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1.
Increases in runoff and erosion after wildfires are often attributed to the development of hydrophobic soils. The potential for increased overland flow depends on the spatial contiguity of the hydrophobicity as well as its overall strength, but there is limited information on the spatial variability of soil hydrophobicity. We conducted spatially intensive hydrophobicity measurements in 225 m2 and 1 m2 plots in forested areas of Montana and Colorado burned at moderate to high severity, and in unburned control plots. Both the burned and unburned 225 m2 plots contained 10–23 hydrophobic soil patches in which hydrophobicity was strongest at the surface and declined rapidly with depth. The hydrophobic patches were closer together and up to 3 times larger in the burned plots. Consequently, 19% to 76% of the burned plots were hydrophobic compared to just 11% of the unburned plots. In five of the six burned plots, the patches were not laterally connected, suggesting that in most cases Hortonian overland flow generated from hydrophobic patches will infiltrate near its point of origin. The 1 m2 plots were smaller than most of the hydrophobic patches, so they did not capture the spatial characteristics of soil hydrophobicity. Characterization of the spatial variability of soil hydrophobicity should be based on measurements conducted at  1 m intervals across areas of > 100 m2. Due to the patchiness of soil hydrophobicity at the 100 to 101 meter scale, overland flow measurements in small ( 1 m2) plots may overestimate the magnitude and variability of runoff from burned catchments.  相似文献   

2.
Gully erosion is an important environmental hazard in the black soil region of northeastern China. It is a primary sediment source in the region which needs appropriate soil conservation practices. Gully incision in rolling hills typical of this region was monitored using real-time kinematic GPS to assess the rates of gully development and the resultant sediment production. From 2002 to 2005, gully heads in the study area retreated between 15.4 and 33.5 m, giving an average retreat rate of 8.4 m yr− 1. Field measurements showed that total sediment production due to gully erosion during the three years ranged between 257 and 1854 m3 yr− 1, which is equivalent to 326 to 2355 t yr− 1, with gully-head retreat accounting for 0 to 21.7% (4.4% in average). The sediment delivery ratio was especially high during the summer rainy season (56% in average). Sediment production by ephemeral gullies and permanent gullies was 1.5 times greater than that from surface erosion. Gully heads retreated faster in the spring freeze–thaw period than in the summer. The stage of gully development could be identified based on short-term changes in the gully erosion rate.  相似文献   

3.
The landscape evolution in Neogene intramontane basins is a result of the interaction of climatic, lithologic, and tectonic factors. When sedimentation ceases and a basin enters an erosional stage, estimating erosion rates across the entire basin can offer a good view of landscape evolution. In this work, the erosion rates in the Guadix–Baza basin have been calculated based on a volumetric estimate of sediment loss by river erosion since the Late Pleistocene. To do so, the distribution of a glacis surface at ca. 43 kyr, characterised by a calcrete layer that caps the basin infilling, has been reconstructed. To support this age, new radiometric data of the glacis are presented. The volume of sediment loss by water erosion has been calculated for the entire basin by comparing the reconstructed geomorphic surface and the present-day topography. The resulting erosion rates vary between 4.28 and 6.57 m3 ha− 1 yr− 1, and are the consequence of the interaction of climatic, lithologic, topographic, and tectonic factors. Individual erosion rates for the Guadix and Baza sub-basins (11.80 m3 ha− 1 yr− 1 and 1.77 m3 ha− 1 yr− 1 respectively) suggest different stages of drainage pattern evolution in the two sub-basins. We attribute the lower values obtained in the Baza sub-basin to the down-throw of this sub-basin caused by very recent activity along the Baza fault.  相似文献   

4.
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 210Pbxs 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.  相似文献   

5.
Debris flows generated during rain storms on recently burned areas have destroyed lives and property throughout the Western U.S. Field evidence indicate that unlike landslide-triggered debris flows, these events have no identifiable initiation source and can occur with little or no antecedent moisture. Using rain gage and response data from five fires in Colorado and southern California, we document the rainfall conditions that have triggered post-fire debris flows and develop empirical rainfall intensity–duration thresholds for the occurrence of debris flows and floods following wildfires in these settings. This information can provide guidance for warning systems and planning for emergency response in similar settings.Debris flows were produced from 25 recently burned basins in Colorado in response to 13 short-duration, high-intensity convective storms. Debris flows were triggered after as little as six to 10 min of storm rainfall. About 80% of the storms that generated debris flows lasted less than 3 h, with most of the rain falling in less than 1 h. The storms triggering debris flows ranged in average intensity between 1.0 and 32.0 mm/h, and had recurrence intervals of two years or less. Threshold rainfall conditions for floods and debris flows sufficiently large to pose threats to life and property from recently burned areas in south-central, and southwestern, Colorado are defined by: I = 6.5D− 0.7 and I = 9.5D− 0.7, respectively, where I = rainfall intensity (in mm/h) and D = duration (in hours).Debris flows were generated from 68 recently burned areas in southern California in response to long-duration frontal storms. The flows occurred after as little as two hours, and up to 16 h, of low-intensity (2–10 mm/h) rainfall. The storms lasted between 5.5 and 33 h, with average intensities between 1.3 and 20.4 mm/h, and had recurrence intervals of two years or less. Threshold rainfall conditions for life- and property-threatening floods and debris flows during the first winter season following fires in Ventura County, and in the San Bernardino, San Gabriel and San Jacinto Mountains of southern California are defined by I = 12.5D−0.4, and I = 7.2D−0.4, respectively. A threshold defined for flood and debris-flow conditions following a year of vegetative recovery and sediment removal for the San Bernardino, San Gabriel and San Jacinto Mountains of I = 14.0D−0.5 is approximately 25 mm/h higher than that developed for the first year following fires.The thresholds defined here are significantly lower than most identified for unburned settings, perhaps because of the difference between extremely rapid, runoff-dominated processes acting in burned areas and longer-term, infiltration-dominated processes on unburned hillslopes.  相似文献   

6.
Using the USPED (Unit Stream Power Erosion Deposition) model, three land use scenarios were analysed for an Italian small catchment (15 km2) of high landscape value. The upper Orme stream catchment, located in the Chianti area, 30 km south of Florence, has a long historical agriculture record. Information on land use and soil conservation practices date back to 1821, hence offering an opportunity to model impacts of land use change on erosion and deposition. For this study, a procedure that takes into account soil conservation practices and potential sediment storage is proposed. The approach was to calculate and model the flow accumulation considering rural and logging roads, location of urban areas, drainage ditches, streams, gullies and permanent sediment sinks. This calculation attempts to assess the spatial variability, especially the impact of support practices (P factor). Weather data from 1980–2003 were taken into account to calculate the R factor. However, to consider the intense pluviometric conditions in terms of the erosivity factor, the 0.75th quantile was used, while the lowest erosivity was modelled using the 0.25th quantile. Results of the USPED model simulation show that in 1821 the mean annual net erosion for the watershed was 2.8 Mg ha− 1 y− 1; in 1954 it was 4.2 Mg ha− 1 y− 1; and in 2004 it was 5.3 Mg ha− 1 y− 1. Conservation practices can reduce erosion processes by ≥ 20 Mg ha− 1 y− 1 when the 1821 practices are introduced in the present management. On the other hand, if the support practices are not considered in the model, soil erosion risk is overestimated. Field observation for the present-day simulation confirmed that erosion and associated sediment deposition predicted by the model depend, as expected, on geomorphology and land use. The model shows limitations that are mainly due to the input data. A high resolution DEM is essential for the delineation of reliable topographic potential to predict erosion and deposition especially in vineyards.  相似文献   

7.
A new and simple method is developed to efficiently quantify erosion and deposition rates based on stock unearthing measurements. This is applicable to spatial scales ranging from plot to hillslopes, and to time scales ranging from single hydrologic events to centennial scales. The method is applied to a plot area on vineyard hillslopes in Burgundy (Monthélie, France), with measurement of 4328 vine plants. A sediment budget established at the plot scale shows a mean soil lowering of 3.44 ± 1 cm over 20 years, involving a minimal erosion rate of 1.7 ± 0.5 mm yr− 1. Locally, erosion rates can reach up to 8.2 ± 0.5 mm yr− 1.This approach allows the sediment redistribution to be mapped and analyzed at 1-m resolution. It provides novel insights into the characterization of erosion patterns on pluri-decennial scales and into the analysis of spatial distribution of erosion processes on cultivated hillslopes.  相似文献   

8.
Wind erosion of soil is an appreciable but unstudied event following fires in cold desert. We examined aeolian transport of sediment for 1 year following fire in semi-arid shrub steppe on loess soils in southern Idaho, USA. Sediment collectors were used to determine horizontal mass transport of soil and saltation sensors and anemometers were used to determine saltation activity (fraction of time having saltation) and threshold wind speed in an area burned in August and an unburned control site. Horizontal mass transport (per 30-day period) was negligible in the unburned area, but in the burned area was 5.40 kg m?1 in October and decreased to 2.80 kg m?1 in November and 0.32 kg m?1 in December. Saltation activity was high enough to determine threshold wind speeds only in the burn site during fall, when values ranged from 10.0 to 10.6 m s?1. Sediment flux and saltation activity in the burned site became much less pronounced following the emergence of herbaceous vegetation in the spring. Post-fire sediment flux in the shrub steppe we examined was of greater magnitude but shorter duration than post-fire fluxes in warm deserts or sandier regions that experience more frequent wind erosion.  相似文献   

9.
Understanding and quantifying sediment load is important in catchments draining highly erodible materials that eventually contribute to siltation of downstream reservoirs. Within this context, the suspended sediment transport and its temporal dynamics have been studied in the River Isábena (445 km2, south-central Pyrenees, Ebro basin) by means of direct sampling and turbidity recording during a 3-year dry period. The average flood-suspended sediment concentration was 8 g l− 1, with maximum instantaneous values above 350 g l− 1. The high scatter between discharge and suspended sediment concentrations (up to five orders of magnitude) has not permitted the use of rating curve methods to estimate the total load. Interpolation techniques yielded a mean annual sediment load of 184,253 t y− 1 for the study period, with a specific yield of 414 t km− 2 y− 1. This value resembles those reported for small torrents in nearby mountainous environments and is the result of the high connectivity between the badland source areas and stream courses, a fact that maximises sediment conveyance through the catchment. Floods dominated the sediment transport and yield. However, sediment transport was more constant through time than that observed in Mediterranean counterparts; this can be attributed to the role of base flows that entrain fine sediment temporarily stored in the channel and force the river to carry high sediment concentrations (i.e., generally in the order of 0.5 g l− 1), even under minimum flow conditions.  相似文献   

10.
We have monitored initiation conditions for six debris flows between May 2004 and July 2006 in a 0.3 km2 drainage basin at Chalk Cliffs; a band of hydrothermally-altered quartz monzonite in central Colorado. Debris flows were initiated by water runoff from colluvium and bedrock that entrained sediment from rills and channels with slopes ranging from about 14° to 45°. The availability of channel material is essentially unlimited because of thick channel fill and refilling following debris flows by rock fall and dry ravel processes. Rainfall exceeding I = 6.61(D)− 0.77, where I is rainfall intensity (mm/h), and D is duration (h), was required for the initiation of debris flows in the drainage basin. The approximate minimum runoff discharge from the surface of bedrock required to initiate debris flows in the channels was 0.15 m3/s. Colluvium in the basin was unsaturated immediately prior to (antecedent) and during debris flows. Antecedent, volumetric moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 4–9%, and 4–7%, respectively. During debris flows, peak moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 10–20%, and 4–12%, respectively. Channel sediment at a depth of 45 cm was unsaturated before and during debris flows; antecedent moisture ranged from 20–22%, and peak moisture ranged from 24–38%. Although we have no measurements from shallow rill or channel sediment, we infer that it was unsaturated before debris flows, and saturated by surface-water runoff during debris flows.Our results allow us to make the following general statements with regard to debris flows generated by runoff in semi-arid to arid mountainous regions: 1) high antecedent moisture levels in hillslope and channel sediment are not required for the initiation of debris flows by runoff, 2) locations of entrainment of sediment by successive runoff events can vary within a basin as a function of variations in the thickness of existing channel fill and the rate of replenishment of channel fill by rock fall and dry ravel processes following debris flows, and 3) rainfall and simulated surface-water discharge thresholds can be useful in understanding and predicting debris flows generated by runoff and sediment entrainment.  相似文献   

11.
Muddy floods, i.e. runoff from cultivated areas carrying large quantities of soil, are frequent and widespread in the European loess belt. They are mainly generated in dry zero-order valleys and are nowadays considered as the most likely process transferring material eroded from cultivated hillslopes during the Holocene to the flood plain. The huge costs of muddy flood damages justify the urgent installation of control measures. In the framework of the ‘Soil Erosion Decree’ of the Belgian Flemish region, a 12 ha-grassed waterway and three earthen dams have been installed between 2002–2004 in the thalweg of a 300-ha cultivated dry valley in the Belgian loess belt. The measures served their purpose by preventing any muddy flood in the downstream village, despite the occurrence of several extreme rainfall events (with a maximum return period of 150 years). The catchment has been intensively monitored from 2005–2007 and 39 runoff events were recorded in that period. Peak discharge (per ha) was reduced by 69% between the upstream and the downstream extremities of the grassed waterway (GWW). Furthermore, runoff was buffered for 5–12 h behind the dams, and the lag time at the outlet of the catchment was thereby increased by 75%. Reinfiltration was also observed within the waterway, runoff coefficients decreasing by a mean of 50% between both extremities of the GWW. Sediment discharge was also reduced by 93% between the GWW's inflow and the outlet. Before the installation of the control measures, specific sediment yield (SSY) of the catchment reached 3.5 t ha− 1 yr− 1 and an ephemeral gully was observed nearly each year in the catchment. Since the control measures have been installed, no (ephemeral) gully has developed and the SSY of the catchment dropped to a mean of 0.5 t ha− 1 yr− 1. Hence, sediment transfer from the cultivated dry valley to the alluvial plain should dramatically decrease. Total cost of the control measures that are built for a 20 year-period is very low (126 € ha− 1) compared to the mean damage cost associated with muddy floods in the study area (54 € ha− 1 yr− 1). Similar measures should therefore be installed to protect other flooded villages of the Belgian loess belt and comparable environments.  相似文献   

12.
Field experiments were conducted in Nellis Dunes Recreational Area (Clark County, Nevada, USA) to investigate emission of dust produced by off-road driving. Experiments were carried out with three types of vehicles: 4-wheelers (quads), dirt bikes (motorcycles) and dune buggies, on 17 soil types characteristic for a desert environment. Tests were done at various driving speeds, and emissions were measured for a large number of grain size fractions. This paper reports the results for two size fractions of emissions: PM10 (particles < 10 μm) and PM60 (particles < 60 μm). The latter was considered in this study to be sufficiently representative of the total suspendable fraction (TSP). Off-road driving was found to be a significant source of dust. However, the amounts varied greatly with the type of soil and the characteristics of the top layer. Models predicting emission of dust by off-road driving should thus consider a number of soil parameters and not just one key parameter. Vehicle type and driving speed are additional parameters that affect emission. In general, 4-wheelers produce more dust than dune buggies, and dune buggies, more than dirt bikes. Higher speeds also result in higher emissions. Dust emitted by off-road driving is less coarse than the parent sediment on the road surface. Off-road driving thus results in a progressive coarsening of the top layer. Exceptions to this are silty surfaces with no, or almost no, vegetation. For such surfaces no substantial differences were observed between the grain size distribution of road dust and emitted dust. Typical emission values for off-road driving on dry desert soils are: for sandy areas, 30–40 g km− 1 (PM10) and 150–250 g km− 1 (TSP); for silty areas, 100–200 g km− 1 (PM10) and 600–2000 g km− 1 (TSP); for drainages, 30–40 g km− 1 (PM10) and 100–400 g km− 1 (TSP); and for mixed terrain, 60–100 g km− 1 (PM10) and 300–800 g km− 1 (TSP). These values are for the types of vehicles tested in this study and do not refer to cars or trucks, which produce significantly more dust.  相似文献   

13.
T.C. Hales  J.J. Roering 《Geomorphology》2009,107(3-4):241-253
In the Southern Alps, New Zealand, large gradients in precipitation (< 1 to 12 m year− 1) and rock uplift (< 1 to 10 mm year− 1) produce distinct post-glacial geomorphic domains in which landslide-driven sediment production dominates in the wet, rapid-uplift western region, and rockfall controls erosion in the drier, low-uplift eastern region. Because the western region accounts for < 25% of the active orogen, the dynamics of erosion in the extensive eastern region are of equal importance in estimating the relative balance of uplift and erosion across the Southern Alps. Here, we assess the efficacy of frost cracking as the primary rockfall mechanism in the eastern Southern Alps using air photo and topographic analysis of scree slopes, cosmogenic radionuclide dating of headwalls, paleo-climate data, and a numerical model of headwall temperature. Currently, active scree slopes occur at a relatively uniform mean elevation ( 1450 m) and their distribution is independent of hillslope aspect and rock type, consistent with the notion that frost cracking (which is maximized between − 3 and − 8 °C) may control rockfall erosion. Headwall erosion rates of 0.3 to 0.9 mm year− 1, measured using in-situ 10Be and 26Al in the Cragieburn Range, confirm that rockfall erosion is active in the late Holocene at rates that roughly balance rock uplift. Models of the predicted depth of frost activity are consistent with the scale of fractures and scree blocks in our field sites. Also, vegetated, paleo-scree slopes are ubiquitous at elevations lower than active scree slopes, consistent with the notion that lower temperatures during the last glacial advance induced pervasive rockfall erosion due to frost cracking. Our modeling suggests temporally-averaged peak frost cracking intensity occurs at 2300 m a.s.l., the approximate elevation of the highest peaks in the central Southern Alps, suggesting that the height of these peaks may be limited by a “frost buzzsaw.”  相似文献   

14.
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.  相似文献   

15.
Rates of sheet and rill erosion in Germany — A meta-analysis   总被引：2，自引：0，他引：2  
K. Auerswald  P. Fiener  R. Dikau   《Geomorphology》2009,111(3-4):182-193
Knowledge of erosion rates under real conditions is of great concern regarding sustainability of landuse and off-site effects on water bodies and settlements. Experimentally derived rates of sheet and rill erosion are often biased by experimental settings, which deviate considerably from typical landuse, by short measuring periods and by small spatial extensions, which do not account for the pronounced spatio-temporal variability of erosion events. We compiled data from 27 studies covering 1076 plot years to account for this variability. Modelling was used to correct for deficiencies in the experimental settings, which overrepresented arable land and used steeper and shorter slopes as well as higher erosivity than typically found in reality. For example, the average slope gradient was 5.9° for all arable plot experiments while it is only 2.6° on total arable land in Germany. The expected soil loss by sheet and rill erosion in Germany after taking real slopes, landuse and erosivity into account averaged 2.7 t ha− 1 yr− 1. Annual crops contributed the largest proportion (90%) but hops despite its negligible contribution to landuse (0.06%) still contribute 1.0% due to its extraordinary rapid erosion, which was even faster than the measured bare fallow soil loss standardized to otherwise identical conditions. Bare fallow soil loss, which is often used as baseline, was 80 t ha− 1 yr− 1 when standardized to 5.1° slope gradient, 200 m flow path length, and average German erosivity.  相似文献   

16.
Beach–dune seasonal elevation changes, aeolian sand transport measurements, bathymetric surveys and shoreline evolution assessments were used to investigate annual and seasonal patterns of dune development on Sfântu Gheorghe beach, the Danube delta coast, from 1997 to 2004. Dune volume increased consistently (1.96 m3 m− 1 y− 1 to 5.1 m3 m− 1 y− 1) over this 7-year period with higher rates in the southward (downdrift) direction. Dune aggradation is periodically limited by storms, each of which marks a new evolutionary phase of the beach–dune system. As a consequence of the variable beach morphology and vegetation density during a year, foredune growth occurs during the April–December interval while between December and April a slightly erosive tendency is present. The pattern of erosion and deposition shown by the topographical surveys is in good agreement with the sand transport measurements and demonstrates the presence of a vigorous sand flux over the foredunes which is 20–50% smaller than on the beach. This high sand flux, due to low precipitation and sparse vegetation cover, creates an aerodynamically efficient morphology on the seaward dune slope. The seaward dune face accretes during low to medium onshore winds (5.5–12 m s− 1) and erodes during high winds (> 12 m s− 1).  相似文献   

17.
Information on post-fire sediment and nutrient redistribution is required to underpin post-fire catchment management decisions. Fallout radionuclide budgets (210Pbxs, 137Cs and 7Be) were derived to quantify soil redistribution and sediment yield in forested terrain following a moderately severe wildfire in a small (89 ha) water supply catchment in SE Australia. Application of these techniques in burnt terrain requires careful consideration of the partitioning of radionuclides between organic and mineral soil components. Beryllium-7 and 210Pbxs were shown to be closely associated with ash, litter and soil organic matter whereas 137Cs was more closely associated with subsurface coarse mineral soil. Comparison of the three tracer budgets indicated that the dominant sediment source areas were ridgetops and steep valley sideslopes, from which burnt surface material was conveyed to the stream network via pre-existing gullies. Erosion was predominantly driven by sheetwash, enhanced by soil water repellency, and modified by bioturbation which both supplies subsurface sediment and provides sinks for erosive overland flow. Footslope and riparian zones were not important sediment source areas. The estimated event-based (wildfire and subsequent rainfall) sediment yield is 58 ± 25 t km− 2, based on fallout 7Be measurements. The upper estimate of total particulate phosphorus yield (0.70 kg ha− 1) is more than 10 times that at equivalent unburnt sites. This illustrates that, soon after fire, burnt eucalypt forest can produce nutrient loads similar to those of agricultural catchments. The tracer budgets indicate that wildfire is an important control on sediment and phosphorus inputs to the stream network over the decadal timeframe and the pulsed nature of this release is an important concern for water quality management.  相似文献   

18.
Distinct rock fragment displacements occur on the ambas, or structurally determined stepped mountains of the Northern Ethiopian Highlands. This paper describes the rock fragment detachment from cliffs by rockfall, quantifies its annual rate, and identifies factors controlling rock fragment movement on the scree slopes. It further presents a conceptual model explaining rock fragment cover at the soil surface in these landscapes. In the May Zegzeg catchment (Dogu'a Tembien district, Tigray), rockfall from cliffs and rock fragment movement on debris slopes by runoff and livestock trampling were monitored over a 4-year period (1998–2001). Rockfall and rock fragment transport mainly induced by livestock trampling appear to be important geomorphic processes. Along a 1500-m long section of the Amba Aradam sandstone cliff, at least 80 t of rocks are detached yearly and fall over a mean vertical distance of 24 m resulting in a mean annual cliff retreat rate of 0.37 mm y− 1. Yearly unit rock fragment transport rates on scree slopes ranged between 23.1 and 37.9 kg m− 1 y− 1. This process is virtually stopped when exclosures are established. Corresponding mean rock fragment transport coefficients K are 32–69 kg m− 1 y− 1 on rangeland but only 3.9 kg m− 1 y− 1 in densely vegetated exclosures. A conceptual model indicates that besides rockfall from cliffs and argillipedoturbation, all factors and processes of rock fragment redistribution in the study area are of anthropogenic origin.  相似文献   

19.
Previous studies of chemical weathering rates for soil developed on glacial moraines generally assumed little or no physical erosion of the soil surface. In this study, we investigate the influence of physical erosion on soil profile weathering rate calculations. The calculation of chemical weathering rates is based on the assumption that soil profiles represent the integrated amount of weathering since the time of moraine deposition. The weathering rate of a surface subjected to denudation is the sum of the weathering loss from the existing soil profile added to the weathering loss in the material removed by denudation, divided by the deposition age. In this study, the amount of weathered material removed since moraine deposition is calculated using the denudation rate estimated from cosmogenic nuclide data and the deposition age of the moraine. Weathering rates accounting for denudation since moraine deposition are compared to weathering rates based on the assumption of no physical erosion and on the assumption of steady-state denudation for the Type Pinedale moraine ( 21 ka) and the Bull Lake-age moraine ( 140 ka) in the Fremont Lake Area (Wind River Mountains, Wyoming, USA). The total weathering rates accounting for denudation are 8.15 ± 1.05 g(oxide) m− 2 y− 1 for the Type Pinedale moraine and 4.78 ± 0.89 g(oxide) m− 2 y− 1 for the Bull Lake-age moraine, which are  2 to 4 times higher, respectively, than weathering rates based on the assumption of no physical erosion. The weathering rates based on denudation since moraine deposition are comparable or smaller than weathering rates assuming steady-state denudation. We find the assumption of steady-state denudation is not valid in depositional landscapes with young deposition ages or slow denudation rates. The decrease in weathering rates over time between the Type Pinedale and Bull Lake-age soils that is observed in the case of no physical erosion is decreased when the influence of denudation on the total weathering rates is taken into account. Fresh unweathered material with high reactive mineral surface area is continuously provided to the surface layer by denudation diminishing the effect of decreasing weathering rate over time.  相似文献   

20.
《Geomorphology》1997,18(2):61-75
In badland areas of the Ebro Basin, in a semiarid climate, two erosion plots (257 m2; 5° slope and 128 m2; 23° slope) on exposed Tertiary clays were monitored over two years (Nov. 1991–Nov. 1993). This material is characterized by high sodium absorption ratios which lead to high soil dispersivity. The dominant erosion processes in both plots are rilling and sheet erosion. Rainfall intensity was recorded at a weather station, connected to a data-logger, sediment production for single events was collected in tanks, and ground lowering was measured every six months by erosion pins and microtopographic profile gauge techniques. Significant runoff was produced only by rainfall events above 5 mm. Another threshold at 20 mm rain was noted. For rainfalls higher than 20 mm, the 23° slope plot shows a greater runoff response than the 5° one. Rainfall events exceeding this threshold showed a higher sediment production for the steeper slope. In the relationship between precipitation and sediment concentration, an envelope curve can be drawn indicating that any rainfall event of a given amount and intensity has a maximum sediment concentration which we speculate to be a function of the runoff sediment transport capacity. Runoff response and sediment yield in the studied plots are controlled by the rainfall and soil characteristics and their seasonal variations. In both plots, the erosion pins show that erosion rates in rill areas are 25–50% higher than in the interrill areas. Sediment yield recorded by collector devices was higher than the rates measured by erosion pins. The erosion rates based on rill cross-sections by profilometers were higher than the ones recorded by collectors.  相似文献   

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