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Comparison of C- and L-band simulated compact polarized SAR in oil spill detection
Xiaochen WANG, Yun SHAO, Fengli ZHANG, Wei TIAN
Front. Earth Sci.    https://doi.org/10.1007/s11707-018-0733-9
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This paper presents the compact polarized (CP) pseudo quad-pol parameters for the detection of marine oil spills and segregation of lookalikes using simulated CP SAR data from full-polarized (FP) SAR imagery. According to the CP theory, 11 polarized parameters generally used for the detection of oil spills were derived from reconstructed pseudo quad-pol data for both C and L bands. In addition, the reconstruction performance between C and L bands was also compared by evaluating the reconstruction accuracy of retrieved polarized parameters. The results show that apart from σHV and RH, other polarized parameters of σHH, σVV, H, α, φH-V, r, ρH-V, and γ can be reconstructed with satisfactory accuracy for both C and L bands. Furthermore, C band has a higher reconstruction accuracy than L band, especially for φH-V. Moreover, the effect of reconstruction of polarized parameters on oil spill classification was also evaluated using the maximum likelihood classification (MLC) method. According to the evaluation of kappa coefficients and mapping accuracy, it is recommended to use σ HH, σVV, H, ρH-V, and γ of the C band CP SAR for marine oil spill classification.

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Terrain relief periods of loess landforms based on terrain profiles of the Loess Plateau in northern Shaanxi Province, China
Jianjun CAO, Guoan TANG, Xuan FANG, Jilong LI, Yongjuan LIU, Yiting ZHANG, Ying ZHU, Fayuan LI
Front. Earth Sci.    https://doi.org/10.1007/s11707-018-0732-x
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The Loess Plateau is densely covered by numerous types of gullies which represent different soil erosion intensities. Therefore, research on topographic variation features of the loess gullies is of great significance to environmental protection and ecological management. Using a 5 m digital elevation model and data from a national geographic database, this paper studies different topographical areas of the Loess Plateau, including Shenmu, Suide, Yanchuan, Ganquan, Yijun, and Chunhua, to derive representative gully terrain profile data of the sampled areas. First, the profile data are standardized in MATLAB and then decomposed using the ensemble empirical mode decomposition method. Then, a significance test is performed on the results; the test confidence is 95% to 99%. The most reliable decomposition component is then used to calculate the relief period and size of the gullies. The results showed that relief periods of the Chunhua, Shenmu, Yijun, Yuanchuan, Ganquan, and Suide gullies are 1110.14 m, 1096.85 m, 1002.49 m, 523.48 m, 498.12 m, and 270.83 m, respectively. In terms of gully size, the loess landforms are sorted as loess fragmented tableland, aeolian and dune, loess tableland, loess ridge, loess hill and loess ridge, and loess hill, in descending order. Taken together, the gully terrain features of the sample areas and the results of the study are approximately consistent with the actual terrain profiles. Thus, we conclude that ensemble empirical mode decomposition is a reliable method for the study of the relief and topography of loess gullies.

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Land use and land cover classification using Chinese GF-2 multispectral data in a region of the North China Plain
Kun JIA, Jingcan LIU, Yixuan TU, Qiangzi LI, Zhiwei SUN, Xiangqin WEI, Yunjun YAO, Xiaotong ZHANG
Front. Earth Sci.    https://doi.org/10.1007/s11707-018-0734-8
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The newly launched GF-2 satellite is now the most advanced civil satellite in China to collect high spatial resolution remote sensing data. This study investigated the capability and strategy of GF-2 multispectral data for land use and land cover (LULC) classification in a region of the North China Plain. The pixel-based and object-based classifications using maximum likelihood (MLC) and support vector machine (SVM) classifiers were evaluated to determine the classification strategy that was suitable for GF-2 multispectral data. The validation results indicated that GF-2 multispectral data achieved satisfactory LULC classification performance, and object-based classification using the SVM classifier achieved the best classification accuracy with an overall classification accuracy of 94.33% and kappa coefficient of 0.911. Therefore, considering the LULC classification performance and data characteristics, GF-2 satellite data could serve as a valuable and reliable high-resolution data source for land surface monitoring. Future works should focus on improving LULC classification accuracy by exploring more classification features and exploring the potential applications of GF-2 data in related applications.

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Analysis of the relation between ocean internal wave parameters and ocean surface fluctuation
Yufei ZHANG, Bing DENG, Ming ZHANG
Front. Earth Sci.    https://doi.org/10.1007/s11707-018-0735-7
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The relation between ocean internal waves (IWs) and surface fluctuation is studied using a quasi-incompressible two-dimensional linear ocean wave model. The main conclusions are as follows: the IW parameters can be obtained by solving the boundary value problem of ordinary differential equations with the frequency, wave number, and amplitude of the surface fluctuation. When the ocean surface fluctuation state is given, the ocean IW presents a different structure, i.e., the uncertainty of the solution, which reflects the characteristics of the inverse problem. To obtain a definite solution, this study proposes constraint conditions for the inverse problem, namely, the relationship among background flow, buoyancy frequency, sea surface height, and geostrophic parameters. The necessary and sufficient conditions for the existence of IWs and external waves (surface wave) can be obtained according to the different constraint conditions. The amplitude of the surface fluctuation is positively correlated with IWs, and they share the same frequency and wave number. We also examined the relationship between the vertical structure, the maximum amplitude, and the constraint conditions. For a certain wave number, when the ocean environment is defined, the natural frequency (characteristic frequency) of IWs can be obtained. If the frequency of the surface fluctuation is similar or equal to the natural frequency, the resonance phenomenon will occur and can result in very strong IWs. The presented theory can serve as a basis for the analytical estimation of IWs.

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Water level variation characteristics under the impacts of extreme drought and the operation of the Three Gorges Dam
Yuanfang CHAI, Yitian LI, Yunping YANG, Sixuan LI, Wei ZHANG, Jinqiu REN, Haibin XIONG
Front. Earth Sci.    https://doi.org/10.1007/s11707-018-0739-3
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Under the influence of a climate of extreme drought and the Three Gorges Dam (TGD) operation, the water levels in the middle and lower reaches of the Yangtze River in 2006 and 2011 changed significantly compared with those in the extreme drought years of 1978 and 1986. To quantitatively analyze the characteristics of water level variations in 2006 and 2011, a new calculation method was proposed, and the daily water level and discharge from 1955–2016 were collected in this study. The findings are as follows: in 2006 and 2011, the water level in the dry season significantly increased, but that in the flood season obviously decreased compared with the levels in 1978 and 1986. Here, we described this phenomenon as “no low-water-level in dry season, no high-water-level in flood season”. Based on the calculation method, the contributions of climate variability and the Three Gorges Dam operation to water level variations in the middle and lower reaches of the Yangtze River were calculated, and the contributions indicated that climate variability was the main reason for the phenomenon of “no low-water-level in dry season, no high-water-level in flood season” instead of flood peak reduction in the flood season and drought runoff implementation in the dry season, which are both induced by TGD.

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Evolution model of a modern delta fed by a seasonal river in Daihai Lake, North China: determined from ground-penetrating radar and trenches
Beibei LIU, Chengpeng TAN, Xinghe YU, Xin SHAN, Shunli LI
Front. Earth Sci.    https://doi.org/10.1007/s11707-018-0740-x
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While deltas fed by seasonal rivers are common in modern sedimentary environments, their characteristics remain unclear as compared to those fed by perennial rivers. This study identifies a small delta discharged by a seasonal stream flowing into Daihai Lake, in northern China, which is driven by ephemeral and high-energy flood events. Detailed 3D facies architecture was analyzed using ground-penetrating radar (GPR) and sedimentary logs from outcrop and trenches. Four types of radar surfaces, including truncations of underlying inclined strata, weak reflections, and depositional surface of downlap and onlap, were identified. Six radar facies (high-angle oblique-tangential, low-angle subparallel, gently plane parallel, plane-parallel, chaotic, and continuous strong reflection) were identified based on distinctive reflections, including amplitude, continuity, dip, and termination patterns. Five depositional units (Unit A to E) were documented from proximal to distal delta. Seasonal discharge signatures include significant grain-size decrease over short distance, abundant Froude supercritical flow sedimentary structures, poorly developed barforms, and small-scale scour and fill structures. Records of lake-level and sediment budget were evaluated over the past 60 years. In highstand stage (1960?1980), amalgamated channel (Units A and B), and delta front (Unit C) were deposited. In slope stage (1980?1996), the lower deposits (Units A, B, C) were eroded by Unit D with a distinct truncation surface. In lowstand stage, most eroded sediments bypassed the incised channel and accumulated in the distal part, in which a new depositional unit was formed (Unit E). The model demonstrates that deltas fed by seasonal rivers tend to accumulate large amounts of sediments carried by high magnitude floods within short periods.

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Numerical modeling of the seasonal circulation in the coastal ocean of the Northern South China Sea
Yang DING, Zhigang YAO, Lingling ZHOU, Min BAO, Zhengchen ZANG
Front. Earth Sci.    https://doi.org/10.1007/s11707-018-0741-9
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The Finite Volume Community Ocean Model (FVCOM) was adapted to the Northern South China Sea (NSCS) to investigate the seasonality of coastal circulation, as well as along-shelf and cross-shelf transport. In fall and winter, southwestward current dominates the NSCS shelf, while the current’s direction shifts to northeast in summer. The circulation pattern in spring is more complicated: both southwestward and northeastward currents are detected on the NSCS shelf. The mean shelf circulation pattern in winter does not show the permanent counter-wind South China Sea Warm Current (SCSWC) along the 100–200 m isobaths. Meanwhile, the model results indicate a northeastward current flowing along 50–100 m isobaths in spring. Southwestward along-shelf transport varies from 0.30–1.93 Sv in fall and winter, and it redirects to northeast in summer ranging from 0.44–1.09 Sv. Onshore transport is mainly through the shelf break segment southeast of the Pearl River Estuary.

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Ecological vulnerability analysis of Beidagang National Park, China
Xue YU, Yue LI, Min XI, Fanlong KONG, Mingyue PANG, Zhengda YU
Front. Earth Sci.    https://doi.org/10.1007/s11707-018-0726-8
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Ecological vulnerability analysis (EVA) is vital for ecological protection, restoration, and management of wetland-type national parks. In this study, we assessed the ecological vulnerability of Beidagang National Park based upon remote sensing (RS) and geographic information system (GIS) technologies. To quantify the ecological vulnerability, 10 indices were collected by the ‘exposure-sensitivity-adaptive capacity’ model and spatial principal component analysis (SPCA) was then applied to calculate the ecological vulnerability degree (EVD). Based on the numerical values, EVD of the study area was classified into five levels: moderate, light, medium, strong, and extreme. Results showed that the average EVD value was approximately 0.39, indicating overall good ecological vulnerability in Beidagang National Park. To be specific, 80.42% of the whole area was assigned to a moderate level of EVD with the highest being the tourism developed areas and the lowest being the reservoirs and offshore areas. Ecological vulnerability of the region was determined to be affected by the natural environment and anthropogenic disturbance jointly. The primary factors included tourism disturbance, traffic interference, exotic species invasion, land use/land cover, and soil salinization. We expected to provide some insights of the sustainable development of Beidagang National Park and would like to extend the results to other wetland-type national parks in the future.

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A fast and simple algorithm for calculating flow accumulation matrices from raster digital elevation
Guiyun ZHOU, Hongqiang WEI, Suhua FU
Front. Earth Sci.    https://doi.org/10.1007/s11707-018-0725-9
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Calculating the flow accumulation matrix is an essential step for many hydrological and topographical analyses. This study gives an overview of the existing algorithms for flow accumulation calculations for single-flow direction matrices. A fast and simple algorithm for calculating flow accumulation matrices is proposed in this study. The algorithm identifies three types of cells in a flow direction matrix: source cells, intersection cells, and interior cells. It traverses all source cells and traces the downstream interior cells of each source cell until an intersection cell is encountered. An intersection cell is treated as an interior cell when its last drainage path is traced and the tracing continues with its downstream cells. Experiments are conducted on thirty datasets with a resolution of 3 m. Compared with the existing algorithms for flow accumulation calculation, the proposed algorithm is easy to implement, runs much faster than existing algorithms, and generally requires less memory space.

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Development of a groundwater flow and reactive solute transport model in the Yongding River alluvial fan, China
Haizhu HU, Xiaomin MAO, Qing YANG
Front. Earth Sci.    https://doi.org/10.1007/s11707-018-0718-8
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The Yongding River in the western suburbs of Beijing has been recharged with reclaimed water since 2010 for the purpose of ecological restoration. Where the reclaimed water is not well treated, it poses a danger to the aquifer underneath the river. To provide a reliable tool which could be used in future research to quantify the influence of reclaimed water in the Yongding River on the local groundwater environment, a transient groundwater flow and reactive solute transport model was developed using FEFLOWTM in the middle-upper part of the Yongding River Alluvium Fan. The numerical model was calibrated against the observed groundwater levels and the concentrations of typical solutes from June 2009 to May 2010 and validated from June 2010 to December 2010. The average RMSE and R2 of groundwater level at four observation wells are 0.48 m and 0.61, respectively. The reasonable agreement between observed and simulated results demonstrates that the developed model is reliable and capable of predicting the behavior of groundwater flow and typical contaminant transport with reactions. Water budget analysis indicates that the water storage in this aquifer had decreased by 43.76×106 m3 from June 2009 to December 2010. The concentration distributions of typical solutes suggest that the middle and southern parts of the unconfined aquifer have been polluted by previous discharge of industrial and domestic sewage. The results underscore the necessity of predicting the groundwater response to reclaimed water being discharged into the Yongding River. The study established a coupled groundwater flow and reactive solute transport model in the middle-upper part of the Yongding River Alluvium Fan, one of the drinking water supply sites in Beijing city. The model would be used for risk assessment when reclaimed water was recharged into Yongding River.

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