Gully and sheet erosion on subtropical mountain slopes: Their respective roles and the scale effect

As most mountains in tropical and subtropical zones, the Western Sierra Madre suffers active present erosion, which may create some constraints to the social and economic development in the area.The objectives of this study of soil degradation in the Western Sierra Madre, are to determine the respective roles of gully and sheet erosion. This research is based on field observations, field measurements of runoff and, soil losses at the plot, as well as the watershed scales as an analysis of an exhaustive census of the few gullies located in an experimental area.Measured soil losses in the Western Sierra Madre are high although there are few gullies. Most of the sediment yield seems to originate in widespread degraded areas where stoniness is the main evidence of a previous stage of erosion. Previously overgrazing and deforestation were determined as the factors of the appearance of new soil surface characteristics which explain the high runoff and sediment productions. The soil compacted by cattle trampling reduces infiltration. The decrease of the vegetation cover triggers a rise in the splash effect and thus, a soil sealing.These processes induce an increase in runoff and soil losses. The main erosion type has been described as sheet erosion: it is characterised by the removal of fine soil particles and the remains of gravels, pebbles and blocks, which constitute a pavement on the soil. Gullies generally appear on the bottom of wide valleys and depressions, where soils are thick. It is shown that sheet erosion is two orders of magnitude higher than gully erosion at the hillslope scale.Due to the spatial distribution of land use and the geological context such as the heavily degraded areas close to the main rivers, the reduction of runoff and soil loss rates within the extension of a considered area, commonly observed in hydrology, only applies up to the elementary catchments scale (1 to 50 km²). Above this area, runoff coefficient and soil loss rates increase.     

Overgrazing effects on vegetation cover and properties of volcanic ash soil in the páramo of Llangahua and La Esperanza (Tungurahua, Ecuador)

Abstract. The páramo is a high-altitude ecosystem of the northern Andes. The vegetation is continuous, with grasses as the dominant groundcover. Because of their high water retention, páramos play a fundamental role in water availability for all the population of the inter-Andean valleys. There are many studies of this specific ecosystem, but very few are focussed on overgrazing and its effect on vegetation and soil properties. Intensive grazing started less than 20 years ago and was studied in a representative area in the western Cordillera of central Ecuador covered by recent volcanic ash deposits. Intensive sheep grazing has led to a strong decline in the number of plant species, the replacement of the tussock grass vegetation by a short carpet grass vegetation, and an increase of bare land. In that area, the upper 50 cm of Andisols are deeply affected by a convergent decrease of Al and Fe oxalate and pyrophosphate in soil extracts, carbon contents decrease from 100 g kg^–1 to less than 50 g kg^–1 in the humid zone, from 70 to 40 g kg^–1 in the dry zone and a reduction of water-retention capacity at −33 kPa matrix potential from 800 g kg^–1 to 200 g kg^–1 in humid zones, from 350 to less than 100 g kg^–1 in drier areas. They showed also a decrease in the macrostructure and the development of a highly water repellent microstructure. All these important transformations favour the development of aeolian erosion in dry areas, runoff on bare surfaces, and gully erosion on slopes. The role of the páramo in water-regulation of this ecosystem seems to have been adversely affected for the future.     

Climate and vegetation determine soil organic matter status in an alpine inner-tropical soil catena in the Fan Si Pan Mountain,Vietnam

High mountain ecosystems are generally considered to be particularly sensitive to global climate change. Studies of pedogenesis associated with altitudinal variation, vegetation type and soil carbon content on the same type of parent rock are very limited in inter-tropical mountain areas. Therefore the altitudinal variation of soil pedogenesis through 9 selected profiles from the altitude of 1340 m to 3143 m asl, the summit of the Fan Si Pan Mountain, in the north of Vietnam was examined. Fan Si Pan Mountain is composed of a homogenous alkaline granite rock and is the highest point of the Inter-tropical Continental Asia. The Soil Organic Matter properties (C, N, δ¹³C and δ¹⁵N contents) of the different grain-size fractions of the topsoil of 4 selected profiles corresponding to different ecosystems were also examined.     

Quantifying suspended sediment sources during runoff events in headwater catchments using spectrocolorimetry

Purpose

Understanding hydro-sedimentary dynamics at the catchment scale requires high temporal resolution data on suspended sediments such as their origin, in addition to the common measurements of sediment concentrations and discharges. Some rapid and low-cost fingerprinting methods based on spectroscopy have recently been developed. We investigated how visible spectra could be used to predict the proportion of various source materials in suspended sediment samples, paying particular attention to the potential alteration of spectrocolorimetric signatures between soils and suspended sediments during transport.

Materials and methods

The 22-km² Galabre catchment, France, is composed of black marls, limestones, molasses, undifferentiated deposits and gypsum. Forty-eight source materials were sampled and 328 suspended sediment samples were collected at the outlet during 23 runoff events. Measurements were taken with a diffuse reflectance spectrophotometer on dried samples. As the erosion processes are particle size selective, five particle size fractions of source material were measured in order to assess the potential alteration of the fingerprint signatures. As the biogeochemical processes occurring in the river could also affect the signatures, source materials were immersed in the river for durations ranging from 1 to 63 days and subsequently measured. Finally, partial least-squares regression models were constructed on 81 artificial laboratory mixtures to predict the proportions of source materials.

Results and discussion

The spectrocolorimetric measurements discriminated the primary source materials but not the Quaternary deposits. As the gypsum was not conservative, only the black marls, molasses and limestones were used in the fingerprinting procedure. The construction of the partial least-squares regression models led to a median absolute error of 1.1%. This error increased to 3.9% when the models were applied to source samples with: (1) different particle sizes; (2) different durations spent in the river; or (3) different origins than those used for their construction. The effect of particle size on the fingerprinting procedure was larger than the effect of biogeochemical reactions or the spatial variability of the spectrocolorimetric signatures. Half of the 23 runoff events analysed exhibited huge variations in the source proportions from one sediment sample to another.

Conclusions

The spectrocolorimetric fingerprinting approach was able to quantify routinely the proportion of primary source materials in all suspended sediment samples collected during runoff events. The high temporal resolution of the predicted proportions revealed that only analysing three or four suspended sediment samples during a runoff event could lead to a misunderstanding of the hydro-sedimentary processes for more than half of the investigated runoff events.     

Comparing alternative tracing measurements and mixing models to fingerprint suspended sediment sources in a mesoscale Mediterranean catchment

Journal of Soils and Sediments - Knowledge of suspended sediment provenance in mesoscale catchments is important for applying erosion control measures and best management practices as well as for...     

Shrinkage and drainage in aggregates of volcanic soils: a new approach combining mercury porosimetry and vacuum drying kinetics

The topsoils of Andisols in the wet high altitude grasslands (páramos) of Ecuador have shrunk after recent changes from permanent grassland to agriculture. The question arises as to whether all Andisols in the region behave in the same way. We have therefore studied the shrinkage on drying of topsoils from four representative types of Andisol in the páramos using a combination of drying kinetics and mercury porosimetry. We aimed to identify and quantify the pore volumes that participate in either drainage or shrinkage or both. The key concept of hierarchical dual porosity of soils used to interpret the shrinkage curves was partly validated, but we also identified more complex relations between drainage, shrinkage and structure. We found that the older was the soil, the more weathered it was, and, consequently, the greater its aggregation and porosity. In turn, the total volumetric shrinkage was controlled by the initial void ratio of the wet soil samples. Our results also show that the solid–pore interfaces of the volcanic soil aggregates were less accessible after shrinkage than before.     

Water repellency of volcanic ash soils from Ecuadorian páramo: effect of water content and characteristics of hydrophobic organic matter

Water repellency of volcanic ash soils from the Ecuadorian páramo was studied by a combination of extraction and analysis of water‐repellent products, Molarity Ethanol Droplet values, water contact‐angle measurements by capillary rise, and N₂ adsorption isotherms. The undried samples studied are hydrophilic, but exhibit water repellency after moderate drying (48 hours at 30°C). The advancing water contact‐angle measured by capillary rise varies from 78° to 89°. These water contact‐angles decrease strongly after extraction of organic materials by an isopropanol–water mixture. Elemental analysis, infrared spectra and gas chromatography‐mass spectrometry analyses were used to characterize the extracts. The results show that long‐chain fatty acids and more complex non‐polar alkyl components (waxes) are the main water‐repellent materials. The deposition of such extracted materials onto hydrophilic sand leads to the rapid increase of water contact‐angle until values close to those measured on the soil samples are achieved. Assuming a coating of the mineral surface by organic hydrophobic products and using Cassie's law, the water contact‐angle of extracted materials was computed. The values ranged from 100° to 157°. Nitrogen specific surface areas of the soils studied were very small, indicating a low adsorbent–adsorbate interaction on hydrophobic surfaces. These results partially validate the hypothesis of water‐repellent materials that occur as coatings at least after a drying process.     

Soil physico-chemical changes following application of municipal solid waste leachates to grasslands

Concerns about the use of residues from municipal solid waste incinerators (MSWI) in construction materials usually focus on the potential for heavy metals and organic chemicals to leach into drainage waters under the influence of rain. We hypothesised that high level of salts in the MSWI leachates may cause more of a problem, particularly on soil physico-chemical properties. Both bottom ash (BA) and Solidified Air Pollution Control residue (SAPCr) leachates were added to experimental grassland plots. The amounts of Na⁺ increased by up to 13% in soils supplemented with each leachate. A decrease of the soil total porosity (?14%) was evidence of a subsequent adverse physical effect of this strong salinity. The potential for the grass cover type (species composition or density) to limit this adverse effect was discussed. Laboratory tests allowed us to determine that undiluted SAPCr induced slaking of aggregates accompanied by a strong decrease of aggregate stability, to 49% of control values. Undiluted BA induced dispersion of clays and others fine particles, which are then dislodged and transported into pores, causing blockage and decreasing total porosity. Clay dispersion followed by aggregate collapse occurred when soil solution contaminated by SAPCr was diluted by rainwater. This work stressed the importance of accounting for mineral contaminants, such as salts, when conducting an assessment of waste reuse scenarios.     

Influence of allophane and organic matter contents on surface properties of Andosols

The aim of this study was to use nitrogen gas adsorption to study the complex surface properties of a wide range of Andosol Ah and Bw horizons; N₂ gas adsorption not only provides specific surface area, SSA, but it also yields complementary information about micropore volume and hydrophilic and/or hydrophobic surface properties. Total SSAs were positively related to micropore SSA which was, in its turn, positively interrelated to the dimensionless parameter C of the Brunauer‐Emmett‐Teller (BET) equation (micropore N₂ filling) and microporous allophane content. The more allophanic the Andosol horizon sample, the larger were its total, micropore and mesopore SSAs. On the other hand, strong negative exponential relationships were obtained between either total or micropore or mesopore SSA and soil organic carbon content, with a SSA threshold at an organic carbon content of c. 8–10%, the SSAs being extremely small at larger organic carbon content values. Both SSA_BET and C_BET decreased non‐linearly as a function of the organic C/allophane ratio, with either a SSA_BET or a C_BET threshold at an organic C/allophane ratio value of 3–5, above which both SSA_BET and C_BET were very small (mostly the aluandic Andosol Ah horizons). The more the soil allophanes are assumed to be coated, the more hydrophobic the soil surfaces become and the smaller both SSA_BET and C_BET become; nitrogen gas has a permanent quadrupole moment and therefore acts as a polar probe when surfaces are sufficiently hydrophobic. The antagonistic roles played by allophanes and organic matter in both the SSAs and the values of the dimensionless parameter C in the BET equation were also highlighted by (i) multiple linear regressions between the SSAs and both allophane and organic carbon contents and (ii) principal‐component analysis of SSA_BET, C_BET and soil constituent (allophane, ferrihydrite and organic C) contents.