Influences Of Climatic Variability And Human Activities On Terrestrial Water Storage Variations Across The Yellow River Basin

Terrestrial water storage change has been recognized as one of the important components of the hydrological cycle because it can reflect the net effect of all hydrological flux variables such as precipitation,runoff and evapotranspiration.In addition,it can be viewed as a key hydrological indicator to evaluate the water resources across the study regions.In recent years,climatic variability and extensive anthropogenic activities not only have profoundly influenced the stability of regional water circulation,water systems and social development but led to obvious changes in the spatial and temporal distributions of terrestrial water storage over regions.Therefore,investigating the impact of climatic variability and human activities on terrestrial water storage change is of great significance to understand the mechanism about how climatic variability influences the hydrological cycle,to predict extreme weather disasters(e.g.drought and flood),to make rational arrangement for agricultural production and to improve the utilization efficiency of water resources.In this thesis,GRACE data and ground observations including precipitation and streamflow are jointly used to estimate monthly evapotranspiration across the Source Region of the Yellow River based on a water balance method with the purpose of investigating the role of terrestrial water storage change played in the hydrological cycle.Then,variations in terrestrial water storage across the entire Yellow River Basin are estimated by the GRACE data.Meanwhile,the contribution of different factors from climatic variability and human activities to terrestrial water storage change across the Yellow River Basin are assessed based on a simple but effective index which is proposed for the first time in this study.To further investigate the effect of human activities especially reservoirs operation on terrestrial water storage change over regions,the relationship between water storage change induced by reservoirs operation and terrestrial water storages change is analyzed.The major works and conclusions are as follows:(1)Monthly evapotranspiration over the Source Region of the Yellow River has been derived from GRACE data and ground observations,including precipitation and streamflow data based on the water balance approach.The results indicate that mean annual water balance-based evapotranspiration over the Source Region of the Yellow River is up to?403.4 mm during 2003-2015,which is more highly correlated with that derived from GLDAS2.0 with r=0.90,RMSE=13.11 mm and BIAS=-11%than that derived from MODIS satellite data with r=0.80,RMSE=21.77 mm and BIAS=33%.The considerable discrepancies between various evapotranspiration products arise from the lack of human intervention simulation in most land surface models or impact of soil moisture on evapotranspiration in MODIS data.Additionally,the neglect of terrestrial water storage change over regions can cause large errors in the water balance-based evapotranspiration calculation,suggesting that terrestrial water storage change may play a critical role in the hydrological cycle.(2)GRACE-based estimates indicate that terrestrial water storage anomalies in the Yellow River Basin have undergone a sharp decrease at a rate of-4.64±1.4 mm/yr during the past decade.This is consistent with the depletion of groundwater storage anomalies,which shows a similar rate of-4.2±1.0 mm/yr as compared to terrestrial water storage anomalies.In addition,there exists a significant correlation between terrestrial water storage anomalies and groundwater storage anomalies with a result of r=0.87 and p<0.01.Given the continuous groundwater depletion estimated by GRACE in the Yellow River Basin,more effective measures should be taken to curb groundwater loss,such as developing a sustainable water resource management strategy,maximizing the reuse of treated wastewater and the conservation of groundwater.(3)In the past decade,tremendous changes have taken place in the Yellow River Basin about terrestrial water storage because of climatic variability.Therefore,the relationships between terrestrial water storage change and various climatic variables including precipitation,potential evapotranspiration and dryness index have been investigated in this study.The results indicate that there is a good consistency between terrestrial water storage change and variations in precipitation and potential evapotranspiration with correlation coefficients of 0.76 and-0.63 respectively in the Yellow River Basin.In addition,there is also a high correlation coefficient of-0.78 with the climatic dryness index,which can effectively reflect the water and energy balance over study regions.(4)The influence of human activities on terrestrial water storage change is represented by two different patterns:the negative contribution of water loss due to water withdrawals and the positive contribution of water gains impounded by reservoirs.This study investigates the relationship between terrestrial water storage change and human activities.The results indicate that terrestrial water storage change in the Yellow River Basin shows a better correlation with reservoir water storage change(r=0.85)than water withdrawals(r=0.4).It is worthy to note that reservoirs operation can make a higher contribution to terrestrial water storage change than climatic variability in some regions(e.g.Subbasin I).(5)A simple but effective index is proposed to assess the contributions of different factors to terrestrial water storage change in this study.The results show that primary causes of the terrestrial water storage change vary from region to region,which mainly depend on the characteristics of climate and the degree of human activities including water withdrawals and reservoir operation in different regions.For the entire Yellow River Basin,the contributions of climatic variability,human activities and reservoirs operation to yearly terrestrial water storage change range from 20.4%?63.8%,-75.5%?-25.8%and-11.8%?14.9%respectively.(6)We have provided observational evidence that the signal of variations in water storage(including increase and decrease)from reservoirs is so significant that can be effectively detected by GRACE satellites from space.Additionally,water storage changes induced by reservoirs operation can play a pivotal role in the regional variations in terrestrial water storage on yearly time scales especially in some regions where major reservoirs are located.The results indicate that there exists a good correlation between regional variations in terrestrial water storage and water storage changes induced by reservoirs operation for the Xiaolangdi Basin(with r=0.90,p<0.01)and the Longyangxia Basin(with r=0.83,p<0.01)respectively.