Showing papers on "Silt published in 2012"

Particle size scales and classification of sediment types based on particle size distributions: review and recommended procedures.

Abstract: This paper provides a review of different particle size scales, size class terminology and particle size distribution (‘textural’) classification schemes which are widely used in sedimentology, geomorphology, soil science, aquatic ecology and civil engineering It is concluded that a revised system of size class nomenclature, based on the Udden (1898) and Wentworth (1922) schemes, provides the most logical and consistent framework for use with sediments and a wide range of other particulate materials A refined scheme is proposed which has five first-order size classes (boulder, gravel, sand, silt and clay), each of which has five second-order subdivisions with limits defined at one phi intervals The scheme is simple and intuitively easy to understand The paper also provides a review of previous schemes that have been proposed to describe and classify sediments on the basis of the proportions of gravel, sand and mud, or sand, silt and clay using trigons (also termed ternary diagrams) Many of these schemes do not have a logical basis and provide limited or uneven resolution New gravel, sand and mud and sand, silt and clay classification systems are proposed that are both more logical and provide greater discriminatory power than previous schemes; they are therefore more suitable for use in environmental and forensic investigations A new Microsoft Excel® program, freely available to download from http://wwwkpalcouk, allows rapid classification of sediments based on the proportions of gravel, sand and mud and sand, silt and clay proportions and graphical comparison of the data for different sample groups

ISRIC-WISE derived soil properties on a 5 by 5 arc-minutes global grid (ver. 1.2)

Abstract: Version 1.2 of describes a harmonized dataset of derived soil properties for the world. It was created using the soil distribution shown on the 1:5 million scale FAO-Unesco Soil Map of the World (DSMW), rasterised at 5 by 5 arcminutes, and soil property estimates derived from the ISRIC-WISE soil profile database, version 3.1. The dataset considers 19 soil variables that are commonly required for agro-ecological zoning, land evaluation, crop growth simulation, modelling of soil gaseous emissions, and analyses of global environmental change. It presents ‘best’ estimates for: soil drainage class, organic carbon content, total nitrogen, C/N ratio, pH (H2O), CECsoil, CECclay, effective CEC, base saturation, aluminium saturation, calcium carbonate content, gypsum content, exchangeable sodium percentage (ESP), electrical conductivity, particle size distribution (i.e. content of sand, silt and clay), content of coarse fragments (> 2 mm), bulk density, and available water capacity (-33 to -1500 kPa). These estimates are presented by FAO soil unit for fixed depth intervals of 20 cm up to 100 cm depth (or less when appropriate) for so-called virtual profiles. The associated soil property values were derived from analyses of some 10,250 profiles held in WISE using a scheme of taxonomy-based taxotransfer rules complemented with expert-rules. The type of rules used to derive the various soil property values have been flagged in the database to provide an indication of the possible confidence in the derived data. Most map units on the DSMW are complex, comprising up to eight different soil units. Assessments and model applications that use the derived soil properties therefore should consider the full map unit composition and depth range. The soil property values presented here should be seen as best estimates based on the current selection of soil profiles in WISE, the procedure for clustering the measured data, taxotransfer scheme used for deriving soil, properties, and the spatial data of the digital Soil Map of the World. The derived information may be used for exploratory assessments at a broad scale (< 1:5 million; 5 by 5 arcminutes and coarser), pending the global update of the information on world soil resources at more detailed scales, upon due consideration of the underlying generalisations and assumptions.

The role of the exopolysaccharides in enhancing hydraulic conductivity of biological soil crusts

Abstract: Biological soil crusts (BSCs) are highly specialized topsoil microbial communities commonly found in arid and semiarid environments, permeated by a polymeric matrix of polysaccharides. BSCs can in principle influence edaphic properties such as texture, pore formation and water retention, which in turn determine water distribution and biological activity in dry lands. This paper investigates the influence of biotic and abiotic factors on BSC hydraulic conductivity, a parameter gauging the ease with which water can move through the pore spaces. Texture, phototroph abundance, microbial composition, and extracellular carbohydrate content were considered as potentially relevant parameters in a correlational study of BSC samples that spanned 1.5 orders of magnitude in hydraulic conductivity. A newly developed, non-destructive extraction method enabled us to directly quantify the specific role of extracellular polysaccharides on soil permeability on a variety of samples. Hydraulic conductivity showed a strongest correlation with texture (positive with sand content, negative with silt and clay). A weaker negative correlation with carbohydrate content, especially with polysaccharides having a molecular weight

Effect of rainfall intensity, slope and antecedent moisture content on sediment concentration and sediment enrichment ratio

Abstract: The transportability of sediment by runoff and its potential for subsequent deposition is dependent on the size and distribution of the transported material and the sediment concentration. The size distribution of the eroded sediment is also expected to vary depending on which erosion process is predominant. This study discusses the results of laboratory analysis to evaluate the effect of rainfall intensity, slope steepness, soil types and antecedent moisture content on sediment concentration, runoff coefficients, and sediment enrichment ratios. To achieve the objectives, laboratory experiments were conducted using FEL-3 rainfall simulator and erosion pan. Rainfall intensity of 120, 70, and 55 mm/h was applied sequentially at 9, 25 and 45% slope for three soil types found in Alemaya watershed, eastern Ethiopia Alemaya Black soil (Soil A), Regosols (Soil B), and Cambisols (Soil C) varied from clay to sandy clay loam in texture with wet and dry antecedent moisture contents. The rate of sediment concentration significantly varied with moisture contents. For Soil A, sediment concentrations (24.3 g/l) from an initially air-dry surface and (18.7 g/l) from an initially wet surface were observed. Wetting decreased sediment concentration of Soil A, B and C by 23, 45.7 and 1.3%, respectively. Sediment concentration didn't show any trend with rainfall intensities. For Soil B, the highest average silt enrichment ratio of 2.15 was observed followed by average clay enrichment ratio of 1.61. For Soil A and Soil C, average clay enrichment ratios of 1.13 and 1.91 were observed, respectively. Initial moisture contents had effect on the enrichment ratio, however, the actual effect varied with soil type and the size of the particle. Results of the study indicated that the actual effect of slope as well as rainfall intensity on sediment concentration and sediment yield vary with soil types and moisture contents.

Experimental study on unfrozen water content and soil matric potential of Qinghai-Tibetan silty clay

Abstract: A new soil moisture content sensor coupled with a new matric potential sensor that can operate in the subfreezing environment was used to measure the moisture content and soil matric potential dynamics of Qinghai-Tibetan silty clay. Combined with nuclear magnetic resonance (NMR) technique and thermal resistor temperature probe, the characteristics of unfrozen water content and soil matric potential, and their relationships with temperature were analyzed. The results show that initial water content has an impact on the freezing point and unfrozen water content. The decrease in the initial water content results in a depression in the freezing point. The Qinghai-Tibetan silty clay has more similar unfrozen water content characteristic to clay than to silt. There is approximately 3% of unfrozen water content retained when the soil temperature drops to −15°C. The change of soil matric potential with temperature is similar to that of the unfrozen water content. The matric potential value of the saturated silty clay is approximately −200 kPa when the soil temperature drops to −20°C. The measured matric potentials are significantly lower than the calculated theoretical values based on the freezing point depression. Moisture migration experiment indicates that soil matric potential controls the direction of moisture movement and moisture redistribution (including ice and liquid water) during the soil freezing.

Effects of long-term compost and fertilizer application on stability of aggregate-associated organic carbon in an intensively cultivated sandy loam soil

Abstract: The study examined the influence of compost and mineral fertilizer application on the content and stability of soil organic carbon (SOC). Soil samples collected from a long-term field experiment were separated into macroaggregate, microaggregate, and silt + clay fractions by wet-sieving. The experiment involved seven treatments: compost, half-compost N plus half-fertilizer N, fertilizer NPK, fertilizer NP, fertilizer NK, fertilizer PK, and control. The 18-year application of compost increased SOC by 70.7–121.7%, and mineral fertilizer increased by 5.4–25.5%, with no significant difference between control soil and initial soil. The C mineralization rate (rate per unit dry mass) in microaggregates was 1.52–2.87 mg C kg−1 day−1, significantly lower than in macroaggregate and silt + clay fractions (P < 0.05). Specific C mineralization rate (rate per unit SOC) in silt + clay fraction amounted to 0.48–0.87 mg C g−1 SOC day−1 and was higher than in macroaggregates and microaggregates. Our data indicate that SOC in microaggregates is more stable than in macroaggregate and silt + clay fractions. Compost and mineral fertilizer application increased C mineralization rate in all aggregates compared with control. However, compost application significantly decreased specific C mineralization rate in microaggregate and silt + clay fractions by 2.6–28.2% and 21.9–25.0%, respectively (P < 0.05). By contrast, fertilizer NPK application did not affect specific C mineralization rate in microaggregates but significantly increased that in silt + clay fractions. Carbon sequestration in compost-amended soil was therefore due to improving SOC stability in microaggregate and silt + clay fractions. In contrast, fertilizer NPK application enhanced SOC with low stability in macroaggregate and silt + clay fractions.

Sources, spatial variation, and speciation of heavy metals in sediments of the Tamagawa River in Central Japan.

Abstract: Sediments of the Tamagawa River in central Japan were studied to explain the spatial variation, to identify the sources of heavy metals, and to evaluate the anthropogenic influence on these pollutants in the river. Sediment samples were collected from 20 sites along the river (five upstream, four midstream, and 11 downstream). Heavy metal concentrations, viz. chromium, nickel, copper, zinc, lead, cadmium, and molybdenum, in the samples were measured using inductively coupled plasma-mass spectroscopy. The chemical speciations of heavy metals in the sediments were identified by the widely used five-step Hall method. Lead isotopes were analyzed to identify what portion is contributed by anthropogenic sources. The total heavy metal concentrations were compared with global averages for continental crust (shale) and average values for Japanese river sediments. The mean heavy metal concentrations were higher in downstream sediments than in upstream and midstream samples, and the concentrations in the silt samples were higher than those in the sand samples. Speciation results demonstrate that, for chromium and nickel, the residual fractions were dominant. These findings imply that the influence of anthropogenic chromium and nickel contamination is negligible, while copper, zinc, and lead were mostly extracted in the non-residual fraction (metals in adsorbed/exchangeable/carbonate forms or bound to amorphous Fe oxyhydroxides, crystalline Fe oxides, or organic matter), indicating that these elements have high chemical mobility. The proportion of lead (Pb) isotopes in the downstream silt samples indicates that Pb accumulation is primarily derived from anthropogenic sources.

Modeling sand-mud transport induced by tidal currents and wind waves in shallow microtidal basins: Application to the Venice Lagoon (Italy)

Abstract: In this study we present a mathematical model for sediment entrainment, transport and deposition caused by the combined action of tidal currents and wind waves in shallow micro-tidal basins. The model uses a bi-granular mixture made up of cohesive and non-cohesive sediments thus considering clay, silt and sand, all of which commonly characterize the sediment bed composition of estuaries and tidal basins. The model also describes the evolution of bed elevation and evaluates the variation of bed sediment composition distinguishing cohesive from non-cohesive behavior. A stochastic approach is proposed to evaluate sediment entrainment close to incipient sediment motion. Particular attention is also given to the problem of reconstructing a reliable initial bed composition as this has a significant impact on sediment entrainment. The model was applied to the test case of the Venice lagoon (Italy) and good agreement was found when comparing model results to a series of turbidity measurements collected inside the lagoon. The model was then used to predict the actual net amount of sand and mud flowing through the inlets and the bottom evolution in terms of elevation and composition.

Clay Dispersibility and Soil Friability—Testing the Soil Clay-to-Carbon Saturation Concept

Abstract: Soil organic carbon (OC) influences clay dispersibility, which affects soil tilth conditions and the risk of vertical migration of clay colloids. No universal lower threshold of OC has been identified for satisfactory stabilization of soil structure. We tested the concept of clay saturation with OC as a predictor of clay dispersibility and soil friability. Soil was sampled three years in a field varying in clay content (~100 to ~220 g kg-1 soil) and grown with different crop rotations. Clay dispersibility was measured after end-over-end shaking of field-moist soil and 1-2 mm sized aggregates either air-dried or rewetted to -100 hPa matric potential. Tensile strength of 1-2, 2-4, 4-8 and 8-16 mm air-dried aggregates was calculated from their compressive strength, and soil friability estimated from the strength-volume relation. Crop rotation characteristics gave only minor effects on clay dispersibility and no detectable effects on soil friability. Dispersed clay correlated to soil content of clay but the correlation increased if subtracting a fraction assumed protected by OC. This trend was less convincing for soil tensile strength and friability. Increased clay dispersibility and reduced soil friability one year of measurements could be ascribed to wet conditions for potato harvest and tillage the preceding year. Literature data indicate soils’ content of clay and silt (Fines20) to be a better predictor of specific surface area than clay. We conclude that a clay/OC ratio of 10 and a Fines20/OC ratio of 20 may serve as corresponding thresholds for clay dispersibility, the latter probably best reflecting organomineral interactions of importance to the soil physical properties.

Bedload Transport of Mud Across A Wide, Storm-Influenced Ramp: Cenomanian–Turonian Kaskapau Formation, Western Canada Foreland Basin

Abstract: Mudstone-dominated shallow-marine rocks of the Kaskapau Formation were deposited on a low-gradient ramp that spanned the foredeep of the Western Canada foreland basin during the late Cenomanian to middle Turonian. Organic-, clay-rich, and silt-rich mudstone accumulated on the flank of the forebulge, > 200 km from the western shoreline. Within this mudstone, a large proportion of the clay minerals are organized into silt- and very fine sand-size aggregate grains. These aggregates were produced both in the water column as marine snow and phytodetritus and also through reworking of previously deposited cohesive mud to form intraclasts. The latter, along with siliceous silt grains, form wave- and combined-flow ripples, graded beds, scour-fills, and ripple-tail lamination. Where sand-size sediment (comprising detrital siliceous, calcareous bioclastic, or phosphatic grains) is present, it is molded into combined-flow ripples, HCS, gutter casts, and lags. Thus all sediment grades indicate storm- wave and current reworking of the sea floor at a distance of > 200 km offshore. The common occurrence of clay minerals in the form of aggregate grains, organized into combined-flow ripples and parallel lamination, implies advective transport of clay minerals as bedload, driven by combined flows across a very low-gradient ramp. On the distal part of this ramp, latest Cenomanian rocks include thin tongues of SW-prograding quartz-rich sandstone that was derived from an emergent forebulge. This source was drowned during the early Turonian eustatic rise when the sediment abruptly changed to organic-rich mudstone dominated by clay-mineral aggregates. This compositional change was mainly a response to a sudden increase in distance to detrital sources in the west, rather than a dramatic increase in water depth. Throughout much of early to middle Turonian time, the sea floor in the forebulge region lay above effective storm wave base for silt, estimated at ~ 70 m. At sea-level lowstands, wave winnowing and erosion of the sea floor concentrated bioclastic lags at the top of siltier-upward sequences; lags are interpreted to correspond to falling-stage, lowstand, and early transgressive systems tract deposits on the western margin of the basin. Previous studies may have substantially overestimated water depth for organic- and clay-rich calcareous mudstones in the Western Interior Seaway.

Plants and biological soil crust influence the hydrophysical parameters and water flow in an aeolian sandy soil / Vplyv rastlín a biologického pôdneho pokryvu na hydrofyzikálne parametre a prúdenie vody v piesočnatej pôde

Abstract: This study tested the hypothesis that the changes in hydrophysical parameters and heterogeneity of water flow in an aeolian sandy soil have the same trend as the process of succession. Three sub-sites were demarcated at the area of about 50 m x 50 m. The first sub-site was located at the pine-forest glade covered with a biological soil crust and represented the initial stage of succession. The second sub-site was located at the grassland and represented more advanced stage of succession. The third sub-site was located at the pine forest with 30-year old Scots pines and represented advanced stage (close to climax) of succession. The sandy soil at the surface was compared to the soil at the pine-forest glade at 50 cm depth, which served as a control because it had a similar texture but limited impact of vegetation or organic matter. It was found that any type of vegetation cover studied had a strong influence on hydrophysical parameters and heterogeneity of water flow in an aeolian sandy soil during hot and dry spells. The changes in some hydrophysical parameters (WDPT, R, k(–2 cm), Sw(–2 cm), ECS and DPF) and heterogeneity of water flow in an aeolian sandy soil had the same trend as the process of succession, but it was not so in the case of Ks and Se(–2 cm), probably due to the higher content of smaller soil particles in grassland soil in comparison with that content at other sub-sites. Both the persistence and index of water repellency of pure sand differed significantly from those of grassland, glade and forest soils. The highest repellency parameter values in forest soil resulted in the lowest value of both the water sorptivity and hydraulic conductivity in this soil in comparison with other soils studied. The highest value of ethanol sorptivity and the lowest value of saturated hydraulic conductivity in the grassland soil in comparison with other soils studied were due to the higher content of fine-grained (silt and clay) particles in the grassland soil. The effective cross section and the degree of preferential flow of pure sand differed significantly from those of grassland, glade and forest soils. The change in soil hydrophysical parameters due to soil water repellency resulted in preferential flow in the grassland, glade and forest soils, while the wetting front in pure sand area exhibited a form typical of that for stable flow. The latter shape of the wetting front can be expected in the studied soils in spring, when soil water repellency is alleviated substantially. The columnar shape of the wetting front, which can be met during heavy rains following long dry and hot spells, was attributed to redistribution of applied water on the surface to a series of micro-catchments, which acted as runon and runoff zones.

The link between grain‐size components and depositional processes in a modern clastic lake

Abstract: It is widely recognized that lake sediment grain-size distributions tend to be polymodal and consist of two or more grain-size components. However, for specific cases, the genesis of each component usually is poorly understood. In this study, the grain-size components of the surface sediments of Hulun Lake, Inner Mongolia, were partitioned using the log-normal distribution function method and the relationship between the identity of each grain-size component and the hydraulic condition of the lake was investigated in order to relate the constituent components to specific depositional processes in the lake. The data indicate that the modern clastic sediments of Hulun Lake contain six distinct unimodal grain-size distributions representing six grainsize components. Each of the components retains its identity including modal size, manner of transportation and environment of deposition, although the relative percentage varies with the hydraulic conditions throughout the lake. These components are specified from fine to coarse modes as long-term suspension clay, offshore-suspension fine silt and medium to coarse silt, and nearshore-suspension fine sand, saltation medium sand and traction coarse sand. The percentage contribution of several grain-size components interpreted as being indicative of nearshore environments is shown to be correlated negatively with water depth across the modern lake bed; this suggests that the proportion of these components in core data might be useful as a proxy for water depth. This possibility was tested using a sediment core from Hulun Lake where high percentages of the nearshore grain-size components were found to be correlated with low regional precipitation reconstructed from the pollen profile of the same core. The coincidence of two independent proxies does not only demonstrate the validity of log-normal distribution function in partitioning polymodal sediments but reveals the potential of lake sediment grain-size components for the research of lake-level fluctuations during the geological past.

Consequences on water retention properties of double porosity features in a compacted silt

Abstract: The paper deals with an experimental investigation aimed at studying microstructural features and their consequences on water retention properties of statically compacted unsaturated silt. The evolution of the microstructure of the aggregate fabric induced by compaction is investigated by studying the pore size distribution changes under different initial conditions (void ratio and water content). The material used is low plasticity silt from Jossigny near Paris, France. A series of mercury intrusion porosimetry tests (MIP) were performed at different void ratios and water contents to provide microstructural information. The arrangement of aggregation/particles and pore network was also investigated with environmental scanning electron microscopy (ESEM). The MIP data were used to determine the water retention curve on drying for the specific pore network configuration induced on compaction. The MIP data were used to formulate and calibrate a multimodal water retention model for a specific pore network configuration, which is obtained by linear superposition of subcurves of a modified van Genuchten type. The study is then complemented with controlled suction oedometer tests on compacted samples to obtain the water retention properties of the material at two different void ratios. Finally, we compare the water retention properties obtained by the simulated progression of the different pore network configurations induced on the hydraulic path with the water retention properties under suction-controlled conditions. Good agreement between the two methods for the drying path is reached.

Siliciclastic Back-Barrier Tidal Flats

Abstract: Back-barrier tidal flats occur along micro- to mesotidal coasts landward of barrier islands and in the shelter of coastal sand spits and bars. Tidal flats are generally flood dominated, the grain size progressively decreasing shoreward. The sediment can be divided into sand, slightly muddy sand, muddy sand, sandy mud, slightly sandy mud, and mud. The mud fraction consists of non-cohesive sortable silt and cohesive flocs and aggregates. Important physical and biological surface structures include wave- and current-generated ripples, ladderback ripples, washed out ripples and other late-stage emergence runoff features, shell pavements, fluid mud sheets, tool marks, crawling, feeding and resting traces of intertidal organisms, as well as the feeding traces and tracks of birds. Internal sedimentary structures range from rare dune cross-bedding to ubiquitous ripple cross-bedding in sand, through flaser, wavy and lenticular bedding in mixed sediment, and homogenous or laminated mud toward the high-water line. Bioturbation may be intense, but the preservation potential depends on the frequency and depth of reworking. The transition from land to sea is typically marked by laminated versicolored microbial mats. The interaction between sea-level rise and sediment supply defines the sediment budget and hence the stratigraphy. Prograding, aggrading or transgressive systems are easily distinguished by their stratigraphic architecture.

Interrill erosion of carbon and phosphorus from conventionally and organically farmed Devon silt soils

Abstract: Globally, between 0.57 and 1.33 Pg of soil organic carbon (SOC) may be affected by interrill processes. Also, a significant amount of phosphorus (P) is contained in the surface soil layer transformed by raindrop impact, runoff and crust formation. In the EU, the P content of a crusted (2 mm) surface layer corresponds to 4 to 40 kg ha− 1 of P on arable land (1.094 mil km2). Therefore, the role of interrill processes for nutrient cycling and the global carbon cycle requires close attention. Interrill erosion is a complex phenomenon involving the detachment, transport and deposition of soil particles by raindrop impacted flow. Resistance to interrill erosion varies between soils depending on their physical, chemical and mineralogical properties. In addition, significant changes in soil resistance to interrill erosion occur during storms as a result of changes in surface roughness, cohesion and particle size. As a consequence, erosion on interrill areas is selective, moving the most easily detached small and/or light soil particles. This leads to the enrichment of clay, phosphorous (P) and carbon (C). Such enrichment in interrill sediment is well documented, however, the role of interrill erosion processes on the enrichment remains unclear. Enrichment of P and C in interrill sediment is attributed to the preferential erosion of the smaller, lighter soil particles. In this study, the P and organic C content of sediment generated from two Devon silts under conventional (CS) and organic (OS) soil management were examined. Artificial rainfall was applied to the soils using two rainfall scenarios of differing intensity and kinetic energy to determine the effects on the P and C enrichment in interrill sediment. Interrill soil erodibility was lower on the OS, irrespective of rainfall intensity. Sediment from both soils showed a significant enrichment in P and C compared to the bulk soil. However, sediment from the OS displayed a much greater degree of P enrichment. This shows that the net P export from organically farmed soils is not reduced by a similar degree than soil erosion compared to conventional soil management. The enrichment of P and C in the interrill sediment was not directly related to SOC, P content of the soil and soil interrill erodibility. A comparison of soil and sediment properties indicates that crusting, P and C content as well as density and size of eroded aggregate fragments control P and C enrichment. Due to complex and dynamic interactions between P, SOC and interrill erosional processes, the nutrient and C status of sediments cannot be predicted based on soil P content, SOC or interrill erodibility alone. Clearly, further research on crust formation and the composition of fragments generated by aggregate breakdown and their transport in raindrop impacted flow under different rainfall conditions is required. Attaining this critical missing knowledge would enable a comprehensive assessment of the benefits of organic farming on nutrient budgets, off-site effects of interrill erosion and its role in the global C cycle.

Identification of mangrove water quality by multivariate statistical analysis methods in Pondicherry coast, India

Abstract: Different multivariate statistical analysis such as, cluster analysis, principal component analysis, and multidimensional scale plot were employed to evaluate the trophic status of water quality for four monitoring stations. The present study was carried out to determine the physicochemical parameters of water and sediment characteristics of Pondicherry mangroves—southeast coast of India, during September 2008–December 2010. Seasonal variations of different parameters investigated were as follows: salinity (10.26–35.20 psu), dissolved oxygen (3.71–5.33 mg/L), pH (7.05–8.36), electrical conductivity (26.41–41.33 ms−1), sulfide (1.98–40.43 mg/L), sediment texture sand (39.54–87.31%), silt (9.89–32.97%), clay (3.06–31.20%), and organic matter (0.94–4.64%). pH, temperature, salinity, sand, silt, clay, and organic matter indicated a correlation at P < 0.01. CA grouped the four seasons in to four groups (pre-monsoon, monsoon, post-monsoon, summer) and the sampling sites in to three groups. PCA identified the spatial and temporal characteristics of trophic stations and showed that the water quality was worse in stations 3 and 4 in the Pondicherry mangroves.

Spatial variability of some soil properties for site specific farming in northern Iran

Abstract: Evaluating agricultural land management practices requires knowledge of soil spatial variability and understanding their relationships. Spatial distributions for fourteen soil physical and chemical properties were examined in a wheat field in Sorkhankalateh district, in Golestan province, Iran. 101 soil samples at the distances of 5m, 10m and 20m as nested grid were collected at the depth of 030cm in early December 2004 just after planting the winter wheat in a plot (1.8 ha area). Data were analyzed both statistically and geostatistically on the basis of the semivariogram. Frequency distribution of all data was normal. The spatial distribution and spatial dependence level varied within location. The range of spatial dependence was found to vary within soil parameters. Nitrate had the shortest range of spatial dependence (23.99m) and K had the longest (93.92m). Eight parameters including pH, EC, sand, silt, clay, P, CaCO3 and organic matter (OM) were moderately spatially dependent whereas saturation percentage (SP), bulk density (Db), K, N, cation exchange capacity (CEC) and exchangeable sodium percentage (ESP) were strongly spatially dependent. The results demonstrate that within the same field, spatial patterns may vary among several soil parameters. Soil nutrients were found to be affected by farmer management. Variography and kriging can be useful tools for designing effective soil sampling strategies and variable rate application of inputs for use in site-specific management.

Fluvial and slope-wash erosion of soil-mantled landscapes: Detachment- or transport-limited?

Abstract: Many numerical landform evolution models assume that soil erosion by flowing water is either purely detachment-limited (i.e. erosion rate is related to the shear stress, power, or velocity of the flow) or purely transport-limited (i.e. erosion/deposition rate is related to the divergence of shear stress, power, or velocity). This paper reviews available data on the relative importance of detachment-limited versus transport-limited erosion by flowing water on soil-mantled hillslopes and low-order valleys. Field measurements indicate that fluvial and slope-wash modification of soil-mantled landscapes is best represented by a combination of transport-limited and detachment-limited conditions with the relative importance of each approximately equal to the ratio of sand and rock fragments to silt and clay in the eroding soil. Available data also indicate that detachment/entrainment thresholds are highly variable in space and time in many landscapes, with local threshold values dependent on vegetation cover, rock-fragment armoring, surface roughness, soil texture and cohesion. This heterogeneity is significant for determining the form of the fluvial/slope-wash erosion or transport law because spatial and/or temporal variations in detachment/entrainment thresholds can effectively increase the nonlinearity of the relationship between sediment transport and stream power. Results from landform evolution modeling also suggest that, aside from the presence of distributary channel networks and autogenic cut-and-fill cycles in non-steady-state transport-limited landscapes, it is difficult to infer the relative importance of transport-limited versus detachment-limited conditions using topography alone. Copyright © 2011 John Wiley & Sons, Ltd.