Lake Level Variations In Southwest China Over The Last Millennium

Knowledge of the historical lake level variations and their driving mechanisms is critical for water resources management,ecology/environment protection,droughts and floods prediction/adaption,as well as understanding of regional/global hydroclimatic forcing.There are many lakes in southwest China.However,the current research on lakes in southwest China mainly focuses on the Holocene climate change,water quality change,eutrophication and heavy metal pollution in the last 100 years.The research on lake level,especially ancient lakes,is still insufficient and the method is relatively simple.In this study we show the last thousand years lake level history based on high-precision ¹⁴C-dated paleo-shorelines at Lake Yangzong,Yunnan province,and summarize lake level changes in southwest China over the past thousand years by comparing with historical literature records and other lake level reconstructions in adjacent areas.In order to further explore the possible driving mechanism of hydroclimate models in southwest China,we made a preliminary analysis on the modern hydrological observation data of Lake Yangzong.We then extend the lake level changes and hydroclimate models in the southwestern region over the past millennium to the entire Indian summer monsoon region for comparative analysis.The results show as follows:（1）The seasonal lake level of Lake Yangzong dropped first and then rose,with the lowest lake level from June to July,and the highest lake level from November to January of the following year.This seasonal lake level variation trend is mainly caused by artificial anti-seasonal regulation.The inter-annual variation of the annual mean lake level in Lake Yangzong indicates that the lake level showed a decreasing trend during1963-2017,which is similar to the long-term decreasing trend of precipitation in the past decade.Further analysis shows that although human activities have a significant influence on seasonal lake level changes,natural climatic processes,especially precipitation,are the main factor that controls the long-term trend of lake level changes in the study area.（2）The AMS ¹⁴C age of the snail shell in the carbonate rock fractures on the east coast of Lake Yangzong and the height of the measured terrace sampling points indicate that the water level of Lake Yangzong was high during AD 500-900,and low in AD900-1350,AD 1350 later the water level rose again and began to fall after AD 1400.（3）The lake level variations in southwestern China can be roughly divided into four stages:high lake levels during AD 500-900,low lake levels during AD 900-1350（Medieval Warm Period）,rising lake levels during AD 1350-1800（Little Ice Age）,and generally falling lake levels in the past～100 years.（4）The lake level variations in southwest China over the last millennium have broadly similar trends to both lake level variations and monsoon precipitation patterns over the extended Indian summer monsoon area.We contend that the centennial lake level changes and the hydroclimatic patterns may be related to the movement of the planetary atmospheric circulations regulated by changes in tropical sea surface temperature.Human activities could have also partly impacted the lake level changes of the last millennium,and may have played a more important role in influencing the rate and magnitude of lake level drops during dry intervals than wet intervals in the study area.