The soil temperature status is of great significance to climate change, the earth's material energy cycle, and the evolution of soil properties. However, there is currently a lack of research on the long-term sequence and spatial changes of soil temperature status on a national scale. Therefore, based on the soil temperature interpolation method and Geographic Weighted Regression (GWR) model, this paper used the observation data of 880 meteorological stations in China from 1951 to 2010 to study the characteristics of the spatio-temporal changes of soil temperature in China and its influencing factors. The results showed that: (1) The overall trend of soil temperature changes in China over the past 60 years increased in the northeast and decreased in a few areas in the southwest; (2) The soil temperature in China can be divided into cold soil temperature status (Northeast China, Qinghai-Tibet Plateau and eastern Inner Mongolia), temperate soil temperature status (southern Xinjiang, Inner Mongolia, southwestern Shanxi and Shandong) and thermal soil temperature status (central China, eastern China, southern China, and Yunnan, Guizhou and Sichuan in Southwest China); (3) Longitude, latitude and air temperature had good responses to and relationships with the soil temperature, of which air temperature was the most important influencing factor; 4) Over the past 60 years, there was a trend of warm soil regime migrating to the north (approximately 46.5 km) and cold soil regime to the south (approximately 43.4 km). The research results can provide a certain reference for in-depth research in related fields such as geography and soil science, and a theoretical basis for the study of soil systematic classification.

The Yarlung Zangbo river basin (YZRB) is sensitive to global climate change. The precipitation changes in the YZRB have a great impact on the evolutions of water system, ecosystem and mountain disaster system on the Qinghai-Tibet Plateau. In this study, we divided three water resources regions of the YZRB into nine subregions through hydrological analysis. Based on precipitation data within the YZRB from 1979 to 2018, we examined the spatio-temporal variations of precipitation at annual, wet and dry seasons, monthly scales, daily and hourly extreme precipitation in the study area and its nine subregions. We further discussed the correlation between precipitation and typical large-scale atmospheric circulation factors. The results showed that: (1) From 1979 to 2018, at the whole basin scale, precipitation at each time scale showed an overall upward trend. Among them, annual precipitation showed the largest increasing trend of 2.5 mm·a^-1. Annual precipitation, dry and wet precipitation, and typical hourly scale extreme precipitation (Rx3hour and Rx12hour) increased significantly at the 95% confidence level. At the regional scale, the variation trend of precipitation at different time scales in each subregion showed more obvious inconsistencies. Except for the increasing trend of hourly scale extreme precipitation, the trends of precipitation in all the subregions were different. (2) There was an obvious spatial heterogeneity of precipitation in the YZRB, and the spatial heterogeneity of precipitation did increase with the shortening of the precipitation time scale. The precipitation at each time scale showed a gradual decreasing trend from the east to the west. The southeastern part of the YZRB (i.e. subregion Ⅲ-2) had been the center of high value, and the central and western parts (i.e. subregions Ⅰ-2 and Ⅱ-1) had a regional high value center. (3) The Northern Hemisphere Subtropical High (NHSH) and the Northern Hemisphere Polar Vortex (NHPV) had significant effects on precipitation changes. The results can help us to understand the characteristics of local precipitation at multiple scales, and provide a scientific basis for water cycle research, water resources development and utilization, and prevention of flash flood disasters in the YZRB and the Qinghai-Tibet Plateau.

High altitude or high latitude mountain areas, especially alpine glaciers and rapidly shrinking permafrost regions, provide suitable topography and material source for the development of debris flow. Under the influence of climate change, such as rising temperature and increasing heavy rainfall events, the potential hazard inducing environment of alpine regions is more likely to trigger hazards. As an important natural hazard and mass flow type, debris flow has become an important driving force of geomorphologic evolution in alpine regions. However, few research reported debris flow processes and its geomorphologic effects in alpine regions due to the difficulty of field monitoring and lack of data. We first analyzed the morphologic effects of alpine debris flow based on a case study on the Parlung Zangbo basin (with three typical debris flow prone gullies, i.e., Zhamunong, Guxiang and Tianmo), located in southeast Tibet, where debris flow occurs frequently. Combined with remote sensing image, DEM data, UAV aerial photography, high-accuracy survey of RTK and field recording, the geomorphic development characteristics of debris flow gully (such as erosion-deposition variation and wandering) and its influence on river morphology of main channels were analyzed. The long-term evolution of river valley morphology under the influence of large-scale debris flow events was also discussed. The debris flow processes in alpine regions strongly shaped gully morphology. The upstream gully channel is eroded and expanded strongly, and the erosion-deposition variation of debris flow deposited fan is determined by the scale of debris flow events and flow intensity. Debris flows significantly affect the river morphology of a main channel, which lead to lateral scouring/silting and wandering of the main channel, and affect the planform channel pattern development of upstream of the landslide dam. In the long term, the river valley morphology would evolve into a wide and narrow alternating planform and a stair-case like longitudinal profile.

The discharge-sediment relation model is an important research technique in river dynamics. Previous studies established a statistical relationship between the sediment transport rate and the flow discharge that follows a power-law form, shown as Q_s=AQ ^b. Studies have also shown that how to determine the model parameters is an important question. However, for heavy sediment-laden rivers, the discharge-sediment relation model is often expressed by a modified power-law relationship between the sediment transport rate and flow discharge, as well as an upstream sediment supply function: Q_s=KS_u^aQ^b, where the model parameters become more complex. Essentially, the modified model reflects a non-equilibrium sediment transport law, and model parameters including coefficient K and exponents a and b are mainly influenced by river boundary geomorphologic conditions. As an important index reflecting the spatial scale along the river, distance has an important impact on the modified model parameters, namely, coefficient K and exponents a and b. Taking the Lower Yellow River as the research object, we studied the variation laws of the model coefficient K and indexes a and b with distance. The results showed that the exponent a decreases exponentially with the increase of downstream distance; the coefficient K decreases with the increase of downstream distance and has a positive exponential correlation with the exponent a; the exponent b increases with the increase of downstream distance and has a negative linear correlation with the exponent a; and the sum of exponents a and b is about 2.0. From that, the calculation expressions of model coefficient K and exponents a and b varying with the distance were established. As such, it can be regarded as the parameter supplementary equations for the discharge-sediment relation model. Simulation results of sediment transport along the Lower Yellow River showed that the trend of calculated sediment concentrations are consistent with that of measured sediment concentrations. The values of the determination coefficient and Nash-Sutcliffe efficiency are 0.96 and 0.93, respectively. This study helps us to have a better understanding of the physical meaning and exploration of calculation methods for the discharge-sediment relation model parameters in heavy sediment-laden rivers.

Based on the latest hydroclimatic reconstructions in peer-reviewed scientific journals, we summarize the multi-scale pattern on hydroclimatic changes and assess whether or not the variability of the 20th century is unusual in the context of the past millennium for different regions of China. The main conclusions are as follows: (1) In the central-eastern China, the dry/wet series reconstructed from historical documents after 1400 have high confidence level, while before 1400, the reconstructions only in half of the period have high confidence due to the shortage of records. In northeastern China and eastern Inner Mongolia, centennial-scale reconstructions from lake sediments at multiple sites are in low agreement in most of periods. In Loess Plateau, Hexi Corridor, central to northern Xinjiang, northeastern and southeastern Tibetan Plateau, hydroclimatic reconstructions from tree rings have robust agreement within the same region. (2) All sub-regions of China show significant cycles with 90% confidence level at inter-annual, inter-decadal and centennial scales. The cycles of 2.5 a, 60-80 a and 110-120 a are detected over all the regions, while the cycles of 3.5-5.0 a and 20-35 a mainly occur in the Loess Plateau, Tibetan Plateau, northeastern and central-eastern China. The cycle of quasi-45 a only occurs in northeastern and central-eastern China. Moreover, the hydroclimatic changes are out of phase in different regions. (3) The 20th century is one of the wettest centuries in the past 3000 years in northeastern Tibetan Plateau. However, most series from other regions show that the inter-decadal hydroclimatic variability of the 20th century does not exceed the amplitude of natural variability, which had ever occurred during the past millennium.

The forcing temperature in spring is the main factor that determines the flowering time of woody plants in the Northern Hemisphere. Global warming has reduced the number of chilling days in winter, which probably alters the thermal requirement of flowering. In the past 50 years, the spatio-temporal changes in the thermal requirement of spring phenology in China remain unclear. Based on the first flowering date (FFD) data of Salix babylonica and Ulmus pumila derived from China Phenological Observation Network during 1963-2018, we used three methods to calculate the thermal requirements of FFD and systematically analyzed their spatial and temporal patterns at representative sites. We also developed chilling days-thermal requirement models to quantitatively simulate the thermal requirement at each site in different years. The results showed that the thermal requirement of FFD exhibited a large spatial difference, with a relatively high value at low latitudes than that at middle latitudes. There was a significant negative exponential relationship between the average thermal requirement and chilling days across sites. The thermal requirements of FFD also changed over time. The trend in thermal requirement of Salix babylonica FFD in Guiyang, Xi'an and Mudanjiang reached 1.28-1.41 °C·d/a (P<0.01), 1.63-1.89 °C·d/a (P<0.01) and 0.12-0.58 °C·d/a (P<0.05) for the growing degree day method, respectively. The thermal requirement of Ulmus pumila FFD also increased significantly in Guiyang and Xi'an, but the trend in Mudanjiang was not significant. The decrease in the number of chilling days was the main reason for the increase in the thermal requirements. Due to the low winter temperature in Mudanjiang, the number of chilling days was large and had a small interannual variation, thus chilling days exerted no significant impact on the thermal requirement. The chilling days-thermal requirement model performed better in simulating the thermal requirement of Salix babylonica FFD, with R ² of 0.54-0.66. In comparison with Salix babylonica, the model showed a relatively low precision in simulating the thermal requirement of Ulmus pumila FFD, with R ² ranging from 0.33 to 0.64. Among the three methods, the thermal requirement could be better simulated by the growing degree days method compared with the growing degree days-sigmoid and growing degree hours methods. This study provides an important scientific basis for quantifying the spatio-temporal variation of the thermal requirement of flowering and for predicting the future flowering date of woody plants under the background of climate change.

Drought is one of the most severe natural disasters that have widespread impacts on eco-environment and agriculture. Great efforts have been made on the study of the non-stationarity of hydrometeorological processes, while few reports are available addressing non-stationarity in drought index. Therefore, in this study, we attempted to develop a non-stationary standardized precipitation evaporation index (NSPEI) based on standardized precipitation evaporation index and non-stationary theory with the aim of investigating drought conditions across China in both space and time under different emission scenarios from 2006 to 2100. The results indicated that: (1) Stations with non-stationary hydrometeorological processes are concentrated in northeast China, the Huang-Huai-Hai Plain, the Yangtze River Delta, the Tibetan Plateau, the Hengduan Mountains, and the southern Xinjiang. The NSPEI has the best fitting performance at 88% of the meteorological stations considered in this study. (2) Compared with other drought indices, SPEI tends to overestimate the intensity and duration of droughts during evaluations of the future drought changes; while NSPEI avoids the weakness of SPEI in overestimation of drought intensity. Therefore, NSPEI can better monitor the meteorological droughts in China and describe the future drought changes across the country. (3) Drought monitoring results based on NSPEI indicated an increasing drought trend in northern China, and southern China is dominated by a wetting trend under the scenario of low and high emissions. Extreme dry and wet duration and occurrence frequency showed an increasing trend in China in the future under medium and high emission scenarios.

The natural river system, which is inherently interconnected, is an essential resource for human and economic society. Beyond water, and the flows of substance (e.g. sediment, contaminant), energy (e.g. streamflow velocity), and information (e.g. livelihood and activity) are exchanged in an interconnected river system. However, under the joint influences of natural evolution and human activities, the natural river system has been and is being changed. Particularly, water-related human activities in recent years are frequent and highly intensified, such as the water allocation projects and man-made reservoirs. Therefore, research on the evaluation of human activities affecting the interconnected river system network (IRSN) is needed to technically support the relevant practices. This study analyzed the affecting factors of IRSN from two perspectives: natural perspective and social perspective. The processes of human activities affecting the IRSN were theoretically explained. On this basis, both the positive and negative influences of human activities on IRSN were summarized in three aspects: interconnected-river-system-network function relationship, rivers-lake-system natural function, and interconnected-river-system-network function. To this end, the study proposed a quantitative evaluation method to assess human activities affecting the IRSN in the above three aspects. As the Huaihe River basin was severely influenced by human activities, we applied the evaluation method in this basin. Results show that human activities contributed to the interconnected-river-system-network function, meaning that possible influences on water scarcity and flood protection were generated in improving the complexity of man-made river system. Meanwhile, slight negative influences on the rivers-lake-system natural function were reported, as downstream aquatic life and surrounding environment were affected by the flow changes of water, substance and information. Together with characteristics of the Huaihe River basin, the overall quantitative evaluation results suggested that stepping up the efforts to protect the environment could further positively influence the IRSN in the Huaihe River basin. This study sheds new light on our understanding and quantitative evaluation of the influence of human activities on the IRSN.

To understand the non-equilibrium morphological adjustment of a river to environmental changes, it is essential to (i) identify accurately how previous water and sediment conditions have impacted the current morphological adjustment of the river to environmental changes and (ii) establish a corresponding simulation method for non-equilibrium conditions. Based on water-discharge and suspended sediment concentration (SSC) data and 82 cross-sectional data for the Huayuankou-Lijin reach of the Lower Yellow River for 1965-2015, the adjustment processes of the main-channel geometry (area, width, depth, and geomorphic coefficient) and their responses to changes in water discharge and SSC for different reaches are analyzed statistically. Then, with the water and sediment conditions as the main controlling factors, a delayed response model (DRM) of the main-channel geometry subjected to previous changes in water discharge and SSC is established using the multi-step analytical model. The results show that the main-channel area, width, and depth decreased initially, then increased, then decreased again, and finally increased again. They were correlated positively with the 4 a moving average discharge and negatively with the 4 a moving average SSC. The main-channel geomorphic coefficient for the Huayuankou-Sunkou reach exhibited a decreasing trend, whereas that for the Sunkou-Lijin reach decreased initially, then increased, then decreased again, and finally increased again. Except for the Huayuankou-Gaocun reach for 1965-1999, the coefficient was correlated negatively with the 4 a moving average discharge and positively with the 4 a moving average SSC. In applying the DRM to the response of the main-channel cross-sectional geometry to previous water and sediment conditions in the Lower Yellow River, the calculated values of the main-channel morphological parameters for all the sub-reaches agree well with the measured values. This indicates that the DRM can be used to simulate the response adjustment process of the main-channel cross-sectional geometry to variations in the water and sediment conditions. The results of the established model show that the adjustment of the main-channel cross-sectional geometry is affected by the current discharge and SSC (30%) and those of the previous seven years (70%), where the numbers in brackets are the respective weight factors. The established model offers insights into the mechanism whereby previous water and sediment conditions influence the current morphological adjustment of the river, and it provides an effective method for predicting the magnitude and trend of the main-channel geometry under different incoming water and sediment conditions.

Under the influence of the changing relationship between the Yellow River and the Huaihe River, the lower reaches of the Huaihe River had gradually drained into the Yangtze River since 1570 AD, and finally became a tributary of the Yangtze River. This transformation added new variations to the evolution of the Yangtze River's fluvial processes by changing the original riverbed features and currents of the Zhenjiang-Yangzhou reach. This article analyzed three surveyed maps, discussed related historical literature and investigated some antique maps to explore the formation and the evolution of the sandbanks outside the estuary of the Huaihe River. Some sandbanks including the Yumin Sandbar and Nanxin Sandbar appeared before the 17th century due to the disintegration of original sandbars such as Tengliao Sandbar,which disappeared during the Chenghua and Hongzhi reigns in the Ming Dynasty (1465-1505 AD). Since the Huaihe River entered into the Yangtze River, the Yumin Sandbar and Nanxin Sandbar had continued to expand and banked to the north along with the newly-formed sandbanks. At last, the northern channel of the Zhenjiang-Yangzhou reach demised and the riverbed transformed from a straightforward bifurcation into a curved one. Accordingly, the formation of meandering flow began to erode Guazhou Town on the upstream concave bank and silt Zhenjiang Town on the convex bank, gradually reshaping the Zhenjiang-Yangzhou reach of the Yangtze River into its current configuration.

Modeling extreme precipitation processes could provide a pathway for a better understanding of the questions concerning how much precipitation is extreme and how that extreme precipitation responds to warming in the climate change sensitive and ecological fragile zone of China over the coming decades. In this perspective, based on daily precipitation and temperature data from 72 meteorological stations released by the National Meteorological Information Center, the spatial-temporal variation of precipitation is investigated in the Qinling Mountains from 1970 to 2017, which is a critical geographical and climatic boundary between northern subtropical and warm temperate zone in China. Then we applied a framework to identify the dominant pattern of EEP in the study region, and time distribution pattern (TDP) of the event-based extreme precipitation (EEP) could be divided into four types, i.e. early, after, balance-phase and single day EEP, here after referred to as TDP1, TDP2, TDP3 and 1-day EEP. More specifically, the relationship between different EEP and local temperature is investigated through the cross wavelet transform and linear correlation. The result showed that from a long-term climate view, precipitation pattern is relatively stable in the Qinling Mountains in the past 48 years, and the 800 mm isohyetal line is still distributed in the south piedmont of Qinling Mountains. Moreover, precipitation analysis showed an obvious synchronous trend in the sub-region, which could be characterized as non-smooth and non-linear, and after 1997, there is an increasing trend in annual precipitation amount, a declining trend in rainy days, and a continuous increase of extreme precipitation intensity. Spatially, according to percentage of total precipitation amount of EEP, a wide distribution of TDP3 is clearly seen over most regions and 1-day EEP does not prevail, which demonstrates that daily precipitation extremes during an EEP could be mainly distributed at both the first and second half parts of the event duration. In north of Qinling Mountains, the dominant pattern combines TDP3 and TDP2. For south piedmont of Qinling Mountains, it witnesses fewer combinations of EEP. There exists difference between the east and west in Hanjiang River Valley, that is, TDP3 and TDP2 in the west is greater than in the east where the dominance of TDP1 and TDP3 is more prominent. However, based on the percentage of total frequency of EEP, TDP1 is observed in southern and northern Qinling Mountains, and the dominant pattern combines TDP1 and TDP2 in Hanjiang River Valley. Furthermore, in the past 48 years, the influence of local climate change, for the decadal variation of extreme precipitation, was more obvious on the time scale of 8-12 years than of on the time scale of 4-8 years. It is worth mentioning that changes in nearly all indices had a strong correlation to temperature in Hanjiang River Valley, especially TDP2 and 1-day EEP. This is particularly true of high temperature related to more precipitation intensity of single day, which in turn raises the expectation of more intense extreme short-duration rainfall events that could be assessed by the wavelet coherence analysis. Between 1998 and 2012, the phenomenon, 'global warming hiatus', occurred in the Qinling Mountains, which led to the decrease of 1-day EEP, and the increases of TDP1 and TDP3 in the north of Qinling Mountains, as well as the increase of TDP2 over the south piedmont of Qinling Mountains. All the above results are closely linked to lower temperature. The response of TDP3 and TDP2 is stronger than other types in Hanjiang River Valley. It should be noted that these results are helpful to understand the relationship between climate warming and extreme precipitation, but the response of different EEP to anthropogenic forcing and atmospheric oscillation is still complex and not explicitly resolved.

The critical karst zone is the central area with the combined action of the carbon cycle in the lithosphere, atmosphere, hydrosphere, and biosphere, and the interaction among different layers forms distinctive reaction systems, where CO₂ plays a vital role. Through monitoring the concentration of CO₂ in the overlying soil of Shuanghe cave, cave water, and atmosphere, the vertical conversion characteristics of CO₂ were systematically analyzed based on the carbonic acid equilibrium theory by adopting statistical analysis. The results show that the vertical conversion process of CO₂ is influenced by the changes in temperature inside and outside the cave, pH value of the dripping water and the process of degassing precipitation, and the source of CO₂ and ion saturation conditions, which are significantly different between the rainy and dry seasons. During the rainy season, the infiltration rate of atmospheric precipitation is high in the soil, forming a relatively stable enclosed environmental soil. The surface karst belt has weak activity in complementing CO₂ in the infiltration water, and the partial pressure of CO₂ (PCO₂) in the infiltration water varies in the range of 0.035~0.126 vol%, and the partial pressure of CO₂ in the water-atmosphere (△lg PCO₂) inside the cave is above 0. The cave water is featured by the corrosion property, and CO₂ in the lower part of the surface karst belt is the primary source of CO₂ in the cave water, whereas during the dry season, due to a small amount of precipitation, the infiltration water has sufficient time to receive CO₂ supplement from the soil and surface karst belt, forming an open system. The infiltration water changes in the range of 0.038~0.129 vol%, which is more conducive to the occurrence of the prior calcite precipitation (PCP). At this time, PCO₂ in the atmosphere inside the cave is smaller than that in cave water (△lg PCO₂ < 0), prompting the recurrence of degassing precipitation inside the cave. Also, both the soil and surface karst belt are the major sources of CO₂ in the cave water.

Water shortage, which can greatly affect both natural systems and social development, has drawn extensive attention from governing authorities around the world, and become an urgent issue to be addressed. How to improve the green efficiency of water resources and alleviate the present situation of water shortage is the content of this study. In the concept of "people-oriented" green development, the evaluation system of green efficiency is constructed, which includes four aspects: nature, social economy, environment, science and technology. This paper, based on the panel data of 31 provincial-level regions of China obtained from 2000 to 2016, aims to calculate the green efficiency of water resources by applying the Super-efficiency SBM model, to analyze its spatial correlation pattern, and to explore the driving mechanism of green efficiency of water resources by making use of the geographical weighted regression (GWR) model. The results show that there was ignificant positive spatial correlation and uneven spatial distribution of the green efficiency of water resources in China. Compared with the OLS model (the regression coefficient obtained by the model just represents the average characteristics of the overall green efficiency of water resources), the GWR model proves that there were spatial differences in the influence of various factors on the green efficiency of water resources. Generally speaking, industrial structure, industrial and agricultural development level, transportation infrastructure construction, environmental protection investment and science and technology conversion rate had a positive effect on the green efficiency of water resources, while the factors, including per capita water resources, population size and pollutant emissions, had a restraining influence. Social factors and environmental factors had a particularly obvious effect on the green efficiency of water resources. In addition, due to the large potential of the technology market turnover, the regions should step up efforts to increase investment of scientific research and focus on the transformation of scientific research results so as to make technology a new growth pole of green efficiency of water resources. In terms of spatial differences, water endowment and population size had significant impact on the green efficiency of water resources in the western region, and industrial structure, water utilization structure and transportation infrastructure posed a positive effect in the eastern region. This study is of great practical significance to solve water shortage and to formulate relevant policies suitable for the development of all regions.

Due to the influence of climate change, population growth, politics, ecological feedback and related factors, the water use system of transboundary basins are characterized by a complicated Water-Energy-Food-Ecology nexus, which constitutes a complex nonlinear system with high uncertainty. Unfortunately, the traditional optimization of water resources is often constrained by such a complex nexus. Despite that integrated modeling approach is often used in the simulation of complex nexus, it remains unsupportive of big data needs, thereby making uncertainty reduction a difficult task. The Bayesian network (BN), which is considered a more robust tool for analyzing complex relationships, was applied in this study to characterize the Water-Energy-Food-Ecology nexus of Syr Darya river basin, a transboundary inland basin which contributed to the Aral Sea ecological crisis for unreasonable water use. The annual scale data was introduced into BN to model the impact of stochastic annual runoff and compare their differences using periods "before" and "after" the disintegration of the Soviet Union as benchmark. Results show that during the before period, the amount of water inflow into the Aral Sea was sensitive to increased irrigation for agricultural development, increased water storage of the upstream reservoir and stochastic runoff. This situation became reverse after the disintegration of the Soviet Union. The reverse situation resulted from unresolvable disputes between power generation in upstream and irrigation water in downstream countries. Comprehensive scenario analysis shows that it is effective to improve the proportion of food crops, the efficiency of water use for salt leaching and irrigation, and prevent drought damage. Based on the increased use of advanced drip irrigation technology from 50% to 80%, it is anticipated that the annual inflow into the Aral Sea will increase significantly, reaching 6.4 km ³ and 9.6 km ³, respectively; and is capable of ameliorating ecological crisis within the basin. Finally, this study shows that the BN is a cost-efficient approach for predicting systematic water usage as decision support in water management with less complexity, and it is effective in modeling casual relationships in the Water-Energy-Food-Ecology nexus.

The goal of constructing artificial islands on coral reefs in China is to build ecological habitable islands. The seawater in the artificial islands is gradually replaced by rainfall infiltration, and the freshwater lens formed is an important water source supporting the island ecosystem. The double-layer geological structure is widespread in coral reefs in the South China Sea. Considering the fact that there exists a great large difference in the permeability coefficients obtained using different measurement methods, this study established numerical simulation models for a coral reef island in the South China Sea to simulate the seawater refreshing process. The results show that different permeabilities of the double-layer geological structure of the artificial islands have no obvious influence on the formation time of fresh water on the artificial islands. The permeability of the Holocene coral sand sediment is the main factor controlling the freshwater lens of the artificial reef. The volume of the forming freshwater lens decreases logarithmically with the increase of the permeability coefficient, and the volume of the freshwater lens decreases more significantly with the increase of the permeability of the Holocene sediment than the thickness of the freshwater lens. After the freshwater lens crossed the unconformity surface and entered the Pleistocene reef limestone, due to the sudden increase of the permeability coefficient of the aquifer, the freshwater lens expanded to the horizontal direction and formed the mustached-shape freshwater body in the reef limestone.

A method has been developed based on the Soil and Water Assessment Tool (SWAT) to quantitatively evaluate the water conservation function of forests and its multi-time scale characteristics in a discontinuous forest watershed. Using this method, we have divided hydrological response units (HRU) based on the spatial distribution of forests, and derived a formula to quantify the water conservation in discontinuous forest watershed based on the water balance method. Here we take the Jinjiang River Basin in southeast coastal China as an example. We constructed SWAT model under land use conditions in 2006 and analyzed the temporal variation of forest water conservation in the study river basin under precipitation conditions from 2002 to 2010. The results show that (1) the SWAT model of the study area is of high accuracy, and the hydrological response unit can accurately reflect the distribution of forest patches when the area threshold is zero. The model provides a new method for evaluation of forest water conservation function in the discontinuous forest watershed using a distributed hydrological model. (2) The annual conservation of forest water in the Jinjiang River Basin was 271.41-565.25 mm. The annual conservation function of forest water is relatively stable, and there was no runoff regulation between consecutive years. The monthly conservation ranged from -29.15 mm to 154.59 mm, which is positive for most months of the year. The forest water conservation was positive in extreme precipitation period, and negative in extremely dry period. This demonstrates the function of forest water conservation in retaining rainwater in wet periods to decrease flood in the river and supplying water in dry periods to supplement the flow at the daily scale was more effective than that at the monthly scale.

The accumulation phenomenon commonly occurs in fluvial processes. Accurately considering the accumulation effects of previous water and sediment conditions is essential for the study of riverbed evolution. To reveal the physical dynamics of the accumulation phenomenon, herein various geometry observations upstream and downstream of dam on several domestic and overseas typical fluvial rivers were analyzed. To do this, the changes in water and sediment conditions were defined as external disturbances, based on assumptions that the probability of an external disturbance conforms to the Poisson distribution and the feedback intensity induced by an individual disturbance decays exponentially with time. In this paper, a mathematical description of the accumulation processes of internal feedback induced by external disturbances is given, and a corresponding theoretical model is proposed for simulating the spatio-temporal adjustment processes of river characteristic variables on the basis of the stochastic theory in statistical mechanics. Further, the above models were then applied to investigate the spatio-temporal adjustment processes of the upper and lower reaches of dams after their construction. Results revealed two key findings. (i) Temporally, the vertical, lateral, and whole reaches' adjustment rates over time are relatively fast in the early period following disturbances but then slow down rapidly, while the accumulated bed degradation, river width and accumulated sedimentation continuously increase until a new dynamic equilibrium state is attained; these phenomena reflect the representative accumulation characteristics of fluvial processes. (ii) Spatially, the erosion intensity downstream of dams decreases nonlinearly along the channel until it eventually diminishes. In fact, the unbalanced distribution of erosion intensity across space arises from the system feedback caused by external disturbances propagating in space yet decaying over time, which is another external manifestation of an accumulation characteristic in fluvial processes. Model applications indicate that the spatio-temporal adjustment processes of cross sections and channel reaches can be accurately described by the unified theoretical formula derived from equation deforming, since the model predictions show good agreement with observed field data: coefficient of determination (R ²) between them attained values of 0.92, 0.93, 0.76, and 0.95. The proposed theoretical models take both the accumulative characteristics of fluvial processes and the spatial propagation characteristics of system feedback into account synthetically. In demonstrating this approach, this study provides the theoretical basis and new calculation method for quantitatively describing the spatio-temporal adjustment processes of non-equilibrium fluvial channels following disturbances.

The valley evolution of the Jinsha River is a research hotspot in the geomorphology, but there is a big dispute on the formation time and the evolution process of the river valley. Fluvial terraces and the correlative sediments contain abundant information about the fluvial valley evolution. Through field investigation combined with Optically Stimulated Luminescence (OSL) and Electron Spin Resonance (ESR) dating, we confirmed that 8 fluvial strath terraces were formed, and then preserved, along the course of the Jinsha River near Hulukou. The formation ages of T6-T1 are 1190±154 ka, 1079±118 ka, 942±92 ka, 816±48 ka, 544±82 ka, and 41±5 ka, respectively, corresponding well with the transition period of MIS. Near the Qinggangba, 5 terraces (T5-T1) composed of the dammed lake sediments are preserved on the hillslope, with ages of 629 ka, 88 ka, 71 ka, 49 ka and 20 ka, respectively, indicating that the Jinsha River valley from Qiaojia to Menggu has been repeatedly dammed by the landslide deposits since the mid-Pleistocene epoch. The valley evolution defers to the model of "cut-landslide-dammed-aggradation-cut". We calculated the incision rate based on the formation age and the height of the strath terraces near Hulukou, and found that the incision rate can be separated into two parts: before 0.82 Ma, the incision rate is 0.56 mm/a; from 0.82 Ma to now, it descended to 0.19 mm/a. We think this transition is induced by the frequent landslide events, which can bring a large amount of sediments to the river valley, and then impound a lake and accumulate sediments upstream, inhibiting the river incision. Synthesized studies of the fluvial terraces indicate that the formation of the modern valley of the Jinsha River may have begun in the late Early Pleistocene.