Climate Change And Land Cover Change Impacts On Runoff In The Source Region Of The Yangtze River

The impacts of climate change on water resources mostly depend on type,trends,and magnitude of the temperature and precipitation.The assessment of trends,types,and magnitude of these variables can assist the watershed managers in the selection of mitigation and adaptation practices.The climate is significantly getting warmer at an unprecedented rate,which is non-uniform at both spatial and temporal scales and could have a different scope of impacts in different regions and various aspects of socio-economic sectors.One needs to understand its evolution at a local scale to better evaluate the potential impacts on water resources.Over the decades,the Qinghai Tibetan Plateau(QTP)has been seen as a sensitive area for climate change.Climate change and its impacts are evident in the Qinghai Tibetan Plateau in the last decade.The warming rate is more significant during the last 50 years on the Qinghai Tibetan Plateau as compared to other regions in the world with the same altitude.The general objective of this study was to separate the impact of climate change and landcover/landuse change on runoff from the permafrost covered areas in the Yangtze River Source Region(YRSR).This research was divided into four parts from Chapter 4 to Chapter 7.In chapter 4(first paper),the trends and variability in the temperature variables were assessed in YRSR while in chapter 5(second paper),the climate and streamflow linkages were explored.In chapter 6,(third paper)the separation of the impact of climate change and landcover/landuse were studied for 1990 and 2005.However,in the last chapter 7(fourth paper),the attribution of changes in streamflow due to climate change and landcover/landuse change from 1985-2015were studied.In Chapter 4,the trends in temperature variables and their elevation dependency during1965-2014 in the headwaters of the Yangtze River is presented.The understanding of temperature trends in high elevation mountain areas is an integral part of climate change research and it is critical for assessing the impacts of climate change on water resources including glacier melt,degradation of soils,and active layer thickness.In this study,climate changes were analyzed based on trends in air temperature variables(T_max,T_min,T_mean),and Diurnal Temperature Range(DTR)as well as elevation-dependent warming at annual and seasonal scales in the Headwaters of Yangtze River(HWYZ),Qinghai Tibetan Plateau.The Base Period(1965-2014)was split into two subperiods;Period-I(1965-1989)and Period-II(1990-2014)and the analysis was constrained over two subbasins;Zhimenda and Tuotuohe.Increasing trends were found in absolute changes in temperature variables during Period-II as compared to Period-I.T_max,T_min,and T_mean had significant increasing trends for both sub-basins.The highest significant trends in annual time scale were observed in T_min(1.15°C decade^-1)in Tuotuohe and 0.98°C decade-1 in Zhimenda sub-basins.In Period-II,only the winter season had the highest magnitudes of T_max and T_min 0.58°C decade^-1 and 1.26°C decade^-1 in Tuotuohe subbasin,respectively.Elevation dependent warming analysis revealed that T_max,T_min and T_meantrend magnitudes increase with the increase of elevations in the middle reaches(4000 m to 4400m)of the HWYZ during Period II annually.The increasing trend magnitude during Period-II,for T_max,is 1.77,0.92,and 1.31°C decade-1,for T_min 1.20,1.32 and 1.59°C decade^-1,for T_mean1.51,1.10 and 1.51°C decade-1 at elevations of 4066 m,4175 m and 4415 m respectively in the winter season.T_mean increases during the spring season for>3681m elevations during Period-II,with no particular relation with elevation dependency for other variables.During the summer season in Period-II,T_max,T_min,T_mean increases with the increase of elevations(3681 m to 4415m)in the middle reaches of HYZ.Climatic Variability and Periodicity for Upstream Sub-Basins of the Yangtze River,China explored in chapter 5.The headwaters of the Yangtze River are located on the Qinghai Tibetan Plateau,which is affected by climate change.Here,streamflow trends for Tuotuohe and Zhimenda sub basins and relations to temperature and precipitation trends during 1961–2015were investigated.The modified Mann–Kendall trend test,Pettitt test,wavelet analysis,and multivariate correlation analysis was deployed for this purpose.The temperature and precipitation significantly increased for each sub-basin,and the temperature increase was more significant in Tuotuohe sub-basin as compared to the Zhimenda sub-basin.A statistically significant periodicity of 2–4 years was observed for both sub-basins in different time spans.Higher flow periodicities for Tuotuohe and Zhimenda sub-basin were found after 1991 and2004,respectively,which indicates that these are the change years of trends in streamflows.The influence of temperature on streamflow is more substantial in Tuotuohe sub-basin,which will ultimately impact the melting of glaciers and snowmelt runoff in this sub-basin.Precipitation plays a more critical role in the Zhimenda streamflow.Precipitation and temperature changes in the headwaters of the Yangtze River will change the streamflow variability,which will ultimately impact the hydropower supply and water resources of the Yangtze Basin.This study contributes to the understanding of the dynamics of the hydrological cycle and may lead to better hydrologic system modeling for downstream water resource developments.Separation of the Impact of Landuse/Landcover Change and Climate Change on Runoff in The Upstream Area of The Yangtze River,China were analysed in chapter 6.Landuse/landcover change(LULCC)and climate change(CC)impacts on streamflow in high elevated catchments are very important for sustainable management of water resources and ecological developments.In this research,a statistical technique was used in combination with the Soil and Water Assessment Tool(SWAT)to the Upstream Area of the Yangtze River(UAYR).Different performance criteria(e.g.,R²,NSE,and PBIAS)were used to evaluate the acceptability of the model simulation results.The model provided satisfactory results for monthly simulations in the calibration(R²;0.80,NSE;0.78 and PBIAS;22.3%)and the validation period(R²;0.89,NSE;0.75 and PBIAS;19.1%).Major landuse/landcover transformations from 1990 to 2005 have occurred from low grassland to medium grassland(2%)and wetlands(0.9%),bare land to medium grassland(0.2%),glaciers to wetland(16.8%),and high grassland to medium grassland(5.8%).The results show that there is an increase in average annual runoff at the Zhimenda station in UAYR by 15 mm of,which approximately 98%is caused by climate change and only 2%by landuse/landcover change.The changes evapotranspiration is larger due to climate change as compared to landuse/landcover change,particularly from August to October.Precipitation and temperature have increased during these months.On the contrary,there has been a decrease in evapotranspiration and runoff from October to March which depicts the intra-annual variations in the vegetation in the study area.Attribution of changes in streamflow to climate change and land cover/land use change in a high elevated catchment,Qinghai-Tibetan Plateau were studied in chapter 7.Quantitative attribution of changes in streamflow to climate change(CC)and land cover/land use change(LCLUC)in a hydrological system of the Yangtze River Source Region(YRSR)in the Qinghai-Tibetan Plateau(QTP)was assessed.A combination of the SWAT model and the statistical techniques One Factor At a Time(OFAT)and Innovative Trend Analysis(ITA)was used to achieve the study objectives.Hydro-climatic data from 1961-2016 and land cover/land use maps of 1985,1990,1995,2000,2005,2010,and 2015 were used.The ITA results showed that there is a monotonic rise in temperature,precipitation,and streamflow.In addition,it was found that temperature and precipitation were positively correlated(P<0.05)in high flow months(July and August),and negatively correlated in low flow months(November-March).The calibrationperiod(1964-989)using LCLU map of 1985 was validated for period 1990-2016.This calibrated model was further validated for all other LCLU maps used in this study.The SWAT model simulation showed that using LCLU of 1985-1990,1990-1995,showed that there is an increase in streamflow due to CC as compared to LCLUC.However,SWAT model simulations for land cover/land use maps of 2000-2005,2005-2010,and 2010-2015 did not result in changes in streamflow,because there have not been any significant changes in land cover/land use in the Yangtze River Source Region after 2000 while mainly contributing factor was climate change.The results of the SWAT model simulations showed that the main driver of changes in streamflow in the Yangtze River Source Region is climate change,while evapotranspiration is also mainly influenced by climate change as compared to land cover/land use change.This study shows that,the individual impacts are more critical than combined impacts for designing hydraulic structures,water resources planning and management,and decision-making policies at the regional/basin scale.