Characteristics of grassland degradation and driving forces in the source region of the Yellow River from 1985 to 2000*

The source region of the Yellow River is located in the middle east of the Tibetan Plateau in northwest China. The total area is about 51,700 km2, mainly covered by grassland (79%), unused land (16%) and water (4%). The increasing land utilization in this area has increased the risk of environmental degradation. The land use/cover data (1985 and 2000) provided by the Data Center of Resources and Environment in the Chinese Academy of Sciences were used to analyze the land cover change in the source region of the Yellow River. DEM (1:250,000) data, roads and settlement data were used to analyze the spatial characteristics of grasslands degradation. The ArcGIS 9 software was used to convert data types and do the overlay, reclassification and zonal statistic analysis. Results show that grassland degradation is the most important land cover change in the study area, which occupied 8.24% of the region’s total area. Human activities are the main causes of the grassland degradation in the source region of the Yellow River: 1) the degradation rate is higher on the sunny slope than on the shady slope; 2) the grassland degradation rate decreases with an increase in the elevation, and it has a correlation coefficient of -0.93; 3) the nearer to the settlements the grassland is, the higher the degradation rate. Especially within a distance range of 12 km to the settlements, the grassland degradation rate is highly related with the distance, with a coefficient of -0.99; and 4) in the range of 4 km, the degradation rate decreases with the increase of distance to the roads, with a correlation coefficient of -0.98. Besides some physical factors, human activities have been the most important driving forces of the grassland degradation in the source region of the Yellow River since 1985. To resolve the degradation problems, population control is essential, and therefore, it can reduce the social demand of livestock products from the grassland. To achieve sustainable development, it needs to improve the management of grassland ecosystem.     

Deriving indicators of soil degradation from soil aggregation studies in southeastern Spain and southern France

Soil degradation is perceived as a major threat in the Mediterranean region due to changes in land-use and possible future climate change. Soil aggregation parameters are used here to demonstrate their potential as a key-indicator for land degradation studies. The monitoring of these indicators offers a means of establishing the vulnerability and resilience of geo-ecosystems. Soil aggregation stability and distribution were studied on soils with an open shrubby vegetation cover, from several places in southeastern Spain and southern France, by applying drop tests and determining aggregate size distributions. Aspect and vegetation cover were incorporated in the soil sampling. Several indices were derived from these analyses to indicate the degree of soil aggregation. This was done by referencing to a base level of aggregation (bare soil aggregation). It was found that soil aggregates were more stable and were often coarser under vegetation, when compared to their immediate surrounding bare areas. A similar, slightly less clear effect was noted on N-facing exposed slopes when compared to S-facing exposed slopes. Long-term changes were found by studying cultivated land, abandoned fields and land covered by semi-natural vegetation, on comparable substrate and comparable land units. It is clear that soil aggregation and aggregate stability increases with time (years). It is argued that soil aggregation indices can be used as a key-indicator for degradation processes at a fine scale with implications for runoff and sediment generating processes at the hillslope scale.     

Assessment of non-point sources of fecal pollution in coastal waters of Puerto Rico and Trinidad

Traditional and molecular methods (PCR) were used to detect, quantify and identify the source of fecal pollution in coastal sites of Puerto Rico and Trinidad. Enterococci and Escherichia coli standard plate counts were used as a general indicator of fecal contamination while the PCR detection of Bifidobacteria adolescentis and human or bovine specific Bacteroidales were used to examine potential sources. Seven of 14 sites in Trinidad including Maracas Bay which is a major public beach contained significant fecal contamination based on enterococci numbers counts exceeding established thresholds for areas of direct contact. Forty six percent of the 27 stations in Puerto Rico were over the established thresholds for enterococci and 49% according to E. coli counts. About 31% of the stations examined in Puerto Rico had evidence of human derived fecal contamination. Human fecal pollution was detected in only one station from Trinidad. Bovine derived contamination was detected only once.     

Analytical modelling of the shear behaviour of rock joints with progressive degradation of two-order roughness

We present an analytical model for the shear behaviour of rock joints with progressive degradation of two-order asperities including waviness and unevenness. Critical waviness and critical unevenness are used to respectively represent the mechanical involvements of waviness and unevenness during shear. The degradation process of two-order asperities are predicted by considering the stepwise relationship among dilation angle, sheared and unsheared asperity areas, and plastic tangential work. The dilation angle of each asperity decreases as plastic tangential work accumulates. The progressive degradation transiting from critical unevenness to critical waviness is realised through examining the dilation angle and the unsheared area of critical unevenness. The model's predictions are compared with the experimental data from direct shear tests on both regular- and irregular-shaped joints. Good agreement between analytical and experimental curves demonstrate the credence of the proposed model. Therefore, the model, after implemented in finite and discrete element codes, is practicable for the stability assessment of rock-engineering structures.     

An analytical model for chemical diffusion in layered contaminated sediment systems with bioreactive caps

An analytical model for contaminant transport in multilayered capped contaminated sediments including the degradation of organic contaminant is presented. The effect of benthic boundary layer was treated as a Robin-type boundary condition. The results of the proposed analytical model agree well with experimental data. The biodegradation of contaminant in bioturbation layer shows a significant influence on the flux at the surface of system. The maximum flux for the case with t_1/2,bio = 0.07 year can be 4.5 times less than that of the case without considering the effect of biodegradation. The thickness of bioturbation layer has a significant effect on the performance of the capped contaminated sediment. The maximum flux for the case with l_bio = 15 cm can be 17 times larger than that of the case without bioturbation layer. This may be because the effective diffusion coefficient of sand cap can be 28 times lower than D_bio. The mass transfer coefficient should be considered for the design of the capping system as the contaminant concentration at the top of system for the case with k_bl = 2.5 × 10⁻⁵ cm/s can be 13 times greater than that of the case with k_bl = 10⁻⁴ cm/s. The proposed analytical model can be used for verification of complicated numerical methods, evaluation of experimental data, and design of the capping contaminated sediment systems with reactive cap layers.     

Impact of heat advection on the thermal regime of roads built on permafrost

In northern regions, transportation infrastructure can experience severe structural damages due to permafrost degradation. Water infiltration and subsurface water flow under an embankment affect the energy balance of roadways and underlying permafrost. However, the quantification of the processes controlling these changes and a detailed investigation of their thermal impacts remain largely unknown due to a lack of available long-term embankment temperature data in permafrost regions. Here, we report observations of heat advection linked to surface water infiltration and subsurface flow based on a 9-year (from 2009 to 2017) thermal monitoring at an experimental road test site built on ice-rich permafrost conditions in southwestern Yukon, Canada. Our results show that snowmelt water infiltration in the spring rapidly increases temperature in the upper portion of the embankment. The earlier disappearance of snow deposited at the embankment slope increases the thawing period and the temperature gradient in the embankment compared with the natural ground. Infiltrated summer rainfall water lowered the near-surface temperatures and subsequently warmed embankment fill materials down to 3.6-m depth. Heat advection caused by the flow of subsurface water produced warming rates at depth in the embankment subgrade up to two orders of magnitude faster than by atmospheric warming (heat conduction). Subsurface water flow promoted permafrost thawing under the road embankment and led to an increase in active layer thickness. We conclude that the thermal stability of roadways along the Alaska Highway corridor is not maintainable in situations where water is flowing under the infrastructure unless mitigation techniques are used. Severe structural damages to the highway embankment are expected to occur in the next decade.     

Response of low flows under climate warming in high‐altitude permafrost regions in western China

The increase in low flows (winter discharge and minimum monthly discharge), caused primarily by permafrost degradation, is common in high‐latitude permafrost regions, whereas the dynamics of low flows in high‐altitude permafrost regions remain largely unknown. Long‐term discharge data from 28 unregulated catchments in western China were analysed, and the findings showed that winter discharge/minimum monthly discharge significantly increased (p ≤ 0.1) in 82/82%, 55/64%, and 0/0% of the catchments in the higher‐latitude mountain permafrost regions (Tienshan Mountains), mid‐latitude mountain permafrost regions (Qilian Mountains), and mid‐ to low‐latitude plateau permafrost regions (the source regions of the Yangtze and Yellow rivers), respectively. The differences in permafrost type and the distribution of permafrost and alpine cold desert (which is similar to tundra) were found to be the main causes for the different responses in the low flows. The rate of change of low flows (winter discharge and minimum monthly discharge) was negatively and linearly correlated with permafrost coverage when coverage was less than 40% of the catchment area, whereas the low flows changed only slightly when the permafrost coverage exceeded 40%. A significant thickening of the active layer increased the low flows in the lower permafrost‐covered catchments, which are dominated by warm permafrost. However, in the higher permafrost‐covered catchments with cold permafrost and a cold climate, only an increase in permafrost temperature (without a notable thickening of the active layer) occurred, resulting in non‐significant changes in low flows.     

黑碳在全球气候和环境系统中的作用及其在相关研究中的意义

燃烧形成的黑碳粒子进入大气中可影响辐射平衡,导致全球气候变暖,其沉降在河流、湖泊、海洋、土壤等环境中对全球生物地球化学循环起到重要的作用,成为当前国际地球科学研究的热点问题。综述了黑碳的定义及排放、沉降、降解过程,并总结了其在现在及过去环境和气候系统中的重要作用与研究意义。黑碳是全球惰性有机碳库的重要组成部分,在全球慢速碳循环中发挥潜在作用。因其具有很强的吸光特性,它在区域气候变暖过程中扮演重要角色。沉降在不同地质载体中的黑碳难以降解,可以保存几百万年,为地质历史时期古气候和古环境重建研究提供重要信息。海洋沉积物过去数百万年的黑碳记录指示了天然火的演化信息,晚第四纪黄土剖面黑碳也指示了天然火的变化信息,最近千年的湖泊和冰芯黑碳记录既反映了天然火的信息,也指示人类活动的信号。未来黑碳研究应进一步关注标准测量方法,以真正理解黑碳在全球气候与环境系统中的作用。     

Modelling alpine permafrost distribution,Val de Rechy,Valais Alps (Switzerland)

The spatial distribution of the alpine permafrost is modelled in the uppermost part of Val de Rechy (Valais Alps, Switzerland) with the use of a geographical information system (GIS). Two empirical-statistical models are compared and their validity tested by a set of bottom temperature of winter snow (BTS) measurements carried out in the field. One model can simulate the spatial distribution of the permafrost facing ground warming consecutive to climatic change. Local/regional maps of simulated permafrost distribution as presented in this paper can be useful in the context of a preventive approach for natural hazards management.     

Permafrost dynamics in the north-east of European Russia at the end of the 20th century

In the major physiographic divisions of the north-east of European Russia, temperatures in the cryolithozone and the active/seasonally frozen layer were monitored during the period 1970 to 1995. Active layer depths increased on average by 11% to 15%, whereas the depths of the seasonally frozen layer decreased by 19% to 40%. The largest positive temperature increments, up to 1.2C, were characteristic for the piedmont and basement plains. Lacustrine-alluvial lowlands demonstrated oppositely directed temperature trends depending on a landform. Numerous sites with ongoing permafrost formation demonstrated negative temperature increments varying from 0.1C to 2.0C.