Evaluation And Projection Of Extreme Climate Changes And Its Impacts On River Runoff And Sediment Load On The Loess Plateau

Extreme climate events are the most important driving factors on the Loess Plateau,one of the regions with the most soil erosion in the world.Therefore,it is critical to identify the current and future variations of climate extremes and river runoff and sediment load on the Loess Plateau,which benefits the ecological construction,and even supports the strategy of ecological environmental protection and high-quality development in the Yellow River Basin.Based on the data of 80 meteorological stations from 1961 to 2017,this study selected 12 representative extreme climate indices to analyze the spatiotemporal variation characteristics of extreme climate events,and further explored the combined effects of 12 atmospheric circulation factors on different time scales.Thereafter,based on the runoff and sediment load data in eight watersheds from 1961 to 2017,this study analyzed the interannual and decadal variation characteristics of runoff and sediment load,and further quantified the contribution of extreme climate events to the variations of runoff and sediment load in major watersheds.Finally,after correcting five regional climate model inputs by the trend-preserving bias correction method,this study projected variation in the climate extremes and runoff and sediment load in eight watersheds under different emission scenarios from 2021 to 2050.The main conclusions are as follows:(1)Most of the extreme climate indices on the Loess Plateau showed a significant change trend and were affected by single or combined atmospheric circulation factors.The intensity and frequency of precipitation extremes tended to increase,while the duration tended to decrease.They overall decreased from southeast to northwest in space.The warm and cold extremes mainly showed a significant upward and downward trend,respectively.They were high in the south and low in the north,with most of the stations showing a significant change trend.In addition,the temperature had a greater correlation with the duration of extreme precipitation events in summer,while the intensity and frequency of extreme precipitation events in winter.The extreme precipitation indices were predominantly affected by the single or multiple combined factors(Arctic Oscillation,the Pacific-North American Oscillation and the El Ni?o-Southern Oscillation),while the extreme temperature indices had a greater relationship with the combination of Arctic Oscillation,East Asian Summer Monsoon,Asian Zonal Index and Qinghai-Tibet Plateau Index.(2)The correlation between precipitation extremes and runoff was stronger than that of sediment load in the watersheds,and human activities were the main factor in reducing runoff and sediment load recently.From 1961 to 2017,the runoff and sediment load in the watersheds on the Loess Plateau showed a significant downward trend,and the abrupt change mostly occurred in the 1990 s.The amount of runoff and sediment load in the watersheds reached the maximum value mainly in the 1960 s.The runoff in the watersheds had rebounded after 2011,but the amount of sediment load still tended to decrease.During the past 57 years,there had been a strong correlation between runoff and the heavy precipitation days in the watersheds,with the highest correlation coefficient of 0.88.There was a stronger correlation between sediment load and the maximum 5-day precipitation,with the highest correlation coefficient of 0.78.Human activities were the main factor in reducing runoff and sediment load in the watersheds.The contribution rates were 68%–129% and 85%–113%,which were greater than the impacts of extreme rainfall.(3)The corrected-RCMs inputs could better simulate the climate change in the historical period and reduce the uncertainty of future climate extremes and runoff and sediment load projections in major watersheds on the Loess Plateau.After using the trend-preserving bias correction method,the RCMs well reproduced the climate means and extremes in the historical period and maintained the changing trend.Compared with the period of 1980–2005,the intensity of precipitation extremes would decrease,while the frequency and duration of precipitation extremes would increase.Furthermore,the warm extremes would increase and the cold extreme events would decrease from 2021 to 2050.The precipitation extremes largely changed in the southeast,while the temperature extremes dramatically changed in the west.Compared with the reference period(before the abrupt change),the runoff in each basin would overall show an increasing trend in the next 30 years under different scenarios,while the sediment load would still show a decreasing trend.This study analyzed the spatiotemporal variation characteristics and main influencing factors of climate extremes and runoff and sediment load on the Loess Plateau during the historical period,and thus predicted the future changes of extreme climate events and runoff and sediment load in major watersheds under various scenarios.This study provided valuable insights into ecological environment construction and the management of runoff and sediment load in the watersheds on the Loess Plateau.