Analysis Of Inter-Relationships Between Climate Change And Runoff Discharge In The Headwater Region Of Yellow River

Runoff discharge in the headwater region is the important resources for ecological and agricultural water usage along the Yellow River.However,there lack systematic studies and conclusive understanding on the extent of impacts of changes in precipitation caused by global climate change on the runoff discharge into the Yellow River.By collecting and synthesizing data on climate,runoff and vegetation during 1958-2017 for the headwater region of Yellow River,with application of multivariate statistical methods and GIS system,this dissertation analyzed the the patterns of changes in climate,vegetation and runoff and their inter-relationships over the past half a century in the headwater region of Yellow River.The objectives were to address the questions of:(1)What were the patterns of changes in climate and runoff discharge in the headwater region of Yellow River over the past half a century?(2)Were changes in runoff discharge and climate synchronized during the study period?(3)If not synchronized,what would be the causes of the discrepancy between changes in runoff discharge and climate?In this study,it was found that during 1958-2017,the annual runoff marked reduced at a rate of 477 million m3/decade.There was an abrupt change point in 1989,with a significant difference between the runoff before and after this point: there were 8-year and 4-year cyclic patterns during 1958-1988,and 6-year and 3-year cyclic patterns during 1989-2017.The annual precipitation displayed a trend of decline in the early part of the study period and increases in the later part,similar to the patterns of runoff;the long-term linear trend weakly decreased at a rate of 0.1 mm/decade.The decadal analysis shows that in the 21 st century the precipitation in the headwater region of Yellow River recovered to or exceeded the long-term average value,with lagged responses in runoff by a decade.After the abrupt change point,there was a mismatch between precipitation and runoff.During the period of study,the mean annual temperature(MAT)continuously increased with a long-term rate of change at 0.38 ?C/decade.The increases in temperature were most apparent after 2000;the average value was 0.97 ?C higher than during 1990's.Within the study region annual evapotranspiration(ET)displayed a temporal pattern of decline-increases: during 1958-1989,ET fluctuated with a long-term trend of decline at 18.37 mm/decade;starting from the mid-1990,the annual ET continuously increased by 77 mm until 2017.The changes in MAT and annual ET were all significant,with abrupt change points in 1997 and 2002,which lagged the abrupt change point in 1988 for runoff.The cyclic pattern of ET changed from a 8-year cycle prior to the occurrence of abrupt change point for runoff to a 6-year cycle thereafter,with the peaks and valleys being inverse to those for runoff and precipitation.At a decadal scale,the mismatch between precipitation and runoff after 2000 are explainable by increases in temperature and ET,which tended to replace precipitation as the major influencing factors.An abrupt change was detected around 1989 in the Standardized Precipitation Index for 12 months(SPI12).Both the frequency and duration of various degrees of drought were smaller before the abrupt change than after,with inverse patterns of differences for flooding.During 1958-1988,there were similar incidents of drought conditions(SPI12 < 0)and flooding conditions(SPI12 > 0);whereas during 1989-2017,60% of the time in the headwater region of Yellow River were under drought conditions.In SPI12,a 48-month cyclic pattern and a 96-month cyclic pattern were detected during 1958-1988,whilst during 1989-2017 a 72-month cyclic pattern and two 24-month cyclic patterns were detected,in consistency with those of runoff.At an annual scale,there was no apparent inter-relationship between changes in temperature and runoff,but the rainfall factor inferred by SPI12 showed highly consistent patterns with changes in runoff.During the period 1989-2017 following the abrupt change point in runoff,there was a consistent improvement in vegetation.NDVI in the areas from headwater to Dari and from Dari to Maqu significantly increased,which to a certain extent affected the runoff discharge.Among the sub-watersheds in the headwater region of Yellow River,the section from the headwater to Dari experienced increases in precipitation,temperature and ET,with overall steady runoff.In the section from Dari to Maqu,there was a significant decline in precipitation associated with significant increases in temperature and ET,leading to most marked decreases in runoff;the precipitation and runoff did not recover to the long-term norm until 2017.The section from Maqu to Tangnaihai experienced increases in precipitation and significant increases in temperature without notable changes in ET;the runoff significantly decreased until reaching a gradual recovery starting in 2010.In all the sub-watersheds,the patterns of changes and occurrence of abrupt change point in SPI12 were highly consistent with patterns of runoff of the corresponding sections,and the cyclic patterns were highly consistent with the entire headwater region of Yellow River.The above results indicate that the mechanism of runoff discharge in the headwater region of Yellow River is changing.The weakening influences of rainfall factors may be the cause of abrupt changes in runoff,resulting in mismatches between changes in precipitation and runoff after the occurrence of abrupt change point;whilst the influences of temperature and ET are strengthening.At the same time,the continuous improvement of vegetation imposes adverse effects on runoff,especially in sub-watershed above Maqu.The climate change imposes profound impacts on runoff production and ecological processes in the headwater region of Yellow River,such that the major influencing factors of runoff discharge are changing from precipitation to coupled temperature-ET changes.This would not only affect the underlying processes of runoff production,but also the vegetation,consequently leading to ecosystem succession.A continuation and intensification of climate change could cause a series of chain-reactions such as degradation of permafrost,changes in water-proof layer,groundwater decline,disassociation between above-and below-ground systems,and degradation of plant community and ecosystems,causing great uncertainties in future runoff discharge,making predictions of water resources in the headwater region of Yellow River more difficult,and leading to ecological risks in the headwater region or the entire reaches of Yellow River.