Hydrological Variations and the Ancient Silk Road in the Northern Tarim Basin between Han and Sui Dynasties

The fluctuation pattern of China’s civilization can be ascribed to climate change and historical geopolitical variations. The ancient Silk Road served as the most prosperous route connecting East Asia and Europe during Han Dynasty(206 BC–220 AD) and Sui-Tang Dynasties(581–907 AD), but was deserted in Wei, Jin, Northern and Southern dynasties(220–580 AD), of which the Tarim Basin was a key area. However, our understanding about the decline of the route during this period remains limited. Here, we present an ~7-year resolution record based on optically stimulated luminescence(OSL) age-depth model(ca. 120 BC–750 AD) from Luntai(LT) profile, about 5 km from the modern Tarim River, which fed the ancient oases, to assess the potential causes on the documented decline of the ancient Silk Road between Late Han and Sui dynasties. In this study, five episodes of hydrological change were identified by combining grain size, magnetic susceptibility, geochemistry and TOC/TN contents. Our reconstruction reveals that cold and wet climate dominated at 120 BC–50 AD and 550–750 AD, respectively, indicated by strong hydrodynamic conditions. Relatively warm and humid climate occurred at 120–550 AD, between Eastern Han and Sui-Tang dynasties, indicating a better and more suitable local environment. A comparison between the studied region and other areas of China demonstrates that the paleoclimatic variations in eastern and western China exhibit rough discrepancies, and the hydrological conditions in arid region is inconsistent with the decline of ancient Silk Road in the northern Tarim Basin. We suggest that political and societal factors are the key issues that caused the interruption of Silk Road during Wei, Jin, Northern and Southern dynasties, such as the co-occurrence of societal crises, turmoil and division in eastern China, rather than the deteriorating climatic condition in the northern Tarim basin.

Impacts of Global Warming on Hydrological Cycles in the Asian Monsoon Region

The hydrologic changes and the impact of these changes constitute a fundamental global-warmingrelated concern. Faced with threats to human life and natural ecosystems, such as droughts, floods, and soil erosion, water resource planners must increasingly make future risk assessments. Though hydrological predictions associated with the global climate change are already being performed, mainly through the use of GCMs, coarse spatial resolutions and uncertain physical processes limit the representation of terrestrial water/energy interactions and the variability in such systems as the Asian monsoon. Despite numerous studies, the regional responses of hydrologic changes resulting from climate change remains inconclusive. In this paper, an attempt at dynamical downsealing of future hydrologic projection under global climate change in Asia is addressed. The authors conducted present and future Asian regional climate simulations which were nested in the results of Atmospheric General Circulation Model （AGCM） experiments. The regional climate model could capture the general simulated features of the AGCM. Also, some regional phenomena such as orographic precipitation, which did not appear in the outcome of the AGCM simulation, were successfully produced. Under global warming, the increase of water vapor associated with the warmed air temperature was projected. It was projected to bring more abundant water vapor to the southern portions of India and the Bay of Bengal, and to enhance precipitation especially over the mountainous regions, the western part of India and the southern edge of the Tibetan Plateau. As a result of the changes in the synoptic flow patterns and precipitation under global warming, the increases of annual mean precipitation and surface runoff were projected in many regions of Asia. However, both the positive and negative changes of seasonal surface runoff were projected in some regions which will increase the flood risk and cause a mismatch between water demand and water availability in the agricultural season.

Application and comparison of coaxial correlation diagram and hydrological model for reconstructing flood series under human disturbance

Intense human activities have greatly changed the flood generation conditions in most areas of the world, and have destroyed the consistency in the annual flood peak and volume series. For design flood estimation, coaxial correlation diagram and conceptual hydrological model are two frequently used tools to adjust and reconstruct the flood series under human disturbance. This study took a typical mountain catchment of the Haihe River Basin as an example to investigate the effects of human activities on flood regime and to compare and assess the two adjustment methods. The main purpose is to construct a conceptual hydrological model which can incorporate the effects of human activities. The results show that the coaxial correlation diagram is simple and widely-used, but can only adjust the time series of total flood volumes. Therefore, it is only applicable under certain conditions(e.g. There is a strong link between the flood peaks and volumes and the link is not significantly affected by human activities). The conceptual model is a powerful tool to adjust the time series of both flood peak flows and flood volumes over different durations provided that it is closely related to the catchment hydrological characteristics, specifically accounting for the effects of human activities, and incorporating expert knowledge when estimating or calibrating parameters. It is suggested that the two methods should be used together to cross check each other.

Changes in hydrological processes in the headwater area of Yellow River,China during 1956-2019 under the influences of climate change,permafrost thaw and dam

Discharge characteristics are crucial for detecting changes in hydrological processes.Recently,the river hydrology)in the Headwater Area of the Yellow River(HAYR)has exhibited erratic regimes(e.g.,monotonously declining/low/high hydrograph,even with normal precipitation)under the effects of climate change,permafrost thaw and changes in dam operation.This study integrates hydroclimatic variables(air temperature,precipitation,and potential evapotranspiration)with anthropogenic dam operation and permafrost degradation impact data to systematically examine the mechanisms of these hydrological process changes during 1956–2019.The results show the following:1)compared with the pre-dammed gauged flow,dam construction(January 1998–January 2000)and removal of dam(September 2018–August 2019)induced monotonously low(−17.2 m^(3) s^(−1);−61%)and high(+54.6 m^(3) s^(−1);+138%)hydrographs,respectively;2)hydroclimatic variables mainly controlled the summer–autumn hydrological processes in the HAYR;3)the monotonous decline of the hydrograph of Yellow River in the HAYR in some hydrological years(e.g.,1977,1979,1990 and 1995)was closely related with unusually high atmospheric demands of evaporation and low-intense rainfall during summer–autumn seasons;and 4)the lengthening of subsurface hydrological pathways and residence time,permafrost degradation reduced the recession coefficient(−0.002 per year)of winter flow and altered the hydrological regimes of seasonal rivers,which resulted in flattened hydrographs that reduced and delayed the peak flow(of 0.05 mm per year and 1.65 d per year,respectively)as well as boosted the winter baseflow(0.01 mm per year).This study can provide updated and systematic understanding of changing hydrological processes in typical alpine catchments on northeastern Qinghai–Tibet Plateau,China under a warming climate.

Impacts of temperature and precipitation on runoff in the Tarim River during the past 50 years

The relationship between climate change and water resources in the Tarim River was analyzed by combining the temperature,precipitation and streamflow data from 1957 to 2007 from the four headstreams of the Tarim River （Aksu,Hotan,Yarkant and Kaidu rivers） in the study area.The long-term trend of the hydrological time series including temperature,precipitation and streamflow were studied using correlation analysis and partial correlations analysis.Holt double exponential smoothing was used to fit the trends between streamflow and the two climatic factors of Aksu River,Hotan River and Yarkant River.The streamflow of the main stream was forecasted by Autoregressive Integrated Moving Average Model （ARIMA） modeling by the method of time series analysis.The results show that the temperature experienced a trend of monotonic rising.The precipitation and runoff of the four headstreams of the Tarim River increased,while the inflow to the headstreams increased and the inflow into the Tarim River decreased.Changes of temperature and precipitation had a significant impact on runoff into the four headstreams of the Tarim River： the precipitation had a positive impact on water flow in the Aksu River,Hotan River and Kaidu River,while the temperature had a positive impact on water flow in the Yarkant River.The results of Holt double exponential smoothing showed that the correlation between the independent variable and dependent variable was relatively close after the model was fitted to the headstreams,of which only the runoff and temperature values of Hotan River showed a significant negative correlation.The forecasts by the ARIMA model for 50 years of annual runoff at the Allar station followed the pattern of the measured data for the same years.The short-term forecasts beyond the observed series adequately captured the pattern in the data and showed a decreasing tendency in the Tarim River flow of 3.07% every ten years.The results showed that global warming accelerated the water recharge process of the headstreams.The special hydrological characteristics of the arid area determined the significant association between streamflow and the two climatic factors studied.Strong glacier retreat is likely to bring a series of flood disasters within the study area.

Relationship between polar motion and key hydrological elements at multiple scales

Changes in the elements of the Earth system are closely related.Finding the key factors linked with hydrological changes is significant for in-depth analysis of hydrological changes.This study chooses polar motion,which is the movement of the Earth’s rotational axis relative to its crust,as a key factor in the investigation of the physical processes of its interaction with several hydrological elements.First,the statistical relationships between polar motion and multi-hydrological elements(i.e.,precipitation,evaporation,runoff,and terrestrial water storage)are investigated,using trend analysis,mutation analysis,cycle analysis,and correlation analysis methods,from basinal to global and from intra-annual to inter-annual scales.Second,their interactions are explored.The study quantifies the effect of hydrological changes on polar motion using the excitation function.It explores the effect of polar motion on hydrological changes based on the theory of equilibrium tides and atmospheric dynamics.The results show that they are significantly correlated and abruptly changed at a similar time.First,regional to global hydrological changes can significantly excite polar motion.From April 2002 to June 2020,the global terrestrial water storage decreased significantly(by approximately−4.68 mm yr^(−1)),which significantly drove polar motion towards the direction of the Greenwich Meridian(by approximately 4.32 mas yr^(−1)).Changes in regional terrestrial water storage also contributed significantly to directional changes in polar motion around 2005 and 2012.Second,polar motion can perturb the Earth’s centrifugal force system and generate equilibrium tides,and thus further cause changes in sea-level pressure,wind,and water vapor transport.Results show that polar motion-induced water vapor flux divergences correlate significantly with actual precipitation and terrestrial water storage changes in the Yangtze River and the Pearl River basins.Their correlations are also significant when trends are removed,and the polar motion-induced changes are 4 to 14 months earlier.This study further demonstrates the relationship between polar motion and hydrological changes and helps to understand the related factors of hydrological changes in other Earth systems.

Projection of China’s future runoff based on the CMIP6 mid-high warming scenarios

The latest Coupled Model Intercomparison Project Phase 6(CMIP6)proposes new shared pathways(SSPs)that incorporate socioeconomic development with more comprehensive and scientific experimental designs;however,few studies have been performed on the projection of future multibasin hydrological changes in China based on CMIP6 models.In this paper,we use the Equidistant Cumulative Distribution Function method(EDCDFm)to perform downscaling and bias correction in daily precipitation,daily maximum temperature,and daily minimum temperature for six CMIP6 models based on the historical gridded data from the high-resolution China Meteorological Forcing Dataset(CMFD).We use the bias-corrected precipitation,temperature,and daily mean wind speed to drive the variable infiltration capacity(VIC)hydrological model,and study the changes in multiyear average annual precipitation,annual evapotranspiration and total annual runoff depth relative to the historical baseline period(1985–2014)for the Chinese mainland,basins and grid scales in the 21st century future under the SSP2-4.5 and SSP5-8.5 scenarios.The study shows that the VIC model accurately simulates runoff in major Chinese basins;the model data accuracy improves substantially after downscaling bias correction;and the future multimodel-mean multiyear average annual precipitation,annual evapotranspiration,and total annual runoff depth for the Chinese mainland and each basin increase relative to the historical period in near future(2020–2049)and far future(2070–2099)under the SSP2-4.5 and SSP5-8.5scenarios.The new CMIP6-based results of this paper can provide a strong reference for extreme event prevention,water resource utilization and management in China in the 21st century.

Carbon fluxes and soil carbon dynamics along a gradient of biogeomorphic succession in alpine wetlands of Tibetan Plateau

Hydrological changes under climate warming drive the biogeomorphic succession of wetlands and may trigger substantial carbon loss from the carbon-rich ecosystems.Although many studies have explored the responses of wetland carbon emissions to short-term hydrological change,it remains poorly understood how the carbon cycle evolves with hydrology-driven wetland succession.Here,we used a space-for-time approach across hydrological gradients on the Tibetan Plateau to examine the dynamics of ecosystem carbon fluxes(carbon dioxide(CO_(2))and methane(CH4))and soil organic carbon pools during alpine wetland succession.We found that the succession from mesic meadow to fen changed the seasonality of both CO_(2) and CH4 fluxes,which was related to the shift in plant community composition,enhanced regulation of soil hydrology and increasing contribution of spring-thaw emission.The paludification caused a switch from net uptake of gaseous carbon to net release on an annual timescale but produced a large accumulation of soil organic carbon.We attempted to attribute the paradox between evidence from the carbon fluxes and pools to the lateral carbon input and the systematic changes of historical climate,given that the wetlands are spatially low-lying with strong temporal climate-carbon cycle interactions.These findings demonstrate a systematic change in the carbon cycle with succession and suggest that biogeomorphic succession and lateral carbon flows are both important for understanding the long-term dynamics of wetland carbon footprints.

Cretaceous desert cycles, wind direction and hydrologic cycle variations in Ordos Basin: Evidence for Cretaceous climatic unequability

Climatic state under greenhouse effect is a currently hot point. Whether greenhouse climate in geological history, especially in Cretaceous, was equable or not has aroused extensive discussion. By analysis on depositional cyclcity, wind direction change and hydrologic cycle variation of Cretaceous desert in the Ordos Basin of China, the unequability of Cretaceous cli-mate is dealt. It is shown that Cretaceous climate was extremely cyclic, not only having long and mid term but also having strong seasonal even instantaneous changes. Therefore, it is sug-gested that Cretaceous climate was not equable.

Holocene hydro-environmental evolution and its impacts on human occupation in Jianghan-Dongting Basin,middle reaches of the Yangtze River,China

Based on the comprehensive analyses of 18 core profiles’sedimentary sequences and lithological characteristics in Jianghan-Dongting Basin of the middle reaches of the Yangtze River and the spatial-temporal distribution of archeological sites in this area,we reconstructed the Holocene hydro-environmental evolution,and its relationship with human occupation.The comparison reveals:11.5–5.5 ka BP,the water level of rivers and lakes in the middle Yangtze River appeared a rising trend,concurrently,under the development of Neolithic culture and rice agricultural activities,human occupation extended from piedmont plain to inner basin plain in the study area.The water level fell in 5.5–4.0 ka BP,meanwhile,the number of human settlements of Qujialing-Shijiahe culture rapidly increased,especially in the inner basin plain.The water level rose again around 4.0 ka BP,floods spread massively in this period,which led to the decline of Shijiahe culture.The main causes for hydro-environmental evolution in the study area are the fluctuation of sea level and the aggradation of fluvio-lacustrine sediments.