In most resource-based regions, their long-term economic growth is slow during the exploitation of natural resources. How to use resource advantages, get rid of resource bottlenecks, and achieve long-term stable economic development are key issues of the resource-based regions. In view of this, this paper takes the panel data of China's key coal cities from 2000 to 2016 as a sample to explore different ways in which natural resources in resource-based areas affect economic growth. The results show that resource-based regions benefit from the "resource bonus" in the short term. However, due to being prone to resource industry dependence behavior, these regions have a "crowding out effect" on other economic activities and resources and environment, which affects the long-term economic growth rate. At the same time, resource-based regions are likely to exhibit the "extensive growth" characteristics of relying on physical capital investment to achieve economic growth. The conclusions of this paper not only enrich the literature on the impact mechanism of natural resources on economic growth, but also have some practical enlightenment on how to achieve sustainable development in resource-based regions.
Tidal channels play important roles in governing landscape succession in the estuary areas. In the present work, tidal channels and landscapes types of the Yellow River Delta (YRD) were extracted by visual interpretation based on remote sensing images combined with field investigation during 1989-2016, aiming to decipher effects of tidal channel development on Phragmites australis, Suaeda salsa and mudflats in the last 20 years through Redundancy Analysis (RDA). Results indicated that area ratio of mudflats was in the fluctuation of increase and decrease during the past 20 years, and the change trend is opposite to that of Phragmites australis and Suaeda salsa. Patch density was larger in the Northern YRD than that in the Southern YRD, and fragmentation is relatively high. RDA analysis showed that the first two sequence axes could explain about 85.7% variations of landscape indexes, which better reflected the correlation between the two. Length, number, network connectivity and curvature had greater effects on landscape features; the effect of fractal dimension was the smallest. Length, number, and fractal dimension were negatively correlated with the area ratio and patch density of Phragmites australis and Suaeda salsa while they were positively correlated with the area ratio of mudflats.
We developed a four-step method to quantify trade-offs among ecosystem services: evaluate the targeted services at different spatial-temporal scales; identify whether a trade-off exists between pairs of ecosystem services; graph the production-possibility frontier curve; and use that curve to calculate the trade-off intensity between services. Using China's Yellow River Delta wetlands as a case study, we examined the potential trade-offs between material production, carbon storage, and habitat quality under three land-use scenarios (business-as-usual, protection, and exploitation). We found significant correlations between all pairs of the three ecosystem services. Trade-offs existed between material production and habitat quality in all scenarios, with the following order of intensity: protection (6.4) < 2015 status quo (21.8) < business as usual (22.5) < exploitation (24.3). Synergies always existed between habitat quality and carbon storage. The material production and carbon storage services were synergistic in the protection and business as usual scenarios, but they exhibited a trade-off in the exploitation scenario, with a trade-off intensity (59.9) comparable to that (60.3) in the 2015 status quo. The methodology can be flexibly used to analyze trade-offs and compare alternative management plans, thereby revealing the optimal management, which provides a scientific basis for achieving sustainable regional development and resource management.
Coastal saline-alkaline soil plays important roles in global carbon and nitrogen cycling and climate change regulation. Biochar (BC), as an eco-friendly soil amendment, shows a promising prospect in terms of alleviating climate change and promoting sustainable agricultural development. However, most of the previous studies focused on the influence of BC application on greenhouse gas emissions and nitrogen availability and loss in coastal saline-alkaline soil, but little information that comprehensively summarized the effect of BC on the soil carbon and nitrogen cycling is available. As a result, the objective of this review is to comprehensively summarize that: (1) The influences of BC on soil carbon pools (vegetation and soil organic carbon) and soil organic carbon mineralization in the coastal saline-alkaline soil; (2) The influences of BC on nitrogen cycling, including biological nitrogen fixation, nitrification, denitrification, nitrogen mineralization, and ammonia volatilization; (3) The underlying mechanisms responsible for the BC-regulated carbon and nitrogen cycling in the soil. At last, we also point out that more efforts should be paid to the investigation of long-term experiments in field circumstances in future, and the explanation of the microbial mechanisms underlying soil carbon and nitrogen cycling affected by BC application using modern molecular biotechnology (e.g., metagenomics). This review would provide useful information for maintaining health and function of the coastal soil ecosystem by incorporation of BC.
Coastal wetland is the transitional area between land and sea, with high habitat complexity, species biodiversity and ecological service functions. However, due to the influence of human activity and climate change, the areas of the wetlands are gradually reduced, and the structure and functions of wetlands such as hydrological connectivity are greatly influenced. In the present work, we selected three typical tidal channels with obvious differences in hydrological connectivity, investigated the macrobenthos community between different tidal channels and analyzed the influence of longitudinal hydrological connectivity on the distribution and diversity (density, biomass and biodiversity) of macrobenthos. A total of 52 species were documented, and the numerically abundant taxonomic groups were Polychaeta and Mollusca; Overall, with the increase of hydrological connectivity, the total density of macrobenthos showed an upward trend; the areas with moderate hydrological connectivity tended to have a higher biodiversity; different groups of macrobenthos showed different density and distribution responding to environmental factors caused by hydrological connectivity; Polychaeta and Oligochaeta were dominant in areas with low hydrological connectivity and Mollusca were dominant in areas with high hydrological connectivity and abundant nutrients; Crustacea were dominant in areas close to the channel source with the highest hydrological connectivity. This study is helpful to understand the mechanism of wetlands hydrological connectivity on biological connectivity, and provide an important reference for the protection and restoration of coastal wetlands.
To explore the relationship between hydrological connectivity of wetlands and the characteristics of stable carbon and nitrogen isotopes in wetland components (plant, soil and water), hydrological connectivity of typical sites and stable carbon and nitrogen isotopes of plant, soil and water suspended particulate (TSS) samples from the Yellow River Delta were analyzed by field investigation and laboratory analysis. There were significant differences of δ13C between the aboveground and underground parts of the studied three plants (Phragmite australis, Typha orientalis and Suaeda salsa) in restored wetlands. The δ13C in TSS of restored wetlands was lower than that in tidal wetlands. The hydrological connectivity degree of restored wetlands and tidal wetlands was 0.0520 and 0.0484, respectively. The hydrological connectivity degree among sites was quite different. There was a significant negative correlation between the hydrological connectivity degree and the content of δ13C in TSS and aboveground part of plants, which indicated the probable effect of hydrological connectivity on the source of suspended particulate matter in water bodies, as well as the carbon metabolism in plant and plant growth.
Salt marshes are increasingly valued for their function of coastal protection. Affected by periodic tidal events, salt marshes are highly dynamic ecosystems, especially for the hydrological characteristics. The dynamic hydrological characteristics have significantly affected the recruitment of plant, resulting in a profound influence on the distribution of salt marsh plants. To reveal the effect of hydrological characteristics on plant recruitment in salt marshes, this study conducted a field experiment on a bare patch of Yellow River Delta coastal salt marshes. A typical tidal inundation gradient was set, by choosing six transects from a main tidal creek to high areas on a bare patch in mid to high salt marshes. And the measurement of hydrological and soil characteristics, seed emergence and seedling survival experiments were conducted on the tidal gradient. Our results showed that: (1) Elevation, tidal hydrodynamics, and the inundation gradient had significant effects along the gradient from the creek to the salt marsh: The elevation of sampling points increased with the distance from the creek; on the temporal scale, there were wet and dry periods of hydrological process, the inundation frequency, water table and cumulative inundation period were lower during November to April, and they started to increase since May; on the spatial scale, the inundation frequency and cumulative inundation period decreased with the increase of elevation, and became stable after the average high water level; the relationship between the maximum water depth and elevation was revealed as a quadratic curve, and the maximum water depth reached the highest values in the elevation of 0.9 to 1.1 m. (2) Soil salinity and moisture content were not correlated with the hydrological gradient, but their changing trends were significant: with the increase of the distance to the tidal creek, soil salinity was increasing, while soil moisture content was decreasing. (3) The captured Suaeda salsa seeds (by seed traps), seed emergence rate and survival rate decreased with the increase of the distance to the tidal creek, which revealed that the recruitment of Suaeda salsa was significantly affected by hydrological characteristics. At the dry end of hydrological gradient (the higher area), the lower inundation frequency went against the dispersal of Suaeda salsa seeds, seeds emergence and seedling growth, which reduced the recruitment success of Suaeda salsa.
Phragmites auatralis wetlands affected by different flooding conditions (TW: tidal flooding wetlands; FW: fresh water restoration wetland; SW: seasonal flooding wetlands) were selected in the Yellow River Estuary to analyze the adsorption kinetics characteristics and influencing factors of phosphorus on surface wetland soils. Our results suggested that the adsorption of phosphorus in the overlying water on reed wetland soils with different water and salt conditions showed an order of FW>TW>SW, which indicated that the phosphorus adsorption capacity of FW soils was the largest. The phosphorus adsorption of soils from these three types of wetlands exhibited the fastest initial stage (0-3 h), and then gradual slowing (3-24 h), after that a slow and balanced state (after 24 h). However, the FW soils had the fastest absorption rate to phosphorus, while the SW soils showed the slowest absorption rate. The difference in water and salt conditions and physicochemical properties of wetland soils (such as pH, salinity, soil texture, Al0, Fe0 and Ca0) were the main factors leading to differences in phosphorus adsorption amount and rate among the three types of wetland soils. The Simple Elovich model and the Power Function model were more suitable to simulate the dynamic characteristics of phosphorus adsorption by Phragmites auatralis wetland soils in the Yellow River Estuary. The freshwater restoration project of degraded wetlands in the Yellow River Estuary can promote the adsorption of phosphorus on wetland soils to a certain extent, and thus reduce the eutrophic risk of the overlaying water of coastal wetlands.
Tamarix chinensis Lour. is a constructive species of the saline-alkali wetland in the Yellow River Delta, and shallow groundwater is the main water source for the growth of T. chinensis in this area. To explore the response mechanism of T. chinensis to groundwater salinity in shallow groundwater area in the Yellow River Delta, parameters of photosynthesis and water consumption were determined under 4 groundwater salinity gradients of fresh (0 g·L-1), brackish (3 g·L-1), saline (8 g·L-1) and salt (20 g·L-1) water at simulated groundwater level of 0.9 m via leaf gas-exchange and stem sap flow (SF) techniques. The results showed that: (1) With increasing groundwater salinity, soil water content, soil salt content and absolute concentration of soil solution increased gradually; (2) Parameters of the max net photosynthetic rate (Pnmax), apparent quantum yield (AQY), light saturation point (LSP), stomatal conductivity (Gs), transpiration rate (Tr), intercellular CO2 concentration (Ci) and stem SF rate increased first and then decreased with the increase of groundwater salinities, reaching the maximum values respectively under brackish groundwater (Gs, AQY, Tr, Ci, and SF rate) and saline groundwater (Pnmax and LSP) treatments, while all values of the above parameters reached the minimum values under salt groundwater treatment; (3) With the increase of groundwater salinity, the water use efficiency (WUE) and stomatal limitation value (Ls) of T. chinensis decreased before increasing, achieving the lowest level under brackish groundwater and the highest under salt groundwater. Under the condition of groundwater depth of 0.9 m, the salinity of groundwater significantly affects the characteristics of soil water and salt contents. The increase of soil water and salt contents, especially the absolute concentration of soil solution, further affects the photosynthetic efficiency and water use strategy of T. chinensis. Moderate groundwater salinity (brackish and saline groundwater) could improve photosynthetic capacity and reduce WUE of T. chinensis, while excessive high groundwater salinity (salt groundwater) would inhibit photosynthesis severely and improve WUE. T. chinensis has high photosynthetic capacity, wide light ecological amplitude and high WUE under saline groundwater conditions at the groundwater level of 0.9 m. This research can be used for the further study of the relationship between plant photosynthetic processes and soil water and salt contents, and can provide theoretical reference for the restoration and reconstruction of soil and water conservation shelterbelts in the Yellow River Delta.
A crucial question in restoration of coastal saltmarsh ecosystem is how to keep sustainable and evenly distributed seed banks, and approaches to enhance the seed settlement and germination. Thus, an urgent need is to develop a method by ameliorating the available conditions for seeds settlement and germination, thereby promoting the effectiveness of coastal vegetation restoration. Here, using an annual saltmarsh vegetation Suaeda salsa as a model species in the Yellow River Delta, we experimentally enveloped a thin layer outside each seed by three kinds of super absorbent materials, thereby testing their difference and providing an optimal selection for the restoration of Suaeda salsa. The materials we selected were polyacrylamide PAM, hydroxyethyl cellulose HEC, and hydroxypropyl methyl cellulose HPMC, which of each harbors the high ability in water absorption. Firstly, we coated the powder of each kind of absorbent material outside the seeds; secondly, we experimentally manipulated a gradient of water salinity to know the coated seed germination vary with the salinity; thirdly, we experimentally released 30 seeds for testing their mean suspension time and 20 seeds for the whole process of settlement; in the end, we investigated the seeds germination with different absorbent materials. Our results showed that, compared to the natural seeds dispersed to the bare ground, seeds that were enveloped by the water retaining materials (i.e. seed product) significantly harbored more water and maintained the water for a longer time. Our findings are that seed product coated by absorbent materials presented shorter suspension time and quick settlement within the sea water; the seed product significantly increased the germination rate and salt tolerance In addition, we observed that PAM, HPMC and HEC differed significantly in making contribution to the seed suspension, settlement, and seed germination, showing that the seed product with PAM is higher than HPMC, HEC and natural seeds, successively. Our study provides an available approach for governing the persistence of the coastal seed banks, with important management implication to use artificial seed products in the restoration of degraded saltmarsh ecosystems.
In order to reveal the response of photosynthesis and water consumption characteristics in Tamarix chinensis leaves to the depth of groundwater table, and define the depth of groundwater table to maintain T. chinensis higher photosynthetic efficiency and suitable growth. By taking three-year-old T. chinensis seedlings as experimental materials, a total of 7 submersible depths of groundwater table (0 m, 0.3 m, 0.6 m, 0.9 m, 1.2 m, 1.5 m and 1.8 m) was designed, and the light response processes of gas exchange parameters and the daily dynamics of sap flow in T. chinensis leaves to different depths of groundwater table were analyzed. The results showed that different depths of groundwater table could significantly change the soil moisture condition, and affect the photosynthetic process and water consumption in T. chinensis leaves. The net photosynthetic rate (Pn), photosynthetic response parameters, water use efficiency (WUE) and sap flow rate in T. chinensis leaves had obvious response to the depth of groundwater table. (1) With the depth of groundwater table rising, the Pn, WUE, transpiration rate, and sap flow rate increased first and then decreased, which showed the maximum value at the depth of groundwater table of 1.2 m with relative soil water content of 40.51%. The stomatal conductance, stomatal limit value increased first and then decreased, while the intercellular CO2 concentration was the opposite; From 1.2 m to 1.8 m depth of groundwater table or from 1.2 m to 0 m depth of groundwater table, the photosynthetic decrease was mainly due to the stomatal limitation. (2) With the depth of groundwater table rising, the maximum net photosynthetic rate, light saturation point, and apparent quantum efficiency in T. chinensis leaves increased first and then decreased, reaching the maximum value (21.15 μmol·m-2·s-1, 1513.4 μmol·m-2·s-1, and 0.06 μmol·mol-1, respectively) at the 1.2 m depth of groundwater table, while the light compensation point was the opposite. (3) The 1.2 m depth of groundwater table was the turning point of photosynthesis from stomatal limitation to non-stomatal restriction. The stomatal regulation made T. chinensis maintain high photosynthetic characteristics under drought stress. The conclusion was that the T. chinensis showed photosynthetic water adaptability to drought-tolerant water and moisture, and the 1.2 m depth of groundwater table under freshwater conditions was suitable for the growth of T. chinensis.
Xiong'an New Area is a millennium plan and a national event. Water shortage and water pollution have become the main problems which restrict its development. Different from the existing research on Xiong'an strategic development interpretation and resource profile description, this paper combines with scenario analysis and directional distance function models so as to show the water resources utilization efficiency and water saving potential development landscape and path of different scenarios in 2035 for the first time and it is innovatively based on the development of Shenzhen. The research shows that Xiong'an presents a change from low-efficiency zone to high-efficiency zone to high-efficiency zone in the Business as Usual, Starting Process and Construction Completion scenarios from the perspective of utilization efficiency. From the perspective of water saving potential, the reduction of efficiency improvement space and the increase in metatechnology ratios indicate that water saving potential of Xiong'an has been fully explored. Avoiding direct sewage discharge, developing sewage treatment technology actively and optimizing the industrial structure of the Xiong'an New Area are effective ways to promote the utilization efficiency of water resources, which is also an important guarantee for Xiong'an to realize green ecological development.
Quantitative assessment and long-term analysis of ecological vulnerability (EVI) are greatly important for understanding the dynamics of regional ecological environment and guiding ecological environmental protection and restoration. However, in the previous studies, there was rare assessment for ecological vulnerability of the special area of a transboundary basin. In this paper, the Amu Darya River Basin was used as the study area and 11 indicators were selected to reflect the vegetation, hydrology, climate, topography, soil and human activities. After the collinearity diagnostics, an ecological vulnerability assessment system for the study area was constructed. Further, subjective weight method and objective weight method was combined to determine the weight of all indexes, and quantitative estimation for the ecological vulnerability and the spatiotemporal characteristics analysis of the study area from 1990 to 2015 were conducted. The results showed that: (1) The ecological environment of the study area showed a deteriorating trend, and most of the areas were in a heavy vulnerable situation; the average proportion of heavy vulnerability during the study period was 46.40%. The proportion of extreme vulnerability also increased from 2.58% in 1990 to 16.97% in 2015, an increase of 14.39% over the past 25 years. (2) The ecological vulnerability of the study area varied greatly among different land cover types. The EVI value of grassland changed the most, the ecological environment of bare land was the most vulnerable, and the ecological vulnerability of forest was the lowest. In all, the ecological vulnerability levels among different land cover types were in an oreder of bare land > shrub > grassland > cropland > urban land > forest. (3) The relationship between EVI and topographical factors indicated that the area with lower elevation and smaller slope or higher elevation and larger slope was the most vulnerable regions, while the area with low vulnerability was mainly distributed in the regions at an altitude of 2500-3500 m or on a slope of 15-25°.
Under the background of the continuous increase of agricultural cost and the wide use of labor-saving technology, in the mountainous areas, cropland fragmentation has gradually become a key factor to curb the increase of farmers' income. The clarification of the degree of cropland fragmentation and its impact on agricultural production cost has guiding value for reducing agricultural costs. Degree of cropland fragmentation was measured from the perspective of land ownership and spatial distribution, and the semi-logarithm econometric models were developed to evaluate the impact of land fragmentation on agricultural costs based on the field survey data of 1015 farm households in counties of Wulong, Wushan and Youyang of Chongqing, China. The results showed that the cultivated land in the study areas was severely fragmented, with the Simpson Index and the Plot Distance Index being 0.71 and 0.19, respectively. The cropland area per farm household was 6.19 mu, the number of parcels was 8.89 and the distance between the parcels and families was about 430 m, the characteristics of cropland fragmentation demonstrated "more parcels, small scale and short distance". The increased degree of cropland fragmentation has improved the production cost of agriculture. When the Simpson Index and the Plot Distance Index increased by one standard deviation, the total cost per unit of production increased by 33.8% and 16.6%, respectively. To be specific, the increased degree of land fragmentation has significantly increased labor, fertilizer and seed costs, while limiting the use of machinery and other factors. The study further found the heterogeneity in the impact of land fragmentation on production costs in different crops. In addition, high-quality cropland can alleviate the cost increase caused by the fragmentation of the plots, while expanding the scale of the plots will help to reduce production costs. The policy enlightenment was that reducing agricultural operation costs was an important measure to alleviate the cropland abandonment in mountainous areas. The government should establish a low-cost cropland transfer network and encourage land exchange between farmers or villages to expand the size of the plots and reduce the distance of cultivation, and the government should also promote the wide use of micro-farming machinery suitable for mountain agriculture.
Systematic understanding and classification of the spatial form of rural settlements is an important basis for rural renovation and scientific implementation of the strategy of Rural Revitalization at the present stage. In this paper, Pinggu district of Beijing is taken as the research area. According to the idea that rural residential areas are regarded as map patches, land use patterns and carriers, the identification indicators of rural residential types are constructed from three dimensions of intensity, structure and function. The intensity suitability, structure rationality and function coordination of each research unit are obtained by using multi-factor comprehensive evaluation method. With the help of ArcGIS and GeoDa, the intensity suitability, structure rationality and function coordination degree of each research unit are obtained. A spatial correlation analysis of the three is carried out, and eight coupling types of rural settlements are classified based on three-dimensional characteristics. It is concluded that: (1) There is a positive spatial correlation among intensity suitability, structural rationality and functional coordination in Pinggu district, i.e. there is a spatial agglomeration characteristic, and the spatial distribution of hotspot analysis of suitability, rationality and coordination degree presents progressive, circle and mosaic patterns respectively. (2) There are significant differences in spatial and regional characteristics in Pinggu district, Beijing, with 62.8% of the villages not suitable for intensity, 51.3% of the villages are unreasonable in structure and 61.4% of the villages are inconsistent in function. (3) The main types of rural settlements in the whole district are unsuitable-reasonable-uncoordinated and unsuitable-unreasonable-uncoordinated, which account for 53.6% of the total rural settlements. Unsuitable-reasonable-uncoordinated rural settlements are mainly distributed in townships close to urban areas and unsuitable-unreasonable-uncoordinated rural settlements are mainly located in the northern, southwestern and eastern hilly areas far from the urban area.
In the background that China strongly advocated the chemical fertilizer use reduction and zero growth, how to improve chemical fertilizer use efficiency by farmers has become a crucial issue. This paper measures the chemical fertilizer use efficiency from the perspective of technical efficiency and analyzes the influence direction and degree of land fragmentation on the chemical fertilizer use efficiency through regression analysis and Shapley value framework based on R2. The results show that: (1) The technical inefficiency is high in the use of chemical fertilizers. In the case of other inputs such as unchanged labor and land, about 52.51% of fertilizer input can be reduced with no effect on the yield. Therefore, this could sufficiently achieve the aim of fertilizer reduction with no effect on the yield by improving the management level of farmer's fertilizer use. (2) The land fragmentation has a significant negative impact on chemical fertilizer use efficiency. All other factors being equal, each block of land increase could lead to a fertilizer use efficiency decrease of 0.002. This conclusion is robust to different variable definitions, estimating methods and winsorized sample. In addition, the Shapley value decomposition result shows that land fragmentation is also an important reason for low chemical fertilizer use efficiency of Chinese farmers. On the whole, land fragmentation can conduct 12.15% of farmer's chemical fertilization efficiency variance which is fourth in all the factors only next to village dummy variables, paddy field ratio and food crop planting ratio. Moreover, the contribution of the land fragmentation to chemical fertilizer use efficiency difference is stronger in the western region where the fragmentation is relatively high. (3) The difference in the chemical fertilization efficiency mainly depends on local village characte-ristics and the characteristics of production and management including the management scale, plantation structure, land quality, and whether or not to join the cooperative. However, the explanation of farmer's individual characteristics influenced by collective decision to chemical fertilizer use efficiency is relatively weak.
Based on the long time series data of annual precipitation, evapotranspiration and sediment transport of Erlangba Hydrological Station in Tongzi River Basin from 1975 to 2015, the evolution process and future trend of sediment transport in the past 40 years were quantitatively analyzed by the methods of cumulative anomaly, Morlet wavelet analysis and Hurst index, and the contribution rates of climate change and human activities to sediment transport in the study area were quantitatively assessed by the cumulative slope rate change comparison method. The results indicated that: (1) In the past 40 years, the precipitation in the basin ranged from 608.10 mm to 1132.70 mm, with an average of 829.00 mm. And the precipitation showed no significant decreasing trend, with an average annual decrease of -3.10 mm/a. (2) The annual sediment transport in the basin ranged from 0.44 to 478.01 million tons, with an average of 646.8 million tons. And it showed a very significant decreasing trend, with a reduction rate of -413000 t/a. In the future, sediment transport in the basin will continue to decrease, but it is likely to increase after 10 years. (3) In 1989, the sediment discharge in the basin changed abruptly. Before 1989, it showed an upward trend. After 1989, it showed a significant downward trend. The sediment discharge after the abrupt year decreased by 76.82% compared with that before the abrupt year. (4) The sediment discharge had a 12-year interdecadal oscillation period, forming two high oscillation periods and a low oscillation center. The high oscillation period was observed in 1976-1979 and 1986-1992, and the low oscillation period was found in 1979-1985. (5) The sediment transport in summer showed an increasing trend after 8 years, while there was no continuous cycle in other seasons. (6) Based on the period from 1975 to 1989, the contributions of precipitation and human activities to the sediment transport of Tongzi River Basin reached 4.8651% and 95.1385%, respectively in the period from 1989 to 2015. If the influence of evaporation on the sediment transport of the study area was taken into account, the contribution of human activities to the change of runoff of this basin would increase to 98.65% from 1989 to 2015. The decrease of sediment transport in the basin from 1990 to 2015 was mainly controlled by human activities, which reduced sediment transport by 157 thousand tons annually.
The use of machine learning method to estimate Soil Moisture Content (SMC) from multi-source remotely sensed data is a hot topic in the SMC inversion research. However, taking no account of the important variables of SMC in the ML method makes the SMC results uncertain. The Sentinel-1 and MODIS image products and the STRM data were obtained and used for extracting 32 SMC variables, such as backscattering coefficient, vegetation index, surface temperature and evapotranspiration. A total of 27 significant (P<0.05) SMC variables were selected as input parameters referring to the correlation analysis result, and the input parameters were assigned to 3 groups. Random forest, Support vector regression and Back Propagation Neural Network were tested with 3 groups parameters. The Random forest with the group with all input parameters showed the best estimation accuracy, with the RMSE being 0.039 m³/m³, and it was used for the inversion of SMC in the farmland in the middle and lower reaches of Syr Darya River Basin during the growing season of 2017. The retrieved SMC gradually increased in the middle to the lower reaches during the growing season, but there were significant temporal and spatial differences: SMC in spring and autumn was higher than that in summer. These differences were mainly caused by seasonal or spatial differences in soil texture, heat conditions (temperature) and vegetation cover. In spring, SMC in the lower part of the plain is higher than that in the upper part, and the main SMC controlling factors were soil texture and vegetation cover. In summer, the main SMC controlling factors were heat condition. Irrigation compensated for the influence of heat condition difference, resulting in no significant spatial difference of SMC between upper and lower parts of the plain. The main SMC controlling factors in autumn were soil texture and heat conditions, the influence of surface temperature compensated for the influence of soil texture on SMC, as a result, there was no significant spatial difference of SMC in autumn. With regard to overcoming the limitation of taking no account of the important variables in estimating SMC, the research method adopted in this study improves the retrieved SMC accuracy to a large extent.
Capillary water and film water are two important forms of soil water. In humid and semi-humid areas, capillary water is the water that moves quickly and is easily absorbed and utilized by plants. However, in arid and semi-arid areas, film water becomes the key factor limiting soil and water conservation and vegetation growth, and nutrient migration and utilization in soil due to the low soil water content. Soil water characteristic curve (SWRC) is an important soil hydraulic property, which represents the functional relationship between soil water content and pressure head, and is an essential parameter of the soil water movement model. Therefore, a model is urgently needed to describe the SWRC with consideration of the characteristics of capillary water and film water simultaneously. Recently, LIAO model has been proposed to represent the SWRC due to both capillary water and film water. However, this model has not been systematically evaluated. In this study, a total of 256 soil samples from coarse- to fine-textured were selected from unsaturated soil hydraulic property database (UNSODA). These samples were then applied to evaluate the soil water retention curve model from saturation to oven-dryness (LIAO model). Finally, pedotransfer functions (PTFs) for predicting LIAO model parameters were developed and validated. The results showed that: (1) Compared with the classic van Genuchten model which can only describe soil water retention characteristics due to capillary flow, LIAO model can better predict water retention curves for different textured soils with the decrease in root mean squared error by 45%. (2) LIAO model parameters were found to have different degrees of correlation with basic soil properties such as sand, silt, clay, organic matter contents and bulk density. θs had the strongest correlation (r=-0.783, P<0.01) with bulk density, while other parameters had the strongest correlation with silt. (3) Stepwise regression based-PTFs can explain 31%-65% of the variance in LIAO model parameters. The highest accuracy was found for θs. The signs of the regression coefficients and the determination coefficients were stable by the double cross-validation method. The results provided supports for soil water movement simulation at regional scale, especially in arid and semi-arid environments.
