The physical geography and hydroclimatic conditions in the arid region of in northwestern China leading to diverse flood-generation mechanisms. Under the influence of global and regional climate change, the spatiotemporal variation of floods and flood-generation mechanism in this region is still unclear and restricts flood prevention and mitigation and the implementation of the Belt and Road Initiative in the major regions. Based on the series of the annual maximum flood peak discharge in 58 river basins in the study area from 1961 to 2017, we analyzed and revealed the mechanisms, spatial distribution and interannual variation characteristics of flood in the basin in the past 60 years based on statistical tests and machine learning approaches. The results show that the frequency of extreme floods and the annual maximum flood peak discharge magnitude are increasing, with the maximum increase in the frequency of extreme floods at about 0.84 times/10 years, and the maximum increase in the annual maximum flood peak discharge magnitude at about 29%/10 years compared with the multi-year average, and the largest increase is mainly observed in the eastern Tianshan Mountains and the Qilian Mountains. There are three main flood generation mechanisms, i.e., rain (R), snow (S) and mix (M), the frequency of R and M floods increased significantly, while the frequency of S floods decreased. The contribution of flood mechanisms transformation to the increase of annual maximum flood peak discharge magnitude can reach up to 38%, which is significantly higher than the contribution of a single hydrometeorological factor such as precipitation. The results of this study emphasize the importance of attributing and predicting the changes of flood characteristics in geographically complex region from the perspective of flood mechanisms. Engineering hydrological design in the changing environment also needs to consider the influence of the heterogeneity of flood samples caused by different flood mechanisms on the flood frequency analysis, so as to provide scientific support for flood risk management and response in the basin.

The morphological characteristics of floods are important indicators for flood prevention and control, and have important indicative significance. Traditional hydrological studies mainly focus on the change of characteristics in flood magnitude under the changing environment, and the characteristics and mechanism of changes in flood morphological parameters under the influence of urbanization and climate change remain to be further explored. Taking the highly urbanized river network area of the Taihu plain as a typical example, this paper proposed a process-based method to identify flood events, and revealed the change in flood morphological characteristics and its driving mechanisms between 1971 and 2020. The results show that not only the flood magnitude (peak water level and water level increment) but also the morphological characteristics of flood fluctuation (such as rate of rising limb and recession) have changed significantly in the Taihu plain under the background of changing environment. Impervious area has a non-linear effect on the flood morphological characteristics. In the study area, the stations with significant breakpoint for peak water level occupy 61.5%, and 76.9% stations showed a significant upward trend. 46.2% of the stations have a significant breakpoint for rate of rising limb, and all stations showed an increasing trend, among which, 38.5% of the stations reach the significance level. A total of 53.8% of the stations have significant mutation and significant trends of recession rate, and the recession rate reaches 0.628 mm·d^-1·a^-1 in the whole period in the study area (p<0.05). The antecedent water level conditions and rainfall characteristics are the main driving factors affecting the flood morphological pattern in the river network area of the Taihu plain. The results of this study could deepen the understanding of flood evolution in the Taihu plain region under a changing environment, and provide reference and support for flood disaster prevention and control in similar plain river network areas.

Palaeoflood events are instantaneous responses of hydrological processes to extreme climate. Through field investigation, a loess-paleosol sedimentary profile containing a set of overbank flood deposits (OFD1, OFD2 and OFD3) that recorded the extraordinary palaeoflood events was found on the platform scarp of the Shahe River, a tributary of the Huaihe River. The sediment samples were collected, and the physical and chemical properties and optically stimulated luminescence (OSL) dating were analyzed. The results showed that the end-members analysis of particle size indicated that OFD1, OFD2, and OFD3 were overbank flood deposits affected by hydrodynamic forces. However, the particle size composition of OFD3 and OFD1 primarily consisted of sand (>60%), while the particle size composition of OFD2, S₀, L_t and L₁ was mainly silt (>70%). The magnetic susceptibility values of OFD3 and OFD1 were significantly higher than those of S₀, L_t and L₁, and the contents of Na₂O, K₂O and SiO₂ were higher, while the contents of Al₂O₃ and Fe₂O₃ were lower in OFD3 and OFD1. The particle size composition, magnetic susceptibility value and geochemical element composition of OFD2 were similar to those of S₀, L_t and L₁, but significantly different from those of OFD1 and OFD3. These differences could be attributed to the varying material sources of OFD2, OFD3, and OFD1. OSL dating and a stratigraphic chronological framework of sedimentary profile indicated that the three extraordinary palaeoflood events were recorded during the late Holocene from 1550 to 1400 a BP in the Huaihe River basin. The analysis of high-resolution climate proxy indicators, atmospheric circulation factors, and global mean temperature demonstrated that the extreme flood events that occurred during the late Holocene from 1550 to 1400 a BP were a direct response to the abrupt climate changes, corresponding to the severe climate deterioration period of the Northern and Southern Dynasties in China. These findings are significant in understanding the regional response of hydrological climate change to global change.

Streamflow from the Lena River is one of the major sources of freshwater in the Arctic Ocean and has a significant impact on the Arctic atmosphere, sea ice thermal processes and ocean thermohaline circulation. In recent years, streamflow in the Lena River basin is changing significantly with intensified global warming. In order to investigate the response of streamflow to climate change in the Lena River basin, the study firstly analyzed the trends of precipitation, air temperature and streamflow in the basin from 1975 to 2014 using the M-K trend test. Then, we constructed the abcd-cr hydrological model by considering a coupled snowmelt and permafrost module. Based on the abcd-cr model, we simulated climatic scenarios and quantitatively estimated the relative changes of annual and seasonal streamflow and the elasticities of annual and seasonal streamflow to changes in air temperature and precipitation respected to different climate scenarios. Results showed that: (1) Both the annual and seasonal air temperatures in the Lena River showed increasing trends from 1975 to 2014; The annual, summer, and autumn precipitation presented increasing trends while the spring and winter precipitation showed decreasing trends; The annual, spring, autumn, and winter streamflow had significant increasing trends while the summer streamflow showed a non-significant decreasing trend. (2) The climate scenario simulation results quantified the variations in annual and seasonal streamflow resulting from changes in precipitation and air temperature. (3) The results of sensitivity analysis showed that annual streamflow increases with the increase of precipitation and decreases with the increase of air temperature; The impact of precipitation change on seasonal streamflow is similar to that of annual streamflow change. However, the impacts of air temperature varied in different seasons. Specifically, the relative change of summer streamflow is the largest, while that of winter streamflow is the smallest. The impact of air temperature on seasonal streamflow is more complex than that of annual streamflow due to the interactions between seasonal evaporation and freeze-thaw processes. With increasing air temperature, spring streamflow increases, while streamflow in other seasons decreases.

Understanding the stoichiometry of carbon (C), nitrogen (N) and phosphorus (P) in lake sediments is of great significance in enhancing the stoichiometric studies of terrestrial water ecosystems and revealing the response trajectory of biogenic elements between macrophyte- and algae-dominated lake ecosystems. Based on 20 sediment cores covering the whole lake, the total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) in the sediments of shallow eutrophic Yilong lake were measured, and the influence of macrophyte- and algae-dominated lake on the carbon, nitrogen and phosphorus stoichiometry was discussed. The results showed that the stoichiometric characteristics of C, N and P in lake sediments presented spatial and temporal heterogeneity. The contents of TOC, TN and TP in the algae-dominated stage of Yilong lake were (4.83±1.47)%, (0.42±0.09)% and (0.04±0.01)%, respectively, which were significantly higher than those in the macrophyte-dominated stage ((3.87±0.98)%, (0.31±0.08)% and (0.03±0.02)%, respectively). The C∶N∶P value of 347∶26∶1 for the algae-dominated lake stage was significantly lower than that of the macrophyte-dominated lake stage (519∶35∶1). This reflected the effect of aquatic plant community succession during the macrophyte- to algae-dominated stage transformation, and the impact of altered water environments on the ecological stoichiometry of low homeostasis organisms. Among different elements, due to the composition of organic matter sources, the function and activity of chemical elements, the C and N elements in sediments of Yilong lake were mainly enriched in the estuary and littoral zone, while the P element was enriched in the deep-water area, resulting in a low coupling between the P element and the C and N elements. In the surface 0~15 cm layer, the burial amounts of TOC, TN and TP in the sediments of Yilong lake were 1829 t/km², 160 t/km² and 16 t/km², respectively. The average burial rate in the top 0~5 cm (algae-dominated stage) was elevated by 30%-36% compared to the bottom 10~15 cm (macrophyte-dominated stage), but burial in the bottom 10-15 cm layer was 1.5 times higher than in the top 0-5 cm layer, as affected by water content. Comparison with other eutrophic lakes revealed that temperature greatly influenced the effective burial of organic matter in lakes and played a key role in regulating the source and sink functions of C, N and P in lake sediments. The high water temperature of Yilong lake made it easier for sediment nutrients to be released into the lake water, which aggravated the difficulty of lake ecological restoration. The ecological stoichiometry and burial characteristics of lake revealed in this study can provide an important scientific basis for deepening the understanding of C, N and P cycles in plateau lakes and evaluating nutrient burial.

Kashin-Beck disease (KBD) control in Xizang has now entered a new stage of consolidation and improvement. Clarifying the spatiotemporal mechanism of its regression is of great significance for promoting the long-term prevention and control of endemic diseases in the plateau. Based on the detection rate data of KBD in Xizang from 2000 to 2015 and by the methods of random forest and spatial statistics, this study analyzed the spatiotemporal variation characteristics of KBD and explored the main factors affecting the regression of KBD and their regional differences. The results showed that: (1) The condition of KBD decreased steadily in all parts of Xizang during 2000-2015. The spatial distribution of KBD gradually converged to the east from the overall pattern of high in the east and low in the west, high in the south and low in the north. The disease was the most serious and had the longest duration in Qamdo wards. (2) The top four major factors affecting the regression of KBD were per capita food consumption of farmers and herdsmen, per capita GDP, per capita disposable income of farmers and herdsmen, and NDVI. Except for NDVI, all the other factors showed negative correlations with the detection rate of KBD. (3) In Ali, the prevalence of KBD was mainly influenced by per capita GDP and the income level of farmers and herdsmen, while in the middle reaches of the Yarlung Zangbo River and Qamdo, wards were more affected by dietary factors. (4) The dominant factors for the prevalence of KBD varied greatly within different years, which specifically manifested as a shift from natural factors to humanistic and social factors. Social economy, dietary nutrition and land use patterns played important roles in the prevention and control of KBD in Xizang. In view of the unique environmental conditions in Xizang, strengthening diversified food supply and adhering to returning farmland to forests and grasslands are still the basic countermeasures for preventing and controlling KBD. Meanwhile, raising the economic development level of endemic areas and enhancing the dietary nutrition of local residents are the fundamental guarantees for the long-term prevention and control of KBD in the plateau.

The hydrochemical and isotopic compositions of water are controlled by natural and anthropogenic factors, therefore, it is of great significance to study the hydrochemical and isotopic compositions, changes and contributing factors for the rational utilization and scientific management of water resources within a watershed. Hydrochemistry, water (δD/δ¹⁸O-H₂O) and carbon (δ¹³C-DIC) isotope samples from surface-groundwater in the Yulong Snow Mountains-Lijiang area, China, were analyzed to reveal the spatial evolution of the above indicators along the water flow direction, to compare the changes of water chemical compositions between 2005 and 2021, and to explore the spatio-temporal differences of regional water environment affected by human activities. Results showed that: (1) In the basin, surface water and groundwater were mainly recharged by local atmospheric precipitation and glacier-snow meltwater, and the water chemistry types were mainly Ca·Mg-HCO₃ and Ca-HCO₃, while the concentrations of Na⁺, K⁺, Cl^- and {\mathrm{S}\mathrm{O}}_4^{2-} increased in some wells due to human activities; (2) Under the influence of natural processes and human activities, δD/δ¹⁸O-H₂O values increased along the direction of the surface-groundwater flow, while the value of δ¹³C-DIC decreased. Concentrations of Na⁺, K⁺, Cl^- and {\mathrm{S}\mathrm{O}}_4^{2-} increased significantly after water flows through the main urban area; (3) Water quality of the Yuhe River was reduced from class III to class IV (NH₄⁺ exceeded the standard) after the river flows through the Dayan ancient town, and significant negative correlation between Na⁺+K⁺ and δ¹³C-DIC, Cl^-+ \mathrm{ } {\mathrm{S}\mathrm{O}}_4^{2-} and δ¹³C-DIC occurred in wells from the ancient town, which revealed the negative impact of tourism activities on the water environment; and (4) Compared with 2005, the growth rates of Na⁺, K⁺, Cl^- and {\mathrm{S}\mathrm{O}}_4^{2-} concentrations in the Yanggong River, which were fed by urban sewage and wastewater, were greater than those of Lashi Lake, Yufengsi Spring and Tuanshan Reservoir waters. Our study revealed that human activities had a certain negative impact on the water environment in the Yulong Snow Mountain-Lijiang area, which provided basic data for water resources protection in the river basin and also confirmed that the combination of isotopes and water chemistry is an important means to study the water environment change and the contributing factors.