Response of soil aggregate disintegration to the different content of organic carbon and its fractions during splash erosion

Aggregate disintegration is a critical process in soil splash erosion. However, the effect of soil organic carbon (SOC) and its fractions on soil aggregates disintegration is still not clear. In this study, five soils with similar clay contents and different contents of SOC have been used. The effects of slaking and mechanical striking on splash erosion were distinguished by using deionized water and 95% ethanol as raindrops. The simulated rainfall experiments were carried out in four heights (0.5, 1.0, 1.5 and 2.0 m). The result indicated that the soil aggregate stability increased with the increases of SOC and light fraction organic carbon (LFOC). The relative slaking and the mechanical striking index increased with the decreases of SOC and LFOC. The reduction of macroaggregates in eroded soil gradually decreased with the increase of SOC and LFOC, especially in alcohol test. The amount of macroaggregates (>0.25 mm) in deionized water tests were significantly less than that in alcohol tests under the same rainfall heights. The contribution of slaking to splash erosion increased with the decrease of heavy fractions organic carbon. The contribution of mechanical striking was dominant when the rainfall kinetic energy increased to a range of threshold between 9 J m⁻² mm⁻¹ and 12 m⁻² mm⁻¹. This study could provide the scientific basis for deeply understanding the mechanism of soil aggregates disintegration and splash erosion.     

Root‐zone moisture replenishment in a native vegetated catchment under Mediterranean climate

The root‐zone moisture replenishment mechanisms are key unknowns required to understand soil hydrological processes and water sources used by plants. Temporal patterns of root‐zone moisture replenishment reflect wetting events that contribute to plant growth and survival and to catchment water yield. In this study, stable oxygen and hydrogen isotopes of twigs and throughfall were continuously monitored to characterize the seasonal variations of the root‐zone moisture replenishment in a native vegetated catchment under Mediterranean climate in South Australia. The two studied hillslopes (the north‐facing slope [NFS] and the south‐facing slope [SFS]) had different environmental conditions with opposite aspects. The twig and throughfall samples were collected every ~20 days over 1 year on both hillslopes. The root‐zone moisture replenishment, defined as percentage of newly replenished root‐zone moisture as a complement to antecedent moisture for plant use, calculated by an isotope balance model, was about zero (±25% for the NFS and ± 15% for the SFS) at the end of the wet season (October), increased to almost 100% (±26% for the NFS and ± 29% for the SFS) after the dry season (April and May), then decreased close to zero (±24% for the NFS and ± 28% for the SFS) in the middle of the following wet season (August). This seasonal pattern of root‐zone moisture replenishment suggests that the very first rainfall events of the wet season were significant for soil moisture replenishment and supported the plants over wet and subsequent dry seasons, and that NFS completed replenishment over a longer time than SFS in the wet season and depleted the root zone moisture quicker in the dry season. The stable oxygen isotope composition of the intraevent samples and twigs further confirms that rain water in the late wet season contributed little to root‐zone moisture. This study highlights the significant role of the very first rain events in the early wet season for ecosystem and provides insights to understanding ecohydrological separation, catchment water yield, and vegetation response to climate changes.     

One-dimensional consolidation of soil under multistage load based on continuous drainage boundary

In engineering practice, a rapid loading rate can result in ground failure when the strength of soft soils is relatively low, and a multistage loading scheme is always utilized to deal with this situation. Firstly, under a multistage load and the continuous drainage boundary, an analytical solution of excess pore-water pressure and consolidation degree is obtained by virtue of the superposition formula of excess pore-water pressure, and a more general continuous drainage boundary under arbitrary time-dependent load is developed. Then, a comparison with existing analytical solutions is conducted to verify the present solution. A preliminary attempt on applying the continuous drainage boundary into the finite element model is made, and the feasibility of the numerical model for the one-dimensional consolidation under the continuous drainage boundary is verified by comparing the results calculated by FEM with that from present analytical solution. Finally, the consolidation behavior of soil is investigated in detail for different int erface parameters or loading scheme. The results show that, in land reclamation projects, a horizontal drain should be placed close to the boundary with a smaller interface parameter to improve the consolidation efficiency. The degree of consolidation is also related to the applied time-dependent load and interface parameters.     

Constitutive modelling of fine-grained gassy soil: A composite approach

Fine-grained marine sediments containing large undissolved gas bubbles are widely distributed around the world. Presence of the bubbles could degrade the undrained shear strength (s_u ) of the soil, when the gas pressure u_g is relatively high as compared with the effective stress in the saturated soil matrix. Meanwhile, the addition of bubbles may also increase s_u when the difference between u_g and pore water pressure u_w becomes smaller than the water entry value, causing partial water drainage from the saturated matrix into the bubbles (bubble flooding) during globally undrained shearing. A new constitutive model for describing the two competing effects on the stress-strain relationship of fine-grained gassy soil is proposed within the framework of critical state soil mechanics. The gassy soil is considered as a three-phase composite material with compressible cavities, which allows water entry from the saturated matrix. Bubble flooding is modelled by introducing an additional positive volumetric strain increment of the saturated clay matrix, which is dependent on the difference between pore gas and pore water pressure based on experimental observations. A modified hardening law based on that of the modified Cam clay model is employed, which in conjunction with the expression for bubble flooding, can describe both the detrimental and beneficial effects of gas bubbles on soil strength and plastic hardening in shear. Only two extra parameters in addition to those in the modified Cam clay model are used. It is shown that the key features of the stress-strain relationship of three fine-grained gassy soils can be reproduced satisfactorily.     

Frequency analysis of nonstationary annual maximum flood series using the time‐varying two‐component mixture distributions

The most popular practice for analysing nonstationarity of flood series is to use a fixed single‐type probability distribution incorporated with the time‐varying moments. However, the type of probability distribution could be both complex because of distinct flood populations and time‐varying under changing environments. To allow the investigation of this complex nature, the time‐varying two‐component mixture distributions (TTMD) method is proposed in this study by considering the time variations of not only the moments of its component distributions but also the weighting coefficients. Having identified the existence of mixed flood populations based on circular statistics, the proposed TTMD was applied to model the annual maximum flood series of two stations in the Weihe River basin, with the model parameters calibrated by the meta‐heuristic maximum likelihood method. The performance of TTMD was evaluated by different diagnostic plots and indexes and compared with stationary single‐type distributions, stationary mixture distributions and time‐varying single‐type distributions. The results highlighted the advantages of TTMD with physically‐based covariates for both stations. Besides, the optimal TTMD models were considered to be capable of settling the issue of nonstationarity and capturing the mixed flood populations satisfactorily. Copyright © 2016 John Wiley & Sons, Ltd.     

In‐situ analysis of trace elements and Sr–Pb isotopes of K‐feldspars from Tongshankou Cu–Mo deposit,SE Hubei Province,China: Insights into early potassic alteration of the porphyry mineralization system

In the Tongshankou porphyry deposit (SE Hubei Province, South China), three types of K‐feldspars are recognized: (I) the phenocryst type in the porphyry that crystalized during the magmatic stage, (II) the megacryst type and (III) the vein type in the altered porphyry and orebody that was produced by hydrothermal fluids. A detailed in‐situ analysis of trace elements and Sr–Pb isotopes was carried out on K‐feldspars in an attempt to unravel their formation processes and to trace the element sources during potassic alteration. The Type III K‐feldspars show lower Sr contents and Sr‐isotope ratios but higher Pb contents and Pb‐isotope ratios than the Type I and II K‐feldspars, possibly reflecting a contribution from the country carbonate rocks with less radiogenic Sr but more radiogenic Pb sources, and indicate that the ore‐forming fluids and materials may have been partially derived from external sources such as the host sedimentary rocks during the early potassic alteration stage.