Temporal Dynamics of Sediment Transport and Transient In‐channel Storage in a Highly Erodible Catchment

Elucidating the links between catchment and channel geomorphic processes helps the understanding of landscape evolution and the geomorphic development of river basins, and then the land degradation processes. In this study, we analyse suspended sediment dynamics and its relationship with rainfall, discharge and in‐channel sediment storage in a highly dynamic Mediterranean montane catchment (the River Isáben, Southern Pyrenees, NE Iberian Peninsula). The aim is to assess hydrological controls on and temporal patterns of the suspended sediment load. High loads in this basin occur as a consequence of intense erosion in badlands located in the middle of the catchment. This study focuses on a reach located downstream from the main badland areas, where rainfall, discharge (Q), sediment transport (SSC) and in‐channel sediment storage were monitored for a 1‐year period. Marked seasonality in water and sediment load was observed; whereas most of the discharge occurred in winter and spring, most of the sediment was transported during summer and autumn. Q‐SSC hysteretic loops revealed the importance of sediment availability (whether stored in the channel or originating from wider catchment source areas) in the river's sedimentary response. Clockwise loops dominated during winter and spring, whereas counter‐clockwise loops occurred mostly in summer and autumn, when in‐channel storage reached its maximum. There were significant correlations between rainfall intensity in the sediment source areas, in‐channel sediment storage and sediment yield. These correlations emphasize the importance of understanding sediment availability when analysing the temporal dynamics of sediment transport, especially in catchments where different source areas (slopes and riverbed) may contribute to the load to differing degrees and at different times of the year. Copyright © 2015 John Wiley & Sons, Ltd.     

Modelling Discharge and Sediment Yield at Catchment Scale Using Connectivity Components

Knowledge about connectivity and what affects it, through space and time, is needed for taking appropriate action at the right place and/or time by stakeholders. Various conceptual frameworks for hydrological and sediment connectivity have been developed in recent years. For most of these frameworks, the objective was to conceptualise connectivity, not necessarily to infer it from measurements. Studies focussing on measurements of connectivity have so far not been done often. Because of lack of data on connectivity, few real‐world data have been used in recent connectivity modelling studies. The aim of this study was to demonstrate that existing data can be used to assess governing factors of connectivity, and how these change over time. Data from three catchments in Navarre, Northern Spain, were used to assess factors that influence hydrologic and sediment connectivity. These factors were used as components in a linear model for discharge and suspended‐sediment yield. Three components of connectivity were distinguished: topographical, biological and soil. Changes in the topographical component for the studied periods were considered relatively small, and, therefore, kept constant. Changes in the biological component were determined using the Normalised Difference Vegetation Index. Changes in the soil component were assessed using an Antecedent Precipitation Index. Nash–Sutcliffe model efficiency coefficients were between 0·49 through 0·62 for the discharge models and between 0·23 through 0·3 for the sediment‐yield models. We recommend applying the model at smaller spatial scales than catchment scale to minimise the lumping of spatial variability in the components. Copyright © 2016 John Wiley & Sons, Ltd.     

Spatial variation and interaction of runoff generation and erosion within a semi-arid, complex terrain catchment: a hierarchical approach

Purpose

Closed erosion plots have been used extensively to investigate soil loss and its spatial variation within a watershed. However, erosion rates measured on closed plots at various locations within a watershed may not reflect the “real world” conditions due to plot boundary problems. The purpose of this study was to identify runoff and sediment sources in a semi-arid, complex terrain catchment by using the data collected from open plots, nested catchments, and tunnel systems.

Materials and methods

The study catchment, in the Loess Plateau of China, was partitioned into various-level geomorphic units. Runoff and sediment discharges were measured from 55 storm events between 1963 and 1968 on open plots and nested catchments. Storm flows were also monitored in 14 rainfall events from the tunnel systems between 1989 and 1990. This study combined the data collected from the two periods to investigate runoff and sediment sources from the different geomorphic units of the catchment.

Results and discussion

On the four open plots (S1, S2, S3, and S4) of the hill slope, total runoff depths of 128.5 mm (S1), 84.3 mm (S2), 101.92 mm (S3), and 141.73 mm (S4) were recorded from all the events over the first period, which correspondingly produced total sediment yields of 3.056 kg m^?2 (S1), 9.058 kg m^?2 (S1), 42.848 kg m^?2 (S3), and 97.256 kg m^?2 (S4). The number of runoff events also varied due to a non-uniformity in runoff generation among the different geomorphic units of the catchment. Tunnel flows generally had higher mean sediment concentrations than catchment outflows. Three nested catchments located from the headwaters (C1) to the mouth of the catchment (C3) generated total runoff depths of 120.02 mm (C1), 143.92 mm (C2), and 149.43 mm (C3), and correspondingly produced sediments yields of 62.01 kg m^?2 (C1), 144.02 kg m^?2 (C2), and 123.92 kg m^?2 (C3) for the first period.

Conclusions

Significant variations in runoff and erosion existed within the catchment. The spatial variation of runoff generation on the hill slopes resulted from the variation of soil infiltration. Sediment produced from the lower hill slope zone was disproportionally higher than that from the upper hill slope zone. Nevertheless, a significant portion of the sediment eroded on the lower slope zone was caused by runoff generated from the upper slope zone. Tunnel erosion also played a significant role in sediment production.     

Evaluation of the Effectiveness of Forest Restoration and Check‐Dams to Reduce Catchment Sediment Yield

Reforestations and check‐dams are two commonly used measures to reduce soil erosion rates and sediment export from highly eroding catchments. Here, we evaluated the impact of the construction of 94 check‐dams and land use changes (caused by agricultural abandonment and reforestations) on sediment yield in the Upper Taibilla catchment (320 km², SE Spain) from 1956 to 2000. We combined land use change analysis, field surveys, and application of the WaTEM‐SEDEM erosion and sediment yield prediction model for nine scenarios combining land use maps (1956, 1987, and 2000) and different numbers of check‐dams throughout the catchment. Land use changes alone reduced sediment yield up to 14%, but in combination with check‐dams, the reduction in sediment yield reached 44 ± 6%. Sediment yield reduction was higher in smaller sub‐catchments, with, on average, a higher transport capacity than larger catchments, illustrating the scale dependency of human impacts on sediment fluxes and the buffer capacity of larger catchments. From an economical perspective, the construction of check‐dams was estimated to be more expensive than reforestation programs in the studied catchment, while adding more check‐dams did not always result in a proportional reduction of sediment yield. This indicates that optimizing check‐dam distribution relative to land use patterns is crucial to decrease catchment sediment yield. Check‐dams have a large and instantaneous impact on sediment yield over a restricted time period, while reforestations have important sustained effects at a lower economic cost. These contrasting effects require a careful evaluation for optimal effective catchment management. Copyright © 2014 John Wiley & Sons, Ltd.     

Comparison of the Suspended Sediment Dynamics in Two Loess Plateau Catchments,China

Changes in runoff and sediment loads are of great importance for the management of river basins and the implementation of soil and water conservation measures. This study compared the suspended sediment dynamics in the Huangfuchuan and Yanhe catchments on the Loess Plateau. Both annual runoff and sediment load displayed significant reductions from 1955 to 2012. The decreasing rates were −0·88 mm a⁻¹ and −2·72 Mg ha⁻¹ a⁻¹ in the Huangfuchuan catchment, respectively, and ‐0.31 mm a⁻¹ and −1·20 Mg ha⁻¹ a⁻¹ in the Yanhe catchment. A total of 183 and 195 events, respectively, were selected to assess the suspended sediment dynamics in both catchments during the periods of 1971–1989 and 2006–2012. The results showed a good linear relationship between the sediment yield and runoff depth in both catchments from 1971 to 1989 and a relatively worse relationship in the Yanhe catchment from 2006 to 2012. The magnitude and frequency of the hyper‐concentrated sediment flow obviously decreased in the 2000s compared with that between 1971 and 1989. A hysteresis analysis suggested that complex and counter‐clockwise loops were the dominant patterns. Various soil and water conservation measures (e.g., afforestation, grassing, terraces, and check dams) played a critical role in runoff and sediment load changes in both catchments. The two catchments showed obvious heterogeneities in runoff and sediment yield because of different lithologies, soil types, and vegetation. The results of this study provide valuable information on suspended sediment dynamics and could be used to improve soil erosion control measures on the Loess Plateau. Copyright © 2016 John Wiley & Sons, Ltd.     

A strategy for fine sediment retention with bioengineering works in eroded marly catchments in a mountainous mediterranean climate (Southern ALPS,France)

In eroded catchments in mountainous and Mediterranean‐type ecosystems, erosion can be responsible for high yields of fine sediment at the exit of catchments, which can cause different types of damage such as reservoir silting or ecological pollution. To avoid this sediment transport, low‐cost rehabilitation actions such as involving bioengineering techniques within small gullies can be used to trap and retain eroded materials before they reach the catchment exit. Based on current results of research carried out in the French Southern Alps, a specific strategy for such interventions is proposed, with rules for bioengineering structure use and methods for determining priority actions. This strategy for fine sediment retention with bioengineering works is currently being applied on eroded marly catchments in the French Southern Alps, in a mountainous Mediterranean climate. Copyright © 2009 John Wiley & Sons, Ltd.     

Relative Contribution of Rill/Interrill and Gully/Channel Erosion to Small Reservoir Siltation in Mediterranean Environments

Reservoir siltation because of water erosion is an important environmental issue in Mediterranean countries where storage of clear surface water is crucial for their economic and agricultural development. The high density of gully systems observed in Mediterranean regions raises the question of their contribution to reservoir siltation. In this context, this study quantified the absolute and relative contributions of rill/interrill and gully/channel erosion in sediment accumulation at the outlet of small Tunisian catchments (0·1–10 km²) during the last 15 years (1995–2010). To this end, a fingerprinting method based on measurements of caesium‐137 and total organic carbon combined with long‐term field monitoring of catchment sediment yield was applied to five catchments in order to cover the diversity of environmental conditions found along the Tunisian Ridge and in the Cape Bon region. Results showed the very large variability of erosion processes among the selected catchments, with rill/interrill erosion contributions to sediment accumulated in outlet reservoirs ranging from 20 to 80%. Overall, rill/interrill erosion was the dominant process controlling reservoir siltation in three catchments whereas gully/channel erosion dominated in the other two catchments. We identified the presence of marly gypsum substrates and the proportion of catchment surface covered by soil management/conservation measures as the main drivers of erosion process variability at the catchment scale. These results provided a sound basis to propose guidelines for erosion mitigation in these Mediterranean environments and suggested to apply models simulating both rill/interrill and gully/channel erosion in catchments of the region. Copyright © 2015 John Wiley & Sons, Ltd.     

Tracing Sediment Sources Using Mid‐infrared Spectroscopy in Arvorezinha Catchment,Southern Brazil

The widespread adoption of the sediment fingerprinting approach to guide catchment management has been limited by the cost and the difficulty to prepare and process samples for geochemical and radionuclide analyses. Spectral properties have recently been shown to provide a rapid and cost‐efficient alternative for this purpose. The current research objective was (i) to quantify the sediment source contributions in a 1∙19‐km² rural catchment of Southern Brazil by using mid‐infrared (MIR) spectroscopy and (ii) to compare these results with those obtained with geochemical approach and near‐infrared and ultraviolet–visible spectroscopy methods. The sediment sources to discriminate were cropland surface (n  = 20), unpaved roads (n  = 10) and stream channel banks (n  = 10). Twenty‐nine suspended sediment samples were collected at the catchment outlet during nine significant flood events. The sources could be distinguished by MIR spectroscopy. Cropland and channel bank sources mainly differed in their clay mineral contents, but their similar organic matter content complicated the MIR‐model predictions. Unpaved road contributions were discriminated from the other sources by their lower organic carbon content. When the results of the current research based on MIR spectroscopy are compared with those obtained using other sediment fingerprinting approaches, based on geochemistry and near‐infrared and ultraviolet–visible spectroscopy, an overestimation of channel banks contribution and an underestimation of cropland and unpaved road contributions is found. These results suggest that MIR spectroscopy can provide a useful tool that is non‐destructive, rapid and cheap for tracing sediment sources in rural catchments and for guiding the implementation of soil and water conservation measures. Copyright © 2016 John Wiley & Sons, Ltd.     

Sediment in Alluvial and Lacustrine Debris Fans as an Indicator for Land Degradation Around Lake Ashenge (Ethiopia)

Sediments deposited by (paleo) flash floods can hold valuable information on processes of environmental change, land degradation or desertification. In order to assess the suitability of flash flood deposits as proxies for land degradation, we monitored a representative gully segment in North Ethiopia (Ashenge catchment), investigated a sequence of alluvial debris fans downstream of this segment and dated a neighbouring subaquatic debris fan using short‐lived ²¹⁰Pb isotope counting. During one rainy season (July–September 2014), we measured daily rainfall, peak discharge, bedload transport, suspended sediment load and sediment deposition rates. The data show that sediment deposition in the debris fans is significantly dependent on micro‐topography (net incision in micro‐channels) (p < 0·1) and position within the sequence (net incision farther away from the lake) (p < 0·05). As sediment transfer to the lake significantly depends on the balance between available water and sediment (ratio rainfall depth/bedload transport) (p < 0·05), we could reconstruct the hydro‐sedimentary evolution of the gully over the past half century and validate it with aerial photographs and semi‐structured interviews. The findings are consistent with the short‐lived isotope count results, indicating increased sediment supply from the 1970s onwards, when little amounts of clay were deposited in the lake (<5%), and a subrecent clear water effect that resulted in increased deposition rates of clay in the lacustrine debris fan. Overall, our analysis indicates that debris fan sediments can be used to estimate past environmental degradation rates, if the contemporary water and sediment behaviour is well understood. Copyright © 2015 John Wiley & Sons, Ltd.     

Impact of no-tillage agricultural systems on sediment yield in two large catchments in Southern Brazil

Purpose

The impact of agriculture on water resources has long been a problem associated with the formation of runoff, the siltation of lakes and reservoirs, and overall depletion of water quality. In Brazil, these problems are mainly related to soil degradation by water erosion. However, studies of catchment-scale erosion are still rare particularly in grain-producing regions which have adopted conservative tillage systems for soil protection. In order to contribute to a better understanding of the impact of conservation agriculture on water resources, this study determined the runoff coefficient and sediment yield for two agricultural catchments.

Materials and methods

Hydrological and sedimentological monitoring was conducted in two catchments: the Conceicao catchment is characterized by grain production in weathered soils and a gently sloping landscape, while the Guapore catchment is characterized by heterogeneous soils and topography. Both catchments have problems associated with water erosion.

Results and discussion

The magnitudes of annual runoff coefficients and sediment yield were high, even if compared to similar agricultural regions, including a catchment with widespread adoption of no-tillage. The sediment yield was 140 t km^?2 year^?1, and the runoff coefficient was 14 % for the Conceicao catchment, while the sediment yield was 270 t km^?2 year^?1, and the runoff coefficient was 31 % for the Guapore catchment. The results indicate that problems such as gullies, soil compaction, runoff, floods, siltation, and water quality depletion associated with the misuse of agricultural areas in terms of soil conservation and water use are still evident and important even in regions with widespread adoption of no-tillage systems.

Conclusions

The magnitudes of both runoff and sediment yield clearly indicate the need to adopt complementary practices of soil conservation measures, such as mechanical runoff control.     

Establishing sediment budgets for two small lowland agricultural catchments in the UK

Sediment budgets have been established for two small (<4 km²), lowland, agricultural catchments, by using ¹³⁷Cs measurements, sediment source fingerprinting and more traditional monitoring techniques to quantify the individual components of the budget. The gross and net erosion rates for the fields on the catchment slopes were estimated using ¹³⁷Cs measurements within selected fields, which encompassed a representative range of slope angles, slope lengths and land use. These estimates were extrapolated over the entire catchment, using a simple topographically driven soil erosion model (Terrain-Based GIS, TBGIS) superimposed on a DEM, to derive catchment average gross and net erosion rates. Suspended sediment yields were measured at the catchment outlets and sediment source fingerprinting techniques were used to establish the relative contributions from the catchment surface, subsurface tile drains and eroding channel banks to the sediment yields. In-channel and wetland storage were quantified using both direct measurements and ¹³⁷Cs measurements. The sediment budgets established for the catchments highlighted the importance of subsurface tile drains as a pathway for sediment transfer, accounting for ca. 60% and 30% of the sediment output from the two catchments. Erosion from channel banks contributed ca. 10% and 6% of the sediment output from the two catchments. Although the suspended sediment yields from these catchments were considered high by UK standards (ca. 90 t km⁻² year⁻¹), the sediment delivery ratios ranged between 14% and 27%, indicating that a major proportion of the mobilised sediment was stored within the catchments. In-field and field-to-channel storage were shown to be of similar magnitude, but storage of sediment in the channel system and associated wetlands was relatively small, representing <5% of the annual suspended sediment yield.     

Validating erosion rate estimates provided by caesium‐137 measurements for two small forested catchments in Calabria,southern Italy

Increasing concern for problems of soil degradation and the off‐site impacts of accelerated erosion has generated a need for improved methods of estimating rates and patterns of soil erosion by water. The use of environmental radionuclides, particularly ¹³⁷Cs, to estimate erosion rates has attracted increased attention and the approach has been shown to possess several important advantages. However, the use of ¹³⁷Cs measurements to estimate erosion rates introduces one important uncertainty, namely, the need to employ a conversion model or relationship to convert the measured reduction in the ¹³⁷Cs inventory to an estimate of the erosion rate. There have been few attempts to validate these theoretical conversion models and the resulting erosion rate estimates. However, there is an important need for such validation, if the ¹³⁷Cs approach is to be more widely applied and reliance is to be placed on the results obtained. This paper reports the results of a study aimed at validating the use of two theoretical conversion models, namely the exponential depth distribution model and the diffusion and migration model, that have been used in several recent studies to convert measurements of ¹³⁷Cs inventories on uncultivated soils to estimates of soil erosion rates. The study is based on data assembled for two small catchments (1.38 and 1.65 ha) in Calabria, southern Italy, for which measurements of sediment output are available for the catchment outlet. The two catchments differ in terms of the steepness of their terrain, and this difference is reflected by their sediment yields. Because there is no evidence of significant deposition within the two catchments, sediment delivery ratios close to 1.0 can be assumed. It is therefore possible to make a direct comparison between the estimates of the mean annual erosion rates within the two catchments derived from ¹³⁷Cs measurements and the measured sediment outputs. The results of the comparison show that the erosion rate estimates provided by both models are reasonably consistent with the measured sediment yields at the catchment outlets. However, more detailed assessment of the results shows that the validity of the erosion rate estimates is influenced by the magnitude of the erosion rates within the catchment. The exponential depth distribution model appears to perform better for the catchment with higher erosion rates and to overestimate erosion rates in the other catchment. Similarly, the basic migration and diffusion model performs better for the catchment with lower erosion rates and overestimates erosion rates in the other catchment. However, the improved migration and diffusion model appears to perform satisfactorily for both catchments. There is a need for further studies to extend such independent validation of the ¹³⁷Cs technique to other environments, including cultivated soils, and to other conversion models and procedures. Copyright © 2003 John Wiley & Sons, Ltd.     

Modelling soil erosion and sediment yield at a catchment scale: the case of Masinga catchment,Kenya

Development of improved soil erosion and sediment yield prediction technology is required to provide catchment stakeholders with the tools they need to evaluate the impact of various management strategies on soil loss and sediment yield in order to plan for the optimal use of the land. In this paper, a newly developed approach is presented to predict the sources of sediment reaching the stream network within Masinga, a large‐scale rural catchment in Kenya. The study applies the revised universal soil loss equation (RUSLE) and a developed hillslope sediment delivery distributed (HSDD) model embedded in a geographical information system (GIS). The HSDD model estimates the sediment delivery ratio (SDR) on a cell‐by‐cell basis using the concept of runoff travel time as a function of catchment characteristics. The model performance was verified by comparing predicted and measured plot runoff and sediment yield. The results show a fairly good relationship between predicted and measured sediment yield (R²=0·82). The predicted results show that the developed modelling approach can be used as a major tool to estimate spatial soil erosion and sediment yield at a catchment scale. Copyright © 2006 John Wiley & Sons, Ltd.     

Using Cs measurements to validate the application of the AGNPS and ANSWERS erosion and sediment yield models in two small Devon catchments

Distributed erosion and sediment yield models are being increasingly used for predicting soil erosion and sediment yields in agricultural catchments. In most applications, validation of such models has commonly been restricted to comparison of the predicted and measured sediment output from a catchment, because spatially distributed information on rates and patterns of soil redistribution within the catchment has been lacking. However, such spatially distributed data are needed for rigorous model testing, in order to validate the internal functioning of a model and its applicability at different spatial scales. The study reported in this paper uses two approaches to test the performance of the agricultural non-point source pollution (AGNPS) and areal non-point source watershed environmental response simulation (ANSWERS) erosion and sediment yield models in two small catchments in Devon, UK. These involve, firstly, comparison of observed and predicted runoff and sediment output data for individual storm events monitored at the basin outlets and, secondly, information on the spatial pattern of soil redistribution within the catchments derived from ¹³⁷Cs measurements. The results obtained indicate that catchment outputs simulated by both models are reasonably consistent with the recorded values, although the AGNPS model appears to provide closer agreement between observed and predicted values. However, the spatial patterns of soil redistribution and the sediment delivery ratios predicted for the two catchments by the AGNPS and ANSWERS models differ significantly. Comparison of the catchment sediment delivery ratios and the pattern of soil redistribution in individual fields predicted by the models with equivalent information derived from ¹³⁷Cs measurements indicates that the AGNPS model provides more meaningful predictions of erosion and sediment yield under UK conditions than the ANSWERS model and emphasises the importance of using information on both catchment output and sediment redistribution within the catchment for model validation.     

Quantifying farmland shelterbelt impacts on catchment soil erosion and sediment yield for the black soil region,northeastern China

As one part of the ‘Three Norths’ forest protection system, dense farmland shelterbelt networks in northeastern China could greatly modify water and sediment flows. In this paper, catchment soil erosion rate and sediment yield (SY) that are impacted by farmland shelterbelts were estimated using WaTEM/SEDEM model. The shelterbelts reduced catchment soil erosion and SY to some extent. The mean soil erosion rate and specific sediment yield (SSY; defined as the ratio of SY to catchment area; t km^?2 yr^?1) of the 25 reservoir catchments decreased from 351.6 and 93.9 t km^?2 yr^?1 under the supposed scenario without shelterbelts to 331.1 t km^?2 yr^?1 and 86.3% t km^?2 yr^?1 under the current situation with shelterbelts. The sediment trap efficiencies (STEs) varied from 0.01% to 23.6% with an average value of 7.6%. The STEs were significantly correlated with shelterbelt density, catchment perimeter, topographic factors, RUSLEP‐factor and land use patterns including patch density (PD), patch cohesion index (COHESION), Shannon's diversity index (SHDI) and aggregation index (AI). The multiple regression equation involving factors of catchment's topography and morphology and land use pattern has a satisfactory performance, and mean slope gradient (MSG) and AI explained most of the variability of shelterbelts’ STE. This information can help land managers to better design shelterbelts and to reduce water‐derived soil loss at catchment scale.     

Dynamics of Runoff and Suspended Sediment Transport in a Highly Erodible Catchment on the Chinese Loess Plateau

Runoff is the key factor to understand the land degradation causing high risk of soil erosion and can reduce the water available for human societies and ecosystems. The dynamics of runoff and suspended sediment transport are not completely understood. In this study, we examined the trends, breaking point and regime changes for the runoff and sediment load at different temporal scales using 50 years of continuous observational data from a highly erodible sub‐catchment with an area of 7,325 km² in the Beiluo River basin on the Loess Plateau, China. At the annual scale, the runoff and sediment load declined significantly (p < 0·05) with decreasing rates of −0·23 mm y⁻¹ and −164·9 Mg km⁻² y⁻¹, respectively. Abrupt changes in the runoff and sediment load series were detected between 1979 and 1999; thus, the data were divided into intervals of 1960–1979, 1980–1999 and 2000–2009. The flow duration curve analysis indicated increasing low‐flow values and decreasing daily runoff and sediment discharge peaks, which suggested that soil and water conservation measures reduced the volume of runoff and the sediment load. This led to a more uniform runoff regime. At the flood event scale, we investigated the relationship between runoff and the suspended sediment load based on 123 flood events, which showed clearly that the magnitude and frequency of hyper‐concentrated sediment flows decreased in 2000–2009 compared with 1960–1999. The annual erosive rainfall exhibited non‐significant changes throughout the entire study period. We conclude that soil and water conservation measures (e.g. afforestation, grassing, terraces and check dams) have played major roles in the changes in runoff and the sediment load in the Beiluo River catchment. Copyright © 2015 John Wiley & Sons, Ltd.     

The use of visible and near-infrared reflectance measurements for identifying the source of suspended sediment in rivers and comparison with geochemical fingerprinting

Purpose

Visible and near-infrared (Vis-NIR) reflectance measurements may be an alternative technique to identify suspended sediment sources in streams of headwater catchments. In this study, we examined if Vis-NIR reflectance measurements are capable of estimating sediment source contributions to sediment yield and compared this technique with a more conventional (i.e. geochemical) technique.

Materials and methods

Two headwater catchments in Ethiopia, Unta (2,052 ha) and Desera (1,657 ha), were analysed with the same techniques in order to find similarities and differences in the results obtained. The first technique used Vis-NIR spectral analysis as a fingerprint, using a partial least squares regression model. The second technique was a quantitative composite fingerprinting technique using geochemical analysis of source materials and suspended sediment samples. As a comparison, the partial least squares model was also used on the geochemical data. In August and September 2009, 30 soil samples of three different land uses (landslides, croplands, and grazing lands) and 21 suspended sediment samples at the catchment outlet were collected. Source samples were sieved to <63 μm. Geochemical analyses consisted of total element concentrations, percentage carbon, percentage nitrogen, and atom percentage ¹⁵N and δ¹³C. Reflectance measurements were taken on dried source samples with a spectrometer.

Results and discussion

Neither technique was able to predict the contributions of the three land use types; they could only distinguish between landslide and topsoil material. The agreement between the results of both techniques was significant for the Unta catchment (R ²?=?0.80) but not for the Desera catchment (R ²?=?0.39). The uncertainty of the technique using Vis-NIR reflectance measurements was slightly higher than with the geochemical approach. Both techniques revealed that topsoil erosion played an important role during storm runoff discharges. Using the partial least squares model for the geochemical data revealed that uncertainty can differ greatly when using other statistical techniques.

Conclusions

The quantitative composite fingerprinting technique using spectral signatures from both source and suspended sediment samples was able to quantify the contribution of two source materials (landslides and topsoil). It provided a faster and more cost effective alternative to the conventional geochemical procedure.     

Temporal Dynamics of Sediment Sources in an Urbanizing Mediterranean Catchment

The identification of sediment sources is important for catchment management, but few studies have been performed in Mediterranean areas. This study uses a multiproxy sediment fingerprinting approach to explore sources and dynamics of fluvial sediments in a rapidly urbanizing Portuguese peri‐urban catchment. X‐ray fluorescence was used to characterize the elemental geochemistry of sediments collected within the stream network after three storm events in 2012 and 2015. A range of statistical techniques, including hierarchical cluster analysis, was used to identify discriminant sediment properties and similarities between fine bed sediment samples of tributaries and downstream sites. Quantification of sediment supply from upstream sub‐catchments was undertaken using established sediment fingerprinting approaches. The geochemistry of the sediments was found to be influenced by both lithology and land‐use changes. In 2012, the sandstone sub‐catchment provided 75% of the <63 μm and 94% of the 63–125 and 125–2000 μm sediment, mostly from an Enterprise Park site undergoing deforestation and construction (covering 5% of the catchment area), with most of the remaining sediment deriving from the construction of a major road (1% of the catchment) in the limestone sub‐catchment. In 2015, planned and accidental retention basins below the Enterprise Park and major road sites, respectively, prevented some sediment from reaching the stream network, thereby reducing their importance as sediment sources. Sediment mobilization in urban areas with paved roads was small in comparison, but characterized by high heavy metal concentrations. The sediment fingerprinting approach adopted shows promise in identification of the main sources of sediments, necessary to underpin improved peri‐urban management strategies. Copyright © 2017 John Wiley & Sons, Ltd.     

Investigating the effects of afforestation on soil erosion and sediment mobilisation in two small catchments in Southern Italy

Annual soil losses in southern Italy can exceed 100–150 t ha^− 1 year^− 1. Where erosion on agricultural land is particularly severe, land use change and afforestation are frequently seen as the most appropriate means of reducing erosion risk. However, the overall effectiveness of afforestation in reducing soil erosion remains uncertain, due to the poor development of the forest cover in some areas, leading to significant areas with sparse tree cover, and the erosional impact of forest harvesting, which commonly involves clearcutting. The study reported here addresses this uncertainty and focuses on two small catchments (W2 and W3) located in Calabria, southern Italy, for which measurements of suspended sediment yield are available. Both the catchments originally supported a rangeland vegetation cover and they were planted with eucalyptus trees in 1968. Currently, only catchment W3 supports a continuous forest cover. In catchment W2 the forest cover is discontinuous and there is a significant area of the catchment (ca. 20%) where the tree cover is sparse and the vegetation cover is dominated by natural grasses. Two additional erosion plots were established within catchment W2 in 1991, in order to explore the effect of the density of the tree cover on soil erosion. Information on the sediment yields from the two catchments and the plots for 10 storm events that occurred during the period December 2005–December 2006 and associated information on the ¹³⁷Cs and excess ²¹⁰Pb of the sediment, have been used to investigate the effectiveness of afforestation in reducing sediment mobilisation and net soil loss from the catchments involved. The results demonstrate that the areas of greatest soil loss are associated with the slopes where the tree cover is discontinuous, and that forest harvesting by clearcutting causes significant short-term increases in sediment mobilisation and sediment yield. These findings, which are consistent with previous work undertaken within the same area, emphasize the importance of vegetation cover density in influencing rates of soil loss in the study catchments. The study also provided a useful demonstration of the potential for using measurements of the ¹³⁷Cs and ²¹⁰Pb_ex content of sediment, in combination with more traditional sediment monitoring, to investigate sediment sources and to compare the sediment dynamics of catchments subjected to different land management practices.