Hydrological Responses To Land Use And Climate Change In The Upper And Middle Yellow River

In recent decades,the sharply reducing of the streamflow and sediment load of the Yellow River widely aroused people's concern.Quantifying the impact of environmental change on streamflow and sediment load is extremely important for the sustainable watershed management strategies making.Numerous studies have been applied in most tributaries and sub-watershed of the Yellow River Basin to explore water-sediment evolution and driving forces under the condition of climate and land use change.However,fewer studies have combined statistics and models to quantify the hydrological processes in the Macroscale watershed.In this study,hydrological responses to land use and climate change in the middle and upper reaches of the Yellow River basin(UMYRB)were quantified by combining statistical experiments and hydrological model SWAT(Soil and Water Assessment Tool).In addition,downscaling Global Climate Models(GCMs)used to predict climate conditions in the region during 2008-2099 under RCP2.6,RCP4.5,and RCP8.5 emissions scenarios,as well as the evolution of hydrological processes in the UMYRB with future climate change.The main research conclusions are as follows:(1)Nonparametric Mann-Kendall and Pettitt's test were used to detect the temporal variation and abrupt trend of streamflow as well as sediment load during 1956-2016.The results showed that the streamflow and sediment load decreased significantly,and the decrease degree of streamflow was lower than that of sediment load.Furthermore,the changes of water and sediment in the lower reaches were greater than that in the upper reaches.The streamflow and sediment load at Huayuankou station,which was in the outlet of the study basin,decreased by about 43%and 86%,respectively.The abrupt change point detection results showed that the time of water and sediment at the downstream stations lagged behind that at the upstream stations,appeared in the mid-1980s and the late 1990s,respectively.(2)The trend of meteorological time series from 1971-2016 showed that precipitation increased insignificant with 0.29mm/year,while temperature increased significant with 0.035?/year.Compared with the base period(1971-1990),the average annual precipitation in the basin decreased slightly(4mm)and the temperature increased significantly(0.9?)during 1991-2016.In this study,the temperature rose significantly,and the slight increase of precipitation and the significant increase of wind speed in the basin contributed to the increase of evapotranspiration and the significant decrease of relative humidity to a certain extent.The mean values of the three atmospheric models under the RCP2.6,RCP4.5,and RCP8.5 emission for the period 2008-2099 relative to the base period 1976-2005,the maximum temperature ranging from 1.1-4.8?and the minimum temperature ranging from 1.1-5.1?,precipitation increased slightly,with an increase rate of 5.3%-19.9%.(3)From 1980 to 2015,grassland and cropland were main land use types,and both of which decreased by 7.9%,but the area of forest and urban land increased mostly,more than 41%and 75%,respectively.During 1982-2015,the normalized difference vegetation index(NDVI)increased in different levels from the upper reaches to the lower reaches of the Yellow River,and the NDVI increased most obviously with 0.0017/year from Toudaoguai to Longmen area in the central Loess Plateau.The results showed that the soil and water conservation implementation since 1950s and the "conversion of farmland to forest" measures started from 1999 had a significant impact on the ecological landscape in the UMYRB.(4)Due to the greater environmental heterogeneity,the SWAT model with two sets of parameters were set to calibrate and validate the monthly streamflow at eight(TNH,LZ,TDG,LM,TG,SMX,XLD,HYK)gaging stations and the monthly sediment load at two(TNH and LZ)gaging stations during 1971-1986 in the UMYRB.R2 and NSE of streamflow were both greater than 0.5 in both the calibration and validation period,respectively.Sediment load during 1971-1978,R2 and NSE were both 0.61 at TNH station and were both 0.56 at LZ station,respectively;during 1979-1986,the simulated mean value was close to the observed value,PBIAS were 0.9%and-15.8%,respectively.The model performance shows that different streamflow parameters in upstream and midstream,and sediment parameters in two periods,respectively,can better understand the hydrological process in the changing environment.(5)The effects of land use and climate change on streamflow and sediment load were set as four scenarios and the attribution analysis was conducted simulated by SWAT.Land use effect reduced streamflow by 1.9%,contributing about 5.8%to total streamflow reduction,while climate reduced streamflow by 4.9%,contributing about 13%to total streamflow reduction.In the area above Lanzhou station(in group a and group b parameters),the sediment load influenced by the land use increased about 12.1%and 8.4%,respectively,and increased by 7.0%and 7.2%,respectively,impact of climate.Rainfall and temperature in GCMs in the future three periods will increase to different degrees.While the streamflow will decrease significantly about 21%-31%,and the sediment load in the area above Lanzhou station decreased by 40%-50%.The results show that the model is suitable for quantifying the variation of streamflow in the alpine and arid areas under dramatic environmental changes,but it needs to be further improved for the process of simulating sediment evolution.Overall,both land use and climate change have greater effects on sediment than streamflow.Land use plays a dominant role in sediment reduction,while the effects of climate variation varied greatly in the upper reaches of the Yellow River.Under the influence of climate change,the hydrological process fluctuates greatly,and the UMYRB may suffer severe drought and water shortage under the future climate change.The research results can provide reference for water resources planning management as well as soil and water conservation in the Yellow River Basin.