Annual Suspended Sediment Yield Research Articles

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.

Climatic and hydrogeomorphic controls on sediment characteristics in the southern Sierra Nevada

The magnitude of sediment yield from headwater catchments is controlled by the interactions among hydrology, geomorphology, and soil disturbance. In montane regions like the Sierra Nevada, snow is one of the main factors controlling the timing and magnitude of hydrological fluxes. However, the role of snow on modulating spatial and temporal variation of sediment yield remains unclear. Using 120-site years of sediment yield data from 10 headwater catchments (drainage area 50–475 ha), we examined the sediment yield characteristics across an elevational gradient (1,777–2,381 m elevation) in the southern Sierra Nevada. Across space and time, we calculated an average annual suspended sediment yield of 62 ± 147 Mg/km2. In contrast, the measured mean annual bedload yield from the study catchments was small, 1.1 ± 2.4 Mg/km2. A linear mixed-effects model showed that maximum annual discharge alone can only explain 24 % (marginal R2 = 0.24) of the variance in sediment yield. Similarly, the hypsometric integral, which is often used as a metric for erosion susceptibility, showed no predictive power (marginal R2 = 0.005). As much as 65 % of the variance in sediment yield can be explained by fixed effects when snow related drivers (i.e., center of flow timing and aspect) were included in the model along with maximum annual discharge, suggesting a strong influence of snowmelt. Moreover, the relationship between area normalized suspended sediment yield (Qs) and unit discharge (Q) was significantly different between rain and snow events (p = 0.001). Both average slope (α) and exponent (β) terms of the Qs=αQβ relationship across the ten catchments were higher for rainfall (α=5.8,β=1.84) than those for snowmelt (α=3.1,β=1.77) events. As the erosion severity and power were higher during rainfall than the snowmelt events, a shift in the precipitation form from snow to rain under a warming climate will likely increase sediment yield. These results provide critical insights on background sediment yield in the southern Sierra Nevada and likely changes under future climate.

Suspended sediment and discharge dynamics in a glaciated alpine environment: identifying crucial areas and time periods on several spatial and temporal scales in the Ötztal, Austria

Abstract. Glaciated high-alpine areas are fundamentally altered by climate change, with well-known implications for hydrology, e.g., due to glacier retreat, longer snow-free periods, and more frequent and intense summer rainstorms. While knowledge on how these hydrological changes will propagate to suspended sediment dynamics is still scarce, it is needed to inform mitigation and adaptation strategies. To understand the processes and source areas most relevant to sediment dynamics, we analyzed discharge and sediment dynamics in high temporal resolution as well as their patterns on several spatial scales, which to date few studies have done. We used a nested catchment setup in the Upper Ötztal in Tyrol, Austria, where high-resolution (15 min) time series of discharge and suspended sediment concentrations are available for up to 15 years (2006–2020). The catchments of the gauges in Vent, Sölden and Tumpen range from 100 to almost 800 km2 with 10 % to 30 % glacier cover and span an elevation range of 930 to 3772 m a.s.l. We analyzed discharge and suspended sediment yields (SSY), their distribution in space, their seasonality and spatial differences therein, and the relative importance of short-term events. We complemented our analysis by linking the observations to satellite-based snow cover maps, glacier inventories, mass balances and precipitation data. Our results indicate that the areas above 2500 m a.s.l., characterized by glacier tongues and the most recently deglaciated areas, are crucial for sediment generation in all sub-catchments. This notion is supported by the synchronous spring onset of sediment export at the three gauges, which coincides with snowmelt above 2500 m but lags behind spring discharge onsets. This points at a limitation of suspended sediment supply as long as the areas above 2500 m are snow-covered. The positive correlation of annual SSY with glacier cover (among catchments) and glacier mass balances (within a catchment) further supports the importance of the glacier-dominated areas. The analysis of short-term events showed that summer precipitation events were associated with peak sediment concentrations and yields but on average accounted for only 21 % of the annual SSY in the headwaters. These results indicate that under current conditions, thermally induced sediment export (through snow and glacier melt) is dominant in the study area. Our results extend the scientific knowledge on current hydro-sedimentological conditions in glaciated high-alpine areas and provide a baseline for studies on projected future changes in hydro-sedimentological system dynamics.

PAST LAND-USE ACTIVITIES IN RELATION TO THE LONG-TERM SUSPENDED SEDIMENT YIELD OF KINTA RIVER, PERAK, MALAYSIA

Soil erosion and the suspended sediment yield (SSY) of Kinta river catchment were investigated using river and sediment discharge data of Kinta River and its tributaries available from 1961-2006. A long-term annual SSY for the Kinta River basin recorded an average of 2417.6 t/km2/year, with a maximum SSY of 8,757.3 t/km2/year in 1973 and a minimum SSY of 720.4 t/km2/year in 2005. The Mann-Kendall test was conducted to analyse the long-term SSY trends, which revealed an overall decreasing trend of SSY for the Kinta River basin. This 46-year period was divided into four phases of different SSY trend. The second phase from 1970–1985 recorded the highest average of 4,062 t/km2/year, as a result of the rapid development process that occurred in the drainage basin. The declining trends in SSY were observed after many mining operations ceased after 1960 and dam construction start in 2003. Some increasing trends in the early 1970s to 80s were due largely to the initial implementation of the New Economic Policy (NEP) and Green Book Plan. It involved a large-scale conversion of forest areas into agricultural lands with the expansion of urban areas and improvement of infrastructures.

A multiple regression model to estimate the suspended sediment yield in Italian Apennine rivers by means of geomorphometric parameters

A new statistical prediction model, aimed to assess the mean annual suspended sediment yield (SY) in ungauged rivers of Italian Apennines, is here presented. A multiple linear regression equation was obtained by the stepwise technique investigating the relationships between a series of hydro-geomorphometric variables and sediment yield data from 41 river basins located along peninsular Italy and Sicily. The model variables were computed from hydrological data records and from vector river network lines and drainage divides derived from official maps and DEM. The analysis revealed a large variance in the observed sediment yield data. Nonetheless, an optimal result in terms of model significance and efficiency (r2adj = 0.91 at p < 0.05 significance level; Nash–Sutcliffe model efficiency NSE = 0.855; Willmott’s indexes of performance: d = 0.965; d1 = 0.862; dr = 0.862) was finally achieved. According to stepwise regression, the catchment relief and perimeter, together with the topological organization of stream network, the bedrock erodibility and the stream gradient, expressed by specific parameters, appeared as determinant features for SY prediction in the catchments here investigated. The developed model could be helpful to practitioners and scholars for rapid assessments of SY at any point of the river network in the geographic area here concerned. However, the same technique can be utilized wherever in the world since the statistical method itself allows to recognize the number and type of parameters to be used in a given geographic area, on the basis of their local significance.

Tropical Montane Forest Conversion Is a Critical Driver for Sediment Supply in East African Catchments

AbstractLand use change is known to affect suspended sediment fluxes in headwater catchments. There is however limited empirical evidence of the magnitude of these effects for montane catchments in East Africa. We collected a unique 4‐year high‐frequency data set and assessed seasonal sediment variation, water pathways, and sediment response to hydrology in three catchments under contrasting land use in the Mau Forest Complex, Kenya's largest tropical montane forest. Annual suspended sediment yield was significantly higher in a smallholder agriculture‐dominated catchment (131.5 ± 90.6 t km−2 yr−1) than in a tea‐tree plantation catchment (42.0 ± 21.0 t km−2 yr−1) and a natural forest catchment (21.5 ± 11.1 t km−2 yr−1) (p &lt; 0.05). Transfer function models showed that in the natural forest and the tea‐tree plantations subsurface flow pathways delivered water to the stream, while in the smallholder agriculture shallow subsurface and surface runoff were dominant. There was a delayed sediment response to rainfall for the smallholder agriculture and the tea‐tree plantations. A slow depletion in sediment supply suggests that the wider catchment area supplies sediment, especially in the catchment dominated by smallholder farming. In contrast, a fast sediment response and depletion in sediment supply in the natural forest suggests a dominance of temporarily stored and nearby sediment sources. This study shows that the vegetation cover of a forest ecosystem is very effective in conserving soil, whereas catchments with more bare soil and poor soil conservation practices generated six times more suspended sediment yield. Catchment connectivity through unpaved tracks is thought to be the main explanation for the difference in sediment yield.

Streambank Legacy Sediment Contributions to Suspended Sediment‐Bound Nutrient Yields from a Mid‐Atlantic, Piedmont Watershed

AbstractWhile streambank legacy sediment has been increasingly recognized as an important contributor of fine sediment to watersheds, little is known about its contributions to suspended sediment‐bound nutrients. This uncertainty is exacerbated by the ubiquitous presence of legacy sediments in valley‐bottoms of the Mid‐Atlantic region of the United States. Using a sediment fingerprinting model, we quantified suspended sediment and sediment‐bound nutrient sources for a 114 km2 agricultural, Piedmont watershed. Sediment sources included agricultural, developed, and forested soils as well as streambanks legacy sediments. Suspended sediment and sediment‐bound nitrogen (N) and phosphorus (P) concentrations were determined for 10 storms over the study period of August 1, 2017 to July 31, 2018. Streambanks contributed 50% of the annual suspended sediment yield of 1,348 kg/ha followed by agricultural (49%) and forested (1%) soils. Streambanks amounted to 26% and 32% of the suspended sediment‐bound N and P exports, respectively, whereas the corresponding loads from agricultural sources were 73 and 67%, respectively. Bank contributions varied with storm magnitude and seasonality, with the highest exports associated with the largest peak flows. These results highlight that streambank sediments constitute a significant fraction of suspended sediment‐bound nutrient exports and must be considered in watershed models, regulatory assessments, and management practices.

A Regression-Based Prediction Model of Suspended Sediment Yield in the Cuyahoga River in Ohio Using Historical Satellite Images and Precipitation Data

Urbanization typically results in increased imperviousness which alters suspended sediment yield and impacts geomorphic and ecological processes within urban streams. Therefore, there is an increasing interest in the ability to predict suspended sediment yield. This study assesses the combined impact of urban development and increased precipitation on suspended sediment yield in the Cuyahoga River using statistical modeling. Historical satellite-based land-cover data was combined with precipitation and suspended sediment yield data to create a Multiple Linear Regression (MLR) model for the Cuyahoga watershed. An R2 value of 0.71 was obtained for the comparison between the observed and predicted results based on limited land-use and land-cover data. The model also shows that every 1 mm increase in the mean annual precipitation has the potential to increase the mean annual suspended sediment yield by 860 tons/day. Further, a 1 km2 increase in developed land area has the potential to increase mean annual suspended sediment yield by 0.9 tons/day. The framework proposed in this study provides decision makers with a measure for assessing the potential impacts of future development and climate alteration on water quality in the watershed and implications for stream stability, dam and flood management, and in-stream and near-stream infrastructure life.

Changes in maximum daily runoff depth and suspended sediment yield on the Loess Plateau, China

The maximum river flow is of important reference significance to flood disaster prevention on the Loess Plateau of central China. However, due to the lack of hydrological data, previous studies have rarely explored spatiotemporal changes in the annual maximum daily runoff depth (Hmax) and annual maximum daily suspended sediment yield (SSYmax) of rivers in this region. This study uses daily discharge data and daily sediment concentration data from 104 hydrological stations on the plateau to compare the Hmax and the SSYmax during period P1 (1971–1987) and period P2 (2008–2016). The results show that 66% of stations exhibited a significant decline in multi-year mean Hmax from P1 to P2 (p < 0.05). However, the multi-year mean SSYmax showed more widespread changes, with 77% of stations seeing a significant decline (p < 0.05). And on average, the runoff on the day of Hmax contributed 6.6% and 5.8% of the annual runoff during P1 and P2, respectively; by contrast, SSYmax contributed 27.9% and 33.5%, respectively, to the total annual suspended sediment yield in P1 and P2. Analyses using Spearman’s rank correlation coefficient indicate that the spatial distribution of Hmax has larger variability than SSYmax during both P1 and P2 and that the spatial distribution pattern of SSYmax during P2 is more unstable than during P1. We found that the suspended sediment concentration (SSC) on the day that Hmax occurs (SSChmax) remained relatively stable for most stations studied within the loess gullied-hilly area across the Loess Plateau, but the stable SSChmax value during P1 differed from the value during P2. We conclude that human activities (e.g., afforestation, terracing, and construction of check dams) may be the cause of the differing SSC values during peak discharge between the two periods. We also found that the average percentage of years in which Hmax and SSYmax occurred on the same day on the plateau was 55%. Many factors can lead to Hmax and SSYmax happening asynchronously, such as heavy runoff events lasting several consecutive days, the characteristics of a particular basin, and the influence of human activities. These results provide information that will be useful for understanding how to manage extreme hydrological events on the Loess Plateau.

An overview of research into conservation practice effects on soil and water resources in the Upper Washita Basin, Oklahoma, United States

The Fort Cobb Reservoir Experimental Watershed (FCREW) and Little Washita River Experimental Watershed (LWREW), located within the Upper Washita Basin (UWB) in Oklahoma, are part of the Agricultural Research Service (ARS) Benchmark Conservation Effects Assessment Project (CEAP)–Watershed Assessment Studies locations. The CEAP was created in 2003 by the USDA Natural Resources Conservation Service (NRCS) in partnership with USDA ARS and many other partners to quantify the environmental effects of conservation practices (CPs) and programs and to develop the science base for managing the agricultural landscape for environmental quality. The ultimate goal of this study was to present CPs that are expected to work at respective spatial scales, based on the findings of research that has been carried out in these CEAP and other watersheds within the UWB in the last 15 years. A summary of research findings indicates that the effects of CPs on soil and water resources were simulated at various spatial scales. At the large watershed scale, average annual suspended sediment yield at the 786 km² (303 mi²) FCREW outlet was reduced by 86% based on multiple CPs implemented from the 1950s to 2008 by NRCS. Specific quantified effects of studied CPs are presented herein. With the exception of red cedar (<i>Juniperus virginiana</i> L.) removal, single CPs were expected to show improvement of soil and water resources in smaller-scale watersheds. Practices that are expected to improve soil and water resources include grassland conservation from red cedar encroachment (brush control), combined streambank stabilization practices, riparian and filter strip buffer practices, and conversion of cropped area to Bermuda grass (<i>Cynodon dactylon</i> [L.] Pers.).

Spatial and temporal patterns of suspended sediment transport in nested urban watersheds

Patterns of sediment yield have been widely investigated across a variety of scales, but few of these studies have been carried out in urban watersheds. The design of this study represented a unique opportunity to use high-frequency sensor data to compare sediment yields across spatial and temporal scales in an urbanized watershed. Near real-time turbidity and discharge data were collected continuously for four years at five stream gages over three nested spatial scales in the highly impervious 14.2-km2 Dead Run watershed, located in Baltimore County, Maryland, USA. Monthly suspended sediment yields varied by over two orders of magnitude across all stations. Two mid-watershed stations on tributaries to the mainstem of Dead Run are located only about 500 m apart and about 500 m upstream of their confluence, yet displayed observable differences in the timing of their highest sediment yields. Average annual suspended sediment yield was higher at the headwater station with older development (78.2 t/km2/yr) than at the headwater station with more recent development (47.0 t/km2/yr). Average annual yields increased with watershed size from the headwater stations to the mid-watershed stations (87.0 t/km2/yr and 60.4 t/km2/yr). Additionally, yields at the station farthest downstream (81.7 t/km2/yr) were slightly larger than the yield calculated from the sum of both mid-watershed stations (74.6 t/km2/yr), which together represented about 80% of the upstream drainage area.Several additional patterns highlighting spatial heterogeneity of the system were observed. The ratio of sediment yields between the headwater stations increased steadily over time while the runoff ratio remained almost constant. Yields at one mid-watershed station were consistently higher than yields at the other mid-watershed station, despite similarity in runoff totals. Upstream watersheds show year-to-year variation in sediment yield. In contrast, yields at the farthest downstream station show minimal variation. This observation is consistent with the possibility that internal storage and remobilization tend to modulate downstream yields even with spatial and temporal variation in upstream sources. Observable inconsistencies in sediment yields between subwatersheds at monthly time scales suggest that there is a substantial degree of heterogeneity in terms of sediment response to large storm events. Despite significant internal variability, the overall range of sediment yield values at each site was consistent with values reported for multiple urban mid-Atlantic Piedmont watersheds. Comparison with global values indicates that sediment yields from Dead Run are in the same general range as the 25% quartile of urban sediment yields.

Long-Term Suspended Sediment Concentrations and Loads from a Relatively Undisturbed Agroforested Catchment in the Northwest of the Iberian Peninsula

The suspended sediment dynamics in small catchments are difficult to estimate accurately because they result from the coupling of complex processes occurring at different scales. In this study, the dynamics of suspended sediment concentrations (SSC) and loads were assessed in an agroforested humid catchment in NW Spain, based on a long-term rainfall, discharge and suspended sediment dataset (12 hydrological years) from high-frequency monitoring. The results highlight the episodic nature of sediment transport in the study area, given that about 78% of SS was exported over 10% of the study period. The SS transport was related to runoff generation and flooding, although sediment availability also played an important role in SS transport. The SS load was mainly driven by high-magnitude rainfall events, while intense rainfall episodes generated high SSC peaks. The mean annual suspended sediment yield was relatively low from a quantitative stand point (10 Mg km−2 y−1); however, during 11% of the monitoring, SS concentrations exceeding the threshold threatened surface water quality (Freshwater Fish Directive 78/659/EEC and Directive 75/440/EEC), mainly during runoff events, indicating the need to adopt management practices in order to reduce or mitigate sediment loss during such episodes.

The impact of extreme El Ni\xf1o events on modern sediment transport along the western Peruvian Andes (1968\u20132012)

Climate change is considered as one of the main factors controlling sediment fluxes in mountain belts. However, the effect of El Niño, which represents the primary cause of inter-annual climate variability in the South Pacific, on river erosion and sediment transport in the Western Andes remains unclear. Using an unpublished dataset of Suspended Sediment Yield (SSY) in Peru (1968–2012), we show that the annual SSY increases by 3–60 times during Extreme El Niño Events (EENE) compared to normal years. During EENE, 82% to 97% of the annual SSY occurs from January to April. We explain this effect by a sharp increase in river water discharge due to high precipitation rates and transport capacity during EENE. Indeed, sediments accumulate in the mountain and piedmont areas during dry normal years, and are then rapidly mobilized during EENE years. The effect of EENE on SSY depends on the topography, as it is maximum for catchments located in the North of Peru (3–7°S), exhibiting a concave up hypsometric curve, and minimum for catchments in the South (7–18°S), with a concave down hypsometric curve. These findings highlight how the sediment transport of different topographies can respond in very different ways to large climate variability.

Suspended sediment transport into a water supply reservoir in southern Ghana

Weija reservoir on the Densu river serves as a source of water supply to parts of Greater Accra and Central regions of Ghana. The Densu river basin is characterised by accelerated land degradation. A number of Integrated Water Resources Management (IWRM) interventions have been implemented in the basin. Reports suggest there has been an improvement of physicochemical parameters of the water resources. However, there is no recent study on fluvial sediment transport in the basin. This paper assessed sediment transport into the Weija reservoir via the Densu River. Weekly suspended sediment concentration and river discharge were monitored over a 1 year period to quantify sediment yield into the reservoir. The results indicated that total annual suspended sediment yield, and annual specific suspended sediment yield were 5375 t yr -1 and 2.0 t km -2 yr -1 , respectively. These were relatively low compared to results obtained by past studies on the Densu basin at upstream Mangoase (6146 t yr -1 and 2.49 t km -2 yr -1 ) and other river basins in Ghana. Parameters established from a yield rating curve indicated that parts of the basin were degraded and sediment transport in the basin was largely due to the availability of sediment in the catchment and not just due to high river discharges. The relatively low sediment transport at Weija may be indicative of the success of the IWRM interventions. It is recommended that these be sustained and/or intensified.

Sediment source identification and load prediction in a mixed-use Piedmont watershed, South Carolina

Many streams in the Piedmont region of the southeastern United States transport a disproportionately large amount of suspended sediment in response to moderately increased streamflows. Transport and deposition of excess sediment affect the stability of the channel and the health of the biological community; therefore, identifying the main source(s) of sediment and assessing the relationships between source, transport, and streamflow are critical to aquatic life and habitat management, dynamic equilibrium preservation, and development of feasible mitigation scenarios. The objectives of this study were to: (1) predict the annual suspended sediment yield and (2) identify significant contributing upland sources of sediment in the Lawsons Fork Creek basin, a 217 km2 mixed-use watershed in the South Carolina Piedmont. A regularly monitored cross-section located in the downstream reach was equipped with a passive sediment sampler, gage-height recorder, and sediment tiles. Streamflow and sediment concentration were measured over a 24-month period under variable hydrologic regimes. Results indicated that the average annual sediment yield (168 t/km2/yr) is significantly higher than yields documented in Piedmont watersheds of comparable size. To identify and prioritize sources of sediment contribution, stable isotopes of nitrogen (δ15N) and carbon (δ13C) were used as tracers. Source material was compared with suspended sediment near the watershed outlet (target material) and SIAR, a Bayesian Inference model, was used to estimate source apportionment. Results of this source study indicate that approximately 60% of the total sediment load in the water column during high flow events is derived from stream bank erosion. Findings are consistent with observed unstable stream bank conditions in the watershed. This study supports the use of a dual-isotopic fingerprinting approach in tandem with traditional sediment monitoring as a cost-effective method to identify and target sediment sources in a mixed-use watershed.

Impact of climate change on sediment yield for Naran watershed, Pakistan

In this paper, the site-specific impact of climate change on sediment yield has been assessed for the Naran watershed, Pakistan. Observed data has been gathered for period 1961–2010 and HaDCM3 GCM predictors of SRES scenarios A2 and B2 have been downloaded. Future precipitation and temperature time series have been statistically downscaled for time horizon 2011–2040 and 2041–2070. Downscaled data show both increasing and decreasing changes with respect to the observation. Potential sediment yield for future related to climate change has been simulated. The results show that the both snowy and monsoon seasonal stream discharges are expected to increase. This will lead to increase in annual suspended sediment yields. Percentage-wise, a less discharge and more sediment yield are expected during the early summer. The study concluded that the climate change and variability are influencing the watershed, and suspended sediment yield is likely to increase in the future.

Suspended-sediment responses after strip thinning in headwater catchments

We examined changes in suspended-sediment yields (SSY) after a 50 % strip thinning in headwater streams draining a Japanese cedar (Cryptomeria japonica) and cypress (Chamaecyparis obtusa) plantation forest. We applied a paired-catchment analysis to treated (KT: 17.1 ha) and control (KC: 8.9 ha) catchments. Annual suspended-sediment yield (SSYan) in the prethinning period in the KT and KC catchments was 110.0 and 142.1 kg/ha per year. For the postthinning period, SSYan in the KT catchment became 5055.6 kg/ha per year, whereas that in the KC catchment increased 893.2 kg/ha per year. The paired-catchment analysis revealed that SSY in the KT catchment increased 17.0-fold compared with the prethinning period. However, the dominant hysteresis pattern remained clockwise in both pre- and postthinning periods. Sequences of large storm events in the postthinning period elevated SSY in both catchments. Increase in suspended sediment in the KT catchment was associated with the combined impacts of thinning and sequences of storm events during the period of thinning operation.

Effects of cultivation and reforestation on suspended sediment concentrations: a case study in a mountainous catchment in China

Abstract. Understanding how sediment concentrations vary with land use/cover is critical for evaluating the current and future impacts of human activities on river systems. This paper presents suspended sediment concentration (SSC) dynamics and the relationship between SSC and discharge (Q) in the 8973 km2 Du catchment and its sub-catchment (4635 km2). In the Du catchment and its sub-catchment, 4235 and 3980 paired SSC–Q samples, respectively, were collected over 30 years. Under the influence of the Household Contract Responsibility System and Grain-for-Green projects in China, three periods were designated, the original period (1980s), cultivation period (1990s) and reforestation period (2000s). The results of a Mann–Kendall test showed that rainfall slightly increased during the study years; however, the annual discharge and sediment load significantly decreased. The annual suspended sediment yield of the Du catchment varied between 1.3 × 108 and 1.0 × 1010 kg, and that of the sub-catchment varied between 6.3 × 107 and 4.3 × 109 kg. The SSCs in the catchment and sub-catchment fluctuated between 1 and 22400 g m−3 and between 1 and 31800 g m−3, respectively. The mean SSC of the Du catchment was relatively stable during the three periods (±83 g m−3). ANOVA (analysis of variance) indicated that the SSC did not significantly change under cultivation for low and moderate flows, but was significantly different under high flow during reforestation of the Du catchment. The SSC in the sub-catchment was more variable, and the mean SSC in the sub-catchment varied from 1058 ± 2217 g m−3 in the 1980s to 1256 ± 2496 g m−3 in the 1990s and 891 ± 1558 g m−3 in the 2000s. Reforestation significantly decreased the SSCs during low and moderate flows, whereas cultivation increased the SSCs during high flow. The sediment rating curves showed a stable relationship between the SSC and Q in the Du catchment during the three periods. However, the SSC–Q of the sub-catchment exhibited scattered relationships during the original and cultivation periods and a more linear relationship during the reforestation period.

Contemporary suspended sediment dynamics within two partly glacierized mountain drainage basins in western Norway (Erdalen and Bødalen, inner Nordfjord)

This paper focuses on environmental controls, spatiotemporal variability and rates of contemporary fluvial suspended sediment transport in the neighboring, partly glacierized and steep Erdalen (79.5km2) and Bødalen (60.1km2) drainage basins in the fjord landscape of the inner Nordfjord in western Norway. Field work, including extended samplings and measurements, was conducted since 2004 in Erdalen and since 2008 in Bødalen. The distinct intra- and inter-annual temporal variability of suspended sediment transport found is mostly controlled by meteorological events, with most suspended sediment transport occurring during pluvial events in autumn (September–November), followed by mostly thermally determined glacier melt in summer (July–August), and by mostly thermally determined snowmelt in spring (April–June). Extreme rainfall events (>70mmd−1) in autumn can trigger significant debris-flow activity that can cause significant transfers of suspended sediments from ice-free surface areas with sedimentary covers into main stream channels and is particularly important for fluvial suspended sediment transport. In years with occurring relevant debris-flow activity the total annual drainage-basin wide suspended sediment yields are strongly determined by these single extreme events. The proportion of glacier coverage, followed by steepness of slopes, and degree of vegetation cover in ice-free surface areas with sedimentary covers are the main controls for the detected spatial variability of suspended sediment yields. The contemporary sediment supply from glacierized surface areas and the Jostedalsbreen ice cap through different defined outlet glaciers shows a high spatial variability. The fact that the mean annual suspended sediment yield of Bødalen is with 31.3tkm−2yr−1 almost twice as high as the mean annual suspended sediment yield of Erdalen (16.4tkm−2yr−1) is to a large extent explained by the higher proportion of glacier coverage in Bødalen (38% of the drainage basin surface area) as compared to Erdalen (18% of the drainage basin surface area) and by a significantly higher sediment yield from the glacierized area of the Bødalen drainage basin compared to the glacierized surface area in Erdalen. When looking at the total annual mass of suspended sediments being fluvially exported from both entire drainage basin systems, the total amount of suspended sediments coming from the ice-free drainage basin surface areas altogether dominates over the total amount of suspended sediments coming from the glacierized surface area of both drainage basins. Drainage-basin wide annual suspended sediment yields are rather low when compared with yields of other partly glacierized drainage basin systems in Norway and in other cold climate environments worldwide, which is mainly due to the high resistance of the predominant gneisses towards glacial erosion and weathering, the altogether only small amounts of sediments being available within the entire drainage basin systems, the stable and nearly closed vegetation cover in the ice-free surface areas with sedimentary covers, and the efficiency of proglacial lakes in trapping sediments supplied by defined outlet glaciers. Both contemporary and long-term suspended sediment yields are altogether supply-limited. Contemporary suspended sediment transport accounts for nearly two-thirds of the total fluvial transport and, accordingly, plays an important role within the sedimentary budgets of the entire Erdalen and Bødalen drainage basins.

Assessment of some homogeneous methods for the regional analysis of suspended sediment yield in the south and southeast of the Caspian Sea

Regional analysis of suspended sediment yield (SSY) is commonly used to estimate sediment at a particular site where little or no information is available on sediment yield. In this research, the efficiency of three input selection and homogenization methods were evaluated in the estimation of SSY. Therefore, 42 sediment measurement stations and their upstream watersheds were selected and sediment rating curve was estimated by using regression models for each station. Mean annual SSY was estimated by using sediment rating curve and daily discharge. In the present study, in order to determine the independent variables in sediment yield, 11 physiographical, one climatic and two hydrologic variables of whole study watersheds were selected. Then the most effective independent variables were selected by using principal component analysis (PCA), Gamma test (GT) and stepwise regression (SR) techniques. After reducing 14 input variables to five (using PCA and GT) and two (using SR techniques), they are divided into homogeneous areas by Andrew curve (AC), cluster analysis (CA) and canonical discriminate function (CDFs) techniques. The watersheds were divided into two (using PCA-AC), three (using PCA-CA, PCA-CDFs and GT-CDFs), four (using GT-CA, GT-AC and SR-CA) and five (using SR-AC) homogenous regions. Multiple regression models to estimate mean annual SSY as a function of five (using PCA and GT) and two (using SR techniques) watershed characteristics were built in each homogeneous region, and compared to actual mean annual SSY in each station using relative error (RE), efficiency coefficients (CE) and relative root mean square error (RRMSE). The results showed that preprocessing the input variables by means of PCA and GT techniques has improved the homogeneous stations determination and the development models. According to the results, the best technique for determining homogeneous watersheds was AC technique with RE =49.24%, RRMSE =43.75% and CE =71.04%.