Impacts of soil properties on phosphorus adsorption and fractions in purple soils

Information on phosphorus (P) adsorption and its impacts on the redistribution of the P fraction in soil profiles are important for environmental management under intensive agricultural practices. To clarify the dominant factors influencing soil phosphorus adsorption in an Entisol (locally known as purple soil), P adsorption experiments were conducted in Sichuan Basin of southwestern China for cropland and woodland soils with acidic, neutral and calcareous origins throughout their profile. After various doses of P were added during incubation experiments, soil P fractions were also analyzed. The results showed that there were no significant differences in Fe-oxides and P adsorption along the vertical gradients. Agricultural practices and lower pH conditions reduced the P adsorption capacity of purple soils throughout the soil profiles. For acidic and neutral purple soil profiles, the P adsorption capability was mainly influenced by Fe-oxides and soil texture. Ca-bound P and Fe-Al-bound P represented the majority of the total inorganic P of calcareous soils. There was a saturation of adsorption capacity by sesquioxide and a high risk of dissoluble reactive P (NH₄Cl-P) being released out of the soil profile in acidic and neutral purple soils after the greatest P addition, indicated by the higher proportions of NH₄Cl-P (over 40%) and decreasing Fe-Al-P fraction. P fractions migrated with greater difficulty in calcareous purple soil profiles as Ca-P fraction peaked over 65% when adding a P dose at or greater than 80 g P kg^-1, indicating the high potential of P adsorption. The X-Ray Diffraction analysis also verified the formation of brushite. Adaptive management practices should be designed to alleviate P losses for acidic and neutral purple soils.     

土壤有机质含量估测及其影响因素的光谱分析

 土壤颗粒大小差异使土壤反射光谱产生相应变化,影响土壤有机质含量等属性的光谱预测精度。本研究准备了颗粒粒径分别为2、0.25和0.15mm的土样,测定土壤有机质(Soil Organic Matter,SOM)含量,并于室内模拟条件下测定其反射光谱。通过分析不同粒径土样的原始(Raw)、多次散射校正(Multiple scattering correction, Msc)、一阶微分(First derivative, Fd)、连续统去除(Continuum removal, Cr)光谱与SOM含量之间的关系,筛选出与SOM含量相关性最强的Fd光谱单波段(2250nm, r=0.82, P<0.01),并建立线性回归模型;利用全波段光谱反射率,以偏最小二乘回归(Partial least square regression, PLSR)方法,确立2mm土样Msc处理光谱的PLSR模型为最优模型(RPD=3.56、R²=0.90、RMSEP=1.96g/kg)。土壤颗粒粒径对土壤光谱反射率变化有明显影响,但二者之间并非简单的线性关系,可能存在一个转折点;单变量(单波段光谱反射率)线性回归模型的预测能力,明显低于全波段反射光谱(Msc处理)-PLSR模型;土样样本容量对SOM含量预测精度有显著影响。因此,根据样本容量大小,选择合适的土壤颗粒粒径与光谱预处理方法组合可以提高预测精度。     

变换光谱数据对土壤氮素PLSR模型的影响研究

光谱数据变换对消除背景、噪音影响以及提取光谱特征有重要的作用,是光谱数据分析过程中的必要步骤。为了研究光谱变换处理对土壤氮素PLSR模型的影响精度,并选择最佳光谱变换处理方法,本文对原始光谱数据进行了15种典型光谱变换,通过比较不同变换光谱与土壤氮素的相关性,实现土壤氮素的PLSR精确诊断,并综合评定最佳光谱数据变换方法。结果表明,涉及微分处理后的光谱变换,尤其是先进行开方（T₈、T₁₁）、对数（T₆、T₁₂）等变换后再进行微分处理,可提高其与土壤氮素的相关性。在引入较少因子变量个数的条件下,该方法使因变量解释量达到了98%。综合考虑模型的校正、验证效果及模型复杂度（模型最佳因子变量个数）,可得出光谱平方根的一阶微分变换处理（T₈）为最佳的土壤光谱变换算法。该条件下的土壤氮素的校正模型表现为R²=0.985、RMSE_C=0.000132、F_n=6,验证模型的表现为R²=0.9853、RMSE_V=0.000162,结果表明基于T₈的光谱数据变换可实现本试验条件下土壤氮素的光谱估算。另外,可以考虑将原始光谱的一阶微分（T₉）、对数和对数倒数的一阶微分（T₆、T₇）以及平方根和对数的二阶微分（T₁₁、T₁₂）作为光谱数据变换方法。本文研究结果可为土壤氮素估算和光谱数据预处理提供技术参考。     

Combining Environmental Factors and Lab VNIR Spectral Data to Predict SOM by Geospatial Techniques

Soil organic matter(SOM) is an important parameter related to soil nutrient and miscellaneous ecosystem services. This paper attempts to improve the performance of traditional partial least square regression(PLSR) model by considering the spatial autocorrelation and soil forming factors. Surface soil samples(n = 180) were collected from Honghu City located in the middle of Jianghan Plain, China. The visible and near infrared(VNIR) spectra and six environmental factors(elevation, land use types, roughness, relief amplitude, enhanced vegetation index, and land surface water index) were used as the auxiliary variables to construct the multiple linear regression(MLR), PLSR and geographically weighted regression(GWR) models. Results showed that: 1) the VNIR spectra can increase about 39.62% prediction accuracy than the environmental factors in predicting SOM; 2) the comprehensive variables of VNIR spectra and the environmental factors can improve about 5.78% and 44.90% relative to soil spectral models and soil environmental models, respectively; 3) the spatial model(GWR) can improve about 3.28% accuracy than MLR and PLSR. Our results suggest that the combination of spectral reflectance and the environmental variables can be used as the suitable auxiliary variables in predicting SOM, and GWR is a promising model for predicting soil properties.     

Carbonates and organic matter in soils characterized by reflected energy from 350-25000 nm wavelength

The soil carbon pool which is the sum of soil organic carbon(SOC) and soil inorganic carbon(SIC) is the second largest active store of carbon after the oceans and it is an important component of the global carbon cycle. Hence, accurate estimation of SOC and SIC as important carbon reservoirs in terrestrial ecosystems using fast, inexpensive and non-destructive methods is crucial for planning different climate change policies. The aim of the current research was to examine the effectiveness of Vis-NIR(visible and near-infrared spectroscopy: 350-2500 nm) and MIR(mid-infrared spectroscopy: 4000-400 cm~(-1)) to characterize and estimate soil organic matter(SOM) and carbonates as main components of soil carbon stocks in Juneqan, Charmahal va Bakhtiari, Iran. To do so, a total of 548 soil samples from this area were collected(October 2015) and analyzed in laboratory(August 2017). In order to develop models capable of predicting SOM and carbonates content, seven spectral preprocessing methods comprising Absorbance(Abs), De-trending(Det), Continuum removal(CR), Savitzky-Golay derivatives(SGD), standard normal variate transformation(SNV), multiplicative scatter correction(MSC) and Normalization by range(NBR) were conducted along with five multivariate methods including Random Forest(RF), Partial Least-Squares Regression(PLSR), Artificial Neural Network(ANN), Support Vector Machine(SVM) and Gaussian Process Regression(GPR). The content of carbonates caused spectral reflectance intensity to augment on several ranges of spectrum and strong absorption feature at 2338 nm in the Vis-NIR and 714, 850, 870, 1796, 2150 and 2510 cm~(-1) in the MIR spectra range. SOM absorbed energy in several ranges, but also showed specific peaks in MIR. Both facts are associated with the structure of carbonates and SOM and its interaction with energy. The best combination of preprocessing and calibration models for carbonates quantification in Vis-NIR spectra was Det/PLSR(R~2= 0.74, RPD= 2.19, RMSE= 6.45). For SOM, it was Det/PLSR(R~2= 0.82, RPD= 2.41, RMSE= 0.75). The Det/RF(R~2= 0.87, RPD= 2.44, RMSE= 0.66) for the quantification of SOM and MSC/RF(R~2= 0.84, RPD= 2.84, RMSE= 5.50) for carbonates in MIR spectra range showed the greatest results. The stronger occurrence of spectral bands in MIR as well as the specificity of the absorption features indicated that this range produced better predictions. The obtained results highlighted the significant role of soil spectroscopy technique in predicting SOC and soil carbonates as key components of soil carbon stocks in the study area. Therefore, this technique can be used as a more cost-effective, time saving and nondestructive alternative to traditional methods of soil analysis.     

Subsurface flow processes in sloping cropland of purple soil

Subsurface flow is a prominent runoff process in sloping lands of purple soil in the upper Yangtze River basin.However,it remains difficult to identify and quantify.In this study,in situ runoff experimental plots were used to measure soil moisture dynamics using an array of time domain reflectometry(TDR) together with overland flow and subsurface flow using isolated collecting troughs.Frequency of preferential flow during rainfall events and the controls of subsurface flow processes were investigated through combined analysis of soil properties,topography,rainfall intensity,initial wetness,and tillage.Results showed that subsurface flow was ubiquitous in purple soil profiles due to welldeveloped macropores,especially in surface soils while frequency of preferential flow occurrence was very low(only 2 cases in plot C) during all 22 rainfall events.Dry antecedent moisture conditions promoted the occurrence of preferential flow.However,consecutive real-time monitoring of soil moisture at different depths and various slope positions implied the possible occurrence of multiple subsurface lateral flows during intensive storms.Rainfall intensity,tillage operation,and soil properties were recognized as main controls of subsurface flow in the study area,which allows the optimization of management practices for alleviating adverse environmental effects of subsurface flow in the region.     

Spectral response of different eroded soils in subtropical china: A case study in Changting County,China

Hyper-spectral data is widely used to determine soil properties. However, few studies have explored the soil spectral characteristics as response to soil erosion. This study analysed the spectral response of different eroded soils in subtropical China, and then identify the spectral characteristics and soil properties that better discriminate softs with different erosion degrees. Two methods were compared： direct identification by inherent spectral characteristics and indirect identification by predictions of critical soft properties. Results showed that the spectral curves for different degrees of erosion were similar in morphology, while overall reflectance and characteristics of specific absorption peaks were different. When the first method is applied, some differences among different eroded groups were found by integration of associated indicators. However, the index of such indicators showed apparent mixing and crossover among different groups, which reduced the accuracy of identification. For the second method, the correlation between critical soil properties, such as soil organic matter （SOM）, iron and aluminium oxides and reflectance spectra, was analysed. The correlation coefficients for the moderate eroded group were primarily between -0.3 to -0.5, which were worse than the other twogroups. However, the maximum value of R2 was obtained as 0.86 and 0.94 for the non-apparent eroded and the severe group. Furthermore, these two groups also showed some differences in the spectral response of iron complex state （Fep）, Aluminium amorphous state （Alo） and the modelling results for soil organic matter （SOM）. The study proved that it is feasible to identify different degrees of soil erosion by hyperspectral data, and that indirect identification by modelling critical soil properties and reflectance spectra is much better than direct identification. These results indicate that hyper-spectral data may represent a promising tool in monitoring and modelling soil erosion.     

土壤粒径的光谱响应特性研究

以实验室制备的5个不同粒径水平的土壤样本和室内高光谱数据为基础,通过对光谱数据进行重采样、数学变换等预处理并进行单因素方差分析、相关性分析和回归分析,探讨土壤粒径的高光谱特性,建立了光谱数据预测土壤粒径的校正模型。结果表明,土壤粒径对反射光谱有显著的影响,波长越长影响越大;在全波段范围内土壤粒径和光谱数据都呈负相关关系,对原始光谱数据进行微分变换能增加其与土壤粒径的相关性;以反射率一阶微分建立的回归模型为反演土壤粒径的最佳模型,其建模决定系数■、预测决定系数■、预测相对偏差RPD分别为0.666,0.653,2.043,预测均方根误差RMSE为0.175。     

Preliminary Mapping and Characterisation of Soils in the Pirin Mountains （Bulgaria）

Soil types, humus types and vegetation as well as their hypsometric variation were analysed in terms of sequences in the northern part of the high mountains of the Pirin National Park at altitudes between looo and 2400 m a.s.1. The study area is characterised by a large variety of natural parameters like petrology （mainly marble and granite）, morphology （different slope deposits, exposition） and the orographic climate gradient. Statistical analyses using these parameters provided a basis for the soil group classification of the sites. Based on a Digital Terrain Model （DTM） and a geological map of the Pirin National Park, the results of these statistical analyses were used to generate a ＂map of potential soil groups＂ （regionalisation using GIS）. Six potential soil groups could be determined. The resulting map exhibits a confidence level of 68 % on 74.4 % of the covered area. Rendzic Leptosols, in combination with Folic Histosols and Histi-lithic Leptosols occur in the alpine and subalpine regions on calcareous substrates. With decreasing altitude they are replaced by a mosaic of Rendzic Leptosols, Phaeozems and an increasing occurrence of Cambic Umbrisols. Umbrisols found on silicatic substrates in the alpine region are replaced by Cambic Umbrisols with decreasing altitude as well. Hence, pedogenesis is characterised by increasing browning and depth of the soil profiles with decreasing altitude. The pH-level is slightly acidic to neutral in lower zones and on calcareous rocky bases. Acidification increases in the subalpine zone. Soil pH decreases down to 4 on silicate subtrates. Typical humic values in mineral topsoils are 10 to 12 %, and in organic layers of the soils above 2000 m a.s.1, they are even more. The C：N ratio closely ranges around 20 （median）.     

A method for surface roughness parameter estimation in passive microwave remote sensing

Surface roughness parameter is an important factor and obstacle for retrieving soil moisture in passive microwave remote sensing.Two statistical parameters,root mean square (RMS) height (s) and correlation length (l),are designed for describing the roughness of a randomly rough surface.The roughness parameter measured by traditional way is independence of frequency,soil moisture and soil heterogeneity and just the ″geometric″ roughness of random surface.This ″geometric″ roughness can not fully explain the scattered thermal radiation by the earth's surface.The relationship between ″geometric″ roughness and integrated roughness (contain both ″geometric″ roughness and ″dielectric″ roughness) is linked by empirical coefficient.In view of this problem,this paper presents a method for estimating integrated surface roughness from radiometer sampling data at different frequencies,which mainly based on the flourier relationship between power spectral density distribution and spatial autocorrelation function.We can obtain integrated surface roughness at different frequencies by this method.Besides "geometric" roughness,this integrated surface roughness not only contains "dielectric" roughness but also includes frequency dependence.Combined with Q/H model the polarization coupling coefficient can also be obtained for both H and V polarization.Meanwhile,the simulated numerical results show that radiometer with a sensitivity of 0.1 K can distinguish the different surface roughness and the change of roughness with frequency for the same rough surface.This confirms the feasibility of radiometer sampling method for estimating the surface roughness theoretically.This method overcomes the problem of ″dielectric″ roughness measurement to some extent and can achieve the integrated surface roughness within a microwave pixel which can serve soil moisture inversion better than the ″geometric″ roughness.     

Preliminary Mapping and Characterisation of Soils in the Pirin Mountains (Bulgaria)

Soft types, humus types and vegetation as well as their hypsometric variation were analysed in terms of sequences in the northern part of the high mountains of the Pirin National Park at altitudes between 1000 and 2400 m a.s.l. The study area is characterised by a large variety of natural parameters like petrology (mainly marble and granite), morphology (different slope deposits, exposition) and the orographic climate gradient. Statistical analyses using these parameters provided a basis for the soil group classification of the sites. Based on a Digital Terrain Model (DTM) and a geological map of the Pirin National Park, the results of these statistical analyses were used to generate a "map of potential soft groups" (regionalisation using GIS). Six potential soil groups could be determined. The resulting map exhibits a confidence level of 68% on 74.4% of the covered area. Rendzic Leptosols, in combination with Folio Histosols and Histi-lithic Leptosols occur in the alpine and subalpine regions on calcareous substrates. With decreasing altitude they are replaced by a mosaic of Rendzic Leptosols, Phaeozems and an increasing occurrence of Cambic Umbrisols. Umbrisols found on silicatic substrates in the alpine region are replaced by Cambic Umbrisols with decreasing altitude as well. Hence, pedogenesis is characterised by increasing browning and depth of the soil profiles with decreasing altitude. The pH-level is slightly acidic to neutral in lower zones and on calcareous rocky bases. Acidification increases in the subalpine zone. Soft pH decreases down to 4 on silicate subtrates. Typical humic values in mineral topsoils are 10 to 12%, and in organic layers of the softs above 2000 m a.s.l, they are even more. The C:N ratio closely ranges around 20 (median).     

克里格法的土壤水分遥感尺度转换

 尺度效应往往会制约着定量遥感反演的精度,对地学信息进行空间尺度转换是生产实践的必然要求,而常用的尺度转换模型多利用光谱数据进行差值计算,不适合升尺度和降尺度转换。由于土壤含水量数据具有区域变化量的随机性和结构性特点,本文以15m分辨率的ASTER图像像元为基本单元,采用点克里格法完成ASTER 15m至7.5m分辨率的土壤含水量数据降尺度转换,从分维数的相似程度上来看,转换结果是合理的;并利用块状克里格法对地面实测样点数据进行点到7.5m分辨率的面数据升尺度转换,将升尺度和降尺度转换结果与实测样点均值相比较,结果表明:7.5m分辨率的实测样点土壤水均值误差在1.5782-5.019之间,块状克里格法获取的升尺度土壤含水量数据与点克里格法获取的降尺度土壤含水量数据之间误差则为1.2825-5.0481,可见克里格法考虑了点与周边的关系,所获得的土壤含水量值要优于未考虑空间异质性的土壤含水量平均值。     

大柳塔采煤塌陷区土壤含水量的空间变异特征分析

通过研究采煤塌陷区土壤水空间分布及动态变化特征,查明了采煤塌陷对土壤含水量的影响,这对矿区生态环境保护、塌陷区土地复垦等具有重要指导意义。以大柳塔双沟采煤塌陷区为试验区,利用传统统计学和地统计学方法分析了采煤塌陷条件下的包气带土壤水空间变异特征。研究结果表明:由于采煤塌陷造成塌陷区土壤层位在垂向上倒置、重组,引起土壤粒度、容重、孔隙度等土壤物理特性的改变,使塌陷区土壤含水量比非塌陷区显著降低,在不同深度层（0~60 cm）分别减少14.2%~21.9%;在垂向分布上塌陷区土壤水分也表现出较强的变异性,其离散系数在不同深度（0~60 cm）与非塌陷区的差值在19.2%~50%之间。根据试验区0 cm、20 cm、40 cm、60 cm四个层面土壤含水量的Kriging插值等值线图显示,土壤含水量低值区均位于塌陷区内的塌陷坑部位,证明地表的地裂缝、塌陷坑、塌陷洞等塌陷形态对土壤持水能力的影响颇为显著。采煤塌陷区土壤含水量降低、空间变异性增强直接导致了地表植被生存环境的恶化,地表景观被严重破坏。     

Response of Soil Moisture to Rainfall Event in Black Locust Plantations at Different Stages of Restoration in Hilly-gully Area of the Loess Plateau,China

Precipitation plays an important role in the water supplies that support ecological restoration by sustaining large-scale artificial plantations in arid and semiarid regions, especially black locust(Robinia pseudoacacia) plantations(RP plantations), which are widely planted due to R. pseudoacacia being an excellent pioneer species. Characterizing the response of soil moisture to rainfall events at different stages of restoration is important for assessing the sustainability of restoration in RP plantations. In this study, we quantified the response of soil moisture to rainfall events at different years of restoration(15, 20 and 30 yr) representing different restoration stages in RP plantations in a typical hilly-gully area, i.e., the Yangjuangou Catchment, of the Loess Plateau, China. Over the growing season(June to September) of 2017, smart probes were placed at nine depths(10, 20, 40, 60, 80, 100, 120, 150, and 180 cm below the soil surface) to obtain volumetric soil water information at 30-min intervals in the three RP plantations. The advance of the wetting front was depicted, and the total cumulative water infiltration was measured. Soil moisture was mainly replenished by eight heavy rainfall events(mean rainfall amount = 46.3 mm), accounting for 88.7% of the rainfall during the growing season. The mean soil moisture content profiles of RP plantations at the three restoration stages were ordered as 30-yr(14.07%) 20-yr(10.12%) 15-yr(8.03%), and this relationship displayed temporal stability. Soil moisture was primarily replenished by rainfall at the 0-60 cm soil depth, and soil moisture remained stable below the 100-cm soil depth. The rainfall regime influenced the advancement of the wetting front. Here, a single rainfall event of 30 mm was the rainfall threshold for infiltration into the 60-cm soil layer. The total infiltration time ranged from 310.5-322.0 h, but no significant differences were found among RP plantations at different restoration stages. Young and old RP plantations had more total infiltration(more than 228.2 mm) and deeper infiltration depths(80-100 cm) than middle-aged plantations. The RP plantation at the intermediate restoration stage exhibited minimal total infiltration(174.2 mm) and a shallow infiltration depth(60 cm) due to the soil physical structure of the plot, which may have limited rain infiltration. More stand conditions that may affect infiltration should be considered for priority afforestation areas.     

Shear strength features of soils developed from purple clay rock and containing less than two-millimeter rock fragments

Soil shear strength is an important indicator of engineering design and an essential parameter of soil precision tillage and agricultural machinery and equipment design. Although numerous studies have investigated the characteristics of different soil shear strengths, only a few of these works have paid attention to soils containing considerable quantities of rock fragments. To date, most studies on the effects of rock fragments on the shear strength have paid attention to the role of rock fragments with sizes >2 mm. The effects of rock fragments <2 mm in soil are generally ignored. Similar to rock fragments >2 mm, the presence of rock fragments <2 mm could also change the mechanical properties of soils. Thus, in the present study we evaluated the potential influence of <2 mm rock fragments on soil shear strength via an unconsolidated undrained (UU) triaxial compression test. Our results were as follows: (1) A certain quantity of <2 mm rock fragments presented in purple soils developed from clay rocks; and an appropriate quantity of <2 mm rock fragments could improve the shear strength of soils. (2) The different PSDs of soils containing <2 mm rock fragments mainly caused variations in the internal friction angle of soils. (3) The shear strengths of the two mudstone-developed red-brown and gray-brown purple soils was more sensitive to water than that of the shale-developed coarse-dark purple soil. As the soil water content increased from 9% to 23%, the changes in the cohesion, internal friction angle, shear strength, and the maximum principal stress difference were smaller in the coarse dark purple soil than in the two other soils. We therefore concluded that <2 mm rock fragments in purple soils exerted important effects on soil shear strength. A better understanding of the differences among the shear strength features of purple soils could help improve the design of agricultural machinery and equipment.     

Numerical simulation and validation of ocean waves measured by an Along-Track Interferometric Synthetic Aperture Radar

A new nonlinear integral transform of ocean wave spectra into Along-Track Interferometric Synthetic Aperture Radar （ATI-SAR） image spectra is described. ATI-SAR phase image spectra are calculated for various sea states and radar configurations based on the nonlinear integral transform. The numerical simulations show that the slant range to velocity ratio （R/V）, significant wave height to ocean wavelength ratio （Hi2）, the baseline （2B） and incident angle （0） affect ATI-SAR imaging. The ATI-SAR imaging theory is validated by means of Two X-band, HH-polarized ATI-SAR phase images of ocean waves and eight C-band, HH-polarized ATI-SAR phase image spectra of ocean waves. It is shown that ATI-SAR phase image spectra are in agreement with those calculated by forward mapping in situ directional wave spectra collected simultaneously with available ATI-SAR observations. ATI-SAR spectral correlation coefficients between observed and simulated are greater than 0.6 and are not sensitive to the degree of nonlinearity. However, the ATI-SARoPhase image spectral turns towards the range direction, even if the real ocean wave direction is 30. It is also shown that the ATI-SAR imaging mechanism is significantly affected by the degree of velocity bunching nonlinearity, especially for high values of R/V and H/2.     

Influences of soil moisture and salt content on loess shear strength in the Xining Basin,northeastern Qinghai-Tibet Plateau

Moisture and salt content of soil are the two predominant factors influencing its shear strength. This study aims to investigate the effects of these two factors on shear strength behavior of loess in the Xining Basin of Northeast Qinghai-Tibet Plateau, where such geological hazards as soil erosion, landslides collapse and debris flows are widespread due to the highly erodible loess. Salinized loess soil collected from the test site was desalinized through salt-leaching in the laboratory. The desalinized and oven-dried loess samples were also artificially moisturized and salinized in order to examine how soil salinity affects its shear strength at different moisture levels. Soil samples prepared in different ways(moisturizing, salt-leaching, and salinized) were measured to determine soil cohesion and internal friction angle. The results show that salt-leaching up to 18 rounds almost completely removed the salt content and considerably changed the physical components of loess, but the soil type remained unchanged. As salt content increases from 0.00% to 12.00%, both the cohesion and internal friction angle exhibit an initial decrease and then increase with salt content. As moisture content is 12.00%, the salt content threshold value for both cohesion and internal friction angle is identified as 3.00%. As the moisture content rises to 16.0% and 20.00%, the salt content threshold value for cohesion is still 6.00%, but 3.00% for internal friction angle. At these thresholds soil shear strength is the lowest, below which it is inversely related to soil salinity. Beyond the thresholds, however, the relationship is positive. Dissimilar to salinity, soil moisture content exerts an adverse effect on shear strength of loess. The findings of this study can provide a valuable guidance on stabilizing the engineering properties of salinized loess to prevent slope failures during heavy rainfall events.     

基于GF-3和Landsat 8遥感数据的土壤水分反演研究

基于我国首颗全极化雷达卫星高分三号（GF-3）和Landsat8数据,研究浓密植被覆盖地表土壤水分反演方法。为了提高浓密植被覆盖地表土壤水分反演精度,首先利用PROSAIL模型、实测植被参数及Landsat8光学数据分析了8种植被指数与植被冠层含水量的相关性,从中优选出归一化差异水指数（NDWI₅）用于反演植被冠层含水量,并通过分析植被含水量和植被冠层含水量的关系,构建植被含水量模型;然后结合植被含水量反演模型和简化MIMICS模型校正了植被对雷达后向散射系数的影响,最后基于AIEM建立裸土后向散射系数模拟数据集,发展一种主动微波和光学数据协同反演浓密植被覆盖地表土壤水分模型,并以山东省禹城市为研究区,实现了玉米覆盖下HH、VV和HH+VV 3种模式土壤水分反演。实验结果表明： ① NDWI₅为最佳植被指数,对于去除植被影响有较好效果;② 基于此方法,利用GF-3和Landsat8卫星数据反演得到的土壤水分具有较高的精度;③ 相比HH和VV两种极化模式,HH+VV双通道模式对土壤水分反演结果更好,决定系数（R2）为0.4037,均方根误差（RMSE）为0.0667 m ³m ^-3。     

基于GNSS+R裸土延迟多普勒图理论研究

侧重散射机理研究,以GPS相关函数和相关功率模型为基础,主要通过对双站雷达散射截面计算模型进行修改,得到裸土的GPS散射信号模型。其中裸土的双站圆极化雷达散射截面采用双站随机粗糙面散射模型计算得到,利用极化合成的方法使模型具备全极化计算功能。通过修改后的模型,模拟分析不同极化下裸土参数（土壤水分和地表粗糙度）对延迟多普勒图DDM（delay Doppler map）的影响。该理论模型研究有助于机载/星载GNSS-R接收机数据解释、GNSS-R实验设计以及后向反演算法的开发。同时,延迟多普勒图模型的研究结果也为GNSS+R土壤水分遥感研究提供了一定的理论依据。     

Stability analysis of an unsaturated soil slope considering rainfall infiltration based on the Green-Ampt model

Based on the principle of saturated infiltration and the Green-Ampt model, an unsaturated infiltration model for a soil slope surface was established for either constant moisture content, or depth-varying moisture content and the slope. Infiltration parameters in the partially saturated slope were revealed under sustained rainfall. Through analysis of the variation of initial moisture content in the slope, the ponding time, infiltration depth, and infiltration rate were deduced for an unsaturated soil slope subject to rainfall infiltration. There is no ponded water on the surface of the slope under sustained low-intensity rainfall. The results show that the infiltration parameters of an unsaturated slope are influenced by the initial moisture content and the wetting front saturation, the soil cohesion and rainfall intensity under sustained rainfall. More short-term slope failures can occur with the decrease of cohesion of the soil of the slope. The ponding time and infiltration depth differ considering constant or different initial moisture content respectively in the soil slope. Then, best-fit curves of the infiltration rate, ponding time, and infiltration depth to the wetting front saturation were obtained with constant or different initial moisture contents. And the slope failure time is roughly uniform when subject to a rainfall intensity I5 mm/h.