Showing papers in "Catena in 2001"

Influence of soil properties on the aggregation of some Mediterranean soils and the use of aggregate size and stability as land degradation indicators

Abstract: Soil aggregation in relation to other soil properties was studied along a climatological transect in the Southeast of Spain. Three sites were selected along this transect ranging from semiarid to subhumid climatological conditions. The aggregate size distribution, the macro and microaggregate stability of the superficial soil horizon and their relations with other soil properties were analysed. Large aggregates (>10, 10–5, 5–2 mm) were present in highest proportions in the most arid of the studied areas. These large aggregates were associated with high values of water-stable microaggregates; however, they did not improve soil structure and are related to high bulk densities and low water retention. Aggregates 1–0.105 mm were positively correlated to medium, fine, very fine sand and silt fractions and to organic matter. Aggregates Water stability of microaggregates showed a positive correlation with clay content while the stability of the macroaggregates depended on the organic matter content when the organic matter content was greater than 5 or 6%. Below that threshold the carbonate content was strongly correlated with aggregate stability. A general conceptual scheme of associations between aggregate size distribution, water-stable microaggregation and textural characteristics for the area was developed. The land use history affecting soil overlaps the pattern of climatological situations and has to be taken into account. Aggregate size distribution and stability can be used as indicator of soil degradation, but not as a unique parameter.

Coupling between hillslopes and channels in upland fluvial systems: implications for landscape sensitivity, illustrated from the Howgill Fells, northwest England

Abstract: The sensitivity of upland fluvial systems depends on the magnitude and frequency of sediment and flood producing events, modified by the internal coupling characteristics of the system. This paper assesses the role of hillslope/channel coupling for the sensitivity of upland geomorphic systems, using evidence from a 30-year monitoring programme of geomorphic change in the Carlingill valley, Howgill Fells, northwest England. In the hillslope zone, there is little sediment supply to the stream system. Locally, slope failures occur in response to extreme events (six such events in 30 years within the ca. 6 km 2 Carlingill valley). In the footslope coupling zone, basally induced gullies are major sediment sources to the stream. Sediment-production events occur ca. 30 times per year, feeding sediment to basal debris cones. Stream floods which can entrain these sediments occur once every ca. 2–5 years. Stream channel morphology is adjusted to this regime. Downstream of gullies, channels are wide and braided; elsewhere they are narrow and single-thread. As the gullies develop, however, this coupling weakens and the eroding slopes eventually stabilise by revegetation. Over the 30-year monitoring period, there has been a progressive trend towards gully stabilisation, and an associated reduction in channel instability. Massive destabilisation may occur in response to rare extreme flood events. Such an event (return period >100 years) occurred in neighbouring Langdale and Bowderdale valleys in June 1982. That event destabilised the system, causing slope failures, fan deposition, and in some places, a switch in channel style to wide braided channels. Since 1982 there has been a progressive recovery by slope stabilisation and single-thread sinuous channels have become reestablished. A different style of extreme event occurred in Carlingill in October 1998, in response to the wettest week in the 30-year period. A slope failure fed debris flows 400 m downslope, almost coupling with the channel system. Future climatic change could render the system prone to destabilisation, through either extreme flood events or through major slope failures.

A simple approach to soil loss prediction: a revised Morgan–Morgan–Finney model

Abstract: A revised version of the Morgan–Morgan–Finney model for prediction of annual soil loss by water is presented. Changes have been made to the way soil particle detachment by raindrop impact is simulated, which now takes account of plant canopy height and leaf drainage, and a component has been added for soil particle detachment by flow. When tested against the same data set used to validate the original version at the erosion plot scale, predictions made with the revised model gave slopes of a reduced major-axis regression line closer to 1.0 when compared with measured values. The coefficient of efficiency, for sites with measured runoff and soil loss, increased from 0.54 to 0.65. When applied to a new data set for erosion plots in Denmark, Spain, Greece and Nepal, very high coefficients of efficiency of 0.94 for runoff and 0.84 for soil loss were obtained. The revised version was applied to two small catchments by dividing them into land elements and routing annual runoff and sediment production over the land surface from one element to another. The results indicate that, when used in this way, the model provides useful information on the source areas of sediment, sediment delivery to streams and annual sediment yield.

Agricultural influence on landscape sensitivity in the Upper Mississippi River Valley

Abstract: Agricultural landscapes are more sensitive to climatic variability than natural landscapes because tillage and grazing typically reduce water infiltration and increase rates and magnitudes of surface runoff. This paper evaluates how agricultural land use influenced the relative responsiveness of floods, erosion, and sedimentation to extreme and nonextreme hydrologic activity occurring in watersheds of the Upper Mississippi Valley. Temporally overlapping stratigraphic and historical instrumental records from southwestern Wisconsin and northwestern Illinois show how agricultural modification of a natural prairie and forest land cover affected the behavior of floods and sedimentation during the last two centuries. For comparison, pre-agriculture Holocene alluvial sediments document the sensitivity of floods and alluvial activity to climate change prior to significant human influences on the natural land cover. High-resolution floodplain stratigraphy of the last two centuries shows that accelerated runoff associated with agricultural land use has increased the magnitudes of floods across a wide range of recurrence frequencies. The stratigraphic record also shows that large floods have been particularly important to the movement and storage of sediment in the floodplains of the Upper Mississippi Valley. Comparison of floodplain alluvial sequences in watersheds ranging in scale from headwater tributaries to the main valley Mississippi River demonstrates that land use changes triggered hydrologic responses that were transmitted nearly simultaneously to all watershed scales. In turn, flood-driven hydraulic adjustments in channel and floodplain morphologies contributed to feedback effects that caused scale-dependent long-term lag responses. There has been a general reduction in magnitudes of flooding, erosion, and sedimentation since the mid-20th century, largely in response to better land conservation practices. The reduction trend is most apparent on tributary watersheds of a few hundred square kilometers and smaller sizes. However, the main-channel Upper Mississippi River, with associated drainage areas between about 100,000–200,000 km2, has experienced increased occurrences of large floods during the second half of the 20th century. Most of these large floods have been associated with snowmelt runoff which is occurring more rapidly and earlier in the season in response to a trend toward warmer winters and springs in the late 20th century. Modification of the natural drainage network through establishment of drainage tiles and channelization has also continued during the late 20th century. Tiling and channelization have increased drainage efficiency and probably have contributed in part to the occurrence of large floods on the Mississippi River, but the magnitudes of their effects are unknown at present. In spite of reduced sediment loads since about 1950 on all watershed scales, the anomalous high frequency of large floods on the Upper Mississippi River continues the accelerated delivery of agriculturally-related sediment to floodplain and backwater environments. The results of this study indicate that agricultural land use has escalated landscape sensitivity to such a degree that modern process rates provide a very distorted representation of process rates that occurred in the geologic past prior to human disturbance.

A critical assessment of the sensitivity concept in geomorphology

Abstract: The landscape sensitivity concept concerns the likelihood that a given change in the controls of a system or the forces applied to the system will produce a sensible, recognisable, and persistent response. The idea is an essential element of the fundamental proposition of landscape stability. This is described as a function of the spatial and temporal distributions of the resisting and disturbing forces and is known as the factor of safety or the stability index. The resistance of a system is defined by the system specifications: its structure, strength properties, transmission linkages, coupling efficiency, shock absorption capacity, complexity and resilience. The disturbing forces include the steady application of energy from the specified tectonic, climatic, biotic, marine and human environmental controls. Change takes place through time and space as a normal process–response function to these specifications and involves material transport, morphological evolution and structural rearrangement. These, in turn, progressively change the system specifications, which alters the performance through time. To make progress with these issues, the nature of waves of aggression, temporal adjustments to disturbing forces, spatial interactions with structure, divergent pathways of change propagation, evolution of ‘barriers to change,’ effects of inheritance, decoupling, and the effects of change on system specifications all need to be understood at all temporal and spatial scales.

Landscape sensitivity in time and space — an introduction

Abstract: Landscape sensitivity may be discussed in terms of the response of landscape systems to perturbation on different time and spatial scales. Unstable systems behave chaotically but may show self organised criticality, while stable systems resist change until threshold values of system parameters are exceeded. Spatial sensitivity is expressed in different rates of change, between landscape components or elements. This leads to divergence between landscape elements, and the inheritance of palaeoforms in present-day landscape mosaics. Temporal sensitivity reflects the magnitude and frequency of individual events nested within patterns of longer term environmental changes occurring on different timescales. The resulting landscape complexity reflects the spatio-temporal sensitivity of earth surface systems over ten orders of scale magnitude. The connectivity within landscapes ensures that site instabilities can be propagated within multievent feedback systems. Landscapes record their own histories in sediments and soils, but interpretation of event stratigraphy may not be straightforward, while soil profiles can absorb individual events without erosion. Although we are increasingly able to model the present, environmental management is dominantly about conserving inherited properties of landscapes: forests, soils, floodplains, coastlines. Landscape sensitivity for landscape management must, therefore, address not only active, largely nonlinear, environmental systems, but also the mosaics and palimpsests that are the inheritance from past environments.

Field soil aggregate stability kit for soil quality and rangeland health evaluations

Abstract: Soil aggregate stability is widely recognized as a key indicator of soil quality and rangeland health. However, few standard methods exist for quantifying soil stability in the field. A stability kit is described which can be inexpensively and easily assembled with minimal tools. It permits up to 18 samples to be evaluated in less than 10 min and eliminates the need for transportation, minimizing damage to soil structure. The kit consists of two 21×10.5×3.5 cm plastic boxes divided into eighteen 3.5×3.5 cm sections, eighteen 2.5-cm diameter sieves with 1.5-mm distance openings and a small spatula used for soil sampling. Soil samples are rated on a scale from one to six based on a combination of ocular observations of slaking during the first 5 min following immersion in distilled water, and the percent remaining on a 1.5-mm sieve after five dipping cycles at the end of the 5-min period. A laboratory comparison yielded a correlation between the stability class and percent aggregate stability based on oven dry weight remaining after treatment using a mechanical sieve. We have applied the method in a wide variety of agricultural and natural ecosystems throughout western North America, including northern Mexico, and have found that it is highly sensitive to differences in management and plant community composition. Although the field kit cannot replace the careful laboratory-based measurements of soil aggregate stability, it can clearly provide valuable information when these more intensive procedures are not possible.

Coastal management and sea-level rise

Abstract: The predicted rise in sea level due to global warming has given rise to much speculation as to the impact on erosion and accretion rates at the coast as well as increases in hazards to coastal users. This paper focuses on the spatial adjustments that coastal landforms will exhibit in response to changing energy gradients both normal to and parallel to the shore. These adjustments, in many cases, will take the form of the migration of landforms in order that they maintain their position within the coastal energy gradient. Prediction of the rates of such migration will be fundamental to the future management of the changing coastal environment. The paper discusses the impact of sea-level rise on the two basic coastal landform assemblages: those in estuaries and those on the open coast, and then goes on to examine the effect on ebb-tidal deltas that are located at the critical junction between estuaries and open coasts. In each case, the rates of landform migration under an accelerated sea-level rise are predicted and compared with existing rates using examples from the east coast of Britain. Assuming a sea-level rise of 6 mm/year, the paper predicts that estuaries will migrate landwards at rates of around 10 m/year, open-coast landforms can exhibit long-shore migration rates of 50 m/year, while ebb-tidal deltas may extend laterally along the shore at rates of 300 m/year. The implication for the management of such dynamic coastal systems, including such issues as coastal defence and conservation, are discussed.

Runoff and soil erosion under rainfall simulation of Andisols from the Ecuadorian Páramo: effect of tillage and burning

Abstract: In northern Ecuador, soils of high altitude grasslands (paramos) are mainly non-allophanic Andisols developed on Holocenic volcanic ash. These soils have a high water retention capacity and are the “water tank” of central Ecuador. To assess the effect of land use (burning and tillage) on soil hydrodynamic properties, rainfall simulation was conducted at two different sites. At Pichincha near Quito, the simulation was conducted on a recent volcanic ash soil comparing natural, tilled and burned plots. At El Angel, the simulation was conducted on a mature non-allophanic Andisol comparing natural, recently tilled and formerly cultivated plots. On natural plots, the infiltration rate was very high and sediment loss very low. Results for infiltration rate and runoff indicated that land use change on paramos increased runoff flow and reduced saturated hydraulic conductivity. Superficial reorganisation of the soil surfaces occurred on tilled plots at both sites. This crusting process was fast and resulted in surfaces with very low conductivity at Pichincha. The same processes seemed to be slower at El Angel. The soil surface of recent Andisols at Pichincha was prone to crusting whereas the mature Andisol, at El Angel, with a lower bulk density, was compacted when the kinetic energy of raindrops was high. Water repellency occurred after burning at Pichincha and following long natural air drying after tillage in the non-allophanic A horizon at El Angel. Water repellency combined with the low bulk density of soil aggregates explain the intensity of sediment losses in the abandoned soils after cultivation (Bare fallow plots). Erosion occurred in these areas through floating hydrophobic and stable aggregates.

Weathering and evolution of soils formed on granitic, glacial deposits: results from chronosequences of Swiss alpine environments

Abstract: Two soil chronosequences of mountainous ecosystems in Switzerland served as the basis to calculate the accumulation of soil organic matter, transformation of pedogenic Fe and Al and net losses of the main elements (Ca, Mg, K, Na, Fe, Al, Mn and Si) by means of mass-balance calculations. Elemental losses due to deglaciation and exposure to the weathering environment were calculated. These mass balance calculations indicate that extensive mineral weathering resulted in significant leaching losses of Si, major base cations, and Al (particularly from the upper horizons). The losses are especially pronounced in the early stages of soil formation. In most cases, the exponential decay model incorporating an asymptotic or logarithmic regression model seems to provide a good description of weathering. The greatest changes in the soil chemistry of these alpine soils on granitic host material occurred within the first 3000–4000 years of soil development. Later, the weathering rates decreased rapidly and the overall depletion of elements nearly reached an asymptote. We also found that the mean ratio of [Al t Fe d ]/[Al d Fe t ] of the fine earth of the A, E and B horizons is closely linked to the duration of soil development. A very rapid decrease of this ratio also occurred at the beginning of soil evolution in order to reach asymptotic values after about 3000 years. Consequently, this ratio could be a good indicator of the age of alpine soils. Furthermore, there is a close relationship between the mass of organic C or N in the whole profile and the soil age: the older the soil the higher the corresponding mass per unit area. The chronofunctions presented give a first attempt of the chemical soil evolution in the Alps. However, only little data has been available up until now regarding alpine soils, and this fact inhibits a more detailed observation of the changes that have occurred over a period of 2000–8000 years of soil formation under similar climatic conditions.

Flood events in Mediterranean ephemeral streams (ramblas) in Valencia region, Spain

Abstract: This article deals with flood events in Mediterranean ephemeral streams typical of the Valencia region. The combination of the basin physical characteristics (steep slopes, sparse vegetation, thin soils and permeable rock) and intense, heavy and irregularly distributed rain, generates flash floods. Runoff generation is sudden, giving rise to flash floods with sharp, narrow hydrographs with short time lags. The high amount of runoff is, nevertheless, only a small fraction of the total rainfall, since the characteristics of the river basins are such that infiltration of large amounts of water occurs. Using daily hydrological data from the Rambla de la Viuda basin and 5-min data from both the Barranc del Carraixet and the Rambla de Poyo basins, the rainfall–runoff processes have been analysed during flood events in these ephemeral streams.

Influence of soil surface features and vegetation on runoff and erosion in the Western Sierra Madre (Durango, Northwest Mexico)

Abstract: In mountainous areas, runoff and soil erosion are closely linked to soil surface features, particularly stoniness. Depending on the size of rock fragments (gravel, pebbles, stones and/or blocks) and especially the way they are integrated into the soil matrix, they may facilitate or hinder infiltration and promote soil losses. The present study examines the role of different soil surface features and their influence on runoff formation and on soil erosion in an area seriously affected by overgrazing. Based on measurements made on hillslopes for 2 years at the plot scale, the results show that grass cover, pebbles and sand content increase runoff and erosion. Inversely, slope value, tree cover percentage, structural stability and organic matter content are negatively correlated with runoff and soil losses. It is shown that the correlations can be explained by the major role played by the surface features on hydrologic behaviour of the hillslopes. Two main surface features were identified and hydraulically characterised, namely: (i) crusted surfaces with embedded gravel widespread on gentle slopes which induce high runoff and erosion rates; and (ii) stony surfaces, where free pebbles and blocks protect the top soil against raindrops and overland flow kinetic energy and lead to reduce runoff and soil losses.

Temporal changes in soil aggregates and water erosion after a wildfire in a Mediterranean pine forest

Abstract: The evolution of soil structure after a forest fire was studied on two zones representatives of a typical Mediterranean Pine forest. These zones were in opposite slope orientation but with similar topographical and pedological characteristics. Changes in soil macro-aggregation and water stable micro-aggregation were monitored seasonally during a year after the fire. The water erosion patterns were also studied from August 1993, immediately after the fire, to the end of 1996. The first 5 cm of soil depth were the most affected by fire temperature, showing clear differences on aggregate distribution and temporal variability between zones. In the north-facing soil, a substantial and gradual recovery on soil aggregation was observed mainly in the fraction greater than 5-mm diameter; this reached an increase of 27% in mass of aggregates. In the south slope, the evolution of aggregation was smooth and restricted to the fraction minor than 0.1 mm. These differences between zones are reflected also in their values on soil cohesiveness and compaction, being lesser in the south-oriented soils. Values of erosion parameters show that both zones have the higher soil losses during the immediate period (4 months) after the fire, being more important in the south zone. This zone produced more sediment and runoff (52.42% and 29.95%, respectively) than north-facing soils for the whole period studied.

The sensitivity of peat-covered upland landscapes

Abstract: Ombrogenous mires, or bogs, are remarkable in that they are organic landforms built from living plants and their partially decayed remains (peat), together with large quantities of water derived directly from precipitation In the uplands and northwest of the British Isles, they tend to dominate landscapes wherever the slope allows The components of ombrogenous mires are highly sensitive to change, especially in hydrology Their vegetation may alter in response to very small changes in water level and/or water chemistry, whereas the underlying peat may undergo total degradation on dewatering The function of intact mire ecosystems incorporates mechanisms which tend to maintain stability when environmental conditions change; observation indicates, however, that the stability threshold may be crossed under some natural as well as some man-induced circumstances Sensitivity is demonstrated by evidence from the plant remains preserved in the peat; from manipulation of management practices (particularly grazing and burning); from long-term (28–68 years) mapping of vegetation change; and from experimentation on the sensitivity of bog plants to components of air pollution The ultimate manifestation of sensitivity is peat erosion, which is widespread in the uplands and may, in places, have been ongoing for several hundred years It is concluded that we may anticipate heightened sensitivity to cultural perturbation of mire ecosystems during times of climate change, and thus that particular care in our approach to management of blanket peat landscapes is indicated at the present time

Lake sediments, erosion and landscape change during the Holocene in Britain and Ireland

Abstract: Lakes are excellent repositories of air-borne and especially stream-borne materials. It has long been recognised that lake sediments contain a strong record of catchment soils via the inputs of minerogenic and chemical erosional products. To these may be added a variety of palaeoecological indicators including pollen, fungi, Sphagnum leaves and faunal remains. Pollen analysis, in particular, enables land use change to be assessed over many millennia and demonstrates the landscape impacts of woodland clearance, grazing and crop cultivation. Radiocarbon dating provides a chronology for environmental and agricultural change and acts as an indicator of erosion in its own right. The use of such indicators to reconstruct past instances and patterns of erosion is demonstrated with reference mainly to 50 British and Irish lake sites and especially those which have produced signals of erosion in the form of accelerated sediment accumulation and inversed or reversed sequences of radiocarbon dates during the mid and late Holocene. Sites displaying even or decreased sedimentation through time are concentrated in northern and northwestern Scotland and have either no clear signs of human impact, or only so towards the latter part of the Holocene. This pattern probably reflects a combination of factors including low population densities, thin soils and the ‘sealing’ effect of blanket peat. In contrast, all sites with accelerated erosion have indications of anthropogenic impact. This is often prior to levels of increased sedimentation showing a delay in system response. While the spread of dates for the start of the rise in sediment accumulation spans 6200–940 BP, three clusters are evident at 5295–4970, 4530–4235 and 2980–2810 BP. Radiocarbon inversions (mostly reversals) are evident for 18 sites. No obvious causal patterning is indicated and they probably demonstrate no more than the fact that catchment soils, including peats, around a particular site were pushed beyond an erosional threshold sufficient to ‘age’ the sediments deposited within the lakes. It is hoped that improvements in data quality will allow the refinement of such exercises in the future and that lakes will be valued as repositories of long-term landscape sensitivity.

Short-term bank gully retreat rates in Mediterranean environments

Abstract: In this study, short-term headcut retreat was monitored from 46 active bank gullies, selected in the Guadalentin and the Guadix basin in Southeast Spain. The measurements were carried out manually using an orthogonal reference system fixed by erosion pins around the gully heads, between April 1997 and April 1999 with a 1-year interval. The average volumetric retreat rate for all gullies was 4.0 m 3 year −1 , corresponding with an average linear retreat rate of 0.1 m year −1 , but more erosion took place during the first monitored year (1997–1998) compared to the second (1998–1999). An interplay of spatial variations in rainfall distribution and tension crack activity is assumed to be responsible for the important difference in annual headcut retreat, compared to the small difference in annual rainfall amounts. Statistical analysis showed that the present drainage-basin area ( A p ) was the most important topographical factor explaining average gully headcut retreat rate, both in terms of annual eroded volume ( V e ) and annual linear retreat ( R l ), and expressed by the power relationships V e =0.04 A p 0.38 ( R 2 =0.39) and R l =0.01 A p 0.23 ( R 2 =0.39). The V e – A p relationship was compared with the relationship between original drainage-basin area ( A o ) and total eroded bank gully volume (Vol), i.e. Vol=1.71 A o 0.60 ( R 2 =0.65). The importance of runoff generation from a drainage basin is shown by the positive correlation of linear headcut retreat and the runoff curve number (CN), representative for the conditions in the drainage basin. High CN values tend to coincide with higher annual eroded volumes in the relationship between present drainage-basin area ( A p ) and annual eroded volume ( V e ), but this effect was not observed in the relationship between original drainage-basin area ( A o ) and total eroded bank gully volume (Vol). Stepwise multiple regression selected the relevant environmental parameters explaining annual eroded volume and linear retreat. In both equations, the present drainage-basin area explained the largest part of the variation. The CN was selected as another common parameter. Height of the headcut was the second most important variable explaining annual eroded volume, indicating the role of energy transfers and undercutting at the headcut. Linear retreat was further explained by the average slope of the present drainage-basin area, representing the effect of decreasing transmission losses and increasing flow velocity with steeper catchment slopes, and by the sand content, decreasing the cohesion of the soil material, promoting soil fall and headcut retreat. Spatial extrapolation of the measured volumetric retreat rate of 4.0 m 3 year −1 revealed that active bank gully heads contribute up to 6% of the sediment yearly filling up the Puentes reservoir. Estimated gully ages (i) based on the ergodic principle, and (ii) by linear extrapolation of actual gully retreat rates in the past, range between 63 years and 1539 and between 64 and 1720 years, respectively. The high correlation between the gully ages estimated by the two methods is attributed to the fact that most gullies have not reached the evolutionary stage of significantly declining retreat rates. Since medium-term gully retreat rates are more dependent on drainage-basin area compared to the short-term retreat rates obtained in this study, the estimated gully ages represent maximum values, assuming that present land-use and climate conditions prevailed over the last two millennia.

Coastal sensitivity to environmental change: a view from the beach

Abstract: The coastal environment is home to a range of landforms and landscapes that include those at either end of the sensitivity scale. The soft coastal landscapes of beaches, sand dunes, and mudflats represent fast-responding and mobile geomorphic systems that are highly sensitive to environmental change. On the other hand, coastal landscapes in areas of hard rock and inherited Quaternary deposits represent relatively slow-responding systems that are, in the main, thought to be relatively insensitive to change. This paper focuses upon both groups of systems and demonstrates not only how soft coasts respond to environmental stimuli over a variety of time scales but also how this inheritance shapes subsequent development. Using mainly Scottish soft coast examples, the changes associated with sensitive coastal landscapes are assessed in terms of the past timing and magnitude of changes in both sea level and sediment economy. Declining abundance of coastal sediment has initiated a process of internal re-organisation that, because of an inherent sensitivity to sea level change and low thresholds for the forcing of change, fundamentally restricts future management options. The paper also attempts to show that hard rock coasts are also responsive to particular types of event on short time scales and examples are discussed that modify the accepted view of hard rock insensitivity to environmental change.

Erodibility and infiltration characteristics of five major soils of southwest Spain

Abstract: Equations used to calculate erodibility in the Water Erosion Prediction Project soil erosion Ž. model WEPP are based on erodibility studies in the USA and may not function well in another Ž. region. This study was conducted to: i analyze erodibility and infiltration characteristics of some Ž. predominant soils of southern Spain, and ii test equations used to calculate interrill erodibility in the WEPP model on these soils. The five soils chosen for this study in Andalusia, southwest Ž. Spain, were: two terrace soils referred to as ‘Red and Yellow Alfisols’ , an alluvial soil

A method for dendrochronological assessment of medium-term gully erosion rates

Abstract: A method based on dendrochronology to estimate gully erosion rates was developed as an alternative of traditional methods for assessing medium-term gully retreat rates, such as field monitoring of headcuts or aerial-photo interpretation of gully retreat. The method makes use of trees or parts of a tree affected by gully erosion revealing information on the history of the erosion process by datable deviations of their normal growth pattern, hence defined as ‘datable objects’. These include roots exposed by erosion; browsing scars made by ungulates on exposed roots or on above-ground parts of fallen trees; exposed and dead root ends; root suckers; stems, branches and leading shoots of fallen trees; and a sequence of trees within a gully. The method is based on the differentiation between three main conditions depending on the relation between the dynamics of the datable object (part of the tree) and the development of the gully. The first condition implies that the datable object was created before erosion of the gully volume to be dated, e.g. exposed tree roots. According to the second condition, the datable object developed as an immediate consequence of the erosion event, e.g. growth reactions of a fallen tree. The third condition implies that the datable object was created some time after the erosion event took place, e.g. trees colonising the gully bed. Each principle has consequences for the accuracy and the correct interpretation of the estimated erosion rate, i.e. whether the true erosion rate is underestimated, exact or overestimated. In spite of methodological limitations and dendrochronological dating problems, the method was successfully applied in southeast Spain. Conservative estimations of gully-head retreat rate resulted in an average medium-term (3–46 years) value of 6 m3 year−1 (n=9). For gully sidewall processes, the average minimum erosion rate per unit sidewall length amounted 0.1 m3 year−1 m−1 (n=9). A strong correlation was found between the headcut retreat rate (vm(ortho), m3 year−1) and the drainage–basin area (A, m2) of the gullies, expressed by vm(ortho)=0.02A0.57 (R2=0.93, n=9). Comparing the findings from this study with those obtained by short-term headcut retreat monitoring suggests a high reliability of the estimated retreat rates, supporting the applicability of the developed dendrochronological method.

Crossing the thresholds: human ecology and historical patterns of landscape degradation

Abstract: In discussions of landscape sensitivity, human activities have generally been regarded as external forces contributing to landscape change, with a focus on the impacts of cultivation methods, fertiliser practices, grazing pressures and atmospheric pollution. However, there has been comparatively little study undertaken that integrates physical and social systems in a historic context to explain the basis of human activity in sensitive landscapes. Where such attempts have been made, the manner of common land management has figured prominently, with ‘tragedy of the commons’ concepts used to explain land degradation and to provide a foundation for policy response. This has also been the case in Southern Iceland and in this paper we assess the extent to which common land domestic grazing pressures were the primary external force causing soil erosion and land degradation during the period of occupation from ca. 874 AD. We first provide field observation of soil erosion, temporally defined by tephrochronology, to highlight the extent of land degradation during this period. The ‘tragedy of the commons’ explanation of degradation is then assessed by evaluating historic documentary sources, and by environmental reconstruction and modeling of historic grazing pressures. These analyses indicate that regulatory mechanisms were in place to prevent overgrazing from at least the 1200s AD and suggest that there was sufficient biomass to support the numbers of domestic livestock indicated from historic sources. We suggest that failure to remove domestic livestock before the end of the growing season and an absence of shepherding were more likely to contribute to land degradation than absolute numbers. Lack of appropriate regulation of domestic livestock on common grazing areas can be attributed to limited cultural knowledge of changing and rapidly fluctuating environmental conditions.

Uncertainty assessment of soil erodibility factor for revised universal soil loss equation

Abstract: Soil erodibility accounts for the influence of the intrinsic soil properties on soil erosion and is one of six factors in the Revised Universal Soil Loss Equation (RUSLE), a most widely used model to predict long-term average annual soil loss. In a traditional soil survey, each of the soil types (classes) is assigned with a soil erodibility value that is assumed to be constant over time. However, heterogeneity of soil in time and in space tends to support the concept that soil erodibility depends dynamically and spatially on the set of properties of a specific soil. This study statistically compared the published soil erodibility values with those from a set of soil samples in terms of their differences. The published values tend to underestimate soil erodibility. This feature is also supported by the uncertainty assessment in difference maps of the published K values versus those from soil samples. Spatial prediction and uncertainty analysis of the soil erodibility from the set of soil samples was carried out using a sequential Gaussian simulation. The results show that the simulation produces a reliable prediction map of soil erodibility and can be recommended as a monitoring strategy to spatially update soil erodibility.

Weathering of a gypsum-calcareous mudstone under semi-arid environment at Tabernas, SE Spain: laboratory and field-based experimental approaches

Abstract: The weathering of a Late Miocene gypsum-calcareous mudstone outcropping in large badland areas of SE Spain, under a semi-arid Mediterranean climate, was studied by means of two experimental approaches Field and laboratory experiments were carried out to reproduce, though in accelerated form, some of the weathering conditions of the consolidated mudstone In the laboratory, three sequences of 5, 10 and 20 wetting–drying cycles were produced on undisturbed blocks of fresh mudstone samples At the end of the three sequences, samples were analysed for their micromorphology, elemental and soluble salt chemistry, and total mineralogy Unweathered dry samples, as blanks, and permanently wet samples were also analysed In the field, two small plots of freshly exposed mudstone were monitored over 3 years for their response to natural weathering in terms of morphological changes and sediment output The porosity was increased by a few wetting–drying cycles, as assessed by significant increases in water absorption capacity of the mudstone A combination of three factors is responsible for mudstone weathering: repeated cycles of wetting–drying, the presence of geologically-induced cracks and fissures, and dissolution–crystallisation of relatively soluble minerals, gypsum being the most abundant within this category A few wetting–drying cycles were sufficient to reveal ion migration (specially Na+, Ca++, Mg++, SO4−, HCO− and Cl−) within the mudstone, explaining mineral dissolution In the field, surface weathering rates from 07 to 8 mm year−1 were measured Weathering rates were found to be proportional to the number of rainfall events during the sampling periods, confirming what was found in laboratory conditions, namely, that the number of wetting–drying cycles has the greatest influence on weathering These weathering rates might be considered as the probable range of incision rates under present semi-arid conditions

Reef-island accretion and soil development on Makin, Kiribati, central Pacific

Abstract: The late Holocene accretionary history of reef islands on Makin, the northernmost table reef of the Gilbert atoll chain, western Kiribati, has been reconstructed based on conventional radiocarbon dating of coral shingle and bulk sand samples, and AMS radiocarbon dating of individual grains. Makin is geomorphologically and sedimentologically simple, comprising one main island with only a minor residual lagoon, and composed of sand dominated by the reef-flat foraminifera Calcarina spengleri, Amphistegina lobifera and Baculogypsina sphaerulata. Deposition commenced in mid platform around 2500 years ago; in situ fossil coral (Heliopora) dated at 2400±80 years BP indicates that sea level was 0.4–0.5 m above present at that time. Progradation occurred progressively to westward at a relatively constant rate of 200–300 m ka−1. However, progradation was interrupted on the eastern side by lagoon encapsulation around 1400 years ago. The soil and vegetation characteristics are uniform over most of the reef islands as a result of clearance and the planting and maintenance of coconuts, and do not reflect reef-island depositional history. Remnant stands of Pisonia forest, with an associated phosphatic soil, are not related to any particular geomorphological feature, or period of deposition. The reef islands are continuing to accrete as a result of production of foraminifera on the reef flat.

Divergent evolution and the spatial structure of soil landscape variability

Abstract: The spatial structure of soil variability at the landscape scale was examined on adjacent geomorphic surfaces dating from 80 to 200 ka in eastern North Carolina. The purpose was to determine whether there is evidence at broader scales (distances of 102–104 m) for the divergent evolution observed in the field at very detailed scales (distances of 100–102 m). The state probability function (SPF), which measures spatial dependence for categorical environmental data along a transect, was applied to soil series mapped at a 1:24,000 scale. The older Talbot Terrace and younger Pamlico Terrace surfaces showed distinctly different patterns of spatial variability. The range of spatial dependence was shorter on the older surface (about 200 vs. 300 m), and the SPF was higher at any given distance, indicating more variability. The SPF for the Pamlico surface also indicates a periodicity related to fluvial dissection of the landscape, which is not readily detectable on the Talbot transect despite its greater degree of dissection. The results confirm earlier field studies which suggest that pedogenesis is marked by divergence, whereby differences in initial conditions or local perturbations persist and increase to produce a more variable soil cover.

Stage dependent variability in tractive force distribution through a riffle–pool sequence

Abstract: High resolution data on spatial and temporal variability in flow hydraulics and sediment transport within riffle–pool sequences are required to improve understanding of how fluvial processes maintain these meso-scale bedforms. This paper addresses this issue by providing velocity and boundary shear stress data over a range of discharges from base flow (0.07 m3 s−1) to just over bankfull (8.52 m3 s−1), from a sequence of four pools and three riffles in the River Rede, Northumberland. The data supports the reversal hypothesis of Keller [Geol. Soc. Am. Bull. 87 (1971) 753.] as the primary explanation for the maintenance of the riffle–pool sequence, although they also indicate that spatial variability in tractive force is highly stage dependent and complex. Section-averaged velocity data indicate reversal to be evident at four out of six riffle–pool units. An equalisation in velocity was found for the other two riffle–pool units close to bankfull stage. The spatial patterns of tractive force exhibited in the study reach as a result of increased discharge demonstrate that riffle–pool units operate independently of one another. Shear stress reversals were observed in individual riffle–pool units at different river stages during a flood hydrograph, and in some instances, two occurred in the same riffle–pool unit during a single flow event. Pools were characterised by coarser bed sediments and narrower channel widths in comparison to riffles, increasing the likelihood of tractive force reversal in the River Rede. Areas of predicted bed sediment entrainment obtained from τo−τc, matched observed channel changes in the upper part of the study reach, but over-estimated change in the middle portion of the reach.

Soil losses due to harvesting of chicory roots and sugar beet: an underrated geomorphic process?

Abstract: Field observations in central Belgium indicate a significant soil degradation process which has hitherto not been considered in assessments of soil erosion rates: i.e., soil losses due to root crop harvesting (SLRH). The objectives of this study were: (1) to assess SLRH for two commonly grown root crops in Belgium, i.e. chicory roots and sugar beet, (2) to investigate some factors controlling the spatial and temporal variation of SLRH, and (3) to evaluate the contribution of this soil degradation process to overall soil loss in the study area. Soil losses due to harvesting of witloof chicory roots were assessed by measuring dirt tare from 43 root samples whereas SLRH for inuline chicory roots and sugar beet were calculated from dirt tare data provided by factories processing these roots. Mean soil loss was 11.8 ton ha−1 harvest−1 for witloof chicory roots, 8.1 ton ha−1 harvest−1 for inuline chicory roots and 9.1 ton ha−1 harvest−1 for sugar beet. Assuming that root crops are grown once in 2 years in the study area, mean annual SLRH equals 5.0 ton ha−1 year−1 (0.33 mm/year). Since these root crops have been grown over a period of at least 200 years in Belgium, this implies a mean soil profile truncation of 66 mm. However, important spatial and temporal variability in SLRH data was observed, depending on soil texture, soil moisture at harvest time and harvesting technique. Given the importance of SLRH, it needs to be incorporated into future assessments of soil degradation processes and sediment budgets.

Human impacts on soil properties and their implications for the sensitivity of soil systems in Scotland

Abstract: Human activities have had pronounced impacts on soil properties. Conifer afforestation in the uplands has caused significant decreases in soil pH and in the quality and turnover of organic matter. Acid deposition has increased soil acidity by a similar amount to conifer afforestation but has been shown to affect soils at greater depths. Acid deposition has also increased the mobility of trace metals in the soil and therefore increased metal concentrations in drainage waters. Applications of sewage sludge to the soil have been shown to increase metal concentrations, although most of the Scottish soils affected have high trace metal binding capacities. Intensification of arable cultivation in the lowlands has reduced organic matter concentrations, structural stability and soil workability, and has had effects on soil erodibility. Human trampling, while highly localised, affects sensitive mountain soils in popular areas, leading to loss of surface organic horizons, and therefore, carbon storage. The future impacts of human activities on the soil may be exacerbated by changing climate, and the need to monitor and predict these will not diminish.

Developing a magnetic tracer to study soil erosion

Abstract: Soil erosion is commonly measured as the quantity of sediment leaving a plot or watershed. The techniques for measuring soil erosion patterns and sediment redistribution within plots or watersheds by direct monitoring are very limited. The objective of this study was to develop a direct and non-intrusive tracer method to study the sources, patterns and rates of erosion and deposition of sediments in erosion plots. The magnetic tracer developed in this study consisted of polystyrene plastic beads embedded with a magnetic powder (magnetite). The “magnetized” beads, with a mean weight diameter of 3.2 mm and particle density of about 1.2 g cm−3, were uniformly mixed with soil and tested in the laboratory using simulated rainfall and inflow studies to simulate the interrill and rill components of soil erosion, respectively. In the interrill and rill experiments, the tracer was transported in the same proportion it was initially mixed with the soil. Given this fact, a magnetometer, which measures the soil's magnetic susceptibility, could be used to identify areas of deposition or detachment. The magnetic susceptibility would be increased or reduced depending on whether deposition or detachment occurs. To simulate detachment and deposition, a magnetometer was tested for different tracer concentrations and different thickness of soil containing the tracer. The magnetometer promises to be a sensitive, accurate, and useful tool to study the spatial variation of soil erosion when magnetic tracers are used.

Geomorphological evidence for fluvial change during the Roman period in the lower Rhone valley (southern France)

Abstract: The hydrological and geomorphological dynamics of the lower Rhone river (southern France) are studied during the Roman period (2nd–1st centuries BC, 2nd–3rd centuries AD). The crossing of archaeological and radiocarbon dating methods allow to study events at a pluridecadal to centennial scale. From the Avignon town to the delta, the 15 sites where Roman fluvial dynamics were recorded show higher flooding frequencies, higher energy levels during floods, rises in the marshes or groundwater levels, and/or active morphological dynamics such as channel migrations from 1st century BC to 1st–2nd centuries AD, with respect to the encircling periods. Although this fluvial change does not reach the amplitude of great climatic events such as the Little Ice Age in the Rhone valley, we show that it is also perceived in other parts of the catchment and could have a climatic origin. However, this event is not recorded in the immediate Mediterranean environment of the lower Rhone, so that the Rhone appears to efficiently transmit a foreign climatic change.

The extent and significance of bioturbation on 137Cs distributions in upland soils

Abstract: Differences between measured 137Cs activity–depth profiles and idealised undisturbed profiles generated from an exponential model suggest that faunal turbation has redistributed 137Cs in mineral and organic upland soils in southern Scotland. Bioturbation is also demonstrated by the vertical displacement of other inputs to the soils of known age (non-native tree pollen and spheroidal carbonaceous particles, SCPs). The causes and mechanisms of bioturbation were further investigated by soil micromorphology. Well-drained mineral soils with active populations of earthworms are the most bioturbated, showing near-complete homogenisation to depths of about 20 cm. Enchytraeids also seem to remobilise 137Cs by the digestion of organic matter and may be the main cause of 137Cs redistribution in organic-rich upland soils. Relative rates of mixing are evaluated by comparing 137Cs depth profiles.