Effects of seasonal water-level fluctuation on soil pore structure in the Three Gorges Reservoir,China

Inundation of the Three Gorges Reservoir has created a 30-m water-level fluctuation zone with seasonal hydrological alternations of submergence and exposure, which may greatly affect soil properties and bank stability. The aim of this study was to investigate the response of soil pore structure to seasonal water-level fluctuation in the reservoir, and particularly, the hydrological change of wetting and drying cycles. Soil pore structure was visualized with industrial X-ray computed tomography and digital image analysis techniques. The results showed that soil total porosity(? 100 ?m), total pore number, total throat number, and mean throat surface area increased significantly under wetting and drying cycles. Soil porosity, pore number and throat numberwithin each size class increased in the course of wetting and drying cycles. The coordination number, degree of anisotropy and fractal dimension were indicating an increase. In contrast, the mean shape factor, pore-throat ratio, and Euler-Poincaré number decreased due to wetting and drying cycles. These illustrated that the wetting and drying cycles made soil pore structure become more porous, continuous, heterogeneous and complex. It can thus be deduced that the water-level fluctuation would modify soil porosity, pore size distribution, and pore morphology in the Three Gorges Reservoir, which may have profound implications for soil processes, soil functions, and bank stability.

Fractal characterization of sediment particle-size distribution in the water-level fluctuation zone of the Three Gorges Reservoir, China

The combined effect of periodic water impoundment and seasonal natural flood events has created a 30 m high water-level fluctuation zone(WLFZ) around the Three Gorges Reservoir(TGR), China, forming a unique eco-landscape. Siltation, eutrophication, enrichment of heavy metals, and methane emissions in the WLFZ have been widely associated with sediment and soil particles generated from the upstream catchment or upland slopes. However, little attention has been paid to the complexity of sediment particle-size distributions in the WLFZ. In the present study, core samples(from a 345 cm thick sediment core from the base of the WLFZ), slope transect surface samples(across/up a WLFZ slope), and along-river/longitudinal surface samples(from the reservoir reaches) were collected. Laser granulometry and a volume-based fractal model were used to reveal the characteristics of sediment particle-size distributions. Results indicate that the alternation of coarse and fine particles in the sedimentary core profile is represented as a fluctuation of low and high values of fractal dimension(D), ranging from 2.59 to 2.77. On the WLFZ slope transect, surface sediment particles coarsen with increasing elevation, sand content increases from 3.3% to 78.5%, and D decreases from 2.76 to 2.53. Longitudinally, surface sediments demonstrate a downstream-fining trend, and D increases gradually downstream. D is significantly positively correlated with the fine particle content. We conclude that D is a useful measure for evaluating sediment particle-size distribution.

A soil quality index for evaluation of degradation under land use and soil erosion categories in a small mountainous catchment, Iran

Soil erosion and land use type have long been viewed as being particularly important drivers of soil degradation.The objectives of this study,therefore,were to select a new soil quality index(SQI)which varies significantly with land use/soil erosion,and to evaluate the new SQI using expert opinion.In total,18 soil physical,chemical,and biochemical properties(indicators)were measured on 56 soil samples collected from four land use/soil erosion categories(rangeland/surface erosion,rangeland/subsurface erosion,cultivated land/surface erosion and dry-farming land/surface erosion).Principal component and classification analysis(PCCA)identified five PCs that explained 77.7%of the variation in soil properties with the biochemical PC varying significantly with land use/soil erosion.General discriminant analysis(GDA)selected urease and clay as the most sensitive properties distinguishing the land use/soil erosion categories.The GDA canonical scores for the new SQI were significantly correlated with expert opinion soil surface summed scores(for soil movement,surface litter,pedestalling,rills and flow pattern)derived using the U.S.Department of the Interior Bureau of Land Management(BLM)method.A forward stepwise general regression model revealed that the new SQI values were explained by soil movement,surface litter,and the summed values of the soil surface factors.Overall,this study confirmed that soil quality in the study area in Iran is controlled by land use and corresponding soil erosion.

Dry cropland changes in China's Three Gorges Reservoir Region during the period 1990 to 2015

Monitoring and analyzing changes in the extent of cultivated land may inform strategic decisions on issues of environmental and food security.The dry cropland area of 12000 km^2in the Three Gorges Reservoir Region(TGRR)of China is essential for feeding the local population of^20 million,but is highly prone to soil erosion,leading to the delivery of excessive amounts of sediment and associated pollutants to the Three Gorges Reservoir(TGR),and causing serious eco-environmental consequences.Against this background,this paper used Landsat images and a digital elevation model to analyze the altitudinal distribution of,and dynamic changes in,the area of dry cropland during the period 1990 to 2015.The results suggest that dry cropland was mainly distributed in the elevation range of 200-600 m.The dry cropland area decreased from 12525.37 km^2to 11796.27 km^2during the 25-year study period,including a particularly significant decrease in the rate of decrease from 6.93 km^2/yr to 43.99 km^2/yr after 2000.The largest decline in the dry cropland area occurred in the elevation range of 600-900 m.The transformations between dry cropland and forest revealed the impact of the TGR operation on the extent of dry cropland.A total of 528.79 km^2of dry cropland with slopes>25°were converted to forest after 2000,whereas a total of 642 km^2of forest was converted to dry cropland during the study period,and these conversions mainly occurred between the elevation of 200–900 m.These spatiotemporal changes in the dry cropland area are likely to raise new issues concerning food security in the TGRR.

A green eco-environment for sustainable development:framework and action

Following its 40-year reform and‘Open Door’policy,China has recently proposed a new approach to green development and rural revitalization—the idea of Agriculture Green Development(AGD),with the key feature of creating a green eco-environment.In this minireview we introduce the definition,theory,framework and major components of a green eco-environment as a key part of the AGD.We define a green eco-environment as including four key elements or measures:(1)a green ecoenvironmental indicator system;(2)environmental monitoring and warning networks;(3)emission standards and environmental thresholds for key pollutants;(4)emission controls and pollution remediation technologies.We have used Quzhou County(a typical county in the center of the North China Plain)as an example to show how detailed air,water and soil monitoring networks,as well as improved farmer practices and pollution control measures(especially ammonia emission mitigation and PM2.5 pollution reduction),can begin to create a green eco-environment in China and that AGD is possible.We conclude by stressing the need to improve the framework and practice for a green eco-environment,especially the importance of linking proposals and practices for a green eco-environment with the United Nations high priority Sustainable Development Goals.

Fingerprinting the sources of water-mobilized sediment threatening agricultural and water resource sustainability:Progress, challenges and prospects in China

Sediment source fingerprinting apportions the sources of sediment produced by water erosion by linking sampled sediment mixtures and landscape source materials using diagnostic and conservative fingerprints.Using this approach,the nature and location of active sediment sources across the catchment can be elucidated,generating information which is a key prerequisite for the design and implementation of catchment management strategies.The science of sediment source fingerprinting continues to attract much research globally,but to date,there have been relatively few fingerprinting studies in China.Here,there remain major challenges for the fingerprinting approach arising from the uniqueness of Chinese landscapes,including for instance,the complex land use configuration with highly fragmented or mosaic patches and the highly dynamic land use conversion rates,generating a need to test the physical basis for the discriminatory power and environmental behavior of both traditional and novel tracers.Future research is needed to investigate the applicability of tracer properties in different physiographic settings and to evaluate the potential strategic utility of the approach for supporting the improved management of soil and water sustainability.Here,the strategic availability of independent observation data for different components of catchment sediment budgets and well-maintained research infrastructure for plots,micro-catchments and drainage basins provides immediate opportunity for testing the approach and refining procedures.Such detailed testing across scales would be invaluable for promoting sediment source fingerprinting as both a scientific and management tool for informing soil and water conservation practices in China.