Annual Suspended Sediment Load Research Articles

Dynamics of suspended sediment load in the Morava River (Serbia) in the period 1967-2007

The study is concerned with determination of the trend of water discharge, suspended sediment concentration and sediment load in the most downstream profile of the Velika Morava River in the period 1967-2007. The gradual trend test (Mann–Kendall test – MK test) and abrupt change test (Pettitt test) have been employed on annual, seasonal and monthly water discharge, suspended sediment concentration and suspended sediment load for the given time series. Both the Mann–Kendall and Pettitt tests indicate that water discharge showed no significant annual trend or abrupt shift. However, annual suspended sediment concentration and sediment load showed significant decreasing trends (α=0.001). The average decrease of suspended sediment load transport amounted to 3.15 t/km2/yr. The Pettitt test results showed that the change-point year was detected in 1982. The average specific sediment load amounted to 134.6 t/km2/yr before the transition year, and 36.5 t/km2/yr after the transition year, i.e., it was reduced by 73 %. In the intra-annual distribution, the MK test results indicate that the most pronounced decreasing trend (α=0.001) of the sediment load is during summer and winter. Strong seasonal and monthly variability in sediment load was found. Sediment was strongly transported during spring months, in the period of frequent flood events. Almost 50% of the annual sediment is transported during March, April and May. Analysis of the discharge and suspended sediment concentration relationship revealed the existence of hysteresis loop in the shape of figure eight. The results of this study confirm the complex and heterogeneous nature of sediment response in the Velika Morava River.

Analyses of suspended sediment loads in Slovenian rivers

ABSTRACTSuspended solids are present in every river, but high quantities can worsen the ecological conditions of streams; therefore, effective monitoring and analysis of this hydrological variable are necessary. Frequency, seasonality, inter-correlation, extreme events, trends and lag analyses were carried out for peaks of suspended sediment concentration (SSC) and discharge (Q) data from Slovenian streams using officially monitored data from 1955 to 2006 that were made available by the Slovenian Environment Agency. In total more than 500 station-years of daily Q and SSC data were used. No uniform (positive or negative) trend was found in the SSC series; however, all the statistically significant trends were decreasing. No generalization is possible for the best fit distribution function. A seasonality analysis showed that most of the SSC peaks occurred in the summer (short-term intense convective precipitation produced by thunderstorms) and in the autumn (prolonged frontal precipitation). Correlations between Q and SSC values were generally relatively small (Pearson correlation coefficient values from 0.05 to 0.59), which means that the often applied Q–SSC curves should be used with caution when estimating annual suspended sediment loads. On average, flood peak Q occurred after the corresponding SSC peak (clockwise-positive hysteresis loops), but the average lag time was rather small (less than 1 day).Editor M.C. Acreman; Associate editor Y. Gyasi-Agyei

Transporte de sedimentos en suspensión en los principales ríos del Caribe colombiano: magnitud, tendencias y variabilidad

Se analizaron series mensuales del transporte de sedimento en suspensión de siete ríos del Caribe colombiano, con el fin de proporcionar un estimativo actualizado, detectar los cambios recientes y los patrones de variabilidad del transporte de sedimento en suspensión. En su conjunto, estos ríos transportan un promedio de ~146,3 x106 t a-1 de sedimentos en suspensión hacia el litoral Caribe colombiano. El mayor aporte proviene del río Magdalena, con 142,6 x106 t a-1, que corresponden a 38 % del total de los sedimentos en suspensión estimados para el litoral Caribe. La tasa anual del transporte de sedimentos en suspensión de estos sistemas fluviales ascendió hasta 36 % entre 2000 y 2010. El análisis espectral basado en la transformada de ondícula (wavelet) evidenció lapsos de intensa variabilidad entre los periodos 1987-1990 y 1994-2002, en los cuales las principales componentes oscilatorias aparecieron de manera simultánea. Las bandas anual, semianual y ‘cuasi-decadal’ aparecieron como las principales componentes oscilatorias en la variabilidad del transporte de sedimento en suspensión, mientras que las bandas ‘cuasibienal’ e interanual constituyeron una fuente de variabilidad de segundo orden. Se considera que los mecanismos forzadores climáticos y oceanográficos asociados con las oscilaciones detectadas mediante el análisis espectral definen un marco de variabilidad de mediano a largo plazo para el transporte de sedimentos en suspensión.

Suspended sediment transport in the Magdalena River (Colombia, South America): Hydrologic regime, rating parameters and effective discharge variability

The Magdalena River has the greatest length (1612km) and drainage area (257400km2) of any river system in Colombia, and its watershed occupies 24% of the Colombian territory. The river originates in the Andean Cordillera at an elevation of 3685m and flows to its mouth in the Caribbean Sea (Bocas de Cenizas). For this study, information on daily flow discharge (Q), suspended sediment transport (Qs) and mean suspended sediment concentration (Cs) from 1990–2010 for the Magdalena River, based on measurements at the Calamar gauging station (the closest station to the mouth), was provided by the Institute of Hydrology, Meteorology and Environmental Studies of Colombia (IDEAM in Spanish). The mean annual suspended sediment load was 169±73×106tyr−1 for the period from 1990–1999 and 145±47×106tyr−1 for the period from 2000–2010, amounting to a 14% reduction in the suspended sediment transport rate between the two periods. The sediment rating curve (Qs=aQb) was also evaluated for these two periods (1990–1999 and 2000–2010). An analysis of the regression coefficients (a, b) for the two periods showed a decrease in b and an increase in a after 1999, which coincides with a decrease in suspended sediment transport. This change might be attributed to natural factors, including the accumulation of sediment in the Depresion Momposina and variation in hydro-climatological patterns in the Caribbean region. Decreases in effective transport were confirmed using three methods (Log-Normal and Normal distribution, and magnitude-frequency histogram) for the study period.

Quantile Regression-Based Probabilistic Estimation Scheme for Daily and Annual Suspended Sediment Loads

Assessing suspended sediment loads in rivers is important since it affects water quality, hydraulic-facility design, and many other sediment-induced problems. Sediment-load estimation heavily depends upon empirical approaches such as a sediment rating curve, which is the empirical relationship between sediment load and river discharge. However, the sediment rating curve is insufficient to describe the inevitable scatter between sediment and discharge. This study aims to develop a probabilistic estimation scheme for daily and annual suspended sediment loads using quantile regression. All recorded daily suspended sediment load and discharge data are employed to construct quantile-dependent sediment rating curves. The empirical probability distribution of daily suspended sediment load is then built by integrating the conditional estimations associated with the corresponding quantiles for a given discharge. The probability distribution of a cumulative sediment load over a longer period can also be derived by the obtained daily sediment-load probability distributions and convolution theorem. The proposed approach is applied to the Laonung station located in southern Taiwan. The results indicate that the proposed approach provides not only the probabilistic description for daily and annual suspended sediment loads, but also the single estimations including the mean, median, and mode of the derived probability distribution. For the 1,110 recorded data of Laonung station during the 1959–2008 period, the proposed mean and median estimation schemes outperform the traditional sediment-rating-curve approach for less mean absolute errors.

Fitting sediment rating curves using regression analysis: a case study of Russian Arctic rivers

Abstract. Published suspended sediment data for Arctic rivers is scarce. Suspended sediment rating curves for three medium to large rivers of the Russian Arctic were obtained using various curve-fitting techniques. Due to the biased sampling strategy, the raw datasets do not exhibit log-normal distribution, which restricts the applicability of a log-transformed linear fit. Non-linear (power) model coefficients were estimated using the Levenberg-Marquardt, Nelder-Mead and Hooke-Jeeves algorithms, all of which generally showed close agreement. A non-linear power model employing the Levenberg-Marquardt parameter evaluation algorithm was identified as an optimal statistical solution of the problem. Long-term annual suspended sediment loads estimated using the non-linear power model are, in general, consistent with previously published results.

Quantifying suspended sediment loads delivered to Cheney Reservoir, Kansas: Temporal patterns and management implications

Cheney Reservoir, constructed during 1962 to 1965, is the primary water sup- ply for the city of Wichita, the largest city in Kansas. Sediment is an important concern for the reservoir as it degrades water quality and progressively decreases water storage capacity. Long-term data collection provided a unique opportunity to estimate the annual suspended sediment loads for the entire history of the reservoir. To quantify and characterize sediment loading to Cheney Reservoir, discrete suspended sediment samples and continuously mea- sured streamflow data were collected from the North Fork Ninnescah River, the primary inflow to Cheney Reservoir, over a 48-year period. Continuous turbidity data also were collected over a 15-year period. These data were used together to develop simple linear regression models to compute continuous suspended sediment concentrations and loads from 1966 to 2013. The inclusion of turbidity as an additional explanatory variable with streamflow improved regression model diagnostics and increased the amount of variability in suspended sediment concentration explained by 14%. Using suspended sediment concentration from the streamflow-only model, the average annual suspended sediment load was 102,517 t (113,006 tn) and ranged from 4,826 t (5,320 tn) in 1966 to 967,569 t (1,066,562 tn) in 1979. The sed- iment load in 1979 accounted for about 20% of the total load over the 48-year history of the reservoir and 92% of the 1979 sediment load occurred in one 24-hour period during a 1% annual exceedance probability flow event (104-year flood). Nearly 60% of the reservoir sedi- ment load during the 48-year study period occurred in 5 years with extreme flow events (9% to 1% annual exceedance probability, or 11- to 104-year flood events). A substantial portion (41%) of sediment was transported to the reservoir during five storm events spanning only eight 24-hour periods during 1966 to 2013. Annual suspended sediment load estimates based on streamflow were, on average, within ±20% of estimates based on streamflow and turbidity combined. Results demonstrate that large suspended sediment loads are delivered to Cheney Reservoir in very short time periods, indicating that sediment management plans eventually must address large, infrequent inflow events to be effective.

Managing flow, sediment, and hydropower regimes in the Sre Pok, Se San, and Se Kong Rivers of the Mekong basin

AbstractThe Lancang/Mekong River Basin is presently undergoing a period of rapid hydropower development. In its natural undeveloped state, the river transports about 160 million metric tons of sediment per year, maintaining the geomorphologic features of the basin, sustaining habitats, and transporting the nutrients that support ecosystem productivity. Despite the importance of sediment in the river, currently little attention is being paid to reservoir sediment trapping. This study is devoted to assessing the potential for managing sediment and its impact on energy production in the Se San, Sre Pok, and Se Kong tributaries of the Mekong River. These tributaries drain a set of adjacent watersheds that are important with respect to biodiversity and ecological productivity, and serve as a significant source of flow and sediment to the mainstream Mekong River. A daily sediment transport model is used to assess tradeoffs among energy production and sediment and flow regime alteration in multiple reservoir systems. This study finds that eventually about 40%–80% of the annual suspended sediment load may be trapped in reservoirs. Clearly, these reservoirs will affect the rivers' sediment regimes. However, even after 100 years of simulated sedimentation, reservoir storage capacities and hydropower production at most reservoir sites are not significantly reduced. This suggests that the strongest motivation for implementing measures to reduce trapped sediment is their impact not on hydropower production but on fish migration and survival and on sediment‐dependent ecosystems such as the Vietnam Delta and Cambodia's Tonle Sap Lake.

An assessment of human versus climatic impacts on large-sized basin erosion: the case of the upper Yangtze River

Riverine sediment load, a reflection of basin erosion and sediment yield, is influenced by both climatic and human factors. Complex interaction between various factors within a basin dampens and counteracts the forces that drive sediment variations. The gross human impact index and the index estimation method have both been proposed to reflect the impacts of human activities on soil erosion and sediment yield. Sediment load and daily rainfall data from 1955 to 2010 in the upper Yangtze basin, and in the Wu, Jialing, Min and Jinsha subbasins, were collected to assess the human versus climatic impacts on sediment yield. From 1955 to 2010, the average annual runoff in the study area was 428.2 billion m(3), and the average annual suspended sediment load was approximately 0.43 billion t. There was a critical point in 1984, 1985, 1991, 1993 and 1999 when the sediment load decreased in the Wu, Jialing, upper Yangtze, Min and Jinsha river, respectively. The annual regional rainfall erosivities in the upper Yangtze basin in most years ranged between 2,500 and 3,500 MJ mm hm(-2) h(-1) year(-1) and fluctuated around 3,000 MJ mm hm(-2) h(-1) year(-1) with a small coefficient of variation of 0.11. In the Jinsha subbasin, the index indicated that increasing rainfall erosivity could not account for the reduction in riverine sediment load and that anthropogenic erosion-control measures played a key role. The index values for the Min, Jialing and Wu subbasins ranged from 76 to 97 % and for the upper Yangtze basin is 95 %, demonstrating the joint effects of precipitation and human activities in all basins, with erosion-controlling measures playing a major role in sediment load reduction.

Towards understanding river sediment dynamics as a basis for improved catchment, channel, and coastal management: the case of the Motueka catchment, Nelson, New Zealand

ABSTRACTThis paper brings together work in the Motueka catchment that has focused on both suspended sediment data and bedload transfers to provide a more holistic understanding of sediment dynamics in the catchment to inform effective river management. The annual suspended sediment load averages 349,000 t and shows considerable temporal variability (49,000 t to 1.7 Mt). Event yields may increase by an order of magnitude in response to single high magnitude storm events. Much of the sediment is generated from high rainfall areas of the catchment under indigenous forest and grassland. Short-term studies show pasture areas have a higher specific sediment yield than production forest, but that forest harvesting leads to a short-term increase in yield. Bedload transfers assessed via morphological budgeting from digital elevation model (DEM) differencing in selected reaches of the upper Motueka reveal similarly highly variable transfers at an annual scale, reflecting the magnitude and frequency of competent flow events. Longer term mean bed-level (MBL) changes demonstrate a high degree of spatial variability in the upper Motueka. Overall, DEMs of difference and longer term MBL changes both reveal a net channel degradation and export of bedload in the mainstem of the upper Motueka. Suspended sediment data also suggest an overall reduction in sediment yield from the catchment, suggesting a catchment-wide limitation of sediment supply, or a period of lower flows reducing sediment mobilization. This understanding has informed on issues such as the role of river channel management and catchment land use on in-stream ecosystems, coastal erosion, and shelf water quality and fisheries. Future river management, if it is to be effective, needs to recognize the history of this system, its likely longer term trajectory, and its linkages with the coast.

Temporal change of runoff and sediment load and their differential response to human activities: A case study for a well-vegetated mountain watershed of southern China

This study examined the temporal trends of runoff and sediment load and their differential response to human activities in the Lishui river, a tributary of the Yangtze river in southern China. The long-term observation data at four gauging stations, generally involving two periods from 1954 to 1985 and from 2007 to 2011, were used. We detected no significant temporal trend for both the annual runoff volume (Q) and the annual suspended Sediment Load (SL) over more than 30 years before 1985. The flow duration curves and the Suspended Sediment Concentration (SSC) also hold constant before 1985. Compared with the period before 1985, SL has decreased by about 80% though Q remains unchanged for the period after 2007. Detailed examination shows that the flow duration curves after 2007 have changed with a significant decrease in the high-flow component, which acts as a major cause for the decreasing SL. In addition, SSC has decreased by several times, which also contributes to the decrease in SL after 2007. Both decreases in high-flow discharges and in SSC can be linked with recent human activities, mainly including vegetation establishment and dam constructions. The constant Q and the decreasing SL are also reported for the main stream of the Yangtze River and other major rivers in southern China, although they are orders of magnitude larger than our study area in drainage area size. The present study highlights the importance of high-flow discharges on SL and suggests that the use of SL is more appropriate to reflect environmental change than Q.

An assessment of sediment-transport processes in the Lower Mekong River based on deposit grain sizes, the CM technique and flow-energy data

The Lower Mekong River has been an important research topic for at least 15years, notably in the fields of hydrology, fluvial geomorphology, and the impact of dams. Recent papers refer mostly to the impact of the Lancang chain of hydroelectric dams constructed on the Chinese section of the river. Among the pending scientific questions are (1) the upstream–downstream variations in the concentration and yield of suspended sediment and (2) the relative importance of sand in the total yield. The general consensus among the scientific community is that the relative importance of sand in suspended load is not the main scientific concern despite its extensive presence in the Mekong River channel, as noted by geomorphologists, and despite its extreme importance for the stability of the delta shoreline in Viet Nam.This paper contradicts the general consensus. Its objective is to present new information on the processes of sand transport in the Lower Mekong channel. Imagery, field observations, sampling on the river banks, and grain size analysis of sand deposits have been carried out between Chiang Sean (downstream of the Chinese border) and the delta. The C–M image technique served to discriminate between the various transport processes (bedload and different types of suspension). This technique helps in understanding the changing processes responsible for downstream sediment transfer in river channels.The results of this study are the following.(1)Discrepancies in the values of annual suspended-sediment load detected by different gauging stations have not yet been satisfactorily explained. The CM image technique indicates that grain size and the quantity of sand transiting in suspension vs. bedload vary from upstream to downstream.(2)These upstream–downstream differences may be related to the energy of floods. Energy helps to explain longitudinal variations in the relative importance of the different modes of transport and serves to distinguish between bedload, graded and uniform suspension, and wash load.(3)The Mekong is seen as a river carrying mostly silt and clay; however, the results suggest that sand plays a major role in downstream sediment transfer during floods. This result is in line with the large amounts of sand present in the channel. Its relative importance in the total yield must be reassessed, as suggested by Walling (2005, 2008). As may be expected, the role of sand in the stability of the delta shoreline is of prime importance.

Geomorphology and environmental management of the Yellow River source zone

628 Geologic, climatic and anthropogenic factors have fashioned the distinctive landscapes and ecosystems of the Yellow River source zone (Brierley and Huang 2013; Figure 1). Located at the north-east corner of the Qinghai-Tibetan Plateau (QTP), at elevations between 2500 and 4500 m, this area has an arid/semi-arid climate. Many of these landscapes retain a significant imprint from Quaternary environmental changes. This mix of factors has induced marked variability in vegetation patterns and land use potential across this vast area, where land use practices have maintained grazing adapted ecosystems for several thousand years (Miehe et al. 2009). Despite the low population density, development pressures and climate change present significant threats to ecosystem values (e.g. Liu et al. 2012; Shapiro 2012). In recent decades, grassland and wetland degradation, alongside desertification, have placed significant stress upon land and water resources and biodiversity (including various iconic and endemic species, many of which are threatened or endangered). A sustainability focus recognizes that the costs of prevention of environmental degradation are significantly less than the costs of repair (if indeed repair is realistically achievable). Effective approaches to environmental protection and rehabilitation of rivers, wetlands and grasslands build upon a landscape platform, recognizing inherent links between geodiversity and biodiversity (Blue et al. 2013; Li et al. 2012). The Yellow River originates from the Bayankala Mountains atop the Qinghai–Tibetan Plateau of Qinghai Province, from which it flows through eight provinces and autonomous regions before emptying into the Bohai Sea north of the Shandong Peninsula. With a length of over 5400 km, it is the second longest river in China and the tenth longest in the world. The basin contains approximately 9 % of China’s population and 17 % of its agricultural area (Pietz and Giordano 2009). From its source at an elevation of around 4600 m, the upper Yellow River flows through a series of deep gorges and basins atop the QTP (Nicoll et al. 2013). Alluvial reaches in wide and shallow valleys with gentle valley slopes are characterized by a wide range of river types, including anabranching, anastomosing, braided and meandering reaches (see Figure 1; Blue et al. 2013; Li et al. 2013; Yu et al. 2013). Upstream from Hekouzhen in Inner Mongolia, the upper Yellow River contributes around 200 × 108 m3 per year (about 56 % of the total annual runoff of the basin). However, the upper Yellow River has a low sediment yield, generating about 1 × 108 t suspended sediment per year (less than 10 % of the annual suspended sediment load of the whole river basin) (Wang et al. 2007). Hence, while the source zone generates the largest portion of the total flow of the Yellow River, the majority of sediment is generated from the Loess Plateau in the middle reaches of its basin. Unlike the middle and lower reaches, which are Geomorphology and Environmental Management of the Yellow River Source Zone

Regional rating curve models of suspended sediment transport for Turkey

Estimations of annual suspended sediment loads are required for various types of water resources studies. Often estimation of the sediment load is needed for ungauged watersheds. Regionalization methods provide a practical solution to solve such problems. The purpose of this study is to classify suspended sediment yields in watersheds into homogeneous regions in order to identify their regional sediment rating curves. This study has been carried out for suspended sediment stations on 26 main basins of Turkey. Long term-scale suspended sediment rating curves of 115 gauging stations in Turkey were classified using cluster analysis on the basis of hydrological homogeneity. An agglomerative hierarchical clustering algorithm is used so that stations from different geographical locations are considered in the same cluster independently of their geographical location. 115 gauging stations were clustered into 4 different homogenous regions and the regional suspended sediment rating curve was developed for each region. The performance efficiencies of the developed regional rating curves were evaluated for 8 test stations and compared to the performances of rating curves in test sites. A regionalization model is developed for estimating suspended sediment rating curves for ungauged sites in Turkey. The developed regional rating curve models result in very close performances to those of their corresponding site rating curves.

Comparison of sediment supply to San Francisco Bay from watersheds draining the Bay Area and the Central Valley of California

Quantifying suspended sediment loads is important for managing the world's estuaries in the context of navigation, pollutant transport, wetland restoration, and coastal erosion. To address these needs, a comprehensive analysis was completed on sediment supply to San Francisco Bay from fluvial sources. Suspended sediment, optical backscatter, velocity data near the head of the estuary, and discharge data obtained from the output of a water balance model were used to generate continuous suspended sediment concentration records and compute loads to the Bay from the large Central Valley watershed. Sediment loads from small tributary watersheds around the Bay were determined using 235 station-years of suspended sediment data from 38 watershed locations, regression analysis, and simple modeling. Over 16 years, net annual suspended sediment load to the head of the estuary from its 154,000 km 2 Central Valley watershed varied from 0.13 to 2.58 (mean = 0.89) million metric t of suspended sediment, or an average yield of 11 metric t/km 2 /yr. Small tributaries, totaling 8145 km 2 , in the nine-county Bay Area discharged between 0.081 and 4.27 (mean = 1.39) million metric t with a mean yield of 212 metric t/km 2 /yr. The results indicate that the hundreds of urbanized and tectonically active tributaries adjacent to the Bay, which together account for just 5% of the total watershed area draining to the Bay and provide just 7% of the annual average fluvial flow, supply 61% of the suspended sediment. The small tributary loads are more variable (53-fold between years compared to 21-fold for the inland Central Valley rivers) and dominated fluvial sediment supply to the Bay during 10 out of 16 yr. If San Francisco Bay is typical of other estuaries in active tectonic or climatically variable coastal regimes, managers responsible for water quality, dredging and reusing sediment accumulating in shipping channels, or restoring wetlands in the world's estuaries may need to more carefully account for proximal small urbanized watersheds that may dominate sediment supply. • Fluvial suspended sediment loads to the tidal zone of San Francisco Bay. • Suspended sediment loads in Central Valley Rivers vary 21-fold between years. • Suspended sediment loads in small tributaries vary 53-fold between years. • Step-changes in sediment loads between wet and dry periods were observed. • Small tributaries covering 5% of the watershed area, supply 61% of the sediment.

Estimating quantiles of annual maximum suspended sediment load in the tributaries of the Lower Mekong River

Sediment is an important factor involved in sustainable development of water resources systems. Suspended sediment is a major portion of total load transported by streams. Quantification of annual maximum suspended sediment load (SSLm) and its quantiles (SSLm*) is the basic requirement for planning, design and management of hydraulic structures and water resources-related projects. Less sediment sampling is very common in developing and remote regions and it is the main constraint for extreme event study. This research attempts to estimate site-specific SSLm* based on discontinuous time series with low sampling frequency and develop regional empirical approaches for predicting SSLm* of ungauged tributaries in the Lower Mekong Basin. Firstly, SSLm were distinguished from the observed records. Due to data insufficiency for fitting probability distribution, two methods were then applied to estimate SSLm for each station, but only the one using regional linear regression was valid. The site-specific SSLm* could be estimated with an accuracy of an average mean absolute percentage error of 35%. Finally, empirical equations associated with catchment area with slope greater than 15% (A15) were established and can be used to estimate SSLm* until 100-year return period with A15 ranging between 120 and 6,540 km2.

石垣島・水田地帯からの浮遊土砂流出量の長期観測およびL-Q式の適用

Suspended sediment runoff from paddy fields is neglected compared with sediment runoff from sugarcane and pineapple fields in the Ishigaki Island, Okinawa. We observed suspended sediment runoff from the double-cropping paddy fields of the island and examined the applicability of L-Q equation for the observations. Firstly, the annual suspended sediment load was 7,130 kg per ha. The runoff load during no-rainfall periods accounted for 54 percentages of the total runoff load, and the runoff load in puddling and transplanting periods accounted for 76 percentages of the total. Secondly, the different specific L-Q equations in puddling and transplanting periods and the other period improved the estimation of the annual suspended sediment runoff load.

Statistical analysis and estimation of annual suspended sediments of major rivers in Japan

We evaluate the spatiotemporal trends of recent suspended sediment conditions in Japanese rivers. Statistical and spatiotemporal trend analysis is conducted on the 92 major rivers in Japan based on water quality monitoring data from 1992 to 2005. The Mann-Kendall non-parametric method was used to investigate the spatial and temporal trends for the suspended sediment indicator. Results show that the mean concentration of suspended sediments in Japanese rivers has generally declined in recent years, although there are still water quality problems at some monitoring sites (Kanto, Chubu, Kinki and Kyushu regions). A positive relationship between observed yearly discharge and suspended sediment load was found. Land use maps with 100 meter spatial resolution were used to apply an empirical model and develop a regression model for estimating annual suspended sediment loads directly from land use and hydrologic data. Rivers were assigned to three groups according to statistical cluster analysis of suspended sediment (SS) concentration. The correlation between the simulation result from the empirical model and the observed data had R(2) values of 0.62 and 0.71 for groups 2 and 3, and the correlation between the simulation result from the regression model and the observed data had R(2) values of 0.48 and 0.34 for groups 2 and 3. Results show that the proposed simulation technique can be used to predict the pollutant loads to river basins in Japan. Results also suggest prioritization methods and strategies that policy-makers can use to address suspended sediment pollution in rivers and water quality management in general.

Contribution of diffuse hillslope erosion to the sediment export of French rivers

Soil erosion is one of the major drivers of landscape evolution in Western Europe. However, depending on the land use characteristics and on the geological and topographical settings, miscellaneous forms of erosion may lead to a very diverse morphological evolution. A key element is the connection between the hillslope and the river. In this context, the objective of this study is to investigate to what extent eroded soil particles contribute to river sediment exports for the major French basins. The hillslope erosion rates are deduced from a distributed pan-European assessment of hillslope, rill, and interrill erosion by water, which was quantified by interpolation of erosion plot data as a function of topographical, land use, and pedological parameters. Sediment fluxes were mainly assessed from the French river quality database using an improved rating curve approach from the daily discharge data (IRCA), which allows estimation of the mean annual suspended sediment load. A similar approach based on the same datasets was used to calculate the mean annual dissolved load over the same period. The resulting mean annual suspended sediment loads show that French rivers export approximately 50 Mt of sediments per year to the seas, including ca. 17 Mt as particulate matter, corresponding to a specific sediment yield of ca. 0.4tha−1yr−1. No relationship was found between the mass of the exported sediment and the size of the drainage basins, but large differences were observed between the different basins. For the Seine river basin and the Rhône river basin, the estimated average gross erosion rates are comparable, with ca. 1.14tha−1yr−1 for the Rhone and ca. 1.80tha−1yr−1 for the Seine. In contrast, the Rhône exports ca. 10 times more than the Seine River. The ratio of gross erosion to sediment exports calculated in this work ranges from 6.8% for the Seine to more than 100% for the Rhône. To explain these variations, we propose the use of indices that represent the landscape patchiness and connectivity.

Analyzing the uncertainty of suspended sediment load prediction using sequential data assimilation

► Estimated snow melt and streamflow using SNOW-17 and SAC-SMA models. ► Applied ensemble data assimilation to improve predictions and estimate uncertainty. ► Developed an approach to dynamically adjust model error variance scaling factors. ► Linked streamflow ensemble to non-linear regression model of suspended sediment. ► Estimated annual suspended sediment load for watershed in the Lake Tahoe Basin. A first step in understanding the impacts of sediment and controlling the sources of sediment is to quantify the mass loading. Since mass loading is the product of flow and concentration, the quantification of loads first requires the quantification of runoff volume. Using the National Weather Service’s SNOW-17 and the Sacramento Soil Moisture Accounting (SAC-SMA) models, this study employed particle filter based Bayesian data assimilation methods to predict seasonal snow water equivalent (SWE) and runoff within a small watershed in the Lake Tahoe Basin located in California, USA. A procedure was developed to scale the variance multipliers (a.k.a hyperparameters) for model parameters and predictions based on the accuracy of the mean predictions relative to the ensemble spread. In addition, an online bias correction algorithm based on the lagged average bias was implemented to detect and correct for systematic bias in model forecasts prior to updating with the particle filter. Both of these methods significantly improved the performance of the particle filter without requiring excessively wide prediction bounds. The flow ensemble was linked to a non-linear regression model that was used to predict suspended sediment concentrations (SSCs) based on runoff rate and time of year. Runoff volumes and SSC were then combined to produce an ensemble of suspended sediment load estimates. Annual suspended sediment loads for the 5 years of simulation were finally computed along with 95% prediction intervals that account for uncertainty in both the SSC regression model and flow rate estimates. Understanding the uncertainty associated with annual suspended sediment load predictions is critical for making sound watershed management decisions aimed at maintaining the exceptional clarity of Lake Tahoe. The computational methods developed and applied in this research could assist with similar studies where it is important to quantify the predictive uncertainty of pollutant load estimates.