Annual sediment budget of a UK mountain torrent

Research into torrent erosion focuses heavily on bedload transport dynamics and debris flow propagation during specific events. As a result, there is limited understanding of the sediment budgets operating in torrent systems over longer timescales. The aim of this study is to construct a sediment budget of the main geomorphological processes operating in a mountain torrent sediment system over a full year.
The study site is Iron Crag which is a small torrent system (catchment area 2.4 ha) situated in the northern Lake District, UK. The site has the characteristic morphology of a torrent: multiple hillslope sediment sources, steep channel, gorges, and a basal alluvial fan. A measurement scheme was designed to monitor process activity, linking the sediment sources and sinks, from December 1998 to December 1999. Over this time period the sediment budget demonstrates that 184 tonnes of sediment was supplied to the alluvial fan (which acted primarily as a sediment sink). Channel (70%) and bank (25%) sources dominated the sediment supply, and surface processes and rockfall on the hillslopes (5%) made only a minor contribution.
Temporal variations in process activity are significant. Surface processes and rockfall display seasonal variations in yield, whilst channel and bank yields are influenced by individual storm events. Site–specific meteorological data are used to explain these observations and freeze–thaw activity and rainfall characteristics are shown to be important controlling factors.     

A sediment budget for a grazed semi-arid catchment in the Burdekin basin, Australia

In catchments adjacent to the Great Barrier Reef World Heritage Area in Queensland, Australia, there is a growing concern that sediments and nutrients being exported from the land are having a detrimental effect on coral reef communities. There is a need to determine the processes and rates of erosion from the major land use types, so that management intervention can be initiated to reduce sediment yields where required. This paper presents a sediment budget for Weany Creek, a 13.5 km² grazed semi-arid sub-catchment of the Burdekin River catchment, Australia. A range of field methods was used to measure erosion from hillslopes, gullies and stream banks, as well as identify the amount of sediment being deposited and remobilised on the bed of gullies and the stream network. The data suggests that at least during drought conditions, the primary erosion source in this catchment is gully erosion. However, the largest source of sediment in the budget is actually associated with the remobilisation of in-channel sediment stores. Overall, the sediment budget is comprised of  81% coarse material and 19% fine sediment and an agreement between the fine sediment yield estimated in the sediment budget and the yield measured at the catchment outlet is within 10%. The total sediment yield estimated for this catchment is  4205 t yr^− 1 and is much lower than expected for a catchment of this size. This may reflect the drought conditions during the measurement period; however, there is also the possibility that the primary erosion sources have been exhausted, and the rates of sediment loss may be much lower now than they may have been in the past. Nonetheless, the results show that stored sediment, which may have been deposited in the channel many decades ago, is an important contributor to end of catchment sediment yields and warrants further investigation.     

Alluvial and colluvial sediment storage in the Geul River catchment (The Netherlands) — Combining field and modelling data to construct a Late Holocene sediment budget

We used a combined approach of a two-dimensional erosion and hillslope sediment delivery model (WATEM/SEDEM) and detailed geomorphological reconstructions to quantify the different components in a sediment budget for the Geul River catchment (southern Netherlands) since the High Middle Ages. Hillslope erosion and colluvium deposition were calculated using the model, while floodplain storage was estimated using field data. Our results show that more than 80% of the total sediment production in the catchment has been stored as colluvium (mostly generated by hillslope erosion), while almost 13% is stored in the floodplain since the High Middle Ages (this situation resembles a capacity-limited system). Model results for the period prior to the High Middle Ages (with a nearly completely forested catchment) show that far less sediment was generated and that most of the sediments were directly transported to the main river valleys or out of the catchment (a supply-limited system). Geomorphological analysis of a large alluvial fan shows the sensitivity of the study area to changes in the percentage of arable land.Our combined field data-modeling study presents an elegant method to calculate a catchment sediment budget for a longer period and is able to identify and quantify the most important sediment storage elements. Furthermore, it provides a valuable tool to calculate a sediment budget while only limited dated fluvial sediment sequences are available.     

The sediment budget of a highly dynamic mesoscale catchment: The River Isábena

The paper presents the sediment budget of the Isábena basin, a highly dynamic 445-km² catchment located in the Central Pyrenees that is patched by highly erodible areas (i.e., badlands). The budget for the period 2007-2009 is constructed following a methodology that allows the interpolation of intermittent measurements of suspended sediment concentrations and enables a subsequent calculation of sediment loads. Data allow specification of the contribution of each subbasin to the water and sediment yield in the catchment outlet. Mean annual sediment load was 235,000 t y^− 1. Specific sediment yield reached 2000 t km^− 2 y^− 1, a value that indicates very high sedimentary activity, especially in the case of Villacarli and Lascuarre subcatchments, were most badlands are located. The specific sediment yield obtained for the entire Isábena is 527 t km^− 2 y^− 1, a high value for such a mesoscale basin. Results show that a small part of the area (i.e., 1%) controls most of the catchment's gross sediment contribution. Sediment delivery ratio (ratio between sediment input from primary sources and basin export) has been estimated at around 90%, while in-channel storage represents the 5% of the annual load on average. The high connectivity between sediment sources (i.e., badlands) and transfer paths (i.e., streamcourses) exacerbates the influence of the local sediment production on the catchment's sediment yield, a quite unusual fact for a basin of this scale.     

In-stream wetlands and their significance for channel filling and the catchment sediment budget, Jugiong Creek, New South Wales

Evidence is presented here of recent and extensive infilling of the incised channel network of the Jugiong Creek catchment, SE Australia. The present channel network resulted from widespread stream and gully incision in the period between 1880 and 1920. Our survey shows that gully floors have been colonised extensively by emergent macrophyte vegetation since before 1944, forming continuous, dense, in-stream wetlands, which now cover 25% of the channel network in the 2175 km² catchment and have so far trapped almost 2,000,000 t of nutrient-enriched, fine sediments. This mass of sediments represents the equivalent of 4.7 years of annual sediment production across the catchment and in some tributaries, more than 20 years of annual yield is stored within in-stream wetlands.Previous work on the late Quaternary stratigraphy of the region has shown that there were repeated phases of channel incision in the past following which the channels quickly stabilised by natural means and then filled with fine-grained sediment to the point of channel extinction, creating unchannelled swampy valley floors. The current formation and spread of in-stream wetlands is interpreted to be the onset of the next infill phase but it is not known whether present conditions will allow complete channel filling and reformation of the pre-existing swampy valley floors. Nevertheless, further spread of in-stream wetlands is likely to increase the sediment trapping capacity and further reduce the discharge of sediments and nutrients into the Murrumbidgee River. The in-stream wetlands may provide a significant capacity to buffer erosion from gullied catchments of considerable size (up to 300 km²) as an adjunct to current riparian management options. They may also assist the recovery of sediment-impacted channels downstream.     

Fallout radionuclide tracers identify a switch in sediment sources and transport-limited sediment yield following wildfire in a eucalypt forest

Fire can alter sediment sources and transport rates in river basins, changing landforms and aquatic habitats and degrading downstream water quality. Variability in the response between environments, between fires, and with time since fire makes predicting the catchment-scale effect of individual fires difficult. This study applies the fallout radionuclides ¹³⁷Cs and ²¹⁰Pb_xs to trace the sources and transport of fine sediment through a river network following a wildfire of moderate to extreme severity in the 629-km² eucalypt-forested Nattai River water-supply catchment near Sydney, Australia. The tracer analysis showed that post-fire erosion caused a switch in fine (< 10 µm) sediment sources from 80% subsoil derived from gully and river bank erosion to 86% topsoil derived from hillslope surface erosion. The fine sediment phosphorus content increased 4–10 fold over pre-fire levels. Annual post-fire sediment yields estimated from suspended solids rating curves were 109–250 times higher than they would have been without fire. A large additional amount of sediment remained stored within the river network for at least four years, particularly in lower-gradient reaches. Analysis of a sediment core showed that surface erosion following a previous fire had supplied at least 29% of total catchment sediment yield over the past 36 years. It is concluded that wildfire can alter catchment sediment budgets in two ways. Firstly, a spatially-diffuse pulse of elevated erosion is associated with moderate or intense rainfall events in post-fire years. Secondly, pulses of elevated catchment sediment yield are driven by the timing and river sediment transport capacity of runoff events. Severe post-fire erosion and high interannual hydrologic variability can result in large sediment stores persisting within the river network for many years. Fallout radionuclide tracers are shown to be useful in quantifying fine sediment sources and transport dynamics following wildfire, and the contribution of wildfire to catchment sediment yield.     

The influence of landsliding on sediment supply and channel change in a steep mountain catchment

Rates of sediment supply by landsliding to an alluvial channel in a small catchment in central Switzerland were estimated over an 11-month study period. Fluvial sediment transport in the channel is independently monitored at the upstream and downstream ends of the study reach, yielding a unique opportunity to quantitatively compare the volume of sediment supplied to the channel with the volume in fluvial transport. Landslide-derived sediment discharge to the channel was greatest during the winter and spring months, while most of the fluvial sediment transport occurred during short, intense summer storms. Approximately 98 m³ of sediment was delivered directly to the study reach by landsliding,  80 m³ was transported into the reach from upstream, and  70 m³ was transported out of the reach. Thus, there was a net accumulation of  100 m³ of sediment during the 11-month study. Decadal-scale channel aggradation was independently assessed by comparing channel longitudinal profiles in 1993 and 2004. During this 11-year period, a total of  1500 m³ of sediment has accumulated in the study reach. Aggradation has occurred largely in two broad zones that correspond with both the locations of major landslide complexes and reaches of high channel slope, indicating that hillslope sediment input left an imprint on the morphology of the channel bed that appears to be stable over at least decadal time scales.     

Sediment budget analysis of slope–channel coupling and in-channel sediment storage in an upland catchment, southeastern Australia

Slope–channel coupling and in-channel sediment storage can be important factors that influence sediment delivery through catchments. Sediment budgets offer an appropriate means to assess the role of these factors by quantifying the various components in the catchment sediment transfer system. In this study a fine (< 63 µm) sediment budget was developed for a 1.64-km² gullied upland catchment in southeastern Australia. A process-based approach was adopted that involved detailed monitoring of hillslope and bank erosion, channel change, and suspended sediment output in conjunction with USLE-based hillslope erosion estimation and sediment source tracing using ¹³⁷Cs and ²¹⁰Pb_ex. The sediment budget developed from these datasets indicated channel banks accounted for an estimated 80% of total sediment inputs. Valley floor and in-channel sediment storage accounted for 53% of inputs, with the remaining 47% being discharged from the catchment outlet. Estimated hillslope sediment input to channels was low (5.7 t) for the study period compared to channel bank input (41.6 t). However an estimated 56% of eroded hillslope sediment reached channels, suggesting a greater level of coupling between the two subsystems than was apparent from comparison of sediment source inputs. Evidently the interpretation of variability in catchment sediment yield is largely dependent on the dynamics of sediment supply and storage in channels in response to patterns of rainfall and discharge. This was reflected in the sediment delivery ratios (SDR) for individual measurement intervals, which ranged from 1 to 153%. Bank sediment supply during low rainfall periods was reduced but ongoing from subaerial processes delivering sediment to channels, resulting in net accumulation on the channel bed with insufficient flow to transport this material to the catchment outlet. Following the higher flow period in spring of the first year of monitoring, the sediment supplied to channels during this interval was removed as well as an estimated 72% of the sediment accumulated on the channel bed since the start of the study period. Given the seasonal and drought-dependent variability in storage and delivery, the period of monitoring may have an important influence on the overall SDR. On the basis of these findings, this study highlights the potential importance of sediment dynamics in channels for determining contemporary sediment yields from small gullied upland catchments in southeastern Australia.     

Holocene sediment budgets in two river catchments in the Southern Upper Rhine Valley, Germany

The Holocene sediments of two catchments in the southern Upper Rhine valley have been quantified as part of the German LUCIFS Programme (RheinLUCIFS), which aims to quantify sediment fluxes in the Rhine catchment since the onset of agriculture in the Neolithic about 7500 years ago.The spatial distribution of the alluvial and colluvial sediments was derived using geological maps, with information on the thickness of these sediments from various sources including auger profiles and data from excavations. The sediments were subdivided into characteristic sedimentary storage types according to the different types of landscapes. For each of the sedimentary storage types an average thickness was assessed so that an integral sediment balance for the Holocene could be derived.For the different types of landscapes in the study area, 32 Holocene sedimentary storage types were determined, 21 in the Elz catchment (1500 km²) and 11 in the Möhlin catchment (230 km²). By adding up the sediment volumes of all single sedimentary storage types the total Holocene sediment volumes for the two catchments were calculated. Erosion depths were determined by dividing the sediment volumes through the potential erosion areas (slope > 2%) and by assuming a sediment delivery ratio (SDR) between 0 and 0.4. The total erosion for the potential erosion areas during the Holocene was calculated as 31–61 cm in the Elz catchment and 44–79 cm in the Möhlin catchment.     

Environmental factors controlling spatial variation in sediment yield in a central Andean mountain area

A large spatial variability in sediment yield was observed from small streams in the Ecuadorian Andes. The objective of this study was to analyze the environmental factors controlling these variations in sediment yield in the Paute basin, Ecuador. Sediment yield data were calculated based on sediment volumes accumulated behind checkdams for 37 small catchments. Mean annual specific sediment yield (SSY) shows a large spatial variability and ranges between 26 and 15,100 Mg km^− 2 year^− 1. Mean vegetation cover (C, fraction) in the catchment, i.e. the plant cover at or near the surface, exerts a first order control on sediment yield. The fractional vegetation cover alone explains 57% of the observed variance in ln(SSY). The negative exponential relation (SSY = a × e^{−b C}) which was found between vegetation cover and sediment yield at the catchment scale (10³–10⁹ m²), is very similar to the equations derived from splash, interrill and rill erosion experiments at the plot scale (1–10³ m²). This affirms the general character of an exponential decrease of sediment yield with increasing vegetation cover at a wide range of spatial scales, provided the distribution of cover can be considered to be essentially random. Lithology also significantly affects the sediment yield, and explains an additional 23% of the observed variance in ln(SSY). Based on these two catchment parameters, a multiple regression model was built. This empirical regression model already explains more than 75% of the total variance in the mean annual sediment yield. These results highlight the large potential of revegetation programs for controlling sediment yield. They show that a slight increase in the overall fractional vegetation cover of degraded land is likely to have a large effect on sediment production and delivery. Moreover, they point to the importance of detailed surface vegetation data for predicting and modeling sediment production rates.     

Estimating the fluvial sediment input to the coastal sediment budget: A case study of Ghana

Knowledge of fluvial sediment supply to the coastal sediment budget is important for the assessment of the impacts on coastal stability. Such knowledge is valuable for designing coastal engineering schemes and the development of shoreline management planning policies. It also facilitates understanding of the connection between rivers in the hinterland and adjoining coastal systems. Ghana's coast has many fluvial sediment sources and this paper provides the first quantitative assessments of their contributions to the coastal sediment budget. The methods use largely existing data and attempt to cover all of Ghana's significant coastal rivers. Initially work was hindered by insufficient direct measured data. However, the problem was overcome by the application of a regression approach, which provides an estimated sediment yield for non-gauged rivers based on data from gauged rivers with similar characteristics. The regression approach was effective because a regional coherence in behaviour was determined between those rivers, where direct measured data were available. The results of the assessment revealed that Ghana's coast is dissected by many south-draining rivers, stream and lagoons. These rivers, streams and lagoons supply significant amounts of sediment to coastal lowlands and therefore contribute importantly to beaches. Anthropogenic impoundment of fluvial sediment, especially the Akosombo dam on the Volta River, has reduced the total fluvial sediment input to the coast from about 71 × 10⁶ m³/a before 1964 (pre-Akosombo dam) to about 7 × 10⁶ m³/a at present (post-Akosombo dam). This sharp reduction threatened the stability of the east coast and prompted an expensive ($83 million) defence scheme to be implemented to protect 8.4 km-long coastline at Keta. Sections of Ghana's coast are closely connected to the hinterland through the fluvial sediment input from local rivers. Therefore, development in the hinterland that alters the fluvial sediment input from those local rivers could have significant effects on the coast. There is the need, therefore, to ensure that catchment management plans and coastal management plans are integrated or interconnected.     

Using cosmogenic beryllium–7 as a tracer in sediment budget investigations

Recent advances in the use of the environmental radionuclides caesium–137 and unsupported lead–210 to quantify medium– and longer–term rates of erosion and sediment accumulation have proved of considerable value in catchment sediment budget investigations. However, there remains a need to explore the potential for using other shorter–lived radionuclides to provide evidence of sediment mobilisation, transport and storage over shorter timescales and particularly for individual events. This contribution reports the results of a study aimed at exploring the potential for using beryllium–7 (⁷Be, t _½= 53.3 days) to meet this requirement. The study investigated the use of ⁷Be as a sediment tracer in three key components of the sediment budget, namely, soil erosion and sediment mobilisation from slopes, the transport, storage and remobilisation of fine sediment in river channels and overbank deposition on river floodplains. The results presented clearly demonstrate the potential for using ⁷Be to obtain information on short–term and event–based sediment redistribution rates for use in catchment sediment budget investigations.     

Prolonged impact of earthquake-induced landslides on sediment yield in a mountain watershed: The Tanzawa region, Japan

To determine for how long a landslide affects sediment discharge, the sediment yields of 15 check-dam basins were compared with the time series of landslide distributions in a mountain basin in the Tanzawa region, central Japan. The distribution of sediment yield was quantitatively estimated from deposition in the sediment pools of check dams. The relationship between the landslide history and sediment discharge in the Nakagawa River basin was examined for an approximately 80-year period. Two major landslide events occurred during this period: the 1923 Kanto Earthquake and the 1972 disaster caused by heavy rainfall. The resulting trend in sediment discharge of the whole basin, estimated using reservoir sedimentation in the Miho Dam at its base, was nearly constant, with high sediment discharge (2897 m³ km^− 2 yr^− 1) in the intervening quarter-century, despite the recovery of vegetation on landslide areas in this period. Comparisons of the landslide distributions resulting from the two disasters, the sediment yields of check-dam basins, and the sediment discharge of the whole basin indicate that recent sediment discharge contains landslide debris that was originated by the Kanto Earthquake that occurred over 80 years ago. Thus, to understand high sediment discharge, it is essential to investigate not only the current basin condition and recent events, but also the landslide history of the basin for at least the previous 100 years.     

Historical trends affecting accumulation of sediment and phosphorus in Lake Pepin,upper Mississippi River,USA

This study was conducted to collect historical land use information that would help explain the historical patterns in accumulation of sediment and phosphorus in Lake Pepin documented by Engstrom et al. (J Paleolimnol, this issue). A wide range of historical factors including cropping systems, phosphorus applications from fertilizer and manure, human and animal populations, river flows and phosphorus discharges from waste water treatment plants were studied using statistical methods. Results showed that sediment losses from the Minnesota River basin are significantly correlated with historical increases in river flows, row crop production acreage and basin population. Total phosphorus accumulations in the sediments of Lake Pepin are significantly correlated with increased phosphorus discharges from metropolitan area wastewater treatment plants, and increases in row crop acreage and river flows. Total phosphorus inflows to Lake Pepin are significantly correlated with increases in river flows, row crop acreage and phosphorus fertilizer applied to agricultural lands. This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this special issue of the Journal of Paleolimnology. D. R. Engstrom served as guest editor of the special issue.     

Geochemical reconstruction of late Holocene drainage and mixing in Kluane Lake,Yukon Territory

The level of Kluane Lake in southwest Yukon Territory, Canada, has fluctuated tens of metres during the late Holocene. Contributions of sediment from different watersheds in the basin over the past 5,000 years were inferred from the elemental geochemistry of Kluane Lake sediment cores. Elements associated with organic material and oxyhydroxides were used to reconstruct redox fluctuations in the hypolimnion of the lake. The data reveal complex relationships between climate and river discharge during the late Holocene. A period of influx of Duke River sediment coincides with a relatively warm climate around 1,300 years BP. Discharge of Slims River into Kluane Lake occurred when Kaskawulsh Glacier advanced to the present drainage divide separating flow to the Pacific Ocean via Kaskawulsh and Alsek rivers from flow to Bering Sea via tributaries of Yukon River. During periods when neither Duke nor Slims river discharged into Kluane Lake, the level of the lake was low and stable thermal stratification developed, with anoxic and eventually euxinic conditions in the hypolimnion.     

The role of channel and floodplain storage in the suspended sediment budget of the River Ouse, Yorkshire, UK

This study attempts to quantify the amount of fine-grained (ca. < 150 μm) sediment stored on the floodplains and on the channel bed of the non-tidal sections of the main channels in the catchment of the River Ouse (3315 km²) and of one of its tributaries, the River Waarfe (818 km²), in Yorkshire, UK. Caesium-137 analyses of floodplain sediment cores were used to quantify the amount of Iloodplain deposition as a result of overbank flooding during the last ca. 40 years. A combination of bulk and sectioned cores were collected along transects perpendicular to the channel at 26 sites throughout the study basins. In general, rates of overbank sedimentation decrease with distance from the channel. The average values for individual transects range between 0.010 and 0.554 g cm⁻² year⁻¹. Floodplain storage along the main channels of the Ouse and Wharfe basins accounts for 60645 and 10325 t year⁻¹, respectively, and represents a net loss from the system. The amount of fine-grained sediment stored on the channel bed was estimated by a survey undertaken in August 1996, during which the fine material deposited on the bed was resuspended and its mass estimated at 16 locations. The average values for the individual locations range between 0.017 and 0.924 g cm⁻² and tend to increase downstream. The total channel bed storage at the time of sampling in 1996 was estimated to be 16076 and 1866 t for the Ouse and Wharfe basins, respectively. It is assumed that channel bed storage is seasonal and that no net loss to the system occurs at the annual timescale. Floodplain storage for the Ouse and Wharfe basins represents 39 and 49%, and channel bed storage equals 10 and 9%, respectively, of the annual suspended sediment load (1995–1996) delivered to the channel system. These results have important implications for the routing of fine-grained sediment and sediment-associated contaminants in drainage basins, and for the interpretation of downstream sediment yields in terms of upstream sediment mobilisation.     

Historical changes in sediment and phosphorus loading to the upper Mississippi River: mass-balance reconstructions from the sediments of Lake Pepin

Long-term changes in sediment and phosphorus loading to the upper Mississippi River were quantified from an array of 25 sediment cores from Lake Pepin, a large natural impoundment downstream of the Minneapolis-St Paul metropolitan area. Cores were dated and stratigraphically correlated using ²¹⁰Pb, ¹³⁷Cs, ¹⁴C, magnetic susceptibility, pollen analysis, and loss-on-ignition. All cores show a dramatic increase in sediment accumulation beginning with European settlement in 1830. Accumulation rates are highest and show the greatest post-settlement increases in the upper end of the lake. Present-day sediment-phosphorus concentrations are roughly twice those of pre-settlement times, and the Fe/Al-bound fraction makes up a greater portion of the total. Diatom assemblages record a marked increase in nutrient availability over the last 200 years, changing from clear-water benthic forms and mesotrophic planktonic taxa in pre-settlement times to exclusively planktonic assemblages characteristic of highly eutrophic conditions today. Lake-water total-phosphorus concentrations, estimated by weighted averaging regression and calibration, increased from 50 to 200 μg l⁻¹ during this period. Sediment loading to Lake Pepin from the Mississippi River has increased by an order of magnitude since 1830. Modern fluxes are about 900,000 metric tons annually, and are more than 80% detrital mineral matter. About 17% of the lake’s volume in 1830 has been replaced by sediment, and at current accumulation rates the remainder will be filled in another 340 years. Phosphorus accumulation in Lake Pepin sediments has increased 15-fold since 1830, rising from 60 to 900 metric tons annually. This rise represents a sevenfold increase in phosphorus loading from the Mississippi River coupled with more efficient retention of phosphorus inflows by bottom sediments. More efficient trapping of phosphorus in Lake Pepin over the last century resulted from higher rates of sediment burial. The most dramatic changes in nutrient and sediment inputs to Lake Pepin have occurred since 1940, although gradual increases began shortly following European settlement. Sediment accumulation rates rose sharply between 1940 and 1970 and then leveled off, while phosphorus inflows record their largest increases after 1970. This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this special issue of the Journal of Paleolimnology. D. R. Engstrom served as guest editor of the special issue.     

Applicative limitations of sediment transport on predictive modeling in geomorphology

Sources of uncertainty or error that arise in attempting to scale up the results of laboratory-scale sediment transport studies for predictive modeling of geomorphic systems include: (i) model imperfection, (ii) omission of important processes, (iii) lack of knowledge of initial conditions, (iv) sensitivity to initial conditions, (v) unresolved heterogeneity, (vi) occurrence of external forcing, and (vii) inapplicability of the factor of safety concept. Sources of uncertainty that are unimportant or that can be controlled at small scales and over short time scales become important in large-scale applications and over long time scales. Control and repeatability, hallmarks of laboratory-scale experiments, are usually lacking at the large scales characteristic of geomorphology. Heterogeneity is an important concomitant of size, and tends to make large systems unique. Uniqueness implies that prediction cannot be based upon first-principles quantitative modeling alone, but must be a function of system history as well. Periodic data collection, feedback, and model updating are essential where site-specific prediction is required.     

9000 years of geochemical evolution of lithogenic major and trace elements in the sediment of an alpine lake – the role of climate, vegetation, and land-use history

A 9000cal. year record of geochemistry was analysed in a sediment core obtained from a Swiss alpine hard-water lake (1937 ma.s.l.) that is located at the present-day tree-line. Geochemical stratigraphies are compared to changes in mineralogy, grain-size, pollen, and macrofossil records. This allows the reconstruction of the effects of changes in vegetation and of 3500 years of land-use in the catchment area on sediment geochemistry. Using principal component analysis, two major geochemical groups are distinguished: (i) Changes in concentrations of Rb, Ti, Zr, Fe, As, and Pb are closely related to corresponding changes in the concentrations of quartz and clay. They are thus considered to represent the silicate fraction which shows an increase from the oldest to the youngest core section. (ii) In contrast, Ca and Sr concentrations are positively correlated with changes in silt, sand, and calcite. They are therefore considered to represent the carbonate fraction which gradually decreased. Based on constrained cluster analysis, the core is divided into two major zones. The oldest zone (A; 9000–6400 cal.BP) is characterised by high concentrations of detrital carbonates. The more open catchment vegetation at that time promoted the physical weathering of these carbonates. The second major zone (B, 6400 cal.BP–1996 AD) is divided into four subsections with boundaries at ca. 3500, 2400, and 160cal. BP. The lower part of this zone, B1, is characterized by a gradual decrease in the carbonate-silt fraction and a pronounced increase in the silicate-clay fraction. This is concurrent with the expansion of Picea in the catchment area, which probably stabilized the soil. The middle part, B2 and B3 (3500–160cal. BP), comprises pronounced fluctuations in all elements, especially Ca, Sr, Mn, and Rb, but also in clay and silt. These changes are related to varying intensities of alpine farming. In the same section, Mn/Fe ratios are highly variable, suggesting changes in the mixing regime of the lake with phases of anoxic bottom water. The uppermost section, B4 (since 160cal. BP), is characterized by a steep decline in the silicate fraction and an increase in Ca and Sr. Despite the decrease in the silicate fraction, Pb increases, due to elevated atmospheric input resulting from early metal pollution, are masked by the high natural variability. Generally, changes in vegetation, which correspond to climate changes in the early Holocene and to human activities since ca. 3700cal. BP, are the controlling factor for variations in the geochemical composition of the sediment of Sägistalsee.     

古尔班通古特沙漠地表辐射收支特征

古尔班通古特沙漠是中国最大的固定、半固定沙漠.利用2017年该沙漠克拉美丽站辐射资料,分析了古尔班通古特沙漠不同时间尺度和不同天气条件下的地表辐射变化特征.结果表明:(1)不同月份沙漠辐射收支各分量月平均日变化均呈单峰型,但极值大小及出现时间存在差异.各分量曝辐量季节变化明显:太阳总辐射表现为生长期(4-9月)＞积雪期...