Dynamics of nebkhas superimposed on a parabolic dune and their effect on the dune dynamics

We report here on a bare barchan dune in Israel that converted over the last 60 years to a shrub-covered parabolic dune due to changes in land use. Thirty nebkhas (sand mounds) that were formed by sand trapped around shrubs growing on the dune were monitored during winter of 2004–2005. The rates of erosion or deposition were measured at five points in the nebkhas by erosion pins. All nebkhas were shown to undergo erosion or deposition activity. Nebkhas on the windward slope of the dune experienced primarily erosion. Those on the lee slope grew slowly by the light accumulation of sand. The largest nebkhas were found on the dune crest; they built up through the accumulation of sand that was eroded from the windward slope. There were no significant differences between the rates of erosion/deposition of the five study points placed in each nebkha. However, there were significant differences between the rates of erosion/deposition of the nebkhas on the three dynamic segments of the dunes (windward, crest and lee). A change in dune dynamics was observed by the emergence of shrubs on the crest. These shrubs trapped sand and increased the crest height. The sand trapped on the crest was not deposited on the lee side. In that case, the dune becomes narrower, higher, with a concave shape (of the windward slope), during the transformation from a barchan to a parabolic dune.     

Tectonic constraints on watershed development on frontal ridges: Mohand Ridge, NW Himalaya, India

Uplifting frontal ridges are one of the most conspicuous geomorphic features that mark the frontal parts of actively converging mountain belts. Growth of these ridges can lead to the simultaneous development of a drainage system that is defined by watersheds, stream network and long profiles of channels. In the present study, shape parameters of watersheds, stream network characteristics and pattern of network growth, shape of long profiles, and the SL index have been investigated in a part of NW Himalaya to understand the relationship between endogenic tectonic processes and exogenic fluvial processes. This explains the tectonic control on drainage systems in the uplifting frontal ridge. This watershed analysis was carried out using a Digital Elevation Model (DEM) and a number of anomalies have been identified and analysed. The most striking is the asymmetric development of watersheds on either side of an almost straight ridge crest. Watershed asymmetry along the ridge crest is characterized by larger area and less elongated watersheds in the southern flank (forelimb) in comparison to the northern flank (backlimb). Drainage network and long profile analysis establishes that the larger watershed area in the forelimb is due to dominance of headward erosion and its impact on drainage network growth. Dominance of headward erosion is due to slope variation in response to forelimb development along a fault-related fold. Even through, headward erosion has shifted the ridge crest; it is parallel with the trace of the Himalayan Frontal Thrust (HFT). The parallel ridge crest with reference to the HFT is indicative of the tectonic control of the HFT on the development of the watersheds. Hence, a well developed linkage between tectonic processes (fold development) and surface processes (headward erosion) is responsible for variation in watershed and drainage network pattern across the ridge crest. The study also investigates the role of planform ridge curvature on watershed development. The effect is more pronounced on an asymmetric ridge, such as the Mohand ridge, than on a symmetric ridge.     

Soil erosion assessment tools from point to regional scales—the role of geomorphologists in land management research and implementation

Geomorphological research has played an important role in the development and implementation of soil erosion assessment tools. Because policy and management approaches include the use of soil erosion assessment tools, soil erosion research directly affects the public in terms of providing information on natural hazards and human impacts, and also as the basis for regulatory policy on land management. For example, soil loss calculations and geomorphological expertise are used to support soil conservation planning, both through agricultural legislation that defines maximum tolerable soil loss rates, and through federal and local legislation that requires soil erosion controls on many construction sites.To be useful for decision makers, soil erosion models must have simple data requirements, must consider spatial and temporal variability in hydrological and soil erosion processes, and must be applicable to a variety of regions with minimum calibration. The growing use of erosion models and Geographic Information Systems (GIS) in local to regional scale soil and water conservation raises concerns about how models are used. This has prompted interest in methods to assess how models function at management scales and with the types of data that are commonly available to users. A case study of a GIS-based soil erosion assessment tool using the process-based Water Erosion Prediction Project (WEPP) shows that using commonly available data rather than research grade data can have (predictably) a significant impact on model results. If model results are then used in management decisions, it is critical to assess whether the scale and direction of variation in results will affect management and policy decisions. Geomorphologists provide unique perspectives on soil erosion and can continue to affect policy through soil erosion research. This research should focus on fundamental processes, but equally important is continued development and evaluation of models that are matched to real world data availability, geomorphic settings, and information needs.     

Wind erosion from military training lands in the Mojave Desert, California, U.S.A.

Military training activities reduce vegetation cover, disturb crusts, and degrade soil aggregates, making the land more vulnerable to wind erosion. The objective of this study was to quantify wind erosion rates for typical conditions at the Marine Corps Air Ground Combat Center, Twentynine Palms, CA, U.S.A. Five Big Spring Number Eight (BSNE) sampler stations were installed at each of five sites. Each BSNE station consisted of five BSNE samplers with the lowest sampler at 0·05 m and the highest sampler at 1·0 m above the soil surface. Once a month, sediment was collected from the samplers for analysis. Occurrence of saltating soil aggregates was recorded every hour using Sensits, one at each site. The site with the most erosion had a sediment discharge of 311 kg m⁻¹ over a period of 17 months. Other sites eroded much less because of significant rock cover or the presence of a crust. Hourly sediment discharge was estimated combining hourly Sensit count and monthly sediment discharge measured using BSNE samplers. More simultaneously measured data are needed to better characterize the relationship between these two and reconstruct a detailed time-series of wind erosion. This measured time-series can then be used for comparison with simulation results from process-based wind erosion models such as the Wind Erosion Prediction System (WEPS), once it has been adapted to the unique aspects of military lands.     

Quantifying the erosional impact of the Fennoscandian ice sheet in the Torneträsk–Narvik corridor, northern Sweden, based on cosmogenic radionuclide data

Despite spectacular landform evidence of a dominant role for glacial action in shaping landscapes under former northern hemisphere ice sheets, there is little quantitative evidence for rates and patterns of erosion associated with specific glaciations. Here we use cosmogenic nuclide data to assess rates of subglacial erosion underneath the Fennoscandian ice sheet. By testing whether there are remnant nuclide concentrations in samples taken from sites that include both relict areas and features and landscapes typically associated with vigorous glacial erosion, we can constrain the level and pattern of modification that resulted from the last glaciation. Cosmogenic ¹⁰Be and ³⁶Cl data from the Torneträsk region confirm the temporal and spatial variability of glacial erosion suggested by geomorphological mapping. At some sites, glacial erosion estimates in what appear to be heavily scoured areas indicate erosion of only c. 2 ± 0.4 m of bedrock, based on cosmogenic nuclide inheritance. This implies that the generation of severely scoured terrain in this study area required multiple glaciations. The overall modification produced by ice sheets along glacial corridors may be more restricted than previously thought, or may have occurred preferentially during earlier Quaternary glacial periods.     

Quantifying the erosional impact of the Fennoscandian ice sheet in the Torneträsk–Narvik corridor, northern Sweden, based on cosmogenic radionuclide data

Despite spectacular landform evidence of a dominant role for glacial action in shaping landscapes under former northern hemisphere ice sheets, there is little quantitative evidence for rates and patterns of erosion associated with specific glaciations. Here we use cosmogenic nuclide data to assess rates of subglacial erosion underneath the Fennoscandian ice sheet. By testing whether there are remnant nuclide concentrations in samples taken from sites that include both relict areas and features and landscapes typically associated with vigorous glacial erosion, we can constrain the level and pattern of modification that resulted from the last glaciation. Cosmogenic ¹⁰Be and ³⁶Cl data from the Torneträsk region confirm the temporal and spatial variability of glacial erosion suggested by geomorphological mapping. At some sites, glacial erosion estimates in what appear to be heavily scoured areas indicate erosion of only c. 2 ± 0.4 m of bedrock, based on cosmogenic nuclide inheritance. This implies that the generation of severely scoured terrain in this study area required multiple glaciations. The overall modification produced by ice sheets along glacial corridors may be more restricted than previously thought, or may have occurred preferentially during earlier Quaternary glacial periods.     

应用EUROSEM模型对三峡库区陡坡地水力侵蚀的模拟研究

三峡库区坡地资源被广泛利用,但水土保持措施没有被很好地利用。坡地,尤其是陡坡地是库区主要泥沙来源,因此,有效评估土壤侵蚀风险、预测径流和侵蚀速率以及选择合理的水土保持措施在该地区显得非常必要。EUROSEM模型是动态分布式模型,可以在单独地块或小流域中预测水力侵蚀强度,其特点比较适合库区土壤侵蚀预测预报。本研究以在三峡库区秭归县王家桥小流域水土保持试验站的标准径流小区的人工降雨资料为基础,应用EUROSEM模型模拟陡坡地中的侵蚀状况。模拟结果表明,EUROSEM对人工降雨中径流模拟效果较好,但对土壤流失的模拟效果相对较差,更精确地模拟库区陡坡地的土壤侵蚀状况则需要作进一步的研究     

Relief-rejuvenation and topographic length scales in a fluvial drainage basin, Napf area, Central Switzerland

This paper explores how, and to what extent, a phase of relief-rejuvenation modifies the mode of surface erosion in an approximately 63 km² drainage basin located at the northern border of the Swiss Alps (Luzern area). In the study area, the retreat of the Alpine glaciers at the end of the Last Glacial Maximum (LGM) caused base level to lower by approximately 80 m. The fluvial system adapted to the lowered base level by headward erosion. This is indicated by knickzones in the longitudinal stream profiles and by the continuous upstream narrowing of the width of the valley floor towards these knickzones. In the headwaters above these knickzones, processes are still to a significant extent controlled by the higher base level of the LGM. There, frequent exposure of bedrock in channels and especially on hillslopes implies that sediment flux is to a large extent limited by weathering rates. In the knickzones, however, exposure of bedrock in channels implies that sediment flux is supply-limited, and that erosion rates are controlled by stream power.The morphometric analysis reveals the existence of length scales in the topography that result from distinct geomorphic processes. Along the tributaries where the upstream sizes of the drainage basins exceed 100,000–200,000 m², the mode of sediment transport and erosion changes from predominantly hillslope processes (i.e., landsliding, creep of regolith, rock avalanches and to some extent debris flows) to processes in channels (fluvial processes and debris flows). This length scale reflects the minimum size of the contributing area for channelized processes to take over in the geomorphic development (i.e., threshold size of drainage basin). This threshold size depends on the ratio between production rates of sediment on hillslopes, and export rates of sediment by processes in channels. Consequently, in the headwaters, erosion rates and sediment flux, and hence landscape evolution rates, are to a large extent limited by weathering processes. In contrast, in the lower portion of the drainage basin that adjusts to the lowered base-level, rates of channelized erosion and relief formation are controlled mainly by stream power. Hence, this paper shows that base-level lowering, headward erosion and establishment of knickzones separate drainage basins in two segments with different controls on rates of surface erosion, sediment flux and relief formation.     

用风洞实验方法计算土壤风蚀量的时距问题

通过对延津县沙质壤土原状土进行风蚀风洞模拟实验，探讨了土壤风蚀量计算中的时距问题。结果表明，前人使用“风蚀模数”概念来计算土壤风蚀量会导致较大误差，原因在于同一风蚀事件中单位时间内的风蚀量随吹蚀时间的延续而递减，在风洞实验计算风蚀量时使用“累积风蚀量”概念将会有更精确的结果。     

How does alluvial sedimentation at range fronts modify the erosional dynamics of mountain catchments?

At the geological time scale, the way in which the erosion of drainage catchments responds to tectonic uplift and climate changes depends on boundary conditions. In particular, sediment accumulation and erosion occurring at the edge of mountain ranges should influence the base level of mountain catchments, as well as sediment and water discharges. In this paper, we use a landform evolution model (LEM) to investigate how the presence of alluvial sedimentation at range fronts affects catchment responses to climatic or tectonic changes. This approach is applied to a 25 km × 50 km domain, in which the central part is uplifted progressively to simulate the growth of a small mountain range. The LEM includes different slope and river processes that can compete with each other. This competition leads to ‘transport‐limited’, ‘detachment‐limited’ or ‘mixed’ transport conditions in mountains at dynamic equilibrium. In addition, two end‐member algorithms (the channellized‐flow and the sheet‐flow regimes) have been included for the alluvial fan‐flow regime. The three transport conditions and the two flow algorithms represent six different models for which the responses to increase of rock uplift rate and/or cyclic variation of the precipitation rate are investigated. Our results indicate that addition of an alluvial apron increases the long‐term mountain denudation. In response to uplift, mountain rivers adapt their profile in two successive stages; first by propagation of an erosion wave and then by slowly increasing their channel gradients. During the second stage, the erosion rate is almost uniform across the catchment area at any one time, which suggests that dynamic equilibrium has been reached, although the balance between erosion and rock uplift rates has not yet been achieved. This second stage is initiated by the uplift of the mountain river outlets because of sedimentation aggradation at the mountain front. The response time depends on the type of water flow imposed on the alluvial fans domains (× by 1.5 for channelized flow regime and by 10 for the sheet flow one). Cyclic variations of precipitation rate generate cyclic incisions in the alluvial apron. These incision pulses create knick‐points in the river profile in the case of ‘detachment‐limited’ and ‘mixed’ river conditions, which could be mistaken for tectonically induced knick‐points. ‘Transport‐limited’ conditions do not create such knick‐points, but nevertheless trigger erosion in catchments. The feedbacks linked to sedimentation and erosion at range front can therefore control catchment incision or aggradation. In addition, random river captures in the range front trigger auto‐cyclic erosion pulses in the catchment, capable of generating incision–aggradation cycles.     

Soil erosion rates in rangelands of northeastern Patagonia: A dendrogeomorphological analysis using exposed shrub roots

Soil erosion is an important process of land degradation in many rangelands and a significant driver of desertification in the world's drylands. Dendrogeomorphology is an alternative to traditional methods for determining soil erosion rate. Specifically, the vertical distance between the upper portion of exposed roots and the actual soil surface can be used as a bioindicator of erosion since plant establishment. In this study, we determined (i) the soil erosion rate from exposed roots of the dwarf shrub Margyricarpus pinnatus [Lam.] Kuntze in two ecological sites in the northeastern rangelands of Patagonia and (ii) the relationship between shrub age and upper root diameter. We selected two ecological sites, a pediment-like plateau and a flank pediment, where the dominant soils were Xeric Haplocalcids and Xeric Calciargids, respectively. The soil erosion rates in the pediment-like plateau and in the flank pediment were 2.4 and 3.1 mm yr^− 1, respectively. Data clearly indicate a high rate of soil erosion during the mean 8-year life span of the dwarf shrubs in degraded patches, which represent ~ 10% of surface cover in the study area. Simple linear regression analysis yielded a highly significant predictive model for age estimation of M. pinnatus plants using the upper root diameter as a predictor variable. The measurement of ground lowering against datable exposed roots represents a simple method for the determination of soil erosion rates. In combination with other soil surface features, it was used to infer the episodic nature of soil erosion. This approach could be particularly useful for monitoring the effects of land management practices on recent soil erosion and for the establishment of records in regions where historical data regarding this process are scarce or absent.     

Decadal-scale variation in dune erosion and accretion rates: An investigation of the significance of changing storm tide frequency and magnitude on the Sefton coast, UK

Monitoring of frontal dune erosion and accretion on the Sefton coast in northwest England over the past 50 years has revealed significant spatial and temporal variations. Previous work has shown that the spatial variations primarily reflect longshore differences in beach and nearshore morphology, energy regime and sediment budget, but the causes of temporal variations have not previously been studied in detail. This paper presents the results of work carried out to test the hypothesis that a major cause of temporal variation is changes in the frequency and magnitude of storms, surges and resulting high tides. Dune toe erosion/accretion records dating from 1958 have been compared with tide gauge records at Liverpool and Heysham. Relatively high dune erosion rates at Formby Point 1958–1968 were associated with a relatively large number of storm tides. Slower erosion at Formby, and relatively rapid accretion in areas to the north and south, occurred during the 1970's and 1980's when there were relatively few major storm tides. After 1990 rates of dune erosion at Formby increased again, and dunes to the north and south experienced slower accretion. During this period high storm tides have been more frequent, and the annual number of hours with water levels above the critical level for dune erosion has increased significantly. An increase in the rate of mean sea-level rise at both Liverpool and Heysham is evident since 1990, but we conclude that this factor is of less importance than the occurrence of extreme high tides and wave action associated with storms. The incidence of extreme high tides shows an identifiable relationship with the lunar nodal tidal cycle, but the evidence indicates that meteorological forcing has also had a significant effect. Storms and surges in the eastern Irish Sea are associated with Atlantic depressions whose direction and rate of movement have a strong influence on wind speeds, wave energy and the height of surge tides. However, preliminary analysis has indicated only a modest relationship between dune erosion/accretion rates and the North Atlantic Oscillation index.     

Spatial assessment of soil wind erosion using WEQ approach in Mongolia

Wind erosion is a major contributor to land degradation and desertification. According to the Global Assessment of Human Induced Soil Degradation, the dryland territories of Mongolia are significantly affected by wind erosion. We used the wind erosion equation model in an ArcGIS environment to evaluate wind erosion across Mongolia. The individual factors of the wind erosion equation were parameterized using the following datasets: (a) monthly climatic data from 45 meteorological stations; (b) 16-day composites of MODIS Normalized Difference Vegetation Index data; (c) a SRTM DEM with a 90 m spatial resolution; and (d) the soil map of Mongolia. The results revealed the significant influence of aridity on wind erosion. The desert and semi-desert ecosystems were more vulnerable to wind erosion, hence more affected. The map of wind erosion revealed three major wind erosion regions where the maximum soil loss of 15–27 t/(hm²?a) was observed. In general, the wind erosion potentials for the entire country of Mongolia are 15–27 t/(hm²?a) in the deserts and semi-deserts, 10–15 t/(hm²?a) in the dry steppes and 5–10 t/(hm²?a) in the steppe regions.     

WAVE ENERGY ESTIMATES AND BLUFF RECESSION ALONG LAKE MICHIGAN'S SOUTHEAST SHORE*

Recession rates for unconsolidated bluffs at 23 sites along Lake Michigan's southeast shore are compared with deep water wave energy probabilities to estimate the relative degradational importance of recurrent 5, 10, 20, 50, and 100-year storms. Recession rates are based on measured bluff crest retreat while wave energies are calculated using standard water wave theory and data interpolated from a meteorologically based hindcast model. Correlation and regression tests suggest the following: (1) although wave energy is certainly a destructive factor, it explains less than half of the variation in bluff crest recession, supporting the interpretation that shorezone erosion here results from the interaction of numerous factors; and (2) over long time periods the total effect of more frequent moderate intensity storms is greater than that from rare, especially high energy events.     

Interplay of eustatic,tectonic and autogenic controls on a Late Devonian carbonate platform,northern Canning Basin,Australia

Frasnian reef complexes along the northern margin of the Canning Basin in northwestern Australia evolved during rifting of the Fitzroy Trough. Geological investigations of the Frasnian Hull platform, which developed on an active tilted fault‐block, reveal significant lateral and vertical facies variations superimposed on prominent metre‐scale cyclicity. This study uses numerical analyses of facies and magnetic susceptibility data from three measured sections along the Hull platform to test whether a tectonic signal can be distinguished from eustatic and other signals. Geostatistical analysis of facies variations reveals an exponential distribution of thin (<3 m) facies, characteristic of stochastic depositional processes. Thick subtidal facies predominate in the Guppy Hills (GH) and southeastern Hull Range (SHR) sections near the hangingwall margin, and thick shallow‐subtidal to intertidal facies dominate the Horse Springs drillcore (HD 14) section near the footwall margin. Power and wavelet spectral analyses indicate a strong periodic component; Average Spectral Misfit and spectral optimisation methods confirm the presence of Milankovitch eccentricity signals and suggest the presence of obliquity and precession signals. However, the results also expose strong temporal and spatial variation providing evidence for tectonic control. Spectral analyses show strongest periodicity is recorded in short intervals that are not correlated across the platform and provide evidence of variations in sedimentation rate and hiatuses. Time series for the neighbouring GH and SHR sections show no overall statistical correlation, and Markov analysis indicates weakly ordered vertical facies transitions that do not correlate across the platform. Subtidal to intertidal facies data from HD 14 core suggest that at least 35% of the section is absent, almost obscuring the Milankovitch signal. The results indicate a complex set of controls on deposition on the Hull platform with local tectonic effects having produced spatio‐temporal moderation of the underlying eustatic signals and autogenic processes adding a localised stochastic response.     

SHORELINE CHANGE AT THOMPSON ISLAND,BOSTON HARBOR,MASSACHUSETTS, 1938-1977

A photogrammetric and sediment analysis is presented to illustrate the relationship between beach erosion and seacliff recession on Thompson Island, Boston Harbor, Massachusetts. Aerial photographs taken in 1938, 1952, 1963, and 1977 were measured to determine rates of shoreline change around the island. The 39-year average rate of beach erosion is 0.3 m/yr ± 2% with an average rate of cliff recession at 0.2 m/yr ± 2%. Rates of beach erosion between six orientations that reflect principal wave approach to the island were not found to be significantly different (0.05) for the 39-year period. The rank order correlation between beach erosion and cliff recession for these six orientations was moderately well correlated (0.63). To determine possible controls of cliff erosion, 58 sediment samples were collected from glacial cliffs along the shoreline. The textural composition was determined, and then tested with discriminant function analysis. Partial correlation analysis between beach erosion and cliff recession holding a surrogate for sediment size constant improved the rank order from 0.63 to 0.84. The results indicate that coarser-grained cliffs recede at faster rates, but with less erosion occurring on adjacent beaches. Alternatively, finer-grained cliffs recede at slower rates, but with greater erosion occurring on adjacent beaches.     

Denudation rates and a topography-driven rainfall threshold in northern Chile: Multiple cosmogenic nuclide data and sediment yield budgets

The quantification of geomorphic process rates on the outcrop- and the orogen-scale is important to describe accurately the interaction between the relative effects of erosion, tectonics and climate on landscape evolution. We report single and paired cosmogenic nuclide (¹⁰Be, ²⁶Al and ²¹Ne) derived erosion rates and exposure ages on hillslope interfluves from the tectonically active western central Andes that show a distinct spatial variation. A positive correlation of erosion rates with elevation and present-day rainfall rates is observed. Erosion rates at lower altitudes–the hyperarid Coastal Cordillera and the Western Escarpment with the northern part of the Atacama Desert–are extremely low and of the order of 10–100 cm/My (nominal exposure ages 1–6 My). In contrast, erosion rates at higher altitudes–the semiarid Western Cordillera–range up to 4600 cm/My (nominal exposure ages 0.02–0.1 My). This latter average long-term bedrock erosion rate record, suggested to be coupled to an orographically controlled pattern of rainfall, is also reflected in the pattern of denudation rates derived from a short-term decadal record of limited sediment yield data. Specifically, denudation rates calculated from sediment flux data are of a similar order of magnitude as erosion rates deduced from long-lived cosmogenic nuclides from bedrock hillslope interfluves of the Western Cordillera. Nevertheless, the production and the supply of sediment from the western Andean slope are very limited.Analysis of multiple cosmogenic nuclides allows simultaneous determination of erosion rates and exposure ages but also reveals complex exposure histories of non-bedrock samples, such as boulders or amalgamated clast samples. Notably, this study shows that saturation of nuclides, usually assumed in studies where only a single nuclide is analyzed, is rather the exception than the rule, as revealed by erosion island plots. Constant erosion that started much later than the formation age of the rocks or episodic erosion by spalling can partially explain non-steady-state concentrations and more complicated exposure scenarios. Furthermore, the use of multiple nuclides with different half-lives allowed us to infer that no significant variations in long-term erosion rates have occurred and that at the Western Escarpment erosion rates have been low and constant for most of the late Neogene. Nevertheless, the time intervals necessary to reach steady-state concentrations for cosmogenic nuclides can be quite different from those needed for landscapes to reach steady state.     

Erosions on the southern Tibetan Plateau: Evidence from in-situ cosmogenic nuclides 10Be and 26Al in fluvial sediments

Investigating topographic and climatic controls on erosion at variable spatial and temporal scales is essential to our understanding of the topographic evolution of the orogen.In this work,we quantified millennial-scale erosion rates deduced from cosmogenic 10Be and 26Al concentrations in 15 fluvial sediments from the mainstream and major tributaries of the Yarlung Zangbo River draining the southern Tibetan Plateau (TP).The measured ratios of 26Al/10Be range from 6.33 ± 0.29 to 8.96 ± 0.37,suggesting steady-state erosion processes.The resulted erosion rates vary from 20.60 ± 1.79 to 154.00 ± 13.60 m Myr-1,being spatially low in the upstream areas of the Gyaca knickpoint and high in the downstream areas.By examining the relationships between the erosion rate and topographic or climatic indices,we found that both topography and climate play significant roles in the erosion process for basins in the upstream areas of the Gyaca knickpoint.However,topography dominantly controls the erosion processes in the downstream areas of the Gyaca knickpoint,whereas variations in precipitation have only a second-order control.The marginal Himalayas and the Yarlung Zangbo River Basin (YZRB) yielded significantly higher erosion rates than the central plateau,which indicated that the landscape of the central plateau surface is remarkably stable and is being intensively consumed at its boundaries through river headward erosion.In addition,our 10Be erosion rates are comparable to present-day hydrologic erosion rates in most cases,suggesting either weak human activities or long-term steady-state erosion in this area.     

利用不同方法估测土壤有机质及其对采样数的敏感性分析

用随机方法从262个采样点中抽取200个点作为已知有机质含量的数据集,将所有采样点的碱解氮作为辅助数据预测有机质的空间分布。利用有机质信息的普通克立格法的方差解释量和预测精度最低,而回归克立格法因在预测过程中加入了回归残差而使方差解释量最大、预测精度最高。为了分析采样数对不同方法预测精度的影响,从上述已知有机质含量的200个点中分别随机抽取40、80、120、160个点构成4个数据集,分别利用它们的有机质信息和不同方法预测了有机质的空间分布,结果表明:对于每个数据集,4种方法的预测精度顺序均为RGK>COK>RG>OK,线性回归法的预测精度随采样点的增加基本不变,而其它三种方法的预测精度却逐渐提高。     

Changes in intensity of wind erosion at different stages of degradation development in grasslands of Inner Mongolia, China

Overgrazing by increasing numbers of livestock in the Horqin Sandy Land of Inner Mongolia, China, has led to extensive degradation of the region's sandy steppes. Degraded grasslands are generally classified into four main types: fixed (least degradation), semi-fixed (light degradation), semi-shifting (moderate degradation) and shifting (severe degradation) sandy lands, representing four stages of degradation development. An experiment was conducted in the Horqin Sandy Land to investigate changes in intensity of wind erosion at different stages of degradation development in sandy grasslands and determine the extent to which surface wind erosion was affected by surface-related soil and vegetation factors through their effects on surface roughness length and wind regimes. Daily wind erosion rate was monitored at four sites of degraded grassland over an erosive period from 1 April to 10 June in 2001. Soil and vegetation properties for these sites were also measured twice: one in mid-April prior to the establishment of vegetation and again in mid-June after the establishment of vegetation. Relationships between surface roughness length and soil and vegetation variables were examined at each of the two stages of vegetation development. This study shows striking differences in the intensity of surface wind erosion among sites. The daily wind erosion rate in the fixed sandy land was, on average, only about 1/5 of the rate in the semi-fixed sandy land, 1/14 of the rate in the semi-shifting sandy land and 1/47 of the rate in the shifting sandy land suggesting a much higher resistance of the fixed sandy land to wind erosion compared to other sites. Differences in rate of wind erosion between sites were attributed to between-site differences in soil and vegetation properties that exerted significant effects on wind regimes by altering surface roughness length. At the pre-establishment stage of vegetation, surface roughness length was determined by a combination of litter amount on the ground, soil surface hardness and soil moisture content, with litter amount explaining the greatest proportion of the variation. At the post-establishment stage of vegetation, the development of the surface roughness effects was mainly governed by vegetation characteristics (vegetation cover in particular), while the effects of soil surface hardness and soil moisture on surface roughness length are likely to be masked by vegetation effects. The findings suggest that better management practices of restoring vegetation in degraded grasslands are required to reduce soil erosion losses and achieve a sustainable livestock production in the Horqin Sandy Land, an ecologically fragile sandy land ecosystem.