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Ecosystem services (ES) can mitigate ecological risks (ER), yet these services can also be affected by ER. Based on land use data, socioeconomic data, landscape index, and spatial analysis techniques such as spatial autocorrelation, we study the spatial and temporal characteristics of ES and ER in the Yangtze River Delta from 2000 to 2019. The conclusions are as follows: (1) During the study period, construction land has been growing rapidly, and the depth, complexity, and intensity of land use have been increasing. The reserve land resources in the Yangtze River Delta are scarce. (2) From 2000 to 2019, the ecosystem service value (ESV) has been declining, with a total loss of 107.562 billion RMB. The area of low ESV continues to grow rapidly, while the area of high ESV continues to shrink. High ecological risk potential exists in rivers, lakes, and coastal areas. (3) Although the ecological environment in economically developed areas is more fragile, the ecological risk potential can be reduced in two ways: strengthening urban clusters, thus improving cooperation and communication among them; and protecting ecosystems with high ES such as water area and forestland. When formulating regional development plans, in addition to socioeconomic benefits, managers and policy makers need to focus on natural benefits as well.

Research on the spatiotemporal evolution and trade-offs of ecosystem services (ESs) is important for optimizing the ecological security barrier system and promoting coordinated socio-economic development. Natural factors, e.g., climate change, and human factors, e.g., unreasonable land use, have impacted and damaged ecosystem structure and function, leading to challenges with ES trade-offs and the spatial identification of priority protected areas. Here, the spatiotemporal evolution characteristics of five ESs (water yield, nitrogen export, soil retention, carbon storage, and habitat quality) in Anhui Province, China, from 2000–2020 were analyzed based on the Integrated Valuation of Ecosystem Services and Trade-offs (InVEST) model. The trade-offs and spatial patterns among different ESs were explored using Pearson correlation and hotspot analyses; the dynamics of natural growth, cultivated land protection, and ecological protection scenarios for ESs in 2030 were simulated and analyzed by coupling InVEST with the patch-generating land use simulation (PLUS) model. The results reveal the following. (1) From 2000–2020, increases in water yield and soil retention occurred, with concurrent declines in the other services; the total nitrogen high-value area was mainly concentrated in the plain, with the other services’ high-value areas mainly concentrated in the Dabieshan and Southern Anhui Mountains, with each ES showing similar spatial distributions across years. (2) The ESs were mainly synergistic, with trade-offs mainly between nitrogen export and other services. (3) Hotspot overlap between water yield and the other ESs was relatively low; no more than 6.53% of ecosystems per unit area provided five ESs simultaneously. (4) Other than water yield, the ecological protection scenario was more conducive to improving ecosystem functions. This study’s results indicate inadequate synergy among ESs in Anhui Province; competition among land types must be further balanced in the future. This study provides a basic reference for implementing ecological projects and constructing ecological security patterns.

Rivers and streams are recognized as potential greenhouse gas (GHGs: CO2, CH4, and N2O) sources, contributing to global warming. However, GHG emissions from rivers and streams have received insufficient attention compared to other ecosystems (forests, grasslands, wetlands, etc.). In this study, dissolved GHG concentrations were measured in the Qingyijiang River, the longest tributary in the lower reaches of the Yangtze River, during two campaigns in September 2020 and April 2021. Our results showed that the Qingyijiang River was oversaturated with dissolved GHGs. The dissolved GHG concentration in the surface river water ranged from 8.70 to 67.38 μM CO2, 0.03 to 2.06 μM CH4, and 12.30 to 32.22 nM N2O. The average diffusive GHG emission rates were 31.89 ± 22.23 mmol CO2 m−2 d−1, 697.22 ± 939.82 μmol CH4 m−2 d−1, and 18.12 ± 7.73 μmol N2O m−2 d−1. The total emissions (CO2-e) were CO2 (58%) dominated, while CH4 (38%) played a moderate role in total emissions. Temporally, average GHG concentrations and fluxes from the studied river in April were higher than those in September. The concentration and flux of CH4 exhibited high spatial variability, similar to those in most rivers. In contrast, we found that there was no obvious spatial variability in CO2 and N2O concentrations but a significant difference among reaches in N2O fluxes. We found that water temperature and flow velocity were the potential drivers for the regulating spatial variability in GHGs. However, no other observed limnological parameters were found in governing the spatial patterns of GHGs, suggesting a complex combination of factors governing GHG fluxes; thus, these inconspicuous mechanisms underscore the need for further research. Overall, our study suggests that this river acts as a minor source of GHGs relative to other rivers, and CH4 cannot be ignored when considering aquatic carbon emissions.

The Quaternary red clay (QRC) is one of the typical terrestrial sediments in southern China which contains abundant palaeoclimatic information. A natural QRC outcrop, named the Jingxian red clay section (JRCS), was developed in Jingxian City on the northern piedmont of Mt. Huangshan, offering a valuable archive for long-term palaeoclimatic study from a southern China perspective. The objective of this study is to reveal palaeoclimatic information of regional precipitation from the JRCS and its correlation with the East Asian summer monsoon (EASM) intensity, using grain-size measurements combined with dating via Electron Spin Resonance (ESR) and palaeomagnetism. The results indicate that: (1) Ten palaeoclimatic wetter periods (PWPs) designated as PWPI to PWPX from top to bottom were detected from the grain-size analysis results of the upper 9.4 m JRCS. These PWPs span a period from approximately 840 ka ago to present; (2) three PWPs (PWPVIII to PWPX) were basically determined to approximately 612–690, 720–762 and 792–822 ka ago, basically corresponding to China loess layers of L6, L7 and L8 and deep-sea isotope stages of MIS16, MIS18 and MIS20, respectively. Data suggests that the PWPs in the study area were observed mainly during glaciations; (3) regional precipitation in the study area should be negatively related to the EASM intensity since approximately 840 ka ago as a result of variations in North Hemisphere solar insolation and uplifts of the Tibetan Plateau; (4) the modern anti-phase of monsoonal rainfall at glacial-interglacial timescales seemed to have existed between northern China and southern China since the late Early Pleistocene (approximately 840 ka ago).

The longitudinal functional connectivity of river systems refers to the process-based connections between upstream and downstream areas and is fundamental to understanding the dynamic and nonlinear hydrological behaviour of river basins. However, the quantification of such connectivity remains a challenge due to the absence of a consensus on the appropriate data and methods, especially in delta plains. In this study, based on the difference between water level fluctuations at adjacent stations, a new and quantitative longitudinal functional connectivity index (LFCI) was developed for delta plains. Focusing on the Taihu Plain, we then analysed the spatial-temporal changes in the LFCI during 1960–2012 and investigated the correlations between the LFCI and climate change and human activities. We found that the decadal, annual and seasonal changes in the average LFCI all presented slightly increasing trends in the recent 50 period, but the annual average LFCI increased significantly after 1978; the average LFCIs in June, July, and August of the flood season were less than those in other months in the Taihu Plain. We also found that the spatial-temporal changes in the average LFCI exhibited larger differences at the subregional and station scales; those in the Wu-Cheng-Xi-Yu subregion were least, and the average LFCIs at stations near the borders of adjacent subregions were less than those at other stations. Moreover, we found that the average LFCI had significant correlations with precipitation, river density and water surface ratio. Our results were consistent with common sense facts, which demonstrated that the indicator developed in this study can quite effectively quantify the longitudinal functional connectivity of river systems in delta plains.

The Qingyijiang River, which originates from the northern fringe of Mt. Huangshan, is an important tributary of the lower Yangtze River. A field investigation focused on the natural outcrops of the Qingyijiang in the Jingxian Basin, where a well-preserved sequence of three gravel beds of fluvial terraces is present. Based on gravel fabric analysis and ESR dating, this paper discusses the genesis of the Qingyijiang on the northern fringe of Mt. Huangshan. The main results show that (1) the diameter of the gravels is mainly medium and coarse, showing that the hydrodynamic conditions were strong in the formation periods of the terrace gravel beds, especially for the T3 terrace gravel bed; (2) the gravel orientations in the gravels of the T3 and T2 terraces are 191° and 250°, which are to the SSW and SWW, respectively, and are similar to the present flow, which is from southwest to northeast; (3) the roundness of the gravels is mainly subrounded or rounded, implying that these gravels were transported significant distances from their provenances; (4) the terrace gravels contain mainly quartz sandstone, sandstone, vein quartz and quartzite, which account for 90% of the rudaceous clasts in each terrace gravel, which shows that there was no significant provenance change; (5) the gravels of the T2 and T1 terraces are typical fluvial sediments, whereas that of the T3 terrace is a product of superimposition of debris flow and fluvial deposits; and (6) the Qingyijiang originated from a braided river on an alluvial (or pluvial) fan, and the timing of the Qingyijiang can be traced back at least to 905.4 ± 272.4 kyr ago when the Yangtze cut through the Three Gorges.

Evolution of river systems under the background of human activities has been a heated topic among geographers and hydrologists. Spatial and temporal variations of river systems during the 1960s–2010s in the Yangtze River Delta (YRD) were investigated based on streams derived from the topographic maps in the 1960s, 1980s and 2010s. A list of indices, drainage density (Dd), water surface ratio (WSR), ratio of area to length of main streams (R), evolution coefficient of tributaries (K) and box dimension (D), were classified into three types (quantitative, structural, and complex indices) and used to quantify the variations of stream structure. Results showed that: (1) quantitative indices (Dd, WSR) presented decreasing trend in the past 50 years, and Dd in Wuchengxiyu, Hangjiahu and Yindongnan have decreased most, about 20%. Structurally, the Qinhuai River basin was characterized by significant upward R, and K value in Hangjiahu went down dramatically by 46.8% during the 1960s–2010s. Decreasing tendency in D was found dominating across the YRD, and decreasing magnitude in Wuchengxiyu and Hangjiahu peaks for 7.8% and 6.5%, respectively in the YRD. (2) Urbanization affected the spatial pattern of river system, and areas with high level of urbanization exhibited least Dd (2.18 km/km2), WSR (6.52%), K (2.64) and D (1.42), compared to moderate and low levels of urbanization. (3) Urbanization also affected the evolution of stream system. In the past 50 years, areas with high level of urbanization showed compelling decreasing tendency in quantitative (27.2% and 19.3%) and complex indices (4.9%) and trend of enlarging of main rivers (4.5% and 7.9% in periods of the 1960s–1980s and the 1980s–2010s). In the recent 30 years, areas with low level of urbanization were detected with significant downward trend in Dd and K. (4) Expanding of urban land, construction of hydraulic engineering and irrigation and water conservancy activities were the main means which degraded the river system in the YRD.

Based on daily precipitation data from 24 stations over the Yangtze River Delta (YRD) region, the long-term trends of daily precipitation are analyzed during the period 1960–2012 using 12 precipitation indices. The results indicate that the following. (1) For the regional average trends of precipitation, nine indices [i.e., RR, SDII, PCI(day), R25mm, R50mm, R25mmT, R50mmT, R25mmTOT, and R50mmTOT] show the increasing trends, while three indices [i.e., RX1day, RR1 and PCI(month)] display the decreasing trends. Furthermore, the annual total precipitation (RR) is correlated strongly with most of the other precipitation indices, especially indices represent the heavy precipitation which is responsible for more than 90 % of the total increase of precipitation. (2) The central and southern parts of the study area become more humid than the northern parts, although the daily precipitation undergo a consistent concentrated trend over almost all of the study area. Moreover, the frequency and contribution of the heavy precipitation also arise dramatically in almost the entire study area, and the precipitation events show the significant trend of extremeness. (3) The abrupt changes for RR, RX1day, SDII, R50mm, R50mmT, and R50mmTOT are detected in 1984, 1988, 1987, 1984, 1988, and 1988, respectively, which indicate the abrupt change of precipitation occurred consistently in the mid-1980s. Three major cycles of precipitation detected are 5–6-year cycle, 13-year cycle, and 20-year cycle, with a nonsignificant 2-year cycle coexisted. In addition, a significant change of cycles occurred in the 1980s, when the short cycle was set to overtake the long cycle, resulting in high-frequency and instability of precipitation changes over the YRD region in recent decades. (4) There are three factors (the global warming, the East Asian summer monsoon change, and the urbanization development) might potentially impact on the frequency and volume of precipitation. The variation of East Asian summer monsoon is the key factor that influences the increase of precipitation, while the global warming and the urbanization development perform the background factor and the local factor, respectively.

River islands are sandbars formed by scouring and silting. Their evolution is affected by several factors, among which are runoff and sediment discharge. The spatial-temporal evolution of seven river islands in the Nanjing Section of the Yangtze River of China was examined using TM (Thematic Mapper) and ETM (Enhanced Thematic Mapper)+ images from 1985 to 2015 at five year intervals. The following approaches were applied in this study: the threshold value method, binarization model, image registration, image cropping, convolution and cluster analysis. Annual runoff and sediment discharge data as measured at the Datong hydrological station upstream of Nanjing section were also used to determine the roles and impacts of various factors. The results indicated that: (1) TM/ETM+ images met the criteria of information extraction of river islands; (2) generally, the total area of these islands in this section and their changing rate decreased over time; (3) sediment and river discharge were the most significant factors in island evolution. They directly affect river islands through silting or erosion. Additionally, anthropocentric influences could play increasingly important roles.

Using analytic hierarchy process and entropy method to determine the weight of index, fuzzy comprehensive evaluation model of risk assessment was constructed in this paper. Flood risk index and risk rating of area along the Yangtze River in Anhui province were drawn by using GIS spatial analysis technology. From disaster causing factors, inducing environment and bearing body, average maximum 3 day rainfall, rainstorm number, vegetation coverage, river network density, standard deviation of the elevation, population density, proportion of rural population, density of GDP, per capita net income of farmers, and cultivated land area ratio were selected as evaluation indicators. The results show that flood risk rating and risk index of area along the Yangtze River in Anhui province has a spatial distribution characteristics from south to north gradually reduced.

As one of the most developed regions in China, the plain of East China is undergoing gradually increased flooding under the obvious urbanization process. This paper mainly analyses the trend of water level time series in the region during the past decades, and assesses the temporal and spatial variation of water level and indicators of hydrological alteration. The results show that there is a trend of increasing water level. Bigger slope and higher significant level can be observed in monthly minimum than in monthly maximum water level, in peri-urban than in urban areas. Meanwhile, it is observed that the mean monthly minimum and maximum water level increased in both urban and peri-urban regions, while decreased coefficients of variation (Cv) in urban and increased Cv in peri-urban regions were calculated. Most indicators of hydrologic alteration in urban stations are concentrated to the range of variability approach target, while most indicators are discrete in peri-urban stations. And the degree of hydrologic alteration is higher in peri-urban than in urban regions.