMD and increased at TTH with decreasing ASW in the rainfall season; we attribute this to the variability in runoff mechanisms. The effects of ASW on runoff and runoff coefficients were more pronounced at ZMD than at TTH. A slightly lower maximum peak flow and a greater frequency of moderate and small peak flows were observed at ZMD. Conversely, maximum peak flow increased and the frequency of maximum and moderate peak flows increased at TTH. Flood events increased at both ZMD and TTH, whereas both the rising time and duration decreased at both sites. Overall, the volume of snowmelt water increased and the ability to conserve water decreased in response to decreased ASW and permafrost were responsible for the observed changes in flood characteristics.