Medieval Climate Anomaly (MCA, 950-1250 A.D.) was the most recent and lasting hundreds of years warm period, which has aroused more attention from climate scholars, because MCA provided similar patterns to research on the impact and adaption of global and regional warming. In this study, we investigate the characteristics of temperature variation on inter-decadal to centennial scale during MCA in four regions (including Northeast, Northwest, Central-east and Tibetan Plateau) in China based on high-resolution temperature reconstructions longer than 1000 years and related warm-cold records from historical documents. Ensemble empirical mode decomposition method is used to analyze the cycles and fluctuations of four regional temperature series. The results show that the longest warm period during the last 2000 years occurred in the 10th-13th centuries over the whole country, although there existed several cold decadal intervals in the mid to late 12th century, different starting-ending warming periods and warming magnitudes in four regions. On quasi-30-year scale, regional temperature variations were similar during 950-1130 A.D. while their amplitudes became smaller and the phases did not match well each other in 1130-1250 A.D. On centennial scale, all the four regions began a warm period in the early 10th century and experienced two cold intervals during MCA. However, temperature variations of Northwest and Central-east kept in phase each other, but out phase in Northeast and Tibetan Plateau, where the ending of the warm period was earlier about 40-50 years than that of Northwest and Central-east. On multi-centennial scale, the mean temperature difference between MCA and Little Ice Age in Northeast and Central-east is significant, but did not appear in Northwest and Tibetan Plateau. Compared with the mean temperature of the 20th century, Central-east had a comparable warming level, Northeast was a little cooler, and both Northwest and Tibetan Plateau had significantly lower temperatures in MCA.
In 1991, the collapse of the Soviet Union and the independence of the five Central Asian countries led to profound changes in land cover and evapotranspiration pattern. This study used the five Central Asian countries as target areas. European Space Agency Climate Change Initiative and Global Land Data Assimilation System data were utilised to analyse the spatio-temporal variation of land cover and evapotranspiration from 1992 to 2015 and to further study the cropland evapotranspiration water consumption. This study investigated the continuous change of land cover, specified the characteristics of and differences in land cover and evapotranspiration, strengthened the understanding of land cover and evapotranspiration in the current situation, and provided data references for water and soil resource management and environmental protection. Results show that the changes in land cover in the five countries were characterised by stages, and cropland expansion modified the land cover pattern. From 1992 to 2003, cropland increased rapidly (1.1×10 4 km 2), whereas forest land and grassland decreased. From 2003 to 2015, cropland increased slowly (0.3×10 4 km 2), whereas forest land and grassland increased slightly. Bare land and water bodies continued to decrease, whereas settlements continued to increase; the annual evapotranspiration was 276.8 mm. Evapotranspiration increased rapidly from 1992 to 2003 (11.3 mm/a) and then slowly from 2003 to 2015 (2.4 mm/a). The evapotranspiration of cropland (352.2 mm) and grassland (322.1 mm) was significantly higher than that of forest land (254.7 mm) and bare land (173.2 mm). The evapotranspiration changes in the five Central Asian countries were consistent with the land cover patterns; the evapotranspiration water consumption of the five countries was affected by the cropland area. From 1992 to 2015, the water consumption of cropland evapotranspiration increased by 3.2%, and the contributions of grassland, forest land and bare land continued to decrease. The cropland of Turkmenistan accounted for only 11% of the total evapotranspiration water consumption, whereas that of the other countries accounted for more than 25%.
The Integrated Multi-satellitE Retrievals for GPM (IMERG) and Global Satellite Mapping of Precipitation (GSMaP) are two high precisely multi-satellite precipitation estimates in the GPM era. In order to evaluate the applicability of both IMERG and GSMaP series products (IMERG_Uncal and IMERG_Cal, GSMaP_MVK and GSMaP_Gauge) over the Sichuan region in China, six statistical indices are used to systematically analyze the error characteristics of these products, benchmarked by a set of ground-based dataset from China Meterological Administration (CMA). Results show that: (1) All products show the dramatic regional difference over Sichuan at both daily and hourly scales. The GSMaP series products overestimate precipitation and the most overestimations occur over the high altitude areas located in the Western Sichuan. GSMaP_Gauge shows relatively higher correlation coefficient and lower relative bias and root mean square error due to the employment of gauge-based adjustments. On the contrary, IMERG_Uncal shows underestimation over the mountainous areas, while the relatively slight overestimation appears in the basin area with lower elevation at both daily and hourly time scales, suggesting that gauge-calibrated dataset IMERG_Cal has effectively improved the relative bias in the mountainous areas but not in the flat basin area. (2) By synthesizing the three classified statistical indices, IMERG series products exhibit better potentials in detecting precipitation events. Although GSMaP_Gauge shows a higher hit rate of precipitation, it has more false alarm ratios of precipitation. All products show better hit rate and lower false alarm rate over basin area and southern Sichuan. Furthermore, it is found that the ground-based dataset has some errors in those areas without meteorological stations, which leads to the apparent uncertainty in assessing the accuracy of satellite precipitation products over the Northwest Sichuan Plateau. (3) IMERG_Cal performs better in capturing the rainfall amounts and events compared with other products, especially for the lowest and highest rainfall intensity ranges, demonstrating its application potential for monitoring the extreme weather events. Overall, both IMERG and GSMaP estimates have relatively high uncertainties over the mountainous areas than ones over the flat basin areas. Additionally, the gauge-calibrated products obviously outperform the uncalibrated datasets. On the basis of the findings, future efforts focus on reducing and correcting the errors and biases of satellite precipitation estimates by considering both spatio-temporal characteristics and the topographical information.
Atmospheric CO2 uptake by carbonate rock weathering is continuously transported from the land to the ocean by rivers in the form of HCO3 -, and it has become an important carbon sink of terrestrial ecosystems. In the existing research, the estimation and distribution of carbonate weathering-related carbon sink in global major river basins are still unclear. In this study, we collect hydrochemical and discharge data of multiyear average (runoff modulus, main ion concentration, and dissolved inorganic carbon) in large river basins over 100, 000 km 2. By using hydrochem-discharge method, we estimate that the CO2 uptake rates (Fv) of carbonate weathering in global major river basins is 0.43 ± 0.15 Pg CO2 yr -1 and the average CO2 uptake flux(F) is 7.93 ± 2.8 t km -2 yr -1. The CO2 uptake F and uptake Fv are substantially different under various climatic zones. The annual uptake Fv of tropical and warm regions accounts for 62.95% of the total annual Fv. The cold temperate zone is widely distributed, and its CO2 uptake Fv accounts for 33.05%, which is second only to the tropics. We also propose the nine critical zones of global CO2 uptake F (four in the middle and low latitudes, two in the western hemisphere and three in the eastern hemisphere). The CO2 uptake F in the intersection of the critical zones is high. The average CO2 uptake F in the karst-outcropped basins is 8.50 t km -2 yr -1, which is approximately three times that in the non-karst basins. Carbonate weathering carbon sinks in global karst-outcropped basins play an important role in the study of global carbon cycle, water cycle, and carbon budget balance estimation. On the basis of river basin scales, various factors (e.g., carbonate composition, exogenous acid, and climatic environment) for carbonate weathering carbon sinks should be considered. The hydrochem-discharge method should be further improved in future research. Moreover, the effects of the photosynthesis of river aquatic organisms on rock weathering carbon sinks should be considered, and carbonate rock weathering carbon sinks should be refined and extrapolated to the world.
Based on the revised First and Second Chinese Glacier Inventory and Landsat OLI remote sensing images during 2015-2016, we analyzed the spatial-temporal variation characteristics of glaciers in the Gangdisê Mountains during 1970-2016. The results of this study provide a scientific basis for the rational use of water resources in this region. The results showed that there were 3953 glaciers with a total area of 1306.45 km 2 and ice volume of ~58.16 km 3 in the Gangdisê Mountains during 2015-2016. The glaciers in sizes of 0.1-5 km 2 and < 0.5 km 2 had the largest area and the greatest number of glaciers in the Gangdisê Mountains, respectively. In the past five decades, the area of glaciers decreased by 854.05 km 2 (-1.09%/a), accounting for 39.53% of the total area of glaciers in the region in 1970. The increase of temperature in the ablation period was the most important cause of glacier retreat. Compared to other mountains in western China, the Gangdisê Mountains was the region with the strongest glacier retreat and had an accelerating tendency in recent years. The decrease of glacier area was mainly concentrated at elevations of 5600-6100 m a.s.l. and there was no change in elevation above 6500 m a.s.l. in the Gangdisê Mountains. Except for the south and southeast orientations, the number and area of glaciers were decreasing in all the orientations. Specifically, the north orientation suffered the largest area loss of glaciers and the northwest orientation witnessed the fastest retreat of glacier area. A significant feature of spatial variation showed that the rate of glacier retreat was faster from west to east in the Gangdisê Mountains. The relative change of glacier rate in the eastern section was high at -1.72%/a, followed by the middle section (-1.67%/a), and that in the western section was only -0.83%/a.
The Tibetan Plateau is sensitive to climate changes induced by interactions of large scale atmospheric circulations, including the East Asian monsoon, Indian monsoon and mid-latitude westerlies. In this paper, we present a high-resolution chemical element dataset covering the past 2000 years from Lake Yamzhog Yumco (28°27′N-29°12′N, 90°08′E-91°45′E, altitude in 4440 m a.s.l.), which is a representative inland lake located in the southern Tibetan Plateau. These data were acquired using an X-ray fluorescence (XRF) core scanner, which is used for in situ, high-resolution, continuous, multi-element analyses. The chronology presented herein is based on 210Pb and AMS 14C dates from the macro-remains of plants. The interpretation of elemental geochemistry, together with magnetic susceptibility and grain-size, enabled the reconstruction of the environmental changes in the southern Tibetan Plateau over the past 2000 years. Reconstructions of the temperatures, precipitation and lake levels indicated that the Medieval Warm Period (MWP) and Current Warm Period (CWP) were associated with low precipitation and high temperatures. In contrast, the Dark Cold Age Period (DCAP) and Little Ice Age (LIA) were associated with high precipitation and low temperatures. Moreover, the level of warmth during the peak of the MWP may equal or slightly exceed the 20th century warming. In addition, the prolonged LIA may have experienced a warm event on a centennial timescale, and the 17th and 18th centuries may be the coldest centuries in the last two millennia. Our reconstructions also indicated that the lake level evolution has been affected by interactions of temperature and precipitation. More specifically, during the cold periods, an increase of precipitation amplified the rise of lake levels, and vice versa. The climate records from Lake Yamzhog Yumco have confirmed a cold-moist/warm-dry climate pattern on the southern Tibetan Plateau over the past 2000 years. Additionally, the temperature variations inferred from the records were strongly correlated with the solar irradiance and northern hemispheric temperature changes, which suggests a possible link between the solar forcing and climate variability in the past 2000 years on the southern Tibetan Plateau. In addition, the enhancement and southward shift of the westerlies was determined to have significantly contributed to the high precipitation conditions during the LIA on the southern Tibetan Plateau.
The intense erosion of a near-shore riverbed is one of the main factors for the slope failure of a bank. During a detailed investigation carried out in August 2017 and May 2018 of underwater topography of the Yangtze Estuary, a large scour pit was recognized near the shore of the Hengsha passage. The morphological and geometrical parameters of the scour pit were measured using the SeaBat7125 multi-beam system. Dual-frequency ADCP was used to collect hydrodynamic data near the scour pit. Further, a historic nautical chart was digitalized to analyze the evolution and formation mechanism of the scour pit. The results indicated that the scour pit is in the shape of an oval, with a length and width of around 430 m and 150 m, respectively; the deepest point is approximately 38 m below the surrounding riverbed. Since the formation of the -20 m isobaths line in 1992, the scour pit area has been expanding continuously, and the average depth has been increasing yearly; in particular, after 2005, the depth increased sharply. From 1984 to 2017, the riverbed around the scour pit underwent the process of erosion-silting-erosion. In the 33 years, the scouring amount is 3.45×10 7 m 3, and the average scouring depth is 4.68 m. The ebb tide from the North Channel flows into the Hengsha passage, forming a circulation flow, thereby eroding the channel near the west bank of the Hengsha island and forming a scour pit. After 2005, the scour pit grew rapidly and expanded southward. This is mainly because the reclamation engineering performed in North Changxing reduced the curvature radius of the bend that resulted in intensified erosion. The reservoir construction engineering performed in Qingcaosha moved the thalweg in the upper and middle sections of the North Channel, and the northward entrance of the Hengsha passage expanded owing to the ebb current, which is also one of the main reasons for rapid erosion. In addition, the construction of reclamation and deep-water channel projects in the vicinity has contributed to the erosion of the channel bed, thereby accelerating the expansion of the scour pit. It can be seen that human engineering activities are the main driving factors for the rapid development of large scour pits in the Changjiang Estuary.
Studies on spatial-temporal characteristics and influencing factors of flash floods are key contents to the assessment and management of flash flood. Based on the historic flash flood data in southwest China from 1960 to 2015, the spatial and temporal distribution of flash flood and its influencing factors were analyzed by using the linear regression model, standard deviation ellipse, spatial autocorrelation, and logistic regression model. The results show that: (1) The annual frequency of flash floods in southwest China shows an exponential growth trend, and the annual variation reveals a stable, slow and rapid increase fluctuation during 1960-1980, 1981-1998 and 1999-2015, respectively; the monthly characteristics are obvious and precipitation is mainly concentrated in June, July and August. (2) The obvious spatial difference of flash flood is observed in the study area, and the high-density areas are mainly concentrated in central Yunnan plateau, Sichuan Basin and their surrounding mountainous areas. The flash flood distribution has a significant spatial positive correlation and spatial agglomeration (Moran's I index is 0.127, Z = 5.784, P = 0.007). (3) The gravity center of historical disasters has a distinct trend of moving to the west during the year. The standard deviation ellipse angle gradually weakens, the long axis gradually lengthens, and the short axis gradually shortens. (4) Rainfall factors have the maximum impact on flash floods, followed by human activity factors, and the minimal is surface environmental factors. Among the rainfall factors, the one-hour rainfall has the strongest impact on flash floods, and its dominant ratio reached 3.654. The results can provide techinical support for research on forming mechanism, monitoring and early warning, and implementation of disaster prevention and mitigation measures of flash floods.
Stream flows are of great importance in maintaining a stable hydrosphere and assessing available water resources of a nation. However, previous satellite-methods are difficult to retrieve stream flows for middle- or small-scale rivers due to the satellite course spatial resolution whereas near-ground measuring methods have too complex procedure, requirement of expensive apparatus, or low-efficiency. These shortcomings hindered them to be used widely in non-gauged areas and situations needing non-contact measurement, e.g., accidental pollution events. This paper presented a novel, non-contact, fast method to calculate streamflow using UAV images which can be easily applied to rivers with different scales of width. Using this method, stream flows can be calculated with or without ground-measured cross-section data. With UAV images it produced point-cloud and DSM (digital surface model) which were then used to calculate values of river-width, roughness, longitudinal water-surface slope and cross-section above water surface. With all these values, the hydraulic method was finally adopted to calculate stream flows. Results show that the method has a satisfactory performance with modelled streamflow values slightly higher than observed ones at high-flow periods (R 2= 0.997, RMSE = 4.55 m 3/s) with ground-observed cross-section data. When the cross-section data were absent, the cross-section under water can be generalized with the UAV measured above-water cross-section data. Errors in estimating stream flows induced by cross-section generalization decreased with increment of water-level and water-width. The maximum accumulated errors accounted for 8.28% of the bankfull streamflow. The errors were resulted from the generalization of river bottom with un-regular cross-sections. All the results and methodologies could be of great help in streamflow measurement in accidental pollution events and in ungauged areas across the globe.
County is the basic unit of land space planning and management research. The identification and reconstruction of county ecological space network, an important way of space optimization, plays a positive role in the demarcation of ecological protection red line. However, the methodology of spatial network analysis is stagnant. Based on the construction and identification of the ecological space network in typical hilly areas and counties, this study expresses the characteristics of change brought about by specific disturbance with robustness analysis method, analyzes node importance degree, and evaluates network stability. The results show that nodes with high importance play a key role in maintaining the stability of ecological space network. The number of such nodes accounts for about 5% and most of them are located in the area of Luoxiao Mountain in Chaling county, and corresponding with woodland, water and grassland, garden and other ecological land landscape. The change value of the stability of the ecological space network is highly consistent with the number of nodes. When the failure rate of the network node is higher than 92%, it will be completely paralyzed; when the failure rate is lower than 20%, the stability can be restored. The change of connectivity robustness is more sensitive in the scenario of sabotage which is dominated by interference of human activities. Based on the distribution of important nodes and the judgment results of stability, combined with the red line demarcation scheme of ecological protection, this paper puts forward suggestions on the differentiated management of regional ecological protection and the reconstruction of ecological space network. The study not only provides effective support for county ecological space planning and ecological protection red line control, but also provides case reference for similar regional ecological space network research.
The ecological risks facing the landscape in the Ningjiang River Basin are influenced by multiple parameters. Landscape optimization is an effective way to assess the detrimental effects on the ecosystem. In this study, an integrated "natural-social-landscape" landscape ecological risk assessment system was produced, and the landscape ecological risk classification of the Ningjiang River Basin was based on spatial principal component analysis. Ecological resistance surfaces were constructed using the results of the landscape ecological risk analysis. The landscape resistance surfaces were produced based on the minimum cumulative resistance model (MCR). The results show that human activity and landscape patterns had a more significant influence on the final risk assessment than natural factors do, such as terrain and distance from water. The landscape ecological risk was generally high, and the high ecology risk region covered 523.99 km 2, constituting the largest area and accounting for 36.06% of the research area. With the aim of decreasing the landscape ecological risk of the Ningjiang River Basin, areas of forestland greater than 50 km 2 and areas of water greater than 0.2 km 2 were selected as the eco-sources. Using interlinked points, lines and surfaces, a regional ecological network was constructed out of 15 ecological corridors. The length of the first level corridors was greater than 30000 m, the length of second level corridors was between 10000 m and 30000 m, and the length of the third level corridors was less than 10000 m. A total of 19 ecological nodes were produced. The landscape connectivity was significantly improved following landscape pattern optimization. The results of this study may help improve the ecological stability level in the study area and provide a scientific basis for both landscape ecological risk assessment and landscape pattern optimization research.
In view of the lack of research on the screening and comprehensive evaluation of vegetation ecological parameters based on remotely sensed date products, this paper firstly proposes a method to evaluate the vegetation eco-environmental quality in Tibet by PCA (principal component analysis). Based on five indicators, including fractional vegetation coverage (FVC), leaf area index (LAI), gross primary productivity (GPP), land surface temperature (LST) and modified wet (Wet) from MODIS satellite data, the law and mechanism of spatial-temporal changes of eco-environmental quality in Tibet in the last 10 years are explored. The experimental results show that: (1) The comprehensive quality of the eco-environmental quality in Tibet is relatively fragile, and the spatial difference in eco-environmental quality is significant. It is excellent in the southeast but poor in the northwest. (2) During the period from 2006 to 2016, the eco-environmental quality in Tibet shows a trend of improving overall. In terms of time series, the proportion of areas improved and degraded in the past 10 years was 31.88% and 4.13% respectively. In terms of spatial distribution, the improved areas are distributed in the northwestwen, central and eastern parts, but but the degraded areas are distributed in the southeastern parts. (3) Improvements in hydrothermal conditions (especially the increase in precipitation) and human protection policies have led to improvements in the eco-environmental quality in the western and central Tibet. The strengthening of social and economic activities is the main reason for the deterioration of the eco-environmental quality in the southeast of Tibet.
It is of fundamental significance to reveal the influencing mechanism of changes in human activities on ecological environment and landscape services, and to promote regional sustainable development that explores spatio-temporal variation and evolution of social-ecological resilience at the most operable landscape scale. Taking the loess hilly-gully region of Mizhi county as an example, this paper constructed an evaluation index system of social-ecological landscape resilience from the three dimensions of ecosystem, society system, and production system. The resilience of spatial-temporal evolution of the social-ecological landscape and subsystems were analyzed in this area along with their mutual relationships. The following results were obtained from this study: (1) During 2000-2015, the spatial distribution of the three subsystems resilience was prominently distinct and significantly strengthened. Among them, the resilience was significantly enhanced in the first stage of "Grain for Green Project" (2000-2009), and the resilience index of the three systems increased by 0.134, 0.048 and 0.028, respectively. (2) The spatial distribution of resilience on social-ecological landscape was also significantly distinct and the resilience index increased by 11.60%. The spatial distribution of resilience on social-ecological landscape presents a pattern of "high in center and low in eastern and western parts". (3) The positive relationships between the changes of the subsystems resilience and landscape system resilience were dominant, and the synergistic relationship between the resilience of the subsystems was obvious. The synergy rate of two or three systems is over 90%. In this study, OWA method was applied based on different weights of interference and adaptive ability indexes to set up situational preferences. Following this, simulation diagrams of social-ecological landscape resilience of the unsustainably oriented scenario, the current situation oriented scenario and sustainably oriented scenario were mapped, which provide a decision support for the adaptive management of regional landscape.
Lake bathymetry can provide abundant information for basin planning and governance, watershed erosion and siltation management, water resource utilization, as well as environmental protection. In particular, lake bathymetry is essential for lake development and conservation, and is also closely related to sediment deposition and removal. A number of sounding techniques, such as single-beam sonar sounding, multi-beam sonar sounding, and unmanned ship sounding, have been applied to extract underwater topography of lakes. These techniques, albeit with high measurement accuracy, are time-consuming and costly. Therefore, it is of great need to develop a simple and cost-effective method. To reach this goal, recent advances of remote sensing technologies provide an alternative means of accurately measuring lake bathymetry information. Taking Dongting Lake as the study area, this paper estimated the lake bathymetry through extracting the boundaries of lake areas using multi-temporal Landsat and MODIS image series. In particular, the water level corresponding to each reference point of the lake boundary is retrieved based on trend surface analysis and kriging interpolation technology. Then the water levels of these points are regarded as the elevation points to retrieve the landform of the lakebed of the Dongting Lake. The reliability and accuracy of the developed methods were assessed through the comparsion with the actual measurements. Results indicate that the kriging interpolation technology performs well, with the average of cross-validated error target less than 0.2 meter, and the retrieval error of lake boundary with more reference points less than 1 meter. This paper suggests that an improved method for accurately and rapidly extracting lake bathymetry information can be achieved through examining the changes of water levels. This method might be of great significance for further study on lake evolution, lake development planning, and water ecological protection.
On April 27, 2019, the 2nd Forum on Rural Vitalization and Rural Science was successfully held in Nanjing Normal University. Nearly 400 scholars from more than 20 provinces gathered to discuss the strategy of Rural Vitalization and the development of rural science. This forum has 8 invited thematic lectures, 14 topic reports and two free discussions. This paper summarized the main viewpoints from six aspects: rural science and rural vitalization, urban-rural integration and rural vitalization, targeted poverty alleviation and rural vitalization, deepening reform and rural vitalization, rural restructuring and rural vitalization, village development and rural vitalization, in order to benefit the rapid development of rural science and the orderly promotion of Rural Vitalization in the new era.
