Suspended Sediment Yield Research Articles

Application of the Modified Universal Soil Loss Equation (MUSLE) for the prediction of sediment yield in Agewmariam experimental watershed, Tekeze River basin, Northern Ethiopia

The study utilized the Modified Universal Soil Loss Equation (MUSLE) to predict sediment loss and evaluate the model's performance in the Agewmariam experimental watershed in order to support the planning, management, and appropriate use of the soil and water resources in the watershed. The natural resources conservation service (NRCS) curve number method was used to model runoff energy factor. By overlaying maps of runoff energy, soil erodibility, slope length and steepness, cover management, and support practice factors with assigned values, the cumulative effect of these parameters for the suspended sediment yield was calculated using the ArcGIS raster calculator. The runoff energy factor was the most sensitive parameter, followed by slope length and steepness factor. To improve the model's fit to the local conditions, the initial abstraction to storage ratio (λ) of the runoff energy factor was reduced to 0.023, and the MUSLE model coefficient and exponent were adjusted to 1 and 0.59, respectively. During calibration, the mean observed and estimated suspended sediment yields were 0.2 and 0.23 ton/ha, respectively, while during validation, they were 0.7 and 0.53 ton/ha, respectively. The model evaluation showed that the MUSLE model, without calibration, was not appropriate for estimating runoff and sediment yield. However, with appropriate calibration, the model showed good performance with a coefficient of determination (R2), coefficient of efficiency (E), and index of agreement (d) of 0.85, 0.85, and 0.96 respectively, during calibration and 0.84, 0.65, and 0.83 respectively, during validation. Based on these findings, this study suggests that the calibrated MUSLE model can be used to prioritize soil and water conservation interventions within the watershed or can be extrapolated to neighboring similar watersheds. Further refinement of model input parameters using more data from the watershed is recommended to increase the prediction accuracy of the model.

Confluence-based suspended sediment fingerprinting: an exploratory approach to determining source to sink sediment contributions in the Tsitsa River catchment, Eastern Cape, South Africa

ABSTRACT In the Tsitsa River catchment of the Eastern Cape Province, South Africa, concerns regarding the sedimentation of a proposed large, multipurpose, in-channel water storage scheme prompted investigations of sub-catchment suspended sediment loads and yields. Confluence-based fingerprinting and source apportionment was undertaken to explore whether this simple approach could identify relative sub-catchment contributions to downstream sediment loads, supplementing modelling and direct sampling. In November 2018, fine sediment sequestered in bed substrates was sampled above and below confluences, using a resuspension technique. The < 63 µm sample fraction underwent analysis of mineral magnetic properties, gamma emitting radionuclide activities and geochemistry to identify suitable tracers. Conservatism and range tests were used to select different combinations of composite fingerprints at seven confluences. Bivariate source apportionment using an unmixing model showed that the approach could be used as a preliminary means of determining relative sub-catchment sediment contributions. Catchment characteristics such as geology and land cover, as well as land tenure and management, were considered as explanatory factors for the difference in sediment contributions. The results for most sub-catchments were broadly similar to those from other studies. Shortcomings of this approach included the reliance on a single sampling effort, and uncertainty associated with sub-catchment sediment transport and residence times.

Daily suspended sediment yield estimation using soft-computing algorithms for hilly watersheds in a data-scarce situation: a case study of Bino watershed, Uttarakhand

Daily suspended sediment yield estimation using soft-computing algorithms for hilly watersheds in a data-scarce situation: a case study of Bino watershed, Uttarakhand

The sedimentology of gravel beds in groundwater‐dominated chalk streams: Implications for sediment modelling and management

AbstractElevated fine sediment accumulation in a river system's gravel bed is known to cause detrimental ecological impacts. Current sediment targets and approaches to mitigation have failed due to the oversimplification of geomorphological processes controlling fine sediment accumulation and the lack of relevant scientific knowledge underpinning them. This is particularly apparent in chalk streams (groundwater‐dominated systems) which regularly exhibit high rates of sediment accumulation despite low suspended sediment yields. A necessary first step is to better characterise their sedimentology; thus, the novelty of this study was to determine the sedimentological characteristics of chalk stream gravel beds, specifically the quantity and distribution of fine sediment with depth. We collated published and unpublished freeze‐core data, encompassing 90 sites across 11 UK chalk streams. Results showed average quantities of fine sediment (&lt;2 mm) in chalk stream gravel beds were 25% by weight, with &gt;75% of beds exceeding thresholds for ecological degradation. Quantities of fine sediment increased with increasing depth into the bed, with an average increase between surface and subsurface layers of 54%, and 89% of the gravel bed over‐saturated with fine sediment. Regional differences were attributed to differences in stream power and local sediment sources, including surficial geology and catchment land use. Additionally, a major contrast was identified between experimental conditions in flume studies used to establish models describing interactions/mechanisms of fine sediment infiltration into immobile gravel beds and the natural conditions observed in chalk streams. As such, the use of such models as a basis to explore sediment management scenarios is unlikely to predict the outcome of such management techniques correctly in a real‐world situation.

Projecting sediment export from two highly glacierized alpine catchments under climate change: exploring non-parametric regression as an analysis tool

Abstract. Future changes in suspended sediment export from deglaciating high-alpine catchments affect downstream hydropower reservoirs, flood hazard, ecosystems and water quality. Yet, quantitative projections of future sediment export have so far been hindered by the lack of process-based models that can take into account all relevant processes within the complex systems determining sediment dynamics at the catchment scale. As a promising alternative, machine-learning (ML) approaches have recently been successfully applied to modeling suspended sediment yields (SSYs). This study is the first, to our knowledge, exploring a machine-learning approach to derive sediment export projections until the year 2100. We employ quantile regression forest (QRF), which proved to be a powerful method to model past SSYs in previous studies, for two nested glaciated high-alpine catchments in the Ötztal, Austria, above gauge Vent (98.1 km2) and gauge Vernagt (11.4 km2). As predictors, we use temperature and precipitation projections (EURO-CORDEX) and discharge projections (AMUNDSEN physically based hydroclimatological and snow model) for the two gauges. We address uncertainties associated with the known limitation of QRF that underestimates can be expected if values in the projection period exceed the range represented in the training data (out-of-observation-range days, OOOR). For this, we assess the frequency and extent of these exceedances and the sensitivity of the resulting mean annual suspended sediment concentration (SSC) estimates. We examine the resulting SSY projections for trends, the estimated timing of peak sediment and changes in the seasonal distribution. Our results show that the uncertainties associated with the OOOR data points are small before 2070 (max. 3 % change in estimated mean annual SSC). Results after 2070 have to be treated more cautiously as OOOR data points occur more frequently, and glaciers are projected to have (nearly) vanished by then in some projections, which likely substantially alters sediment dynamics in the area. The resulting projections suggest decreasing sediment export at both gauges in the coming decades, regardless of the emission scenario, which implies that peak sediment has already passed or is underway. This is linked to substantial decreases in discharge volumes, especially during the glacier melt phase in late summer, as a result of increasing temperatures and thus shrinking glaciers. Nevertheless, high(er) annual yields can occur in response to heavy summer precipitation, and both developments would need to be considered in managing sediments, as well as e.g., flood hazard. While we chose the predictors to act as proxies for sediment-relevant processes, future studies are encouraged to try and include geomorphological changes more explicitly, e.g., changes in connectivity, landsliding, rockfalls or vegetation colonization, as these could improve the reliability of the projections.

Sediment yield assessment and suggesting mitigation measures—A case study of Deme watershed, Omo‐Gibe Basin, Ethiopia

AbstractSoil erosion is a big challenge of natural resource development in Ethiopia due to the high expansion of agricultural activity and poor land‐use management. Deme watershed is one of the watersheds found in the Omo‐Gibe Basin, Ethiopia with soil erosion problems. It covers 1284.4 km2. The Soil and Water Assessment Tool (SWAT) integrated with ArcGIS was used to assess the sediment yield of the Deme watershed and develop mitigation measures for soil erosion. The purpose of this study was to assess the sediment yield and provide mitigation measures for the Deme watershed. Ground Control Point (GCP) data collection and field observation were conducted for land‐use land cover classification. The performance of the model was evaluated by coefficient of determination (R2), Nash Sutcliffe simulation efficiency (NSE), and percent bias (PBIAS). SWAT adequately reproduced the observed daily flows and sediment loads temporal evolution with R2 and NSE ≥ .7 and PBIAS &lt; 15% both for the calibration and validation periods. The sediment‐rating curve measured an annual average suspended sediment yield of 12.79 tons per hectare per year, whereas the SWAT model predicted an annual average sediment yield of 14.35 tons per hectare per year. The spatial distribution map of sediment showed that 9 out of 17 sub‐watersheds generate a higher amount of sediment yield (15.1–45 tons/h/year) and are found to be erosion‐prone areas. For proper management of watershed terracing, filter strips, contouring, and reforestation scenarios were developed. Each scenario reduces the sediment yield by 74.29%, 58.34%, 47.78%, and 69.76%, respectively. The proper combination of these scenarios resulted in a 90% reduction in sediment yield. Policymakers and land‐use planners of different organization will benefit from this research by implementing soil and water conservation mechanisms and practices that strengthen and mitigate the natural systems of Deme watershed.

Estimation of Long-Term Suspended Sediment Yield from a Small Agricultural Catchment

Estimation of Long-Term Suspended Sediment Yield from a Small Agricultural Catchment

The Signature of Climate in Fluvial Suspended Sediment Records

AbstractArid regions are often characterized by exceptionally high rates of fluvial sediment transport, but the processes responsible for this apparent connection between climate and sediment transport remain unclear. We examined decades of continuous flow records and suspended sediment concentrations from 71 rivers across the United States by comparing the suspended sediment rating curve behavior, quantified using power law coefficients and exponents, to an aridity index. Results indicate that higher aridity correlates with both greater overall suspended sediment concentration and lower sensitivity of concentration to changes in discharge, demonstrating that rivers in arid locations on average have greater suspended sediment transport efficiency across most discharges, and achieve high transport rates at a relatively lower discharge than rivers in temperate climates. Furthermore, based on additional analyses of the Normalized Difference Vegetation Index, specific suspended sediment yield, and a hydrograph flashiness index, we attribute the relationships between sediment transport and climate primarily to differences in vegetation density, precipitation, and runoff, variables that all influence both sediment supply and riverbed grain sorting. Finally, we note that the observed contrasts in sediment transport behavior likely represent climate‐driven differences in the magnitude and frequency of sediment supply rather than annual suspended sediment load, which does not depend significantly on climate. This study highlights a critical connection between multiple interrelated climatic factors and sediment transport, an important finding for future hazard mitigation in a changing climate with rapidly shifting vegetation patterns and hydrology.

Soil erosion, sediment sources, connectivity and suspended sediment yields in UK temperate agricultural catchments: Discrepancies and reconciliation of field-based measurements

Robust understanding of the fine-grained sediment cascades of temperate agricultural catchments is essential for supporting targeted management for addressing the widely reported sediment problem. Within the UK, many independent field-based measurements of soil erosion, sediment sources and catchment suspended sediment yields have been published. However, attempts to review and assess the compatibility of these measurements are limited. The data available suggest that landscape scale net soil erosion rates (∼38 t km−2 yr−1 for arable and ∼26 t km−2 yr−1 grassland) are comparable to the typical suspended sediment yield of a UK catchment (∼44 t km2 yr−1). This finding cannot, however, be reconciled easily with current prevailing knowledge that agricultural topsoils dominate sediment contributions to watercourses, and that catchment sediment delivery ratios are typically low. Channel bank erosion rates can be high at landscape scale (27 km−2 yr−1) and account for these discrepancies but would need to be the dominant sediment source in most catchments, which does not agree with a review of sediment sources for the UK made in the recent past. A simple and robust colour-based sediment source tracing method using hydrogen peroxide sample treatment is therefore used in fifteen catchments to investigate their key sediment sources. Only in two of the catchments are eroding arable fields likely to be important sediment sources, supporting the alternative hypothesis that bank erosion is likely to be the dominant source of sediment in many UK catchments. It is concluded that the existing lines of evidence on the individual components of the fine sediment cascade in temperate agricultural catchments in the UK are difficult to reconcile and run the risk of best management interventions being targeted inappropriately. Recommendations for future research to address paucities in measured erosion rates, sediment delivery ratios and suspended sediment yields, validate sediment source fingerprinting results, consider the sources of sediment-associated organic matter, and re-visit soil erosion and sediment cascade model parameterisation are therefore made.

Separating impacts of storm characteristics and mitigation activities on suspended sediment: Implications for sampling to evaluate mitigation success

Mitigation activities are often implemented to reduce anthropogenic sediment loads to sensitive ecosystems, including coral reefs, but effectiveness is rarely documented due to a lack of pre-mitigation data and to the difficulty of separating the impacts of mitigation from those of storm size and intra- and inter-storm variability. Here we present a method using stormwise metrics to quantify the effectiveness of a sediment mitigation project in American Samoa, where an aggregate quarry covering <1 % of a small watershed (drainage area 1.8 km2) generated approximately half of the annual suspended sediment load to the coast. Rainfall, runoff, turbidity and suspended sediment concentrations (SSC, mg L–1), yield (SSY, tons km−2 per unit time) and loads (SSL, tons per unit time) were monitored periodically over a five-year period upstream and downstream of the quarry. Mitigation included revegetation and placement of gravel on roads and processing platforms followed by installation of retention ponds. Storms were identified using automated storm separation, and a simple regression model using maximum area-normalized storm discharge (Qmax) estimated SSC, SSY and SSL for each mitigation stage (post-gravel, post-ponds) for a reference storm sequence. Regression parameters, and SSL and SSC for the reference storm sequence were calculated for a range of storm separation parameters. Monte Carlo sampling of the regression errors was used to calculate a mean and standard deviation of SSL and its change following mitigation. SSC was nearly 100x higher during large storms than during small storms, demonstrating the need to account for storm size to detect impacts of mitigation. After mitigation, SSL downstream of the quarry decreased by 63–82 % for all storms and by 88–92 % for small storms, and SSY downstream of the quarry was similar to SSY from the forested watershed upstream. Mitigation of sediment loads can target a small fraction of a watershed area that accounts for the majority of the human impact. Combining stormwise metrics with simple regression modeling and uncertainty analysis is a simple and novel way to detect and quantify the effectiveness of mitigation activities on sediment loads.

Elevation‐Dependent Periglacial and Paraglacial Processes Modulate Tectonically‐Controlled Erosion of the Western Southern Alps, New Zealand

AbstractExamining the links and potential feedbacks between tectonics and climate requires understanding the processes and variables controlling erosion. At the orogen scale, tectonics and climate are thought to be linked through the influence of mountain elevation on orographic precipitation and glaciation; the only documented erosional processes capable of balancing rapid rock‐uplift rates are glacial erosion or coupled river incision and landsliding. Our 20 new 10Be derived catchment‐averaged denudation rates from the Western Southern Alps of New Zealand generally range between 0.6 and 9 mm/yr, within the same order of magnitude as fault‐throw rates, exhumation rates, and erosion rates estimated from suspended sediment yields and landslide inventories. Combining our data with previously published 10Be denudation rates, we find that the proportion of catchment area in the 1,500–2,000 m elevation window is the variable that best explains denudation rate variability and the disparity between rock‐uplift rates and denudation rates. This correlation indicates that enhanced erosion likely occurs at 1,500–2,000 m above sea level, where periglacial and paraglacial processes have been proposed to be most active. We find that these temperature‐controlled erosional processes, which are also elevation‐dependent, can play a greater role in modulating erosion during interglacials than precipitation or glaciation. Our data suggest that temperature‐controlled peri‐ and paraglacial erosion could be efficient enough to balance some of the fastest rock‐uplift rates on Earth. Hence, temperature‐controlled erosion could contribute to limiting orogen elevations and modulating the erosion rates dictated by rock‐uplift, playing an essential role in linking tectonics and climate.

Using high-frequency monitoring data to quantify city-wide suspended-sediment load and evaluate TMDL goals

Excess sediment is a common reason water bodies in the USA become listed as impaired resulting in total maximum daily loads (TMDL) that require municipalities to invest millions of dollars annually on management practices aimed at reducing suspended-sediment loads (SSLs), yet monitoring data are rarely used to quantify SSLs and track TMDL progress. A monitoring network was created to quantify the SSL from the City of Roanoke, Virginia, USA (CoR), to the Roanoke River and Tinker Creek and help guide TMDL assessment and implementation. Suspended-sediment concentrations were estimated between 2020 and 2022 from high-frequency turbidity data using surrogate linear-regression models. Sixty-one percent of the total three-year SSL resulted from five large storm events. The average suspended-sediment yield from the CoR (58.1 metric tons/km2/year) was similar to other urban watersheds in the Eastern United States; however, the yield was nearly five times larger than the TMDL allocation (12.2 metric tons/km2/year). The TMDL allocated load was modeled based on a predominantly forested reference watershed and may not be a practical target for highly impervious watersheds within the CoR. The TMDL model used daily input data which likely does not capture the full range of SSLs during storm events, particularly from flashy urban streams. The average SSL following the five large storm events doubled that of the CoR’s annual allocated load from the TMDL. The results of this study highlight the importance of using high-frequency monitoring data to accurately estimate SSLs and evaluate TMDLs in urban areas.

Towards multi-model soil erosion modelling: An evaluation of the erosion potential method (EPM) for global soil erosion assessments

Soil erosion is expected to increase in the future due to climate change. Soil erosion models are useful tools that can be used by decision makers and other stakeholders to deal with soil erosion problems or the implementation of soil protection measures. Most of the modelling applications are using Universal Soil Loss Equation (USLE)-type models. In this study, we evaluate the applicability of the Erosion Potential Model (EPM) and its modified version (mEPM) for the estimation of the gross and net erosion rates at a global scale. The sensitivity analysis shows that the model results have the highest variability due to the soil protection (land cover) coefficient followed by the soil erodibility parameter. The models’ evaluations indicate that that the EPM cannot be applied to cold regions while the mEPM overcomes this issue. The erosion rates based on the EPM were 1.5–2.5 times larger than the ones obtained from the mEPM. Increasing the number of catchment properties as inputs to the model may help in improving the performance of the tested EPM and mEPM. Moreover, a comparison of net soil losses by mEPM with long-term suspended sediment yield data for 116 catchments located around the globe indicates a median bias of less than 10%, although the bias for around 1/3 of catchments was above 100%. Furthermore, a direct comparison with other soil erosion models such as USLE-type models is not possible since the EPM and mEPM do take into consideration other processes such as soil slumps and gully erosion and not just sheet and rill erosion. Therefore, as expected, the gross erosion rates by the EPM and mEPM are higher compared to the USLE-type models. Hence, the mEPM, despite its limitations, could be regarded as an interesting approach for the describing erosion processes around the globe and should be further tested using small- and medium-sized catchments from various climate zones.

Preliminary assessment of the suspended sediment dynamics in the Sikkim–Darjeeling Himalayan river

The Sikkim–Darjeeling Himalaya region receives the highest amount of rainfall along the whole southern Himalayan margin and is known for the occurrence of extreme hydrometeorological and geomorphological events. The massive amounts of water and sediment transported each year through the mountain part of the Teesta River drainage system (∼8,150 km2)––the largest river in the region––have been severely impacted by dam construction in recent decades. The aim of the current study was to determine, for the first time in this part of the Himalaya region, the dynamics of suspended sediment transfer at a number of points distributed through the mountainous part of the Teesta River catchment prior to dam construction and preliminarily assess the impact of dam operations on the suspended sediment. Sediment sources were identified using a database of landslide inventories from 1965 to 2019, combined with visual interpretation of satellite imagery from the U.S. Corona programme and Google Earth. Hydrological and sediment data up to the second half of the 1990s were used to reconstruct the discharge and suspended sediment dynamics before direct human intervention in the river channels. The beginning and end of the construction of the reservoirs was determined by analyzing satellite images. The impact of dam operations on the suspended sediment was compiled from the available literature. The results of the current study indicate that the primary sources of sediment are landslides caused by the interaction of rainfall and road undercutting of slopes as well as channel erosion. During extreme rainfall events, the influence of deforested areas in the mobilization and delivery of sediment to the river network increases. The current analysis reveals that reconstruction of the suspended sediment dynamics should take into account the course of extreme events responsible for supplying material to the river network, as well as the long-term remobilization of already deposited sediment in the river channel. It was found that the mean suspended sediment load (SSL) following extreme rainfall, flooding, and landslides in the Teesta River catchment can be up to four times higher than its average values for the same catchment unaffected by such an event, and the effects can be observed for more than a decade afterwards. Under these conditions, the mean suspended sediment yield can reach 12,000 and up to 20,000 t/(km2·y) in individual years, which is among the highest in the Himalaya region and, indeed, the world. The construction of 13 dams in the last 30 years has disrupted the hydrological regime and sediment transport in the Teesta River catchment along 70% of its main course and largest tributaries, and this has resulted in the selective retention of coarser material in the reservoirs and a reduction in the SSL in the Himalayan piedmont. The high density of the dams suggests that further transport of suspended sediment will depend on the efficiency of the water and sediment management at the reservoirs, which may be affected by irregular natural extreme events.

Development of Suspended Sediment Monitoring of the Tisza Using an Indirect Measurement Method

The Tisza River experienced successive flood peaks between 1998 and 2010. The reasons of the increased flood height are various, one of them is overbank floodplain accumulation. The aim of the present study is to apply and test an alternative sediment measurement method to evaluate the sediment transport of the Tisza River, Hungary. The new method could help practitioners to better understand the fluvial processes. A significant problem with the current sediment measurement practice is that it does not consider or just to a limited extent, the hydrological conditions of a river. The data measured at the turbidity measuring station installed in the Middle Tisza at Szolnok are evaluated to determine whether or not this measurement procedure can be applied for the highly various sediment transport conditions of the Tisza. In measurement campaigns and based on continuous data from the turbidity measuring probe, a close relationship between near-bank turbidity and suspended sediment concentration was established. The suspended sediment concentrations calculated from the near-bank turbidity were compared with the results of suspended sediment yield from a few cross-section measurements. The results are encouraging; despite the limited number of measurements, the relationship between the parameters is close. In order to make the method more precise, additional series of measurements are needed, which also cover the high water range.

Suspended sediment yield under alternating dry/wet cycles in a Mediterranean river catchment: the case of the Ofanto River, southern Italy

ABSTRACT This paper investigates how hydrological drought affected suspended sediment yield (SSY) in the Mediterranean Ofanto River basin, southern Italy, and in its five sub-basins from 1951 to 1989. The Standardized Precipitation Evaporation Index over a scale of 12 months (SPEI12) has been used to compute the hydrological drought in the investigated span of time. SPEI12, mean and maximum monthly discharge (Qmean and Qmax), monthly rainfall erosivity, monthly simple daily intensity index and monthly SSY were used to assess the relationships between dry/wet cycles, streamflow and SSY through the Pearson correlation matrix. Qmean and Qmax are significantly correlated with SSY, while SPEI12 and rainfall intensity do not show good correlation with SSY. Furthermore, from the overall analyses it emerges that sediment yield estimations were higher mainly during the wet period following a period of drought or during a drought period.

EUSEDcollab: a network of data from European catchments to monitor net soil erosion by water

As a network of researchers we release an open-access database (EUSEDcollab) of water discharge and suspended sediment yield time series records collected in small to medium sized catchments in Europe. EUSEDcollab is compiled to overcome the scarcity of open-access data at relevant spatial scales for studies on runoff, soil loss by water erosion and sediment delivery. Multi-source measurement data from numerous researchers and institutions were harmonised into a common time series and metadata structure. Data reuse is facilitated through accompanying metadata descriptors providing background technical information for each monitoring station setup. Across ten European countries, EUSEDcollab covers over 1600 catchment years of data from 245 catchments at event (11 catchments), daily (22 catchments) and monthly (212 catchments) temporal resolution, and is unique in its focus on small to medium catchment drainage areas (median = 43 km2, min = 0.04 km2, max = 817 km2) with applicability for soil erosion research. We release this database with the aim of uniting people, knowledge and data through the European Union Soil Observatory (EUSO).

Statistical Analysis and Modeling of Suspended Sediment Yield Dependence on Environmental Conditions

Statistical Analysis and Modeling of Suspended Sediment Yield Dependence on Environmental Conditions

Comparative analysis of discharge and sediment flux from two contiguous glacierized basins of Central Himalaya, India

In the present study, suspended sediment load (SSL), sediment yield and erosion rates in Pindari Glacier basin (PGB) and Kafni Glacier basin (KGB) have been estimated using daily discharge and suspended sediment concentration (SSC) data for three ablation seasons (2017-2019). For this, one meteorological observatory and two gauging sites have been established at Dwali (confluence point), and water samples have been collected twice in a day for high flow period (July to September) and daily for lean period (May, June and October). An area-velocity method and stage-discharge relationship has been established to convert water level into discharge (m3 s-1). For estimating SSC (mg/l), collected water samples have been filtered, dried, analysed and confirmed with an automatic suspended solid indicator. Further, SSL, sediment yield and erosion rates have been computed using SSC data. The results reveal that mean annual discharge in PGB (35.06 m3s-1) has been found approximately 1.7 times higher than KGB (20.47 m3s-1). The average SSC and SSL in PGB have been observed about 396.07mg/l and 1928.34 tonnes, and in KGB, it is about 359.67mg/l and 1040.26 tonnes, respectively. The SSC and SSL have followed the pattern of discharge. A significant correlation of SSC and SSL has been found with discharge in both the glacierized basins (p < 0.01). Interestingly, average annual sediment yield in PGB (3196.53 t/km2/yr) and KGB (3087.23 t/km2/yr) have been found almost identical. Likewise, the erosion rates in PGB and KGB have been witnessed about 1.18 and 1.14mm/yr, respectively. Sediment yield and erosion rates in PGB and KGB have been found in correspondence with other basins of Central Himalaya. These findings will be beneficial for engineers and water resource managers in the management of water resources and hydropower projects in high-altitude areas and in the planning and designing of water structures (dams, reservoirs etc.) in downstream areas.

Reconstructing five decades of sediment export from two glacierized high-alpine catchments in Tyrol, Austria, using nonparametric regression

Abstract. Knowledge on the response of sediment export to recent climate change in glacierized areas in the European Alps is limited, primarily because long-term records of suspended sediment concentrations (SSCs) are scarce. Here we tested the estimation of sediment export of the past five decades using quantile regression forest (QRF), a nonparametric, multivariate regression based on random forest. The regression builds on short-term records of SSCs and long records of the most important hydroclimatic drivers (discharge, precipitation and air temperature – QPT). We trained independent models for two nested and partially glacier-covered catchments, Vent (98 km2) and Vernagt (11.4 km2), in the upper Ötztal in Tyrol, Austria (1891 to 3772 m a.s.l.), where available QPT records start in 1967 and 1975. To assess temporal extrapolation ability, we used two 2-year SSC datasets at gauge Vernagt, which are almost 20 years apart, for a validation. For Vent, we performed a five-fold cross-validation on the 15 years of SSC measurements. Further, we quantified the number of days where predictors exceeded the range represented in the training dataset, as the inability to extrapolate beyond this range is a known limitation of QRF. Finally, we compared QRF performance to sediment rating curves (SRCs). We analyzed the modeled sediment export time series, the predictors and glacier mass balance data for trends (Mann–Kendall test and Sen's slope estimator) and step-like changes (using the widely applied Pettitt test and a complementary Bayesian approach). Our validation at gauge Vernagt demonstrated that QRF performs well in estimating past daily sediment export (Nash–Sutcliffe efficiency (NSE) of 0.73) and satisfactorily for SSCs (NSE of 0.51), despite the small training dataset. The temporal extrapolation ability of QRF was superior to SRCs, especially in periods with high-SSC events, which demonstrated the ability of QRF to model threshold effects. Days with high SSCs tended to be underestimated, but the effect on annual yields was small. Days with predictor exceedances were rare, indicating a good representativity of the training dataset. Finally, the QRF reconstruction models outperformed SRCs by about 20 percent points of the explained variance. Significant positive trends in the reconstructed annual suspended sediment yields were found at both gauges, with distinct step-like increases around 1981. This was linked to increased glacier melt, which became apparent through step-like increases in discharge at both gauges as well as change points in mass balances of the two largest glaciers in the Vent catchment. We identified exceptionally high July temperatures in 1982 and 1983 as a likely cause. In contrast, we did not find coinciding change points in precipitation. Opposing trends at the two gauges after 1981 suggest different timings of “peak sediment”. We conclude that, given large-enough training datasets, the presented QRF approach is a promising tool with the ability to deepen our understanding of the response of high-alpine areas to decadal climate change.