Impact Of Human Activities And Climate Change On Water And Sediment Process In Upper Yellow River Basin

Global climate change combined with the impact of human activities make an unprecedented change of the basin water cycle characteristics and then alter the spatial and temporal distribution of runoff and sediment severely.As a mother river in China,the Yellow River Basin showed a significant reduction in runoff and sediment in recent decades.How to evaluate the effects and contribution of climate change and human activities on runoff and sediment,quantify its impact,has become hot spots and problems.Based on the background above,takes the Upper Yellow River Basin(UYRB)as the study area,the hydro-meteorological data from 1957 to 2010 of 34 meteorological stations and Lanzhou hydrological station as data support,combined with the soil type and land use data of the study area,this paper studied the evolution of land use and climate change in UYRB during the past 50 years and then predicted its future change characteristics.Furthermore,the unique SWAT model of UYRB was built to analyze and quantify the impact of land use and climate change on the process of water and sediment cycle.At the same time,water and sediment process in UYRB under future land use and climate change scenarios were also predicted.The main conclusions are as follows:1.Annual precipitation in the study area showed indistinctively increase trend during the measured periods.During the year,precipitation increased from January to June(except April),decreased from July to November(except October)while there was no obvious change in December.Precipitation in UYRB is increasing from northwest to southeast while the trend decreasing from positive to negative.Results of abrupt change test showed that the change of precipitation in UYRB has one mutation point in year 1992,and the annual mean value of precipitation before and after this point decreased indistinctively.Periodic tests showed that precipitation changes during the measured periods exist two time scale cycle vibration,which are 4?7a and more than 30a.The trend of the highest and the lowest temperature during the measured periods presented a stair type rise with a significance level of more than 99%.Among which,the highest temperature had a mutation pointin year 1997,and the mean annual value significantly increased before and after this mutation point.The mutation point of the lowest temperature was also in year 1997.Periodic test showed that three time scale cycle vibration occurred in the highest temperature change during the measured periods,which are 4?7a?10?20a and more than 30a;and two time scale cycle vibration occurred in the lowest temperature,which are 8?15a and 23?30a.2.Future climate change in UYRB among three dififerent emission scenarios were forecasted,the results showed that the mean annual value of precipitation among three emission scenarios in different forecast periods were all higher than that in the measured periods except in 2025s of A1B scenario.Out of which,the most significant increase presented in A2 scenario which increased by about 19.4 up to 2085s compared with the mean annual precipitation during the measured periods.The annual variation of precipitation showed an increase trend first and then decrease from southwest to northeast in the study area under A1B scenario,an increase trend first and then decrease from northwest to southeast under A2 scenario while a continuous increase trend from southwest to northeast under B1 scenario.The prediction results of the highest and the lowest temperature under three different emission scenarios were all presented increase trend compared with that in the measured periods with change ranges A2>A1B>B1.Spatial distribution of the highest and the lowest temperature change trends showed difference and significant seasonal characteristics due to the difference of emission scenarios.3.Land use in UYRB is dominated by grassland,woodland,cultivated land and bare land.From year 1990 to year 2000,there was a significant increase of wetland while a large area of decrease of grassland.From year 2000 to year 2005,land use in UYRB showed the most dramatic change with the transfer rate of up to 31817.9km2.During this time,residential land increased greatly while grassland reduced.A large area of bare land transferred into grassland during the periods of 2005?2014.During the whole statistical periods of 1990?2014,the study area experienced a huge decrease in both bare land and cultivated land that were transferred into grassland and woodland respectively.The spatial distribution of land use change during the whole statistical periods showed a concentration of bare land transferred into grassland in the southwest part of the basin,some grassland transferred into bare land in the middle-upper and south parts while there were no obvious land use change in the restparts of the basin.The annual change rate of land use reached the maximum during the period of 2000?2005,up to about 1.26%.4.The mean annual value of natural runoff during the measured periods inUYRB is 307.18×108m3,mainly comes from May to October.The mean annual value of sediment load is 0.61×108t,mainly concentrates from July to August.Both runoff and sediment load experienced significant decreases during the whole measured periods.Results of M-K test showed that there existed two mutation point of runoff change that were in years 1982and 1997,while the mutation point of sediment change was in year 1968.The results of quantifying the impact of climate change and human activities on runoff and sediment load in UYRB showed that the contribution rate of meteorological factors to runoff change is 48.61%,while the contribution value of human activities is 51.39%;the contribution rate of precipitation change to sediment load is about 16.84,while the contribution value of human activities is 83.16%.5.SWAT model was applied to simulate the runoff and sediment load process in UYRB,the results showed that R2 are all more than 0.7,and NSE are all more than 0.5 except in verification period where the annual scale is relatively small,which achieved satisfactory results.Under future climate change scenarios,the mean annual values of runoff and sediment load decreased by about 6.67%and 18.75%respectively compared with that of the base period under A1B emission scenario;the mean annual values of runoff and sediment load increased by about 7.54%and 13.56%respectively compared with that of the base period under A2 emission scenario;both the simulated runoff and sediment load presented decrease trends under B1 emission scenario compared with that of the base period.Under future land use change scenarios,the simulated mean annual values of both runoff and sediment load are all less than that of the base period.Under the combined climate change and land use scenarios,the simulated mean annual values of both runoff and sediment load are also less than that of the base period,with the significant decrease under L2-B1 scenario.