Identifying the driving factors of sediment delivery ratio on individual flood events in a long-term monitoring headwater basin

The Sediment Delivery Ratio(SDR) has multi-fold environmental implications both in evaluating the soil and water losses and the effectiveness of conservation measures in watersheds. Various factors, including hydrological regime and watershed properties, may influence the SDR at interannual timescales. However, the effect of certain important dynamic factors, such as rainfall peak distribution, runoff erosion power and sediment bulk density, on the sediment delivery ratio of single flood events(SDRe) has received little attention. The Qiaogou headwater basin is in the hilly-gully region of the Chinese Loess Plateau, and it encompasses a 0.45 km^2 catchment. Three large-scale field runoff plots at different geomorphological positions were chosen to obtain the observation data, and the 20-year period between 1986 and 2005 is presented. The results showed that the SDRe of the Qiaogou headwaters varied from 0.49 to 2.77. Among the numerous influential factors, rainfall and runoff were the driving factors causing slope erosion and sediment transport. The rainfall erosivity had a significant positive relationship with the sediment transport modulus(R^2=0.85, P<0.01) but had no significant relationship with SDRe. The rainfall peak coefficient was significantly positively correlated with the SDRe(R^2=0.64, P<0.05), indicating the influence of rainfall energy distribution on the SDRe. The runoff erosion power index was not only significantly related to the sediment transport modulus(R^2=0.84, P<0.01) but also significantly related to the SDRe(R^2=0.57, P<0.01). In addition, the relative bulk density was significantly related to the SDRe, indicating that hyper-concentrated flow characteristics contributed to more transported sediment in the catchment. Thus, the rainfall peak coefficient, runoff erosion power and sediment relative bulk density could be used as dynamic indexes to predict the SDRe in the hilly areas of the Chinese Loess Plateau.

Sediment Delivery across Multiple Spatio-temporal Scales in an Agriculture Watershed of the Chinese Loess Plateau

There is a consensus that sediment delivery ratio in the Chinese Loess Plateau is close to 1at the inter-annual timescale. However, little information is available about the sediment delivery at finer timescales. We evaluated the sediment delivery from plots to watersheds at the event or intra-annual, annual, and inter-annual timescales within the Wudinghe river basin, a 30,261 km2 basin in the Loess Plateau. We calculated the ratio of sediment output to sediment input and presented the temporal change of the channel morphology to determine whether sediment deposition occurs.Although a single flood event frequently has a sediment yield exceeding 10,000 t km-2, sediment deposition rarely occurs except during some small runoff events(sediment yield < 5000 t km-2) or dry years(sediment yield < 10,000 t km-2) when moving from slopes up to the main channels of the Wudinghe River. This observation suggests a sediment delivery ratio close to 1 even at the event or intra-annual and the annual timescales, but not necessarily at the interannual timescale. Such a high sediment delivery ratio can be related to hyper-concentrated flows, which have very strong sediment transport capacity even at low flow strength. Because hyper-concentrated flows are well-developed in the whole Loess Plateau, a sediment delivery ratio close to 1 below the interannual timescale possibly remains true for other rivers in the Loess Plateau.

Modelling sediment delivery using connectivity components at the experimental SPA2 basin, Sicily(Italy)

Sediment delivery ratio can be used as a measure of sediment connectivity and it can be linked to the structural connectivity(morphological unit, slope length, slope steepness, travel time) of a basin and to the functional connectivity(rainfall-runoff processes at morphological unit scale). In this paper the sediment connectivity approach was applied at basin scale both using Sediment Delivery Distributed(SEDD) model, which takes into account the hillslope sediment transport, and sediment yield measurements carried out at SPA2 experimental basin(Sicily, Italy). The expression of the sediment delivery ratio SDRi of a morphological unit was modified for highlighting two components corresponding to the structural(SDR_(L,i)) and functional(SDR_(F,i)) sediment connectivity, respectively. For SPA2 basin the frequency distribution of the travel time of each morphological unit was used to estimate the coefficient βL of the structural component of the sediment delivery ratio of each morphological unit. Then, using the sediment yield measurements carried out at the outlet of the experimental SPA2 basin in the period April 2000-March 2015, the SEDD model was calibrated at event scale for estimating the coefficient βF of the functional component of the sediment delivery ratio. At event scale the developed analysis stated that the functional connectivity is dependent on the magnitude of erosion events. Intermediate and high events, which were characterized by the lowest values of the functional coefficient, determine a more high functional connectivity and are characterized by a more efficient sediment transport along the hillslopes. Finally, at annual scale, the model was calibrated for the period 2000-2015 and relationships for estimating the coefficient βF,a of the functional component of the sediment delivery ratio taking into account the intensity of erosion events occurring in each year were determined. At annual scale, the analysis demonstrated that the functional coefficient was always greater than the landscape coefficient and the sediment connectivity was always controlled by the low values of the functional component.

Improving cross drain systems to minimize sediment delivery using GIS

A well developed network of roads must exist as a necessary infrastructure system in modem forestry to facilitate forest op- erations. But forest roads have the potential to disrupt the drainage characteristics of watersheds and lead to negative impacts on the environment with increased erosion and sediment yields. Numerous factors affect surface erosion of roads and sediment production potential; determining and ranking them could be a guide for management decisions to erosion control. In this study, the CULSED model （as an extension of ArcGIS） was used to estimate sediment delivery and the distribution of a road network, given the exist- ing culverts. Using the model, some culverts were added to the road network around places with high sediment delivery in order to minimize it. After a correlation analysis and adjustment between sediment production and the factors, i.e., road width, road gradient, age of road and vegetation cover, the trend of changes in sediment delivery with model changes in the input was investigated with a sensitivity analysis of the model. The results show that adding new culverts to the road resulted in a significant reduction of sediment delivery. The most important factor affecting sediment delivery was road width, followed by road gradient, vegetation cover and age of road. Road width and gradient were positively correlated with sediment delivery, while vegetation cover and age of road were neg- atively correlated. The best model to show the relation between sediment delivery and road width as well as with road gradient was a linear model, for vegetation cover a cubic equation and for road age a power model. The results of sensitivity analysis showed that sediment delivery had the greatest sensitivity to changes of road width and was least sensitive to changes in the age of the road. This model can help to estimate sediment delivery with its spatial distribution, which can be used for optimization of cross drain systems and strategies of sediment control. Application of the model requires field trials to acquire the necessary input data. The reliability of our results is a function of the accuracy of inputs, especially digital elevation model.

Estimation of soil erosion and sediment yield in Wadi El Hachem watershed(Algeria)using the RUSLE-SDR approach

One of the most common types of soil degradation is water erosion.It reduces soil quality at the erosion site and may cause sedimentation issues at the deposition site.This phenomenon is estimated using a variety of models.The Revised Universal Soil Loss Equation(RUSLE)model is the most often used,due to its consistence and low data requirement.It is useful for estimating annual soil loss at the watershed scale.To investigate the relationship between soil erosion and sediment deposition,the combined RUSLE and Sediment Delivery Ratio(SDR)models are used.The Wadi El Hachem watershed is a coastal and mountainous Mediterranean basin with rugged topography and high degree of climatic aggressiveness.Both of these characteristics can have an immediate effect on soil erosion and sediment yield.This research includes estimating the Average Annual Soil Loss(A)and Sediment Yield(SY)in the Wadi El Hachem watershed,mapping different RUSLE factors as well as A and SY,and studying the influence of rainfall erosivity(R)on A and SY in dry and rainy years.The A results vary from 0 to 410 t·ha^(-1)·yr^(-1)with an annual average of 52 t·ha^(-1)·yr^(-1).The Renfro's SDR model was selected as the best model for estimating SY,with standard error,standard deviation,coefficient of variation,and Nash–Sutcliffe efficiency(NSE)values of 0.38%,0.02,0.07%,and 1.00,respectively.The average SY throughout the whole watershed is around 27 t·ha^(-1)·yr^(-1).The SY map for the entire Wadi El Hachem watershed revealed that sediment production zones are mainly concentrated in the Northeast of the basin,at the basin’s outlet,and in the tributaries of the dam.The simulation results of soil loss and sediment yield in dry and rainy years revealed that R is one of the main factors affecting soil erosion and sediment deposition in the Wadi El Hachem watershed.The mean difference in R factor between dry year and rainy year is 671 MJ·mm·ha^(-1)·h^(-1)·yr^(-1).As a result of this fluctuation,the soil loss and sediment yield have increased by 15 and 8 t·ha^(-1)·yr^(-1),respectively.The results of this research can be used to provide scientific and technical support for conservation and management strategies of the Wadi El Hachem watershed.

Using ^(137)Cs Tracing Methods to Estimate Soil Redistribution Rates and to Construct a Sediment Budget for a Small Agricultural Catchment in the Three Gorges Reservoir Region,China

Soil erosion and associated off-site sedimentation are threatening the sustainable use of the Three Gorges Dam. To initiate management intervention to reduce sediment yields, there is an increasing need for reliable information on soil erosion in the Three Gorges Reservoir Region （TGRR）. The purpose of this study is to use 137Cs tracing methods to construct a sediment budget for a small agricultural catchment in the TGRR. Cores were taken from a pond and from paddy fields, for laTCs measurements. The results show that the average sedimentation rate in the pond since 1963 is 1.50 g cm-2 yr-1 and the corresponding amount of sediment deposited is 1,553 t. The surface erosion rate for the sloping cultivated lands and the sedimentation rate in the paddy fields were estimated to be 3,770 t km-2 yr-1 and 2,600 t km-2 yr^1 respectively. Based on the estimated erosion and deposition rates, and the area of each unit, the post 197o sediment budget for the catchment has been constructed. A sediment delivery ratio of 0.5 has been estimated for the past 42 years. The data indicate that the sloping cultivated lands are the primary sediment source areas, and that the paddy fields are deposition zones. The typical land use pattern （with the upper parts characterized by sloping cultivated land and the lower parts by paddy fields） plays an important role in reducing sediment yield from agricultural catchments in the TGRR. A 137Cs profile for the sediment deposited in a pond is shownto provide an effective means of estimating the land surface erosion rate in the upstream catchment.

Changes in sediment discharge in a sediment-rich region of the Yellow River from 1955 to 2010: implications for further soil erosion control

The well-documented decrease in the discharge of sediment into the Yellow River has attracted considerable attention in recent years. The present study analyzed the spatial and temporal variation of sediment yield based on data from 46 hydrological stations in the sediment-rich region of the Yellow River from 1955 to 2010. The results showed that since 1970 sediment yield in the region has clearly decreased at different rates in the 45 sub-areas controlled by hydrological stations. The decrease in sediment yield was closely related to the intensity and extent of soil erosion control measures and rainstorms that occurred in different periods and sub-areas. The average sediment delivery modulus（SDM） in the study area decreased from 7,767.4 t/（km^2·a） in 1951–1969 to 980.5 t/（km^2·a） in 2000–2010. Our study suggested that 65.5% of the study area with the SDM below 1,000 t/（km^2·a） is still necessary to control soil deterioration caused by erosion, and soil erosion control measures should be further strengthened in the areas with the SDM above 1,000 t/（km^2·a）.

Soil Erosion and Sediment Control Effects in the Three Gorges Reservoir Region, China

The Three Gorges Reservoir, the world’s largest hydropower reservoir, receives a significant sediment yield from soil erosion. Sloping farmland is the main source, exacerbated by changes in land use from relocating the inhabitants, and from engineering projects related to dam construction. Related geo-hazards, including landsliding of valley-side slopes, will further increase the sediment yield to the completed reservoir. Integrated watershed management, begun extensively in 1989, has effectively controlled soil erosion and sediment delivery to date. What is described here as the Taipinxi Mode of integrated watershed management, based on its application in the 26.14 km2 watershed of that name in Yiling District, has been successful and is recommended for the entire region. The effects of this set of erosion-mitigation measures are assessed, using experienced formulas for soil and water conservation and information from remote sensing. The amount of soil erosion, and of sediment delivery to the reservoir were reduced by 43.75–45.94 × 106 t y-1, and by 12.25–12.86 × 106 t y-1, respectively, by 2005, by which time the project had been operative for 16 years.

Assessment of Soil Erosion and Sediment Yield in Liao Watershed, Jiangxi Province, China, Using USLE, GIS, and RS

Soil erosion by water is a serious problem all over the world. In China, about 1 790 000 km2 of land suffers from water erosion, which accounts for 18.3% of China＇s total area. This study was conducted in the Liao （潦） watershed in Jiangxi （江西） Province to assess annual soil erosion and sediment yield using the Universal Soil Loss Equation （USLE）. A geographic information system （GIS） was used to generate maps of the USLE factors, which include rainfall erosivity （R）, soil erodibility （K）,slope length and steepness （LS）, cover （C）, and conservation practice （P） factors. By integrating these factors in a GIS, a spatial distribution of soil erosion over the Liao watershed was obtained. The soil erosion was found to vary from nil for flat and well-covered areas to more than 500 t/ha/a in mountainous places with sparse vegetation. The average soil erosion is 18.2 t/ha/a with a standard deviation of 109.3 t/ha/a. The spatial distribution of erosion classes was estimated. About 39.5% of the watershed is under the tolerant erosion rate, and 60.5% of the study area experienced erosion to different extents. A spatially distributed sediment delivery ratio （SDR） module was developed to account for soil erosion and deposition. It was found that the SDR value at the outlet of the Liao watershed was 0.206, and the sediment yield was 1.32 million t/a, which was 20% higher than the measured sediment. The results can be used to identify the soil erosion hot spots and develop the best soil erosion management practices and help estimate the quantity of soil that was transported into the downstream Poyang （鄱阳） Lake.

Enhanced lakebed sediment erosion in Dongting Lake induced by the operation of the Three Gorges Reservoir

Based on field-survey hydrological series in the Dongting Lake watershed from 1951 to 2011, the variations of lakebed sediment siltation/erosion（S/E） regimes of the Dongting Lake after the operation of the Three Gorges Reservoir（TGR） were analyzed. Significantly positive correlations were found between the flow rate from Zhicheng and the Three Outlets（r^2=0.859, p〈0.0001）, and between the flow rate and sediment delivery rate at the Three Outlets（r^2=0.895, p〈0.0001）. This indicated that the flow rate and sediment delivery rate at the Three Outlets were largely determined by the flow rate from the upstream Yangtze River. Sediment deposition amount in the Dongting Lake basin dropped from ＋4796.4×10^4 t during the period before the operation of TGR（1999–2002） to ＋684.1×10^4 t, ＋449.8×10^4 t and –559.6×10^4 t during the impoundment Phases I, II and III of TGR. The S/E regimes changed from a siltation-dominant to an erosion-dominant state under the pre-discharge, water-storage and water-supplement dispatch over the impoundment from Phase I to III. The sediment deposition amount decreased dramatically under the flood-storage dispatch over the impoundment Phase I to III. The estimated annual mean flow rate, sediment delivery rate and sediment concentration thresholds were respectively 970.81 m3/s, 466.82 kg/s and 0.481 kg/m3 for the upstream Three Outlets to maintain an erosion-dominant state in the downstream linked the Dongting Lake.

Estimating landscape susceptibility to soil erosion using a GIS-based approach in Northern Ethiopia

Soil erosion is a very critical form of land degradation resulting in the loss of soil nutrients and downstream sedimentation of water storages in the highlands of Ethiopia.As it is technically and financially impossible to conserve all landscapes affected by erosion,identification of priority areas of intervention is necessary.Spatially distributed erosion models can help map landscape susceptibility to erosion and identify high erosion risk areas.Integration of erosion models with geographic information systems(GIS)enables assessing evaluate the spatial variability of soil erosion and plan implementing conservation measures at landscape levels.In this study,the Revised Universal Soil Loss Equation adjusted for sediment delivery ratio was used in a GIS system to assess landscape sensitivity to erosion and identify hotspots.The approach was applied in three catchments with size being 10–20 km^(2) and results were compared against quantitative and semi-quantitative data.The model estimated mean soil loss rates of about 45 t ha^(−1) y^(−1) with an average variability of 30%between catchments.The estimated soil loss rate is above the tolerable limit of 10 t ha^(−1) y^(−1).The model predicted high soil loss rates at steep slopes and shoulder positions as well as along gullies.The results of the study demonstrate that knowledge of spatial patterns of high soil loss risk areas can help deploy site-specific conservation measures.