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1.
Sediment yield can be a sensitive indicator of catchment dynamics and environmental change. For a glacierized catchment in the High Arctic, we compiled and analyzed diverse sediment transfer data, spanning a wide range of temporal scales, to quantify catchment yields and explore landscape response to past and ongoing hydroclimatic variability. The dataset integrates rates of lake sedimentation from correlated varve records and repeated annual and seasonal sediment traps, augmented by multi‐year lake and fluvial monitoring. Consistent spatial patterns of deposition enabled reconstruction of catchment yields from varve‐ and trap‐based fluxes. We used hydroclimatic data and multivariate modeling to examine annual controls of sediment delivery over almost a century, and to examine shorter‐term controls of sediment transfer during peak glacier melt. Particle‐size analyses, especially for annual sediment traps, were used to further infer sediment transfer mechanisms and timing. Through the Medieval Warm Period and Little Ice Age, there were no apparent multi‐century trends in lake sedimentation rates, which were over three times greater than those during the mid‐Holocene when glaciers were diminished. Twentieth‐century sedimentation rates were greater than those of previous millennia, with a mid‐century step increase in mean yield from 240 to 425 Mg km?2 yr?1. Annual yields through the twentieth century showed significant positive relations with spring/summer temperature, rainfall, and peak discharge conditions. This finding is significant for the future of sediment transfer at Linnévatnet, and perhaps more broadly in the Arctic, where continued increases in temperature and rainfall are projected. For 2004–2010, annual yields ranged from 294 to 1330 Mg km?2 yr?1. Sediment trap volumes and particle‐size variations indicate that recent annual yields were largely dominated by spring to early summer transfer of relatively coarse‐grained sediment. Fluvial monitoring showed daily to hourly sediment transfer to be related to current and prior discharge, diurnal hysteresis, air temperature, and precipitation. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

2.
The suspended sediment yield and the transfer of polluted sediment are investigated for the Puyango river basin in southern Ecuador. This river system receives metal (Cd, Cu, Hg, Pb and Zn) and cyanide pollution generated by mining, and is associated with large‐scale hydrological variability, which is partly governed by El Niño events. Field sampling and statistical modelling methods are used to quantify the amount of mine tailings that is discharged into the basin. Annual suspended sediment yields are estimated using a novel combination of the suspended sediment rating method and Monte Carlo simulations, which allow for propagation of the uncertainties of the calculations that lead to final load estimates. Geochemical analysis of suspended and river bed sediment is used to assess the dispersion and long‐term fate of contaminated sediment within the river catchment. Knowledge of the inter‐ and intra‐annual variation in suspended sediment yield is shown to be crucial for judging the importance of mining discharges, and the extent to which the resultant pollution is diluted by river flows. In wet years, polluted sediments represent only a very small proportion of the yield estimates, but in dry years the proportion can be significant. Evidence shows that metal contaminated sediments are stored in the Puyango river bed during low flows. Large flood events flush this sediment periodically, both on an annual cycle associated with the rainy season, and also related to El Niño events. Therefore, environmental impacts of mining‐related discharges are more likely to be severe during dry years compared with wet years, and in the dry season rather than the wet season. The hydrological consequences of El Niño events are shown to depend upon the extent to which these events penetrate inland. It is, thus, shown that the general conclusion that El Niño events can significantly affect suspended sediment yields needs evaluation with respect to the particular way in which those events affect a given catchment. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

3.
Although the importance of ENSO on hydrological anomalies has been recognized, variations in sediment fluxes caused by these extreme events are poorly documented. The effect of ENSO is not limited to changes in sediment mobilization. Since ENSO events can affect terrestrial ecosystems, they may have important effects on sediment production and transport in river basins over time spans that are longer than the duration of the event itself. The Catamayo‐Chira basin is an interesting casestudy for investigating these geomorphic implications. The objectives were: (i) to study the effect of ENSO on stream flow and sediment yields in the basin, (ii) to investigate if ENSO events affect sediment yields in the post‐ENSO period and (iii) to understand which factors control the ENSO and post‐ENSO basin response. During strong negative ENSO periods, mean annual stream flow discharge at the inlet of the Poechos reservoir in the lower basin was 5.4 times higher than normal annual discharges, while average sediment fluxes exceeded those of normal years by a factor of about 11. In two heavily affected periods, 45.9% of the total sediment yield in the 29 years observation period was generated. Sediment fluxes in the post‐ENSO period are lower than expected, which proves post‐ENSO event dynamics are significantly different from pre‐event dynamics. Our analysis indicates the increase of vegetation growth in the lower basin is not the main reason explaining considerable sediment flux decrease in post‐ENSO periods. During strong ENSO events, sediment in alluvial stores in the lower part of the basin is removed due to enlarging and deepening of channels. In post‐ENSO periods, normal discharges and persisting sediment supplies from the middle/upper basin lead to river aggradation and storage of large amounts of sediment in alluvial plains. The decrease in sediment export will last for several years until the equilibrium is re‐established. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

4.
The Loess Plateau has been experiencing large‐scale land use and cover changes (LUCCs) over the past 50 years. It is well known about the significant decreasing trend of annual streamflow and sediment load in the catchments in this area. However, how surface run‐off and sediment load behaved in response to LUCC at flood events remained a research question. We investigated 371 flood events from 1963 to 2011 in a typical medium‐sized catchment within the Plateau in order to understand how LUCC affected the surface run‐off generation and sediment load and their behaviours based on the analysis of return periods. The results showed that the mean annual surface run‐off and sediment load from flood events accounted for 49.6% and 91.8% of their mean annual totals. The reduction of surface run‐off and associated sediment yield in floods explained about 85.0% and 89.2% of declines in the total annual streamflow and sediment load, respectively. The occurrences of flood events and peak sediment concentrations greater than 500 kg/m3 showed a significantly downward trend, yet the counterclockwise loop events still dominated the flood event processes in the catchment. The results suggest that LUCC over the past 50 years resulted in significant changes in the water balance components and associated soil erosion and sediment transportation in the catchment. This was achieved mainly by reducing surface run‐off and sediment yield during floods with return period of less than 5 years. Run‐off–sediment load behaviour during the extreme events with greater than 10‐year return periods has not changed. Outcomes from this study are useful in understanding the eco‐hydrological processes and assisting the sustainable catchment management and land use planning on the Loess Plateau, and the methodologies are general and applicable to similar areas worldwide.  相似文献   

5.
A rating curve provides a reasonable estimate of the suspended sediment concentration at a given discharge. However, analysis of a detailed 9‐year time‐series of suspended sediment concentration (SSC) and discharge Q of the Meuse River in The Netherlands indicates that SSC is (besides discharge) controlled by exhaustion and replenishment of different sediment sources. Clockwise hysteresis and other effects of sediment exhaustion can be observed during and after flood events, and the effects of stockpiling of sediment in the river bed during low‐discharge periods are obvious in the SSC of the next flood. In a single regression equation we have implemented a parameter that represents the presence or absence of stock for sediment uptake. In comparison with a rating curve of SSC and Q, adding this parameter is shown to be a more reliable and comprehensive method to predict SSCs at all discharge regimes with all preceding discharge conditions, for single‐peaked and multi‐peaked runoff events as well as for low flow conditions. The method is probably applicable to other small‐ to medium‐scaled river basins. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

6.
Agricultural zones are significant sediment sources, but it is crucial to identify critical source areas (CSAs) of sediment yield within these zones where best management practices (BMPs) can be applied to the best effect in reducing sediment delivery to receiving water bodies rather than the economically nonviable alternative of randomly or sweepingly implementing BMPs. A storm event of a specific magnitude and hyetograph profile may, at different times, generate a greater or lesser sediment yield. The widely used agricultural nonpoint source (AGNPS) model was used to identify CSAs for sediment losses in Southwestern Ontario's agriculture‐dominated 374‐ha Holtby watershed. A storm threshold approach was adopted to identify critical periods for higher sediment losses. An AGNPS model for the Holtby watershed was set up, calibrated, and validated for run‐off volume, peak flow rate, and sediment yield for several storms. The calibrated and validated model was run for storms of increasing return periods to identify threshold storm events that would generate sediment yield greater than an acceptable value for early and late spring, summer, and fall seasons. Finally, to evaluate the potential impacts of climate change, we shifted shorter duration summer storms into spring conditions and quantified the changes in sediment yield dynamics. A 6‐hr, 7.5‐year early spring storm would generate sediment losses exceeding the acceptable limit of 0.34 t ha?1 for the season. However, summer storms (2 hr, up to 100 years) tended to generate sediment yields below those of an identifiable threshold storm. If such shorter duration summer storms occurred in spring, the sediment yield would increase by more than fivefold. A 5‐year future storm would generate an equivalent effect of a 100‐year current spring event. The high sediment delivery to be expected will have significant implications regarding the future management of water quality of receiving waters. Appropriate placement of BMPs at CSAs will thus be needed to reduce such high sediment delivery to receiving waters.  相似文献   

7.
Interannual variations in seasonal sediment transfer in two High Arctic non‐glacial watersheds were evaluated through three summers of field observations (2003–2005). Total seasonal discharge, controlled by initial watershed snow water equivalence (SWE) was the most important factor in total seasonal suspended sediment transfer. Secondary factors included melt energy, snow distribution and sediment supply. The largest pre‐melt SWE of the three years studied (2004) generated the largest seasonal runoff and disproportionately greater suspended sediment yield than the other years. In contrast, 2003 and 2005 had similar SWE and total runoff, but reduced runoff intensity resulted in lower suspended sediment concentrations and lower total suspended sediment yield in 2005. Lower air temperatures at the beginning of the snowmelt period in 2003 prolonged the melt period and increased meltwater storage within the snowpack. Subsequently, peak discharge and instantaneous suspended sediment concentrations were more intense than in the otherwise warmer 2005 season. The results for this study will aid in model development for sediment yield estimation from cold regions and will contribute to the interpretation of paleoenvironmental records obtained from sedimentary deposits in lakes. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

8.
A sediment budget for an upland catchment–reservoir system at Burnhope Reservoir, North Pennines, UK has been developed. This provides a framework for quantifying historic and contemporary sediment yields and drainage basin response to disturbance from climate change and human activities in the recent past. Bathymetric survey, core sampling, 137Cs dating and aerial photographs have been used to assess sediment accumulation in the reservoir. The average reservoir sedimentation rate is 1·24 cm yr?1 (annual sediment yield 33·3 t km?2 yr?1 ± 10%, trap efficiency 92%). Mean annual reservoir sedimentation over the 67 year period has been estimated at 592 t ± 10%. Inputs of suspended sediment from direct catchwater streams account for 54% of sediment supply to the budget (best estimate yield of 318 t yr?1 ± 129%), while those from actively eroding reservoir shorelines contribute 328 t yr?1 ± 92%. Sediment yield estimates from stream monitoring and reservoir sedimentation are an order of magnitude lower than those reported from South Pennine reservoirs of comparable drainage basin area. Analysis of historical rainfall series for the catchment shows fluctuations in winter and summer rainfall patterns over the past 62 years. From 1976 to 1998 there has been a diverging trend between winter and summer rainfall, with a large increase in winter and a gradual decrease in summer totals. Periods of maximum variation occur during the summer drought events of the late 1970s, early 1980s and mid‐1990s. Analysis of the particle size of core sediments highlights abrupt increases in sand‐sized particles in the top 20 cm of the core. Based on the 137Cs chronology, these layers were deposited from the late 1970s onwards and relate to these diverging rainfall records and rapidly fluctuating reservoir levels. This provides evidence of potential sediment reworking within the reservoir by rapid water‐level rise after drought. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

9.
Fluvial sediment transport in the high mountain Partnach River (Reintal Valley, Bavarian Alps) was investigated during a 10‐year observation period (2001–2010). During this period, the downstream reach of the Partnach River was decoupled from upstream sediment throughput by a rockslide deposit until 2005. In August 2005, the dam was partially breached during a flood event resulting in renewed sediment coupling between the upstream and downstream reaches. A comparison of pre‐ and post‐dambreak river sediment load data showed that the dissolved load dominated sediment transport prior to August 2005 with a switch to the dominance of bedload transport, post‐dambreak. The higher post‐dambreak bedload rates were particularly evident during the first years after the dam failure due to significant coarse material coupling between active sediment sources (undercut banks/talus cones) and the Partnach River. In the last years of the observation period (2009 and 2010) the dominance of dissolved load transport was re‐established. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

10.
Reconstruction of high‐frequency erosion variability beyond the instrumental record requires well‐dated, high‐resolution proxies from sediment archives. We used computed tomography (CT) scans of finely laminated silt layers from a lake‐sediment record in southwest Oregon to quantify the magnitude of natural landscape erosion events over the last 2000 years in order to compare with palaeorecords of climate, forest fire, and seismic triggers. Sedimentation rates were modeled from an age–depth relationship fit through five 14C dates and the 1964 AD 137Cs peak in which deposition time (yr mm‐1) varied inversely with the proportion of silt sediment measured by the CT profile. This model resulted in pseudo‐annual estimates of silt deposition for the last 2000 years. Silt accumulation during the past 80 years was strongly correlated with river‐discharge at annual and decadal scales, revealing that erosion was highly responsive to precipitation during the logging era (1930–present). Before logging the frequency–magnitude relationship displayed a power‐law distribution that is characteristic of complex feedbacks and self‐regulating mechanisms. The 100‐year and 10‐year erosion magnitude estimated in a 99‐year moving window varied by 1.7 and 1.0 orders of magnitude, respectively. Decadal erosion magnitude was only moderately positively correlated with a summer temperature reconstruction over the period 900–1900 AD. Magnitude of the seven largest events was similar to the cumulative silt accumulation anomaly, suggesting these events ‘returned the system’ to the long‐term mean rate. Instead, the occurrence of most erosion events was related to fire (silt layers preceded by high charcoal concentration) and earthquakes (the seven thickest layers often match paleo‐earthquake dates). Our data show how internal (i.e. sediment production) and external processes (natural fires or more stochastic events such as earthquakes) co‐determine erosion regimes at millennial time scales, and the extent to which such processes can be offset by recent large‐scale deforestation by logging. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

11.
Previous studies have identified unpaved roads as the primary source of erosion on St John in the US Virgin Islands, but these studies estimated road erosion rates only as annual averages based primarily on road rill measurements. The goal of this project was to quantify the effect of unpaved roads on runoff and sediment production on St John, and to better understand the key controlling factors. To this end runoff and sediment yields were measured from July 1996 to March 1997 from three plots on naturally vegetated hillslopes, four plots on unpaved road surfaces and two cutslope plots. Sediment yields were also measured from seven road segments with contributing areas ranging from 90 to 700 m2. With respect to the vegetated plots, only the two largest storm events generated runoff and there was no measurable sediment yield. Runoff from the road surface plots generally occurred when storm precipitation exceeded 6 mm. Sediment yields from the four road surface plots ranged from 0·9 to 15 kg m−2 a−1, and sediment concentrations were typically 20–80 kg m−3. Differences in runoff between the two cutslope plots were consistent with the difference in upslope contributing area. A sprinkler experiment confirmed that cross‐slope roads intercept shallow subsurface stormflow and convert this into surface runoff. At the road segment scale the estimated sediment yields were 0·1 to 7·4 kg m−2 a−1. Road surface runoff was best predicted by storm precipitation, while sediment yields for at least three of the four road surface plots were significantly correlated with storm rainfall, storm intensity and storm runoff. Sediment yields at the road segment scale were best predicted by road surface area, and sediment yields per unit area were most strongly correlated with road segment slope. The one road segment subjected to heavy traffic and more frequent regrading produced more than twice as much sediment per unit area than comparable segments with no truck traffic. Particle‐size analyses indicate a preferential erosion of fine particles from the road surface and a rapid surface coarsening of new roads. Published in 2001 by John Wiley & Sons, Ltd.  相似文献   

12.
Post‐fire rehabilitation treatments are commonly implemented after high‐severity wildfires, but few data are available about the efficacy of these treatments. This study assessed post‐fire erosion rates and the effectiveness of seeding, straw mulching, and contour felling in reducing erosion after a June 2000 wildfire northwest of Loveland, Colorado. Site characteristics and sediment yields were measured on 12 burned and untreated control plots and 22 burned and treated plots from 2000 to 2003. The size of the hillslope plots ranged from 0·015 to 0·86 ha. Sediment yields varied significantly by treatment and were most closely correlated with the amount of ground cover. On the control plots the mean sediment yield declined from 6–10 Mg ha?1 in the first two years after burning to 1·2 Mg ha?1 in 2002 and 0·7 Mg ha?1 in 2003. Natural regrowth caused the amount of ground cover on the control plots to increase progressively from 33% in fall 2000 to 88% in fall 2003. Seeding had no effect on either the amount of ground cover or sediment yields. Mulching reduced sediment yields by at least 95% relative to the control plots in 2001, 2002, and 2003, and the lower sediment yields are attributed to an immediate increase in the amount of ground cover in the mulched plots. The contour‐felling treatments varied considerably in the quality of installation, and sediment storage capacities ranged from 7 to 32 m3 ha?1. The initial contour‐felling treatment did not reduce sediment yields when subjected to a very large storm event, but sediment yields were significantly reduced by a contour‐felling treatment installed after this large storm. The results indicate that contour felling may be able to store much of the sediment generated in an average year, but will not reduce sediment yields from larger storms. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

13.
Subglacial water flow drives the excavation of a variety of bedrock channels including tunnel valleys and inner gorges. Subglacial floods of various magnitudes – events occurring once per year or less frequently with discharges larger than a few hundred cubic metres per second – are often invoked to explain the erosive power of subglacial water flow. In this study we examine whether subglacial floods are necessary to carve bedrock channels, or if more frequent melt season events (e.g. daily production of meltwater) can explain the formation of substantial bedrock channels over a glacial cycle. We use a one‐dimensional numerical model of bedrock erosion by subglacial meltwater, where water flows through interacting distributed and channelized drainage systems. The shear stresses produced drive bedrock erosion by bed‐ and suspended‐load abrasion. We show that seasonal meltwater discharge can incise an incipient bedrock channel a few tens of centimetres deep and several metres wide, assuming abrasion is the only mechanism of erosion, a particle size of D=256 mm and a prescribed sediment supply per unit width. Using the same sediment characteristics, flood flows yield wider but significantly shallower bedrock channels than seasonal meltwater flows. Furthermore, the smaller the shear stresses produced by a flood, the deeper the bedrock channel. Shear stresses produced by seasonal meltwater are sufficient to readily transport boulders as bedload. Larger flows produce greater shear stresses and the sediment is carried in suspension, which produces fewer contacts with the bed and less erosion. We demonstrate that seasonal meltwater discharge can excavate bedrock volumes commensurate with channels several tens of metres to a few hundred metres wide and several tens of metres deep over several thousand years. Such simulated channels are commensurate with published observations of tunnel valleys and inner gorges. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

14.
Jiongxin Xu 《水文研究》2013,27(18):2623-2636
Fenwei Graben is a famous sediment sink. The Longmen‐Sanmexia sediment sink of middle Yellow River is located in the middle part. Using the sediment budget based on annual data from the period 1920–2006 and flood‐event data from 154 flood events from the period 1950–1985, the variations in sediment storage, release and transport have been analysed. Data from different methods and sources indicate that, during an 1800‐year period, the variation of sedimentation rate in this sink has undergone a cycle from increase to decline; the cause for this can be found in the changes in the manner and intensity of human activities. Over 87 years, sediment storage in this sink can be separated into four stages which showed different trends, depending on changing human activities, such as reservoir construction, soil and water conservation and water diversion. Stepwise multiple regression shows that the runoff and sediment yield from three major source areas have differing influences on sediment storage in the sink. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

15.
In debris‐flow‐prone channels, normal fluvial sediment transport occurs (nearly exclusively in suspended mode) between episodic debris‐flow events. Observations of suspended sediment transport through a winter season in a steepland gully in logged terrain revealed two event types. When flows exceeded a threshold of 270 l s−1, events yielded significant quantities of sediment and suspended sediment concentration increased with flow. Smaller events were strongly ‘supply limited’; sediment concentration decreased as flow increased. Overall, there is no consistent correlation between runoff and sediment yield. Within the season, three subseasons were identified (demarcated by periods of freezing weather) within which a pattern of fine sediment replenishment and evacuation occurred. Finally, a signature of fine sediment mobilization and exhaustion was observed within individual events. Fine sediment transport occurred in discrete pulses within storm periods, most of the yield occurring within 5 to 15% of storm runoff duration, so that it is unlikely that scheduled sampling programs would identify significant transport. Significant events are, however, generally forecastable on the basis of regional heavy rainfall warnings, providing a basis for targeted observations. Radiative snowmelt events and rain‐on‐snow remain difficult to forecast, since the projection of temperatures from the nearest regular weather station yields variable results. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

16.
After the Valley Complex Fire burned 86 000 ha in western Montana in 2000, two studies were conducted to determine the effectiveness of contour‐felled log, straw wattle, and hand‐dug contour trench erosion barriers in mitigating postfire runoff and erosion. Sixteen plots were located across a steep, severely burned slope, with a single barrier installed in 12 plots (four per treatment) and four plots left untreated as controls. In a rainfall‐plus‐inflow simulation, 26 mm h?1 rainfall was applied to each plot for 1 h and 48 L min?1 of overland flow was added for the last 15 min. Total runoff from the contour‐felled log (0·58 mm) and straw wattle (0·40 mm) plots was significantly less than from the control plots (2·0 mm), but the contour trench plots (1·3 mm) showed no difference. The total sediment yield from the straw wattle plots (0·21 Mg ha?1) was significantly less than the control plots (2·2 Mg ha?1); the sediment yields in the contour‐felled log plots (0·58 Mg ha?1) and the contour trench plots (2·5 Mg ha?1) were not significantly different. After the simulations, sediment fences were installed to trap sediment eroded by natural rainfall. During the subsequent 3 years, sediment yields from individual events increased significantly with increasing 10 min maximum intensity and rainfall amounts. High‐intensity rainfall occurred early in the study and the erosion barriers were filled with sediment. There were no significant differences in event or annual sediment yields among treated and control plots. In 2001, the overall mean annual sediment yield was 21 Mg ha?1; this value declined significantly to 0·6 Mg ha?1 in 2002 and 0·2 Mg ha?1 in 2003. The erosion barrier sediment storage used was less than the total available storage capacity; runoff and sediment were observed going over the top and around the ends of the barriers even when the barriers were less than half filled. Published in 2007 by John Wiley & Sons, Ltd.  相似文献   

17.
This study provides data on the fluvial sediment transport at the Eastern Mediterranean, an area in which the regional importance for comparative study has often been raised by investigators but the data are rather scarce. We analysed long‐ and short‐term hydrologic and sedimentological data from one of the largest coastal streams of Israel, Qishon River (1100 km2), and its estuarine environment. The results indicate that during 65 years (1944–2009), a total 140 floods have contributed to the sea an amount of approximately 2.58 × 106 tons of sediment. During this period, (i) the number of floods with a return period of more than 10 years has almost doubled during the last 30 years, and (ii) the mean annual discharge during last 10 years increased by approximately 175%. The analysis of the short (2 years) hydrological and sediment data revealed that approximately 30% of the upstream channel loads do not reach the river mouth and are deposited along the channel bed, even during major flood events. This observation was attributed largely to the facts that the lower river bed is incised below sea level, to the very low slopes and to the correspondingly low stream power and transport capacity. The results of this study highlight the effect of interchannel dynamics as well as the constraints of interaction between fluvial system and estuarine processes on sediment transport. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

18.
Few investigations link post‐fire changes to sediment sources and erosion processes with sediment yield response at the catchment scale. This linkage is essential if downstream impacts on sediment transport after fire are to be understood in the context of fire effects across different forest environments. In this study, we quantify changing source contributions to fine sediment (<63 µm) exported from a eucalypt forest catchment (136 ha) burnt by wildfire. The study catchment is one of a pair of research catchments located in the East Kiewa River valley in southeastern Australia that have been the subject of a research program investigating wildfire effects on runoff, erosion, and catchment sediment/nutrient exports. This previous research provided the opportunity to couple insights gained from a range of measurement techniques with the application of fallout radionuclides 137Cs and 210Pbex to trace sediment sources. It was found that hillslope surface erosion dominated exports throughout the 3·5‐year post‐fire measurement period. During this time there was a pronounced decline in the proportional surface contribution from close to 100% in the first six months to 58% in the fourth year after fire. Over the study period, hillslope surface sources accounted for 93% of the fine sediment yield from the burnt catchment. The largest decline in the hillslope contribution occurred between the first and second years after fire, which corresponded with the previously reported large decline in sediment yield, breakdown of water repellency in burnt soils, substantial reduction in hillslope erodibility, and rapid surface vegetation recovery. Coupling the information on sediment sources with hillslope process measurements indicated that only a small proportion of slopes contributed sediment to the catchment outlet, with material derived from near‐channel areas dominating the post‐fire catchment sediment yield response. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

19.
Rivers of South and Southeast Asia disgorge large suspended sediment loads, reflecting exceptionally high rates of erosion promoted by natural processes (tectonic and climatic) and anthropogenic (land‐use change) activities that are characteristic of the region. While particulate carbon and nitrogen fluxes have been characterized in some large Asian rivers, less is known about the headwater systems where much sediment and organic material are initially mobilized. This study, conducted in the 74‐km2 Mae Sa Experimental Catchment in northern Thailand, shows that the Sa River is an important source for particulate organic carbon (POC) and particulate organic nitrogen (PON) transported to larger river systems and downstream reservoirs. However, the yields during three years of investigation varied greatly: 5.0–22.3 Mg POC km?2 y?1 and 0.48–2.02 Mg PON km?2 y?1. The 22.3 Mg POC km?2 y?1 yield is the highest reported for any river on the Asian continent. Stream samples collected during 12 storms showed that almost 3% of the total suspended solid load is POC 0.7 µm to 2.0 mm in size. This percentage is higher than other values for most large rivers on the continent. Further, we documented a strong pulse hysteretic behaviour in the stream, whereby peak fluxes of POC and PON are often delayed (anticlockwise hysteresis) or accelerated (clockwise hysteresis) relative to stream flow peaks (or are complex), complicating the prediction of storm‐based or annual particulate carbon and nitrogen fluxes. Stream turbidity and total suspended sediment are reasonable proxies for POC and PON concentrations, while stream discharge is not a good predictor variable. Observed C:N ratios for measured particulate samples range from 3 to 83, with the high‐end values likely associated with fresh (non‐decomposed) vegetative material greater than 2 mm in diameter. The C:N ratio (weighted based on three sediment sizes) for 12 events ranges from 7.5 to 15.3. These modest values reflect the relatively low C:N ratios for small size fractions (0.7–0.63 µm) that comprise 50–90% of the TSS load in the events. Overall, organic material <0.63 µm contribute about 75% of the total POC load and 80% of the PON load. The annual C:N ratio for the river is approximately 10–11. Collectively, our findings indicate the occasionally high yields make the Sa River—and potentially other similar headwater rivers—a hot spot for POC and PON transported to downstream water bodies. Complex hysteresis patterns and high year‐to‐year variability hinders our ability to calculate and predict these yields without continuous, automated monitoring of discharge and turbidity. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

20.
This study investigates critical run‐off and sediment production sources in a forested Kasilian watershed located in northern Iran. The Water Erosion Prediction Project (WEPP) watershed model was set up to simulate the run‐off and sediment yields. WEPP was calibrated and validated against measured rainfall–run‐off–sediment data. Results showed that simulated run‐off and sediment yields of the watershed were in agreement with the measured data for the calibration and validation periods. While low and medium values of run‐off and sediment yields were adequately simulated by the WEPP model, high run‐off and sediment yield values were underestimated. Performance of the model was evaluated as very good and satisfactory during the calibration and validation stages, respectively. Total soil erosion and sediment load of the study watershed during the study period were determined to be 10 108 t yr?1 and 8735 t yr?1, respectively. The northern areas of the watershed with dry farming were identified as the critical erosion prone zones. To prioritize the subwatersheds based on their contribution to the run‐off and sediment production at the watershed's main outlet, unit response approach (URA) was applied. In this regard, subwatersheds close to the main outlet were found to have the highest contribution to sediment yield of the whole watershed. Results indicated that depending on the objective of land and water conservation practices, particularly, for controlling sediment yield at the main outlet, critical areas for implementing the best management practices may be identified through conjunctive application of WEPP and URA. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

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