Estimating suspended sediment concentrations in areas with limited hydrological data using a mixed‐effects model

Sediment rating curves are commonly used to estimate the suspended sediment load in rivers and streams under the assumption of a constant relation between discharge (Q) and suspended sediment concentrations (SSC) over time. However, temporal variation in the sediment supply of a watershed results in shifts in this relation by increasing variability and by introducing nonlinearities in the form of hysteresis or a path‐dependent relation. In this study, we used a mixed‐effects linear model to estimate an average SSC–Q relation for different periods of time within the hydrologic cycle while accounting for seasonality and hysteresis. We tested the performance of the mixed‐effects model against the standard rating curve, represented by a generalized least squares regression, by comparing observed and predicted sediment loads for a test case on the Chilliwack River, British Columbia, Canada. In our analyses, the mixed‐effects model reflected more accurate patterns of interpolated SSC from Q data than the rating curve, especially for the low‐flow summer months when the SSC–Q relation is less clear. Akaike information criterion scores were lower for the mixed‐effects model than for the standard model, and the mixed‐effects model explained nearly twice as much variance as the standard model (52% vs 27%). The improved performance was achieved by accounting for variability in the SSC–Q relation within each month and across years for the same month using fixed and random effects, respectively, a characteristic disregarded in the sediment rating curve. Copyright © 2012 John Wiley & Sons, Ltd.     

Use of turbidometry to characterize suspended sediment and phosphorus fluxes in the Lake Tahoe basin,California, USA

The efficacy of in‐stream nephelometric turbidometry as a surrogate for total suspended solids (TSS) and total phosphorus (TP) concentrations was evaluated for use in low turbidity (<50 NTU) subalpine watersheds at Lake Tahoe, California–Nevada, USA. Continuous turbidity records for the 1999, 2000 and 2001 snowmelt seasons and data from water quality samples (1982–2000) were examined to determine watershed sediment delivery dynamics. Strong correlations were found between turbidity and both TSS and TP concentration. The strong correlation indicates that turbidity can serve as a good surrogate for direct measurement in these watersheds. The watersheds displayed clockwise hysteresis: sediment flushing and depletion, on daily, seasonal and decadal time‐scales. The hysteresis curves had strong concave shapes, indicating a sensitive response to peak flow. A pronounced seasonal trend was observed for the ratio of suspended sediment concentration (SSC)/discharge over time, indicating early season flushing of available sediment. Significant linear relationships (p < 0·05) were found for 12 of 17 years. Comparison of annual sediment rating curve coefficients indicated smaller coefficients during high sediment loading years and in the years following. The smaller coefficients are evidence of sediment depletion during high flow years. The effect of hysteresis on monitoring methods was illustrated by comparing turbidity estimates of TSS load with sediment rating curve estimates of SSC. After accounting for differences in SSC/TSS methods of analysis, daily loads calculated with turbidity methods were 58–98% of rating curve estimates for the spring snowmelt seasons of 1999–2001. Copyright © 2006 John Wiley & Sons, Ltd.     

Suspended sediment dynamics associated with snowmelt runoff in a small mountain stream of Lake Tahoe (Nevada)

The Lake Tahoe basin is experiencing an environmental decline that is partly due to sediment intakes from its tributaries. Many studies have estimated suspended sediment loads in these streams with a discrete sampling programme by collecting water samples and using a rating technique. However, the relationship between stream discharge and suspended sediment concentration (SSC) in these tributaries is known to differ during the rising and falling limbs of the snowmelt‐dominated hydrograph. Because of this hysteresis effect, sediment rating curves are poor predictors of suspended sediment dynamics in the stream. In this study, suspended sediment transport was investigated using a turbidity meter to provide a continuous record of sediment concentration during the snowmelt period. Hysteresis in suspended sediment transport was also investigated and is quantified with an H index, which is the ratio of the areas under the curve at different stages of the hydrograph. The temporal lag between the peak of SSC and the peak of stream discharge was quantified using cross‐correlation analysis. For almost all events, SSCs were higher during the rising limb of the hydrograph for a given discharge, with SSC peaks occurring before discharge peaks, resulting in clockwise hysteresis (H > 1). The H indices increased (looser hysteresis loop) as the availability of sediments increased and as the lag between peaks in SSC and discharge was larger. A restriction of the proposed H index was that it could only be computed when stream discharge increased by more than 30% during a melt event. Copyright © 2005 John Wiley & Sons, Ltd.     

Time‐varying,nonlinear suspended sediment rating curves to characterize trends in water quality: An application to the Upper Hudson and Mohawk Rivers,New York

This work proposes two modelling frameworks for diagnosing temporal variations in nonlinear rating curves that describe suspended sediment–discharge relationships. A variant of the weighted regression on time, discharge, and season model is proposed and is compared against dynamic nonlinear modelling, a newly developed nonlinear time series filter based on sequential Monte Carlo sampling. Both approaches estimate a time series of rating curve parameters, with uncertainty, that can be used to diagnose variability in the sediment–discharge relationship over time. We evaluate the models with a variety of synthetic scenarios to highlight their ability to estimate signals of known rating curve change. Results reveal important bias‐variance trade‐offs unique to each approach, and in general, suggest that dynamic nonlinear modelling is better suited for rapid rating curve changes, whereas the weighted regression on time, discharge, and season variant more precisely estimates slow change. The techniques are then applied in two case studies in the Upper Hudson and Mohawk Rivers in New York. We conclude with a discussion of the implications of dynamic rating curves for the management of water quality in riverine and estuary systems.     

Suspended sediment dynamics in a large drainage basin: the River Rhine

The behaviour of suspended sediment in rivers is often a function of energy conditions, i.e. sediment is stored at low flow and transported under high discharge conditions. The timing of maximum sediment transport can, however, also be related to mixing and routing of water and sediment from different sources. In this study suspended sediment transport was studied in the River Rhine between Kaub and the German–Dutch border. As concentrations decrease over a runoff season and as the relationship between water discharge and suspended sediment concentrations during most floods is characterized by clockwise hysteresis, it is concluded that sediment depletion occurs during a hydrological year and during individual floods. However, analyses of the sediment contribution from the River Mosel indicate that clockwise hysteresis may result from sediment depletion as well as from early sediment supply from a tributary. Thus, although the suspended sediment behaviour in the downstream part of the River Rhine is partly a transport phenomenon related to energy conditions, mixing and routing of water from different sources also plays an important role. Suspended sediment transport during floods was modelled using a ‘supply‐based’ model. Addition of a sediment supply term to the sediment rating curve leads to a model that produces better estimates of instantaneous suspended sediment concentrations during high discharge events. A major constriction of the model is that it cannot be used to predict suspended sediment concentrations as long as the amount of sediment in storage and the timing of sediment supply are unknown. Copyright © 1999 John Wiley & Sons, Ltd.     

Predicting suspended sediment concentrations in the Meuse river using a supply‐based rating curve

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

Controls on suspended‐sediment transfer at a high‐arctic glacier,determined from statistical modelling

Simple linear regression models have been widely employed in the analysis of suspended‐sediment concentration (SSC) time series from glacierized catchments, although they have many limitations. This paper builds regression models which address these shortcomings and permit inferences concerning the controls on suspended‐sediment transfer from a glacier at 78°N in the Svalvard archipelago. A bivariate regression model, deterministically predicting SSC from discharge alone, explained less than 15 per cent of the variance in SSC. A multivariate model, incorporating additional potentially explanatory variables, offered little improvement. Diurnal hysteresis in the data gives rise to quasi‐autocorrelation in the residual series from regression models. This was effectively removed by incorporating dummy diurnal variables into the multivariate model. The presence of a first‐order autoregressive, stochastic process gives rise to true autocorrelation in the residual series from regression models. This was accommodated by incorporating an ARIMA (1,0,0) term into a multivariate autoregression model. The model‐building process yielded a systematic progression in the explanation of variance in SSC, stripping away pattern in the autocorrelation function of the residual series; mean model error was reduced from 54 per cent to 6 per cent. The dependence of SSC on the magnitude of discharge is weak and highly variable, whereas the dependence of current SSC on recent values of SSC, revealed through the stochastic term, is an order of magnitude greater and relatively constant during the melt season. The dominant control on SSC throughout the melt season is therefore short‐term sediment availability. The simple and largely unchanging stochastic process generally responsible for generating the observed SSC series implies a simple and unchanging glacier drainage system. Copyright © 1999 John Wiley & Sons, Ltd.     

Assessment of suspended sediment transport in four alpine watersheds (France): influence of the climatic regime

High‐frequency water discharge and suspended sediment concentration (SSC) databases were collected for 3 years on four contrasted watersheds: the Asse and the Bléone (two Mediterranean rainfall regime watersheds) and the Romanche and the Ferrand (two rainfall–snowmelt regime watersheds). SSCs were calculated from turbidity recordings (1‐h time step), converted into SSC values. The rating curve was calculated by means of simultaneous SSC measurement taken by water sampling and turbidity recording. Violent storms during springtime and autumn were responsible for suspended sediment transport on the Asse and the Bléone rivers. On the Ferrand and the Romanche, a large share of suspended sediment transport was also caused by local storms, but 30% of annual fluxes results from snowmelt or icemelt which occurred from April to October. On each watershed, SSC up to 50 g l^?1 were observed. Annual specific fluxes ranged from 450 to 800 t km^?2 year^?1 and 40–80% of annual suspended sediment fluxes occurred within 2% of the time. These general indicators clearly demonstrate the intensity of suspended sediment transport on these types of watersheds. Suspended sediment fluxes proved to be highly variable at the annual scale (inter‐annual variability of specific fluxes) as well as at the event scale (through a hysteresis loop in the SSC/Q relationship) on these watersheds. In both cases, water discharge and precipitations were the main processes involved in suspended sediment production and transport. The temporal and spatial variability of hydro‐meteorological processes on the watershed provides a better understanding of suspended sediment dynamics. Copyright © 2009 John Wiley & Sons, Ltd.     

River suspended sediment modelling using a fuzzy logic approach

Correct estimation of sediment volume carried by a river is very important for many water resources projects. Conventional sediment rating curves, however, are not able to provide sufficiently accurate results. In this paper, a fuzzy logic approach is proposed to estimate suspended sediment concentration from streamflow. This study provides forecasting benchmarks for sediment concentration prediction in the form of a numerical and graphical comparison between fuzzy and rating‐curve models. Benchmarking was based on a 5‐year period of continuous streamflow and sediment concentration data of Quebrada Blanca Station operated by the United States Geological Survey. The benchmark results showed that the fuzzy model was able to produce much better results than rating‐curve models. The fuzzy model proposed in the study is site specific and does not simulate the hysteresis effects. Copyright © 2006 John Wiley & Sons, Ltd.     

Dynamics of suspended sediment transport and yield in a large agricultural catchment,southwest France

The dynamics of suspended sediment transport were monitored continuously in a large agricultural catchment in southwest France from January 2007 to March 2009. The objective of this paper is to analyse the temporal variability in suspended sediment transport and yield in that catchment. Analyses were also undertaken to assess the relationships between precipitation, discharge and suspended sediment transport, and to interpret sediment delivery processes using suspended sediment‐discharge hysteresis patterns. During the study period, we analysed 17 flood events, with high resolution suspended sediment data derived from continuous turbidity and automatic sampling. The results revealed strong seasonal, annual and inter‐annual variability in suspended sediment transport. Sediment was strongly transported during spring, when frequent flood events of high magnitude and intensity occurred. Annual sediment transport in 2007 yielded 16 614 tonnes, representing 15 t km^?2 (85% of annual load transport during floods for 16% of annual duration), while the 2008 sediment yield was 77 960 tonnes, representing 70 t km^?2 (95% of annual load transport during floods for 20% of annual duration). Analysis of the relationships between precipitation, discharge and suspended sediment transport showed that there were significant correlations between total precipitation, peak discharge, total water yield, flood intensity and sediment variables during the flood events, but no relationship with antecedent conditions. Flood events were classified in relation to suspended sediment concentration (SSC)–discharge hysteretic loops, complemented with temporal dynamics of SSC–discharge ranges during rising and falling flow. The hysteretic shapes obtained for all flood events reflected the distribution of probable sediment sources throughout the catchment. Regarding the sediment transport during all flood events, clockwise hysteretic loops represented 68% from river deposited sediments and nearby source areas, anticlockwise 29% from distant source areas, and simultaneity of SSC and discharge 3%. Copyright © 2010 John Wiley & Sons, Ltd.     

An evaluation of sediment rating curves for estimating suspended sediment concentrations for subsequent flux calculations

In the absence of actual suspended sediment concentration (SSC) measurements, hydrologists have used sediment rating (sediment transport) curves to estimate (predict) SSCs for subsequent flux calculations. Various evaluations of the sediment rating‐curve method were made using data from long‐term, daily sediment‐measuring sites within large (>1 000 000 km²), medium (<1 000 000 to >1000 km²), and small (<1000 km²) river basins in the USA and Europe relative to the estimation of suspended sediment fluxes. The evaluations address such issues as the accuracy of flux estimations for various levels of temporal resolution as well as the impact of sampling frequency on the magnitude of flux estimation errors. The sediment rating‐curve method tends to underpredict high, and overpredict low SSCs. As such, the range of errors associated with concomitant flux estimates for relatively short time‐frames (e.g. daily, weekly) are likely to be substantially larger than those associated with longer time‐frames (e.g. quarterly, annually) because the over‐ and underpredictions do not have sufficient time to balance each other. Hence, when error limits must be kept under ±20%, temporal resolution probably should be limited to quarterly or greater. The evaluations indicate that over periods of 20 or more years, errors of <1% can be achieved using a single sediment rating curve based on data spanning the entire period. However, somewhat better estimates for the entire period, and markedly better annual estimates within the period, can be obtained if individual annual sediment rating curves are used instead. Relatively accurate (errors <±20%) annual suspended sediment fluxes can be obtained from hydrologically based monthly measurements/samples. For 5‐year periods or longer, similar results can be obtained from measurements/samples collected once every 2 months. In either case, hydrologically based sampling, as opposed to calendar‐based sampling is likely to limit the magnitude of flux estimation errors. Published in 2003 John Wiley & Sons, Ltd.     

Suspended sediment transport in a small Mediterranean agricultural catchment

The aim of this study is to analyze suspended sediment transport in a Mediterranean agricultural catchment under traditional soil and water conservation practices. Field measurements were conducted in Can Revull, a small ephemeral catchment (1.03 km²) on the island of Mallorca. This study uses continuous turbidity records to analyse suspended sediment transport regimes, construct and interpret multiple regression models of total suspended sediment concentration (SSC) and of SSC related to stormflow discharge, and assess the sediment loads and yields of three hydrological years (2004–2005 to 2006–2007). An annual average SSC of 17.3 mg l^?1, with a maximum of 2270 mg l^?1, was recorded in the middle of the winter period when rainfall intensities are high and headwater slopes are ploughed and thus bare. Strong seasonal contrasts of baseflow dynamics associated with different degrees of dilution provide a large scatter in SSC and in the derived rating curves, reflecting that other factors control the supply of suspended sediment. Multiple regression models identify rainfall intensity as the most significant variable in sediment supply. However, under baseflow conditions, physical and biological processes generate sediment in the channel that is subsequently removed during high flow. In contrast, when baseflow is not present, rainfall intensity is the only process that supplies sediment to the channel, mostly from hillslopes. Considering the study period as average in terms of total annual rainfall and intensities, suspended sediment yields were an order of magnitude lower than those obtained in other Mediterranean catchments, a factor that can be related to the historical use of soil conservation practices. Copyright © 2009 John Wiley & Sons, Ltd.     

Suspended sediment dynamics at high and low storm flows in two small watersheds

A record spanning almost 20 years of suspended sediment and discharge measurements on two reaches of an agricultural watershed is used to assess the influence of in‐channel sediment supplies and bed composition on suspended sediment concentrations (SSC). We analyse discharge‐SSC relationships from two small streams of similar hydrology, climate and land use but widely different bed compositions (one dominated by sand, the other by gravel). Given that sand‐dominated systems have more fine sediment available for transport, we use bed composition and the relative proportion of surface sand and gravel to be representative of in‐channel sediment supply. Both high flow events and lower flows associated with onset and late recessional storm flow (‘low flows’) are analysed in order to distinguish external from in‐channel sources of sediment and to assess the relationship between low flows and sediment supply. We find that SSC during low flows is affected by changes to sediment supply, not just discharge capacity, indicated by the variation in the discharge‐SSC relationship both within and between low flows. Results also demonstrate that suspended sediment and discharge dynamics differ between reaches; high bed sand fractions provide a steady supply of sediment that is quickly replenished, resulting in more frequent sediment‐mobilizing low flow and relatively constant SSC between floods. In contrast, SSC of a gravel‐dominated reach vary widely between events, with high SSC generally associated with only one or two high‐flow events. Results lend support to the idea that fine sediment is both more available and more easily transported from sand‐dominated streambeds, especially during low flows, providing evidence that bed composition and in‐channel sediment supplies may play important roles in the mobilization and transport of fine sediment. In addition, the analysis of low‐flow conditions, an approach unique to this study, provides insight into alternative and potentially significant factors that control fine sediment dynamics. Copyright © 2007 John Wiley & Sons, Ltd.     

Estimation of suspended sediment concentration and yield using linear models,random forests and quantile regression forests

For sediment yield estimation, intermittent measurements of suspended sediment concentration (SSC) have to be interpolated to derive a continuous sedigraph. Traditionally, sediment rating curves (SRCs) based on univariate linear regression of discharge and SSC (or the logarithms thereof) are used but alternative approaches (e.g. fuzzy logic, artificial neural networks, etc.) exist. This paper presents a comparison of the applicability of traditional SRCs, generalized linear models (GLMs) and non‐parametric regression using Random Forests (RF) and Quantile Regression Forests (QRF) applied to a dataset of SSC obtained for four subcatchments (0·08, 41, 145 and 445 km²) in the Central Spanish Pyrenees. The observed SSCs are highly variable and range over six orders of magnitude. For these data, traditional SRCs performed inadequately due to the over‐simplification of relating SSC solely to discharge. Instead, the multitude of acting processes required more flexibility to model these nonlinear relationships. Thus, alternative advanced machine learning techniques that have been successfully applied in other disciplines were tested. GLMs provide the option of including other relevant process variables (e.g. rainfall intensities and temporal information) but require the selection of the most appropriate predictors. For the given datasets, the investigated variable selection methods produced inconsistent results. All proposed GLMs showed an inferior performance, whereas RF and QRF proved to be very robust and performed favourably for reproducing sediment dynamics. QRF additionally provides estimates on the accuracy of the predictions and thus allows the assessment of uncertainties in the estimated sediment yield that is not commonly found in other methods. The capabilities of RF and QRF concerning the interpretation of predictor effects are also outlined. Copyright © 2008 John Wiley & Sons, Ltd.     

Inverse flood risk modelling under changing climatic conditions

One of the most significant anticipated consequences of global climate change is the increased frequency of hydrologic extremes. Predictions of climate change impacts on the regime of hydrologic extremes have traditionally been conducted using a top‐down approach. The top‐down approach involves a high degree of uncertainty associated with global circulation model (GCM) outputs and the choice of downscaling technique. This study attempts to explore an inverse approach to the modelling of hydrologic risk and vulnerability to changing climatic conditions. With a focus targeted at end‐users, the proposed approach first identifies critical hydrologic exposures that may lead to local failures of existing water resources systems. A hydrologic model is used to transform inversely the main hydrologic exposures, such as floods and droughts, into corresponding meteorological conditions. The frequency of critical meteorological situations is investigated under present and future climatic scenarios by means of a generic weather generator. The weather generator, linked with GCMs at the last step of the proposed methodology, allows the creation of an ensemble of different scenarios, as well as an easy updating, when new and improved GCM outputs become available. The technique has been applied in Ontario, Canada. The results show significant changes in the frequency of hydro‐climatic extremes under future climate scenarios in the study area. Copyright © 2007 John Wiley & Sons, Ltd.     

Sediment dynamics of an allogenic river channel in a very arid environment

Despite a growing interest in the sediment dynamics of dryland rivers, most research has been based on ephemeral streams in endogenic hydrological systems (runoff and sediment transport determined by local precipitation). Less attention, however, has been paid to allogenic and perennial dryland rivers. Here, we report a case study on the suspended sediment dynamics of the Tarim River, an allogenic and perennial river flowing in a very arid environment in China, based on mean daily discharge and mean daily suspended sediment concentration (SSC) over the last 5 decades (1960–2011). Results reveal that discharge and SSCs are predominantly low and have distributions with large positive skewness. The SSC–discharge relationships can be fairly well generalized by power functions, with quite large scatter at extreme (low and high) flow conditions. Marked temporal and spatial variations were observed in the effective discharge for sediment transport. The frequency of the effective discharge ranged from 0.5% to 2% (or an average flow duration of 2–7 days/year), implying that moderate to high flows play an important role in sediment transport. The sediment rating curves show strong hysteresis effects, with 3 types of hysteresis loops observed, clockwise (the most predominant), anticlockwise, and figure of eight. The high potential for bank collapse near the peak and falling limb of the flood hydrograph is the major cause of anticlockwise and figure‐of‐eight hysteresis loops.     

Evaluating spatial variation of suspended sediment rating curves in the middle Yellow River basin,China

Abstract The suspended sediment load in the middle Yellow River basin (YRB) cannot be well predicted by capacity‐based transport formulas because a large fraction of suspended sediment load is composed of wash load. This study evaluated the spatial variations of sediment rating curves (SRCs) in the middle YRB. Both power and linear SRCs were used to fit daily flow and suspended sediment concentration (SSC) historical data at 49 gauging stations throughout the middle YRB. The spatial variation in regression coefficients was investigated, and the relationship between regression coefficients and the physical characteristics of watersheds was discussed. The results indicate that SRC regression coefficients vary with drainage area and basin slope, but their responses to these parameters are remarkably different in watersheds with different underlying surfaces, which indicates the significance of sediment availability, erodibility, and grain size distribution. For power SRCs representing sediment transport in unsaturated flows, the regression coefficients are more closely correlated with the drainage area in loess regions and with the basin slope in rock mountain regions. For linear SRCs representing sediment transport in saturated flows, saturated SSCs vary with coarse (particle size > 0.05 mm) and fine (particle size < 0.01 mm) fractions in suspended sediment. The maximum saturated SSC among the different gauging stations is associated with the optimal grain size composition of suspended sediment, which has been proposed for loess regions in previous studies. This study provides theoretical support for estimating the regression parameters for sediment transport modelling, especially in ungauged basins.     

Seasonal patterns of suspended sediment transport in an abandoned farmland catchment in the Central Spanish Pyrenees

The suspended sediment response of a small catchment subjected to farmland abandonment and subsequent plant recolonization was studied in relation to its hydrological functioning. The analysis of data over a seven‐year period demonstrated that suspended sediment yield was greatly influenced by the occurrence of intense, low‐frequency events. Greater amounts of suspended sediment were exported during spring, when the catchment was hydrologically more active. Rainfall intensity and baseflow at the start of a flood event had a strong influence on the sediment response, suggesting that several hydrological processes were active within the catchment. SSC (suspended sediment concentration)‐Q hysteretic loop analysis at the event scale aided understanding of the sedimentological and hydrological behaviour of the catchment. During the study period the SSC‐Q loops showed a high degree of seasonality and two main patterns strongly related to catchment wetness were distinguished. When the catchment was dry (mainly during summer and the beginning of autumn) the predominant process was infiltration excess runoff over sparsely vegetated areas close to the main channel. Under these conditions, floods exhibited a counter‐clockwise hysteretic loop and were characterized by a small streamflow response, short duration and high SSC. Under wet conditions (mainly during winter and spring), saturation excess runoff was increasingly dominant over vegetated areas. Under these conditions, floods exhibited a clockwise hysteretic loop, and were characterized by a larger streamflow response, longer duration and higher suspended sediment yield. The lower SSC during the falling stage of the hydrograph is likely to be due to dilution effects related to the contribution of clean water resulting from enlargement of the saturated areas, together with an increase in the baseflow discharge. Copyright © 2009 John Wiley & Sons, Ltd.     

Integrating downscaled CMIP5 data with a physically based hydrologic model to estimate potential climate change impacts on streamflow processes in a mixed‐use watershed

Climatic changes have altered surface water regimes worldwide, and climate projections suggest that such alterations will continue. To inform management decisions, climate projections must be paired with hydrologic models to develop quantitative estimates of watershed scale water regime changes. Such modeling approaches often involve downscaling climate model outputs, which are generally presented at coarse spatial scales. In this study, Coupled Model Intercomparison Project Phase 5 climate model projections were analyzed to determine models representing severe and conservative climate scenarios for the study watershed. Based on temperature and precipitation projections, output from GFDL‐ESM2G (representative concentration pathway 2.6) and MIROC‐ESM (representative concentration pathway 8.5) were selected to represent conservative (Δ_C) and severe (Δ_S) change scenarios, respectively. Climate data were used as forcing for the soil and water assessment tool to analyze the potential effects of climate change on hydrologic processes in a mixed‐use watershed in central Missouri, USA. Results showed annual streamflow decreases ranging from ?5.9% to ?26.8% and evapotranspiration (ET) increases ranging from +7.2% to +19.4%. During the mid‐21st century, sizeable decreases to summer streamflow were observed under both scenarios, along with large increases of fall, spring, and summer ET under Δ_S. During the late 21st century period, large decreases of summer streamflow under both scenarios, and large increases to spring (Δ_S), fall (Δ_S) and summer (Δ_C) ET were observed. This study demonstrated the sensitivity of a Midwestern watershed to future climatic changes utilizing projections from Coupled Model Intercomparison Project Phase 5 models and presented an approach that used multiple climate model outputs to characterize potential watershed scale climate impacts.     

Suspended sediment and discharge relationships to identify bank degradation as a main sediment source on small agricultural catchments

Variability of suspended sediment concentration (SSC) versus discharge relationships in streams is often high and illustrates variable particle origins or availability. Particle availability depends on both new sediment supply and deposited sediment stock. The aim of this study is to improve SSC–discharge relationship interpretation, in order to determine the origins of particles and to understand the temporal dynamics of particles for two small streams in agricultural catchments from northwestern France. SSC and discharge were continuously recorded at the outlets and data were examined at different time‐scales: yearly, monthly, with distinction between flood periods and non‐flooding periods, and individual flood events. Floods are classified in relation to SSC–discharge hysteresis, and this typology is completed by the analysis of SSC–discharge ranges during rising and falling flow. We show that particles are mainly coming from channel, banks, either by hydraulic erosion or by cattle trampling. Particle availability presents a seasonal dynamics with a maximum at the beginning of autumn when discharge is low, decreasing progressively during autumn to become a minimum in winter when discharge is the highest, and increasing again in spring. Bank degradation by cattle is the determining factor in the suspended sediment dynamics. Cattle bank‐trampling produces sediment, mostly from spring to autumn, that supplies the deposited sediment stock even outside floods. This hydrologically independent process hides SSC–discharge correlation classically linked to hydraulic erosion and transport. Differences in SSC–discharge relationships and suspended sediment budgets between streams are related to differences in transport capacity and bank degradation by cattle trampling and channelization. Copyright © 2007 John Wiley & Sons, Ltd.