The effect of forestry drainage operations on upland sediment yields: A study of two peat-covered catchments

The suspended sediment yields of two adjacent, paired, catchments on blanket peat in mid-Wales were compared before and after preafforestation drainage works. Catchment A was ploughed whilst an adjacent catchment (B) was left unploughed. After eight months Catchment B was also ploughed. The ploughing strategy in both catchments was designed to minimize sediment loss. However, suspended sediment loss increased 2·5 times following ploughing in Catchment A, whilst no such increase occurred at this time in Catchment B. Subsequently, suspended sediment yields in Catchment B increased 4·8 times after it was ploughed. Organic sediment was lost mainly from furrow sides, and erosion pins showed maximum ground recession in summer, due to peat wastage. Summer desiccation prepared sediment for transport, and organic suspended sediment loads were highest in the autumn. Sediment loads were limited by vegetation colonization and some reduction in sediment loss was due to the presence of unploughed strips adjacent to stream courses.     

The hydrology of glacier-bed overdeepenings: Sediment transport mechanics,drainage system morphology,and geomorphological implications

Evacuation of basal sediment by subglacial drainage is an important mediator of rates of glacial erosion and glacier flow. Glacial erosion patterns can produce closed basins (i.e., overdeepenings) in glacier beds, thereby introducing adverse bed gradients that are hypothesized to reduce drainage system efficiency and thus favour basal sediment accumulation. To establish how the presence of a terminal overdeepening might mediate seasonal drainage system evolution and glacial sediment export, we measured suspended sediment transport from Findelengletscher, Switzerland during late August and early September 2016. Analyses of these data demonstrate poor hydraulic efficiency of drainage pathways in the terminus region but high sediment availability. Specifically, the rate of increase of sediment concentration with discharge was found to be significantly lower than that anticipated if channelized flow paths were present. Sediment availability to these flow paths was also higher than would be anticipated for discrete bedrock-floored subglacial channels. Our findings indicate that subglacial drainage in the terminal region of Findelengletscher is dominated by distributed flow where entrainment capacity increases only marginally with discharge, but flow has extensive access to an abundant sediment store. This high availability maintains sediment connectivity between the glacial and proglacial realm and means daily sediment yield is unusually high relative to yields exhibited by similar Alpine glaciers. We present a conceptual model illustrating the potential influence of ice-bed morphology on subglacial drainage evolution and sediment evacuation mechanics, patterns and yields, and recommend that bed morphology should be an explicit consideration when monitoring and evaluating glaciated basin sediment export rates.     

Twentieth century agricultural drainage creates more erosive rivers

Rivers in watersheds dominated by agriculture throughout the US are impaired by excess sediment, a significant portion of which comes from non‐field, near‐channel sources. Both land‐use and climate have been implicated in altering river flows and thereby increasing stream‐channel erosion and sediment loading. In the wetland‐rich landscapes of the upper Mississippi basin, 20th century crop conversions have led to an intensification of artificial drainage, which is now a critical component of modern agriculture. At the same time, much of the region has experienced increased annual rainfall. Uncertainty in separating these drivers of streamflow fuels debate between agricultural and environmental interests on responsibility and solutions for excess riverine sediment. To disentangle the effects of climate and land‐use, we compared changes in precipitation, crop conversions, and extent of drained depressional area in 21 Minnesota watersheds over the past 70 years. Watersheds with large land‐use changes had increases in seasonal and annual water yields of >50% since 1940. On average, changes in precipitation and crop evapotranspiration explained less than one‐half of the increase, with the remainder highly correlated with artificial drainage and loss of depressional areas. Rivers with increased flow have experienced channel widening of 10–40% highlighting a source of sediment seldom addressed by agricultural best management practices. Copyright © 2013 John Wiley & Sons, Ltd.     

Factors affecting connectivity and sediment yields following wildfire and post-fire salvage logging in California's Sierra Nevada

Sediment delivery following post-fire logging is a concern relative to water quality. While studies have assessed the effect of post-fire logging on sediment yields at different spatial scales, none have explicitly identified sediment sources. Our goal was to quantify post-fire and post-salvage logging sediment yields and use rill patterns to identify sediment sources. We measured the extent and type of logging disturbance, length of rills per unit area or “rill density”, ground cover, and sediment yields in nine logged and five control small catchments or “swales”, 0.09 to 0.81 ha, for 5 years after the 2013 Rim Fire in California's Sierra Nevada. The logged swales had a mean ground disturbance of 31%. After the first wet season following logging, there was no difference in either mean rill density (0.071 and 0.088 m m⁻², respectively) or mean transformed, normalized sediment yields between the control and logged swales. Untransformed mean sediment yields across three sites ranged from 0.11–11.8 and 1.1–3.2 Mg ha⁻¹ for the controls and salvage-logged swales, respectively. Rill density was strongly related to sediment yield and increased significantly with the amount of high-traffic skid trail disturbance in logged swales. Rill density was not significantly related to the amount of bare soil despite a significant relationship between sediment yields and bare soil. Rills usually initiated in bare soil and frequently connected high traffic skid trails to the drainage network after being diverted by waterbars. Rill connectivity and sediment yields decreased in control and logged swales where vegetation or other surface cover was high, suggesting this cover disconnected rills from the drainage network. Increasing ground cover on skid trails and between areas disturbed by post-fire logging and stream channels may reduce sediment yields as well as the hydrologic connectivity between hillslopes and the drainage network.     

Interpreting sediment yield scaling

Terrestrial sediment yield – often measured as suspended sediment load in stream channels – commonly scales with drainage area within homogeneous land surface regions. But the effect of drainage area has not usually been recognized in comparative sediment yield analyses, rendering most comparisons of sediment yield from disparate source areas invalid. The procedure to discount scale differences for comparative purposes is presented. Mathematical scaling varies according to landscape condition and provides a physical interpretation of that condition. The results open the way for rational construction of a ‘sediment delivery ratio’. Copyright © 2017 John Wiley & Sons, Ltd.     

Runoff and sediment generation on bench‐terraced hillsides: measurements and up‐scaling of a field‐based model

Despite widespread bench‐terracing, stream sediment yields from agricultural hillsides in upland West Java remain high. We studied the causes of this lack of effect by combining measurements at different spatial scales using an erosion process model. Event runoff and sediment yield from two 4‐ha terraced hillside subcatchments were measured and field surveys of land use, bench‐terrace geometry and storage of sediment in the drainage network were conducted for two consecutive years. Runoff was 3·0–3·9% of rainfall and sediment yield was 11–30 t ha⁻¹ yr⁻¹ for different years, subcatchments and calculation techniques. Sediment storage changes in the subcatchment drainage network were less than 2 t ha⁻¹, whereas an additional 0·3–1·5 t ha⁻¹ was stored in the gully between the subcatchment flumes and the main stream. This suggests mean annual sediment delivery ratios of 86–125%, or 80–104% if this additional storage is included. The Terrace Erosion and Sediment Transport (TEST) model developed and validated for the studied environment was parameterized using erosion plot studies, land use surveys and digital terrain analysis to simulate runoff and sediment generation on the terraced hillsides. This resulted in over‐estimates of runoff and under‐estimates of runoff sediment concentration. Relatively poor model performance was attributed to sample bias in the six erosion plots used for model calibration and unaccounted covariance between important terrain attributes such as slope, infiltration capacity, soil conservation works and vegetation cover. Copyright © 2005 John Wiley & Sons, Ltd.     

Fluvial suspended sediment transport and mechanical erosionin the Maghreb (North Africa)

Abstract

Available data on suspended sediment transported by rivers in the Maghreb are reviewed for 130 drainage basins. These data allow a new estimate to be proposed for the delivery of river sediment to both the Atlantic Ocean and the Mediterranean Sea from the Maghreb region. The influences of several environmental factors (precipitation, runoff, drainage area size and lithology) on mechanical erosion and fluvial sediment transport are analysed. Finally, a multiple regression model is proposed to estimate the river sediment yields in the Maghreb.     

Development of a SWAT extension module to simulate riparian wetland hydrologic processes at a watershed scale

Using a mass balance algorithm, this study develops an extension module that can be embedded in the commonly used Soil and Water Assessment Tool (SWAT). This module makes it possible to assess effects of riparian wetlands on runoff and sediment yields at a watershed scale, which is very important for aquatic ecosystem management but rarely documented in the literature. In addition to delineating boundaries of a watershed and its subwatersheds, the module groups riparian wetlands within a subwatershed into an equivalent wetland for modelling purposes. Further, the module has functions to compute upland drainage area and other parameters (e.g. maximum volume) for the equivalent wetland based on digital elevation model, stream network, land use, soil and wetland distribution GIS datasets. SWAT is used to estimate and route runoff and sediment generated from upland drainage area. The lateral exchange processes between riparian wetlands and their hydraulically connected streams are simulated by the extension module. The developed module is empirically applied to the 53 km² Upper Canagagigue Creek watershed located in Southern Ontario of Canada. The simulation results indicate that the module can make SWAT more reasonably predict flow and sediment loads at the outlet of the watershed and better represent the hydrologic processes within it. The simulation is sensitive to errors of wetland parameters and channel geometry. The approach of embedding the module into SWAT enables simulation of hydrologic processes in riparian wetlands, evaluation of wetland effects on regulating stream flow and sediment loading and assessment of various wetland restoration scenarios. Copyright © 2008 John Wiley & Sons, Ltd.     

Distributed empirical algorithms to estimate catchment scale sediment connectivity and yield in a subtropical region

The intensity of soil loss and sediment delivery, representing hydrologic and geomorphic processes within a catchment, accelerates with rapid changes in land cover and rainfall events. An underlying component of sustainable management of water resources is an understanding of spatial and temporal variability and the adverse influences of regional parameters involved in generating sediment following widespread changes in land cover. A calibrated algorithm of soil loss coupled with a sediment delivery ratio (SDR) was applied in raster data layers to improve the capability of a combined model to estimate annual variability in sediment yields related to changes in vegetation cover identified by analyses of SPOT imagery. Four catchments in Kangaroo River State forest were assessed for annual changes in sediment yields. Two catchments were selectively logged in 2007, while the two other sites remained undisturbed. Results of SDR estimates indicated that only a small proportion of total eroded sediment from hillslopes is transported to catchment outlets. Larger SDR values were estimated in regions close to catchment outlets, and the SDR reduced sharply on hillslopes further than 200–300 m from these areas. Estimated sediment yield increased by up to 30% two years after land cover change (logging) in 2009 when more storm events were recorded, despite the moderate density of vegetation cover in 2009 having almost recovered to its initial pre‐logging (2005) condition. Rainfall had the most significant influence on streamflow and sediment delivery in all catchments, with steeply sloping areas contributing large amounts of sediment during moderate and high rainfall years in 2007 and 2009. It is concluded that the current scenario of single‐tree selection logging utilized in the study area is an acceptable and environmentally sound land management strategy for preservation of soil and water resources. Copyright © 2013 John Wiley & Sons, Ltd.     

Making connections: changing sediment sources and sinks in an upland catchment

The effect of changes in catchment processes and conditions can be studied by using connectivity as a framework for understanding the feedbacks and interactions occurring within the system. The sediment record preserved in reservoirs can be a useful archive of catchment changes, but needs to be considered in conjunction with the different elements that compose and act on the system to take into account its complexity. Changing patterns of connectivity have been studied in the Ingbirchworth Catchment (Yorkshire, UK), using a multiple methodology approach combining the analysis of reservoir‐sediment records with knowledge of recent land‐use history, high resolution rainfall records, catchment characteristics and management aspects. Sedimentation rates inferred from reservoir‐sediment cores from two reservoirs in the Ingbirchworth catchment show sedimentation peaks which coincide with periods of significant changes in the catchment, such as the introduction of arable crops, the establishment of land drainage and the widespread intensification and mechanization of agriculture. Rainfall patterns, including combinations of events such as droughts and increased precipitation, contribute to increased sediment transfer under catchment conditions in which more sediment and/or new pathways are made available due to catchment changes. Sediment fingerprinting supports the notion that changes in sedimentation rates are not just related to increased/reduced erosion and transport in the same areas, but also to the establishment of different pathways increasing sediment connectivity. The results demonstrate that typical calculations of catchment‐area yields are not sufficient as sediment‐contributing areas vary as a consequence of changing conditions. The study provides insights into the complex interactions influencing connectivity, such as the relation between catchment changes and climatic inputs, and the subsequent effect on catchment conditions and transfer networks. Copyright © 2011 John Wiley & Sons, Ltd.     

Hillslope sediment production after wildfire and post-fire forest management in northern California

High severity wildfires impact hillslope processes, including infiltration, runoff, erosion, and sediment delivery to streams. Wildfire effects on these processes can impair vegetation recovery, producing impacts on headwater and downstream water supplies. To promote forest regeneration and maintain forest and aquatic ecosystem functions, land managers often undertake active post-fire land management (e.g., salvage logging, sub-soiling, re-vegetation). The primary objective of our study was to quantify and compare sediment yields eroded from (a) burned, (b) burned and salvage logged, and (c) burned, salvage logged, and sub-soiled plots following the 2015 Valley Fire in the northern California Coast Range. We distributed 25 sediment fences (~75 m² contributing area) across four hillslopes burned at high severity and representative of the three management types. We collected eroded sediment from the fences after precipitation events for 5 years. We also quantified precipitation, canopy cover, ground cover, and soil properties to characterize the processes driving erosion across the three management types. Interestingly, during the second year after the fire, sediment yields were greater in the burned-only plots compared with both the salvage logged and sub-soiled plots. By the third year, there were no differences in sediment yields among the three management types. Sediment yields decreased over the 5 years of the study, which may have occurred due to site recovery or exhaustion of mobile sediment. As expected, sediment yields were positively related to precipitation depth, bulk density, and exposed bare soil, and negatively related to the presence of wood cover on the soil surface. Unexpectedly, we observed greater sediment yields on the burned-only plots with greater canopy closure, which we attributed to increased throughfall drop size and kinetic energy related to the residual canopy. While these results will aid post-fire management decisions in areas with Mediterranean climates prone to low intensity, long duration rainstorms, additional research is needed on the comparative effects of post-fire land management approaches to improve our understanding of the mechanisms driving post-fire erosion and sediment delivery.     

Changing Contributions of Suspended Sediment Sources in Small Basins Resulting from European Settlement on the Canadian Prairies

Lake sediments provide an integrated record of the sediment yields and sources in the contributing basin. In the research area on the prairies of western Canada, the earliest sediments deposited in the larger lakes predate European settlement, allowing direct evaluation of basin response to settlement. Lake sediment cores were collected from an unnamed lake in the Stony Creek drainage basin in the aspen parkland region of eastern Saskatchewan. Pre- and post-settlement sediments in a central core were separated on the basis of an increase in Populus pollen associated with the southward advance of the aspen parkland ecotone caused by fire suppression following settlement. A wet chemical extraction procedure was used to separate the operationally defined organic fraction, the acid-soluble authigenic fraction, and biogenic silica from the clastic, non-carbonate, allogenic fraction of the lake sediment. Changes in the mineralogy and geochemistry of the clastic, allogenic fraction indicate that settlement resulted in an increased contribution of topsoil to the sediment load of Stony Creek. Elemental ratios, however, show that topsoil did contribute to the allogenic lake sediment fraction prior to settlement. Post-settlement changes in deposition rates of the allogenic fraction resulted from changes in land use rather than from climatic variability. Allogenic deposition rates reached a maximum in the 1950s and 1960s owing to an increase in the area under field crops and the increased use of high-powered agricultural machinery. Allogenic deposition rates decreased in more recent years because of a more extensive application of soil conservation measures. Post-settlement changes in deposition rates of individual elements within the allogenic fraction indicate that various sediment sources respond differently to changes in land use. Over the most recent 100 years, since the onset of European settlement, the erosional response of the basin appears to be controlled by land use changes rather than by climatic variability. © 1997 John Wiley & Sons, Ltd.     

Gravel sediment routing from widespread,low- intensity landscape disturbance,Current River Basin,Missouri

During the last 160 years, land-use changes in the Ozarks have had the potential to cause widespread, low-intensity delivery of excess amounts of gravel-sized sediment to stream channels. Previous studies have indicated that this excess gravel bedload is moving in wave-like forms through Ozarks drainage basins. The longitudinal, areal distribution of gravel bars along 160 km of the Current River, Missouri, was evaluated to determine the relative effects of valley-scale controls, tributary basin characteristics, and lagged sediment transport in creating areas of gravel accumulations. The longitudinal distribution of gravel-bar area shows a broad scale wave-like form with increases in gravel-bar area weakly associated with tributary junctions. Secondary peaks of gravel area with 1·8–4·1 km spacing (disturbance reaches) are superimposed on the broad form. Variations in valley width explain some, but not all, of the short-spacing variation in gravel-bar area. Among variables describing tributary drainage basin morphometry, present-day land use and geologic characteristics, only drainage area and road density relate even weakly to gravel-bar areal inventories. A simple, channel network-based sediment routing model shows that many of the features of the observed longitudinal gravel distribution can be replicated by uniform transport of sediment from widespread disturbances through a channel network. These results indicate that lagged sediment transport may have a dominant effect on the synoptic spatial distribution of gravel in Ozarks streams; present-day land uses are only weakly associated with present-day gravel inventories; and valley-scale characteristics have secondary controls on gravel accumulations in disturbance reaches. Copyright © 1999 John Wiley & Sons, Ltd.     

Spatial analysis of bacteria in brackish lake sediment

The spatial distribution of bacterial communities inhabiting sediment is heterogeneous at different spatial scales,but mostly unexplored.Here,it is postulated that the heterogeneity of the bacterial community composition varies at the same scale of the heterogeneity of sediment chemical properties.The large spatial scale(km) diversity in sediment from a brackish water lagoon(Chilika Lake,India) is studied,considering the large scale physical and chemical characteristics of land cover,climate,pH,and salinity.Seventy-two samples(24 stations,3 seasons:winter,rainy,and summer) of sediment from Chilika Lake were analyzed by 16 S ribosomal ribonucleic acid(rRNA) gene sequencing,and the relations with land cover and other physico-chemical paramete rs are discussed.Mo re samples were collected after a severe cyclonic storm(Phailin) passed near the lagoon in 2013,to see the impact of the tropical storm on the spatial and tempo ral distribution of bacteria in the sediment.The results demonstrate clear spatial relations between physico-chemical parameters(e.g.,salinity),land surfaces(e.g.,drainage area),and the distribution of sediment microbial communities.     

Temporal variation of suspended sediment transport in the Koga catchment,North Western Ethiopia and environmental implications

Event sediment transport and yield were studied for 45 events in the upstream part of the 260 km² agricultural Koga catchment that drains to an irrigation reservoir. Discharge and turbidity data were collected over a period of more than a year, accompanied by grab sampling. Turbidity was very well correlated with the sediment concentrations from the samples (r = 0.99), which allowed us to estimate the temporal patterns of sediment concentrations within events. The hysteresis patterns between discharge and sediment concentrations were analysed to provide insight into the different sediment sources. Anticlockwise patterns are the dominant hysteresis patterns in the area, suggesting smaller contributions of suspended sediment from the river channels than from the hillslopes and agricultural areas. Complicated types of hysteresis patterns were mostly observed for long events with multiple peaks. For a given discharge, sediment yields in August and September, when the catchment was almost completely covered with vegetation, were much smaller than during the rest of the rainy season. The hysteresis patterns and timing suggest that the sediment availability from the agricultural areas and hillslopes affects sediment yields more strongly than does peak discharge. Two distinct types of sediment rating curves were observed for the season when the agricultural land was covered with vegetation and when it was not, indicating the dominating contribution of land use/cover to sediment yields in the catchment. The rate of suspended sediment transport in the area was estimated as 25.6 t year^?1 ha^?1. Copyright © 2013 John Wiley & Sons, Ltd.     

Nutrients accumulation in drainage channel sediments

The drainage channel network in Vojvodina,northern part of the Republic of Serbia,in total length of around 20,000 km,transfers excessive(under)ground waters from around 2.15 million ha of lowlands.Channels are mostly in direct connection with the surrounding arable agricultural land and are exposed to different run-off,leaching and/or wind erosion processes.Close to urban areas,some channel sections serve as recipients of unrefined sewage and industrial waste waters.Water flows and velocities,as well as the transportable capacity of fluvial materials(sediments) are relatively low.This,in combination with other natural and anthropogenic impacts,contributes to sediment generation in the drainage channel network.Based on around 100 sediment samples from 46 channels,concentrations of primary nutrients(N,P and K) were elaborated in this study.Detected concentrations of macronutrients in the channel sediments(e.g.N 1-1.2%,P 100-265 and K 100-380 mg 100 g~(-1)) exceeded their content in surrounding arable land by a few fold.Also,significantly higher nutrient concentrations(in average by 50%) were detected in downstream(vs.upstream) channel sections.An excessive presence of observed elements in channel sediments,due to interactive processes between water and sediment material,can adversely influence the water quality and life conditions for channel biota and caused other negative environmental impacts such as eutrophication.These results clearly confirm that the processes of nutrient accumulation in channel sediments are greater than those in the surrounding,mostly intensively arable land areas.The erosion of unprotected agricultural areas and sediment transport as the most important pollution pathways from the drainage basin to channel network may be essential factors responsible for detected condition of nutrient accumulation.     

Sediment routing analyses based on chronological changes in hillslope and riverbed morphologies

Recent studies of sediment delivery and budgets in the United States indicate that upland erosion rates at a given time may not explain contemporaneous sediment yields from a drainage basin. This suggests temporal discontinuities in sediment delivery associated with hillslope and channel storage processes. Integration of sediment production, storage and transport is essential to understand sediment routing in basins. We analysed each process chronologically using aerial photographs, monitoring data of sediment movement and annual tree-rings, and then compared estimated temporal changes in sediment production from hillslopes, floodplain disturbance areas and sediment transport in river channels. Toeslopes, floodplains and alluvial fans together contained 59 per cent of sediment eroded from uplands over the last 30 years. Monitoring results of riverbed changes showed that the volume of stored sediment on floodplains decreased exponentially with succeeding floods. The age distribution of floodplain deposits reflected the disturbance history of a river channel, and followed an exponential decrease with age. The results of this study may have important implications for sediment control plans for watersheds in steep regions.     

Spatial patterns of suspended sediment yields in a humid tropical watershed in Costa Rica

An Erratum has been published for this article in Hydrological Processes 16(5) 2002, 1130–1131. Humid tropical regions are often characterized by extreme variability of fluvial processes. The Rio Terraba drains the largest river basin, covering 4767 km², in Costa Rica. Mean annual rainfall is 3139±419_sd mm and mean annual discharge is 2168±492_sd mm (1971–88). Loss of forest cover, high rainfall erosivity and geomorphologic instability all have led to considerable degradation of soil and water resources at local to basin scales. Parametric and non‐parametric statistical methods were used to estimate sediment yields. In the Terraba basin, sediment yields per unit area increase from the headwaters to the basin mouth, and the trend is generally robust towards choice of methods (parametric and LOESS) used. This is in contrast to a general view that deposition typically exceeds sediment delivery with increase in basin size. The specific sediment yield increases from 112±11·4_sd t km^?2 year^?1 (at 317·9 km² on a major headwater tributary) to 404±141·7_sd t km^?2 year^?1 (at 4766·7 km²) at the basin mouth (1971–92). The analyses of relationships between sediment yields and basin parameters for the Terraba sub‐basins and for a total of 29 basins all over Costa Rica indicate a strong land use effect related to intensive agriculture besides hydro‐climatology. The best explanation for the observed pattern in the Terraba basin is a combined spatial pattern of land use and rainfall erosivity. These were integrated in a soil erosion index that is related to the observed patterns of sediment yield. Estimated sediment delivery ratios increase with basin area. Intensive agriculture in lower‐lying alluvial fans exposed to highly erosive rainfall contributes a large part of the sediment load. The higher elevation regions, although steep in slope, largely remain under forest, pasture, or tree‐crops. High rainfall erosivity (>7400 MJ mm ha^?1 h^?1 year ^?1) is associated with land uses that provide inadequate soil protection. It is also associated with steep, unstable slopes near the basin mouth. Improvements in land use and soil management in the lower‐lying regions exposed to highly erosive rainfall are recommended, and are especially important to basins in which sediment delivery ratio increases downstream with increasing basin area. Copyright © 2001 John Wiley & Sons, Ltd.     

Reach‐scale contributions of road‐surface sediment to the Honna River,Haida Gwaii,BC

Gravel road surfaces can be a major source of fine sediment to streams, yet their contribution to channel reach sediment balances remains poorly documented. To quantify the input of road surface material and to compare this input with natural sediment sources at the reach scale, suspended sediment dynamics was examined and a 16‐month sediment balance was developed for a ~35 channel‐width (approx. 425 m) reach of the Honna River, a medium‐size, road‐affected stream located in coastal British Columbia. Of the 105 ± 33 t of suspended material passing through the reach, 18 ± 6% was attributed to the road surface. The high availability of sediment on the road surface appears to limit hysteresis in road run‐off. During rainstorms that increase streamflow, road surface material composed 0.5–15% of sediment inputs during relatively dry conditions from April to the end of September and 5–70% through wetter conditions from October to the end of March, but our data do not show evidence of major sediment accumulation on the riverbed in the reach. A comparison of modelled sediment production on the road surface with observed yields from drainage channels suggests that (1) during low intensity rainfall, ditches and drainage channels may trap sediment from road run‐off, which is subsequently released during events of greater intensity, and/or (2) production models do not effectively describe processes, such as deposition or erosion of sediment in ditches, which control sediment transport and delivery. Our findings further emphasize the risk of unpaved roads in polluting river systems and highlight the continued need for careful road design and location away from sensitive aquatic environments. Copyright © 2016 John Wiley & Sons, Ltd.     

Thresholds of geographic environmental elements in sediment yield of drainage basins

Threshold of environmental elements in drainage basin sediment yield refers to, under effect of climate, underlying surface and human activity, a turning point of abrupt changes in drainage sediment yield related to environmental element characteristics. Previous studies on threshold of sediment yield of relevant drainage basins were mainly concentrated on impact of natural zones with a few researches on impact of other environmental elements. Particularly studies on compound environmental element threshold in drainage basin sediment yield remain blank today. Studies indicate that sediment yield in drainage basins is affected by compound interactions and complex actions. Based on single element analysis, the present paper gives quantitatively compound threshold of environmental elements affecting sediment yield of the drainage basin between Hekouzhen and Tongguan in the middle Yellow River by the method of multi-variant, polynomial formula regression analysis.