Total Suspended Sediment Concentration Research Articles

Estimating Suspended Sediment Concentration in Streams by Diffuse Light Attenuation

AbstractA light attenuation sensor system (LASS) for measurements in waters is described in this paper. The LASS records irradiance at multiple levels in the water column to provide a measure of the diffuse light attenuation coefficient which is strongly affected by suspended sediment. Dimensional analysis and geometric optical theory are used to relate the irradiance attenuation to sediment properties through a dimensionless product. The latter is termed as a light attenuation number for suspended sediment in waters. The LASS and dimensional analysis results are validated in the laboratory using fluvial sediments collected from a third-order stream and monodisperse quartz sediment. The attenuation coefficient estimated with LASS data varied nonlinearly with total suspended sediment concentration due to particle shadowing and multiple scattering at large optical depths. The light attenuation coefficient for each sediment type is well described as a function of total suspended sediment concentration by emp...

Characterization of spatial and temporal variation of suspended sediments in the Yellow and East China Seas using satellite ocean color data

Total suspended sediment (TSS) data concentrations are retrieved from two sets of satellite ocean color data (the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua and the Korean Geostationary Ocean Color Imager (GOCI)) using an existing regional model to characterize spatial and temporal variation of TSS in the Yellow and East China Seas. MODIS-derived TSS maps show that TSS concentrations are, in general, high along the Korean and Chinese coasts including the Bohai Sea and the Yangtz River estuary, and lower in the middle of the Yellow Sea and the southeastern area of the East China Sea. The monthly average of 10-year MODIS data reveals that TSS values are highest during winter (January to February) and lowest in summer (July to August). Short-term TSS concentrations retrieved from GOCI data showed the dominant influence of semi-diurnal tidal changes on sediment dynamics through temporal (hourly) and spatial distribution in coastal zones of the Yellow sea. The results presented here demonstrate that the satellite-derived TSS products can be utilized as an application tool for future studies on long- and short-term sediment dynamics of turbid coastal waters. In particular, GOCI observations provide unique important capabilities to characterize and quantify the water properties at high temporal (hourly) and spatial (0.5 km) resolutions in the turbid coastal waters of the Yellow Sea and its vicinities.

Atrazine Incorporation and Soil Erosion: Balancing Competing Water Quality Concerns for Claypan Soils

<abstract> <bold><sc>Abstract.</sc></bold> In the U.S. Corn Belt, claypan soils are vulnerable to both erosion and transport of unincorporated herbicides. Thus, there is a need to identify tillage practices that can achieve a balance between herbicide transport and soil erosion for these soils. The objectives of this research were to compare the effect of three tillage systems on sediment and atrazine transport in surface runoff for a claypan soil and to compare their agronomic utility. The tillage treatments were: (1) rotary harrow with atrazine incorporated to ~5 cm (RH), (2) no-till without incorporation (NT), and (3) minimum till with atrazine incorporated to ~10 cm (MT). Three main tillage plots were established on a Leonard silt loam (2% to 3% slopes), a typical claypan soil series. For each treatment, two sets of runoff subplots, with three replicates each, were established within the main plots. Runoff was generated with a rainfall simulator at an average rate of 27 mm h^-1. Runoff samples were collected at unequal time intervals from 1 to 90 min after runoff initiation and analyzed for total suspended sediment (TSS) and dissolved-phase atrazine concentrations. The RH treatment had significantly lower mean TSS concentrations and load (0.25 g L^-1 and 67 kg ha^-1) than MT (0.58 g L^-1 and 155 kg ha^-1), but it did not significantly increase erosion compared to NT (0.16 g L^-1 and 42 kg ha^-1). The RH treatment also significantly decreased atrazine concentrations and relative loads compared to NT, but it did not significantly increase atrazine transport compared to MT. Average atrazine concentrations (in μg L^-1) and relative loads (as a percentage of applied) were: 2100 and 22.2% for NT, 850 and 10.6% for RH, and 405 and 6.4% for MT. The RH treatment also resulted in comparable or better grain yield than the other treatments. Results indicated that the RH treatment successfully managed the trade-off between erosion and atrazine transport and, thus, holds promise as a best management practice for claypan and related restrictive layer soils.

Chromaticity analysis of the Chilika lagoon for total suspended sediment estimation using RESOURCESAT-1 AWiFS data — A case study

The chromaticity analysis of the Chilika lagoon has been attempted using RESOURCESAT-1 AWiFS data (10-bit radiometric resolution) of 26 November 2003. The total suspended sediment (TSS) concentration of surface waters of the lagoon was analyzed using a satellite-based chromaticity technique. The chromaticity coordinates of the entire lagoon were computed. As the lagoon is rich in biodiversity, five chromatically different regions have been identified based on different reflectance signatures. The technique is validated using the ground-truth data of high turbid water, low turbid water, aquatic vegetation, short grasses, and shallow turbid water. Different features clearly separate out on a chromaticity plot. The x-coordinate of the chromaticity shows better correlation with TSS in comparison to y-coordinate of chromaticity. The proposed approach is valuable for a quick estimate of TSS, an important geophysical parameter, which accounts for the water quality of the lagoon. The technique can be applied to compute the moderate TSS (e.g. up to 42gm−3) in a lagoon or any inland water body given the chromaticity image.

Tidal effects on ecosystem variability in the Chesapeake Bay from MODIS-Aqua

Eight-year ocean color observations between 2002 and 2009 from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the satellite Aqua are used to quantitatively assess the tidal effects on variability of ocean's optical, biological, and biogeochemical properties in the Chesapeake Bay. We demonstrate that tidal variation in the lower Chesapeake Bay is one of the important ocean processes that drive significant changes of the ocean's optical and biogeochemical properties. Normalized water-leaving radiance spectra (nLw(λ)), water diffuse attenuation coefficient at the wavelength of 490nm (Kd(490)), and total suspended sediment (TSS) concentration show significant tidal variations in the lower Chesapeake Bay. Indeed, in the lower Bay region, Kd(490) increases from 0.9m−1 under low tidal current conditions at 0.10–0.15m/s to 1.35m−1 under high tidal current conditions at 0.45–0.50m/s, while chlorophyll-a concentration in the lower Bay region decreases slightly with an increase of the tidal current. Tide-driven variability in Kd(490) and TSS in the lower Bay is similar to and slightly larger than the seasonal variability in the region. On the other hand, different from the variability in the lower Bay, the tidal effects on the ecosystem are small and negligible in the middle and a large part of upper Chesapeake Bay regions.

Total Suspended Sediments Mapping by Using ALOS Imagery Over the Coastal Waters of Langkawi Island, Malaysia

Remote sensing has been extensively used for water delineation and has played an important role in water quality evaluation and environmental management strategies. Suspended sediments are important determinants of water quality in coastal zones. Remote sensing enables the effective monitoring of total suspended sediments (TSS) and the detection of areas with critical water quality issues. This study aims to develop and implement regression models for estimating and mapping TSS concentrations from Advanced Land Observation Satellite (ALOS) images over the coastal waters of Langkawi Island, Malaysia. The algorithm was developed based on the water reflectance model, which is a function of the inherent optical properties of water. Such properties can then be linked to TSS concentration. In this study, an ALOS Advanced Visible and Near Infrared Radiometer type 2 device was used as the imaging sensor system. Concurrent complementary in-situ water samples were collected within the area coverage of the sensor, and digital numbers (DN) for each band corresponding to the sea-truth locations were determined. The extracted DN values were converted into reflectance values and then regressed with their respective sea-truth data. An algorithm was proposed to obtain the regression coefficient. This algorithm can estimate TSS concentrations with a high correlation coefficient (R2 = 0.96) and low root-mean-square error (RMSE = 1.98 mg/l). Finally, a map of the TSS concentration was generated by using the proposed algorithm. This study found that TSS mapping can be conducted by using ALOS data over the coastal waters of Langkawi Island, Malaysia.

FIELD INVESTIGATION OF SIZE DISTRIBUTION OF SUSPENDED SEDIMENT USING LISST-100

To investigate size distributions of suspended sediments in the surf zone, a field measurement using LISST (Laser In-Situ Scatter and Transmissometer) was conducted at Hasaki, Japan. The time series of suspended sediment concentration (SSC) for a grain size in the sand range (63 to 500 ï­m) had strong correlations with those for other sand grain sizes, and the strong correlations were also observed in the silt range (2.5 to 28 ï­m). However, at zero time lag, the time series of SSC for sand grain sizes had little correlations with those for silt sizes. With considering time lag, the time series of SSC for sand sizes had weak correlations with those for silt sizes, but which time series lagged behind the others, sand particles or silt particles, was not clear. When the total SSC CLISST was larger than 0.2 g/l, the median sediment diameter d50 was scattered around 185 ï­m. However, at CLISST &lt; 0.2 g/l, d50 increased as CLISST increased. With the increase in d50 from 150 to 200 ï­m, the sorting coefficient decreased from 2.4 to 1.2 and the skewness increased from 0.7 to 1.0.

Short-Term Sedimentation Processes and Accretion Rates in the Sunchon Bay Estuarine Marsh, South Korea

Park, E.-J.; Yu, K.B.; Ku, C.Y.; Psuty, N.P.; Kim D., and Shin, Y.H., 2012. Short-term sedimentation processes and accretion rates in the Sunchon Bay estuarine marsh, South Korea. Spatial variations in sedimentation during individual tidal cycles were investigated using filter paper traps. The mean mass of sediment deposited in each tidal cycle was higher at the levee edge than at the inner marsh. The mass of sediment decreased with distance from the head of the estuary. There were weak relationships between tidal regime and sediment accumulation. Spatial variations of annual accretion rates were investigated with marker plots. The annual accretion rate at the inner marsh was higher (1.5 to 3.5 cm/y) than that at the tidal river levee edge (0.8 to 3.0 cm/y). Total suspended sediment (TSS) concentrations in the tidal water column indicated that the accretion rate was affected by vegetation. TSS concentrations in the water column leaving the vegetation cover during ebb tidal flows dramatically decreased. There were strong relationships between the concentration of suspended load and accretion rate. The spatial pattern of sedimentation processes at different timescales was mostly due to erosion of the levee edge by tidal flow. The major findings are: (1) The spatial patterns of sedimentation processes at various timescales are different, (2) vertical accretion rates ranged from 0.9 cm/y to 3.5 cm/y, (3) the study area has a depositional regime over the timescale of the present data, (4) the mean and minimum deposition rates in the study area are higher than other cited systems, and (5) the high accretion rate of this area is related to human-induced environmental change, and the sedimentation trajectory is now evolving to a new equilibrium.

Human induced turbidity changes in Poyang Lake between 2000 and 2010: Observations from MODIS

A robust retrieval algorithm to estimate concentrations of total suspended sediments (TSS) in Poyang Lake (the largest freshwater lake in China) was developed using Moderate Resolution Imaging Spectroradiometer (MODIS) medium‐resolution (250 m) data from 2000 to 2010 and in situ data collected during two cruise surveys. The algorithm was based on atmospherically corrected surface reflectance at 645 nm, with 1240 nm data serving as a reference for aerosols and a nearest‐neighbor method was used to avoid land adjacency effect. The algorithm showed an uncertainty of 30–40% for TSS ranging between 3 and 200 mg L−1. Long‐term TSS distribution maps derived from MODIS data and the customized TSS algorithm showed significant variations in both space and time, with low TSS (&lt;10 mg L−1) in wet seasons and much higher TSS (&gt;15–20 mg L−1) in dry seasons for the south lake, and generally higher TSS in the north lake. The TSS difference between the north and the south increased significantly after 2002, with mean TSS often reaching &gt;40 mg L−1in the north. While the TSS seasonality was attributed to the seasonal changes of the lake's circulation, the inter‐annual variations were primarily driven by sand dredging activities, regulated by management policies. The case study here provides baseline water quality information for future restoration efforts in Poyang Lake, and more generally, an approach to assess water quality changes in similar water bodies, which have resulted from either climate variability or human activities.

A note on using a digital camera to measure suspended sediment load in Maputo Bay, Mozambique

This article explores the possibility of using digital photography to measure the concentration of suspended sediments in coastal water. Photographs of water colour were taken with a commercially available camera at 21 stations in Maputo Bay, Mozambique, using a short tube with one end immersed in the water to remove surface reflection. Surface water samples were collected at the same time and analysed to determine concentrations of chlorophyll-a and phaeopigments (‘total’ pigments), coloured dissolved organic matter, and total and mineral suspended sediment (MSS) concentration. The optics of the water in the bay is dominated by MSS, which absorbs 75% or more of the light in the red, green and blue parts of the spectrum. As the concentration of MSS increases, the proportion of blue light in the water leaving signal decreases and the proportion of red light increases. The ratio of red to blue ‘counts’ measured with the camera correlates well with the MSS concentration (R 2 = 0.71). There is a weaker but statistically significant relationship between total pigments and the ratio of blue to green counts. We conclude that digital photography is an effective and inexpensive method of quantifying suspended material in these waters although the relationship proposed is likely to be site specific.

Water properties in Chesapeake Bay from MODIS-Aqua measurements

This study evaluates the performance of ocean color products derived from measurements of the Moderate Resolution Imaging Spectroradiometer (MODIS) on the satellite Aqua using the standard near-infrared (NIR) and the shortwave infrared (SWIR)-based atmospheric correction algorithms in the Chesapeake Bay. The MODIS-Aqua-derived normalized water-leaving radiances, nLw(λ), and chlorophyll-a (Chl-a) data are compared with in situ radiometric measurements from the NASA SeaWiFS Bio-optical Archive and Storage System (SeaBASS) database and Chl-a data from the Chesapeake Bay Water Quality Database. Results show that, using the NIR-SWIR combined ocean color data processing, improved nLw(λ) and Chl-a data products can be produced in the Chesapeake Bay. However, Chl-a data are still overestimated in some Chesapeake Bay regions, in particular, in the upper bay region where waters are strongly influenced by the total suspended sediment (TSS) concentration. Specifically, using the NIR-SWIR approach, mean ratios of MODIS-derived and in situ-measured nLw(λ) at wavelengths of 412, 443, 488, 531, 551, and 667nm for the Chesapeake Bay are 1.288, 1.093, 0.998, 0.946, 0.908, and 0.865, respectively, while mean Chl-a values over the region from satellite-derived and in situ-measure data are 11.14 and 10.28mg·m−3, respectively. Based on a strong correlation relationship between TSS and water diffuse attenuation coefficient, a regional TSS algorithm for the Chesapeake Bay has been developed and validated, with mean ratio of 1.064 between MODIS-derived and in situ-measured TSS data. Therefore, using the NIR-SWIR algorithm for MODIS-Aqua ocean color data processing, nLw(λ), Chl-a, and TSS data from 2002 to 2010 for the Chesapeake Bay have been generated and used for characterizing the water properties in the region, showing strong seasonal and interannual variability, as well as important spatial variations in the region.

Quantifying total suspended sediment export from the Burdekin River catchment using the loads regression estimator tool

The loads regression estimator (LRE) was introduced by Wang et al. (2011) as an improved approach for quantifying the export of loads and the corresponding uncertainty from river systems, where data are limited. We extend this methodology and show how LRE can be used to analyze a 24 year record of total suspended sediment concentrations for the Burdekin River. For large catchments with highly variable discharge such as that of the Burdekin River, it is important to quantify loads and their uncertainties accurately to determine the current load and to monitor the effect of changes in catchment management. The extended methodology incorporates (1) multiple discounted flow terms to represent the effect of flow history on concentration, (2) a term that captures sediment trapping and spatial sources of flow in terms of the ratio of flow from above the Burdekin Falls Dam, and (3) catchment vegetation cover. Furthermore, we validated model structure and performance in relation to the application tested. We also considered errors in gauged flow rates of 10% that were consistent with the literature. The results for the Burdekin site indicate substantial variability in loads across years. The inclusion of vegetation cover as a predictor had a significant impact on total suspended sediment (TSS) concentration, with values up to 2.1% lower noted per increasing percentage of vegetation cover. TSS concentration was up to 38% lower in years with greater proportions of flow from above the dam. The extended LRE methodology resulted in improved model performance. The results suggest that management of vegetation cover in dry years can reduce TSS loads from the Burdekin catchment, and this is the focus of future work.

Assessment of total suspended sediment concentrations in Poyang Lake using HJ-1A/1B CCD imagery

We explored the potential of the environment and disaster monitoring and forecasting small satellite constellations (HJ-1A/1B satellites) charge-coupled device (CCD) imagery (spatial resolution of 30 m, revisit time of 2 days) in the monitoring of total suspended sediment (TSS) concentrations in dynamic water bodies using Poyang Lake, the largest freshwater lake in China, as an example. Field surveys conducted during October 17–26, 2009 showed a wide range of TSS concentration (3–524 mg/L). Atmospheric correction was implemented using the Fast Line-of-sight Atmospheric Analysis of Spectral Hypercubes (FLAASH) module in ENVI with the aid of aerosol information retrieved from concurrent Terra/Moderate Resolution Imaging Spectroradiometer (MODIS) surveys, which worked well at the CCD bands with relatively high reflectance. A practical exponential retrieval algorithm was created between satellite remote sensing reflectance and in-situ measured TSS concentration. The retrieved results for the whole water area matched the in-situ data well at most stations. The retrieval errors may be related to the problem of scale matching and mixed pixel. In three selected subregions of Poyang Lake, the distribution trend of retrieved TSS was consistent with that of the field investigation. It was shown that HJ-1A/1B CCD imagery can be used to estimate TSS concentrations in Poyang Lake over synoptic scales after applying an appropriate atmospheric correction method and retrieval algorithm.

Sauter mean diameter determination for the fine fraction of suspended sediments using a LISST-25X diffractometer

The LISST-25X ability for measuring the Sauter mean diameter of the fine fraction of sediment samples, SMDf, is evaluated. A simple algorithm to determine the SMDf is proposed. The algorithm is based on the standard output variables from the LISST-25X diffractometer: Sauter mean diameter of the complete sample, SMDt; Sauter mean diameter of the coarse fraction, SMDg; total suspended sediment concentration, SSCt, and coarse suspended sediment concentration, SSCg. Validation is performed in the laboratory by contrasting algorithm results from complete sediment samples against LISST SMDt measurements made on the fine fraction of the samples. A second validation is performed by comparison between algorithm results and Malvern measurements on the same samples. The samples are from the Salado River (Argentina) and the Paraná River (Argentina). Their SMDt range from 10μm to 70μm.Results indicate that the algorithm determines the Sauter mean diameter of the fine fraction in a reliable way, with average differences of 1.4μm (algorithm-LISST comparison) and 3.4μm (algorithm-Malvern comparison). The observed differences are attributed to operational conditions rather than to algorithm limitations. A correlation between calculated SMDf and d50 of the fine fraction is also established, with a determination coefficient of R2=0.46.

Organic carbon fluxes from the upper Yangtze basin: an example of the Longchuanjiang River, China

AbstractTo investigate the effects of anthropogenic activity, namely, land use change and reservoir construction, on particulate organic carbon (POC) transport, we collected monthly water samples during September 2007 to August 2009 from the Longchuanjiang River to understand seasonal variations in the concentrations of organic carbon species and their sources and the yield of organic and inorganic carbon from the catchment in the Upper Yangtze basin. The contents of riverine POC, total organic carbon and total suspended sediment (TSS) changed synchronously with water discharge, whereas the contents of dissolved organic carbon had a small variation. The POC concentration in the suspended sediment decreased non‐linearly with increasing TSS concentration. Higher molar C/N ratio of particulate organic matter (average 77) revealed that POC was dominated by terrestrially derived organic matter in the high flows and urban wastewaters in the low flows. The TSS transported by this river was 2.7 × 105 t/yr in 2008. The specific fluxes of total organic carbon and dissolved inorganic carbon (DIC) were 5.6 and 6 t/km2/yr, respectively, with more than 90% in the high flow period. A high carbon yield in the catchment of the upper Yangtze was due to human‐induced land use alterations and urban wastes. Consistent with most rivers in the monsoon climate regions, the dissolved organic carbon–POC ratio of the export flux was low (0.41). Twenty‐two percent (0.9 t/km2/yr) of POC out of 4 t/km2/yr was from autochthonous production and 78% (3.1 t/km2/yr) from allochthonous production. The annual sediment load and hence the organic carbon flux have been affected by environmental alterations of physical, chemical and hydrological conditions in the past 50 years, demonstrating the impacts of human disturbances on the global and local carbon cycling. Finally, we addressed that organic carbon flux should be reassessed using adequate samples (i.e. at least two times in low‐flow month, four times in high‐flow month and one time per day during the flood period), daily water discharge and sediment loads and appropriate estimate method. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Modelling impact of northerly wind-generated waves on sediments resuspensions in the Dover Strait and adjacent waters

A numerical approach based on the coupling of the wave propagation module SWAN (Simulating WAves Nearshore) with the three-dimensional circulation module COHERENS (COupled Hydrodynamical-Ecological model for RegioNal and Shelf seas) is developed to simulate the impact of a northerly wind-generated storm on the resuspensions of heterogeneous sediments in the Dover Strait and adjacent waters. Suspended sediment transport is computed for the four grain-size classes of silts ( d 1 = 25 μ m ) , very fine sands ( d 2 = 75 μ m ) , fine sands ( d 3 = 150 μ m ) and medium sands ( d 4 = 350 μ m ) . Predicted times histories of wave variables and the near-bottom mean current and total suspended sediment concentration (SSC) are compared with field data collected off Merlimont Beach, in the south of Boulogne-sur-Mer. Mappings of the growth of the near-bottom total SSC during the storm are produced. The total SSC increases by four at the Merlimont site. The multiplying factor reaches 40 along the exposed northern French coastline revealing the Flandres bed features. This increase is limited to five in deep waters of the southern Dover Strait with local strengthening on the top of the Vergoyer and the Bassure de Baas sandbanks. The storm modifies the granulometric composition of suspended materials by increasing the contribution of medium sands. Main modifications appear at the south-western entrance of the Dover Strait and in the exposed southern North Sea. The near-bottom resuspensions remain elsewhere dominated by the local resuspensions of fine sands added to remote resuspended silts advected by currents.

A review of sediment and nutrient concentration data from Australia for use in catchment water quality models

Land use (and land management) change is seen as the primary factor responsible for changes in sediment and nutrient delivery to water bodies. Understanding how sediment and nutrient (or constituent) concentrations vary with land use is critical to understanding the current and future impact of land use change on aquatic ecosystems. Access to appropriate land-use based water quality data is also important for calculating reliable load estimates using water quality models. This study collated published and unpublished runoff, constituent concentration and load data for Australian catchments. Water quality data for total suspended sediments (TSS), total nitrogen (TN) and total phosphorus (TP) were collated from runoff events with a focus on catchment areas that have a single or majority of the contributing area under one land use. Where possible, information on the dissolved forms of nutrients were also collated. For each data point, information was included on the site location, land use type and condition, contributing catchment area, runoff, laboratory analyses, the number of samples collected over the hydrograph and the mean constituent concentration calculation method.A total of ∼750 entries were recorded from 514 different geographical sites covering 13 different land uses. We found that the nutrient concentrations collected using “grab” sampling (without a well defined hydrograph) were lower than for sites with gauged auto-samplers although this data set was small and no statistical analysis could be undertaken. There was no statistically significant difference (p<0.05) between data collected at plot and catchment scales for the same land use. This is most likely due to differences in land condition over-shadowing the effects of spatial scale. There was, however, a significant difference in the concentration value for constituent samples collected from sites where >90% of the catchment was represented by a single land use, compared to sites with <90% of the upstream area represented by a single land use. This highlights the need for more single land use water quality data, preferably over a range of spatial scales. Overall, the land uses with the highest median TSS concentrations were mining (∼50,000mg/l), horticulture (∼3000mg/l), dryland cropping (∼2000mg/l), cotton (∼600mg/l) and grazing on native pastures (∼300mg/l). The highest median TN concentrations are from horticulture (∼32,000μg/l), cotton (∼6500μg/l), bananas (∼2700μg/l), grazing on modified pastures (∼2200μg/l) and sugar (∼1700μg/l). For TP it is forestry (∼5800μg/l), horticulture (∼1500μg/l), bananas (∼1400μg/l), dryland cropping (∼900mg/l) and grazing on modified pastures (∼400μg/l). For the dissolved nutrient fractions, the sugarcane land use had the highest concentrations of dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP). Urban land use had the highest concentrations of dissolved inorganic phosphorus (DIP). This study provides modellers and catchment managers with an increased understanding of the processes involved in estimating constituent concentrations, the data available for use in modelling projects, and the conditions under which they should be applied. Areas requiring more data are also discussed.

Long‐term patterns and short‐term dynamics of stream solutes and suspended sediment in a rapidly weathering tropical watershed

The 326 ha Río Icacos watershed in the tropical wet forest of the Luquillo Mountains, northeastern Puerto Rico, is underlain by granodiorite bedrock with weathering rates among the highest in the world. We pooled stream chemistry and total suspended sediment (TSS) data sets from three discrete periods: 1983–1987, 1991–1997, and 2000–2008. During this period three major hurricanes crossed the site: Hugo in 1989, Hortense in 1996, and Georges in 1998. Stream chemistry reflects sea salt inputs (Na, Cl, and SO4), and high weathering rates of the granodiorite (Ca, Mg, Si, and alkalinity). During rainfall, stream composition shifts toward that of precipitation, diluting 90% or more in the largest storms, but maintains a biogeochemical watershed signal marked by elevated K and dissolved organic carbon (DOC) concentration. DOC exhibits an unusual “boomerang” pattern, initially increasing with flow but then decreasing at the highest flows as it becomes depleted and/or vigorous overland flow minimizes contact with watershed surfaces. TSS increased markedly with discharge (power function slope 1.54), reflecting the erosive power of large storms in a landslide‐prone landscape. The relations of TSS and most solute concentrations with stream discharge were stable through time, suggesting minimal long‐term effects from repeated hurricane disturbance. Nitrate concentration, however, increased about threefold in response to hurricanes then returned to baseline over several years following a pseudo first‐order decay pattern. The combined data sets provide insight about important hydrologic pathways, a long‐term perspective to assess response to hurricanes, and a framework to evaluate future climate change in tropical ecosystems.

A Numerical Model for the Transport of Chromium and Fine Sediments

A mathematical model which represents the transport processes of heavy metals and fine sediments in a fluvial stream was developed. The model consists of a three-equation system: the first one for total chromium concentration in the water column, CTw, the second one for total suspended sediment concentration, Sw, and the third one for chromium concentration in bed sediments, r. The third equation represents the chromium exchange between the water column and bed sediments by two processes: diffusion of soluble chromium and erosion/deposition of chromium sorbed to sediments. The basic assumption of the model is the instantaneous equilibrium. The main parame- ters are the partition coefficients in the water column and bed sediments, the depth of the active bed sediment layer, and the mass transfer coefficient between the water column and sediment pore water. The numerical model approx- imates the equations of advection-dispersion for chromium in water and suspended sediments by using a Eulerian third-order scheme. Numerical vs. analytical solutions were considered satisfactory for different initial, boundary, and sedimentological conditions. In order to estimate the impact of a chromium side discharge, the model was implemented for the Salado River in a reach of 65.6-km long (Santa Fe, Argentina). The results showed the effect of chromium discharge on almost the whole reach, then the vulnerability of the water quality in the Salado River when the flow was low was evidenced. When comparing the computed and measured results, the former showed a reasonable repre- sentation of the presence of chromium in water and bed sediments.

Watershed-level analysis of exceedance frequencies for different management strategies

Exceedance of water-quality standards is important in assessing water quality. The effectiveness of soil conservation Beneficial Management Practices (BMPs) should be measured according to the BMPs' impact on exceedance frequencies. However, estimating exceedance frequencies for different management scenarios with field measurements is practically impossible due to difficulties in obtaining adequate data for analysing different combinations of BMPs. The objective of this modeling research was to analyse exceedance frequencies for different management strategies applied in the Black Brook Watershed (BBW). Daily concentrations of total suspended sediments (TSS) and soluble phosphorous (sol-P) were predicted with the Soil and Water Assessment Tool (SWAT) and assessed against water-quality standards from the Canadian Council of Ministers of the Environment (CCME) and National Agri-Environmental Standards Initiative-Ideal Performance Standards (NAESI-IPS). The investigated BMPs included conservation tillage, reduced fertilizer application, crop rotation, flow diversion terraces (FDT) and the combination of all four BMPs. The results indicated that FDT was the most effective at reducing exceedance frequencies of TSS and sol-P. Under the different management scenarios, we calculated the annual exceedance frequencies of TSS and sol-P concentrations above the CCME (20–45% and 10–26%) and NAESI-IPS (32–55% and 20–38%).