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Long-term rates of escarpment retreat in sedimentary tablelands have been rarely determined and their possible variability in relation to major environmental changes has often been speculative, especially in older contributions. Moreover, rates reported so far were almost exclusively from arid regions. In this paper we present a dataset of 20 cosmogenic exposure ages, obtained for cliffs and boulders produced by rock-slope disintegration, left in front of a retreating 150-m-high escarpment of a sandstone mesa in SW Poland. The most distant boulders are >400 m away from the cliffs. Following previous research in the region, boulders are mainly products of in situ disintegration and hence, their present-day position on the slope approximates the past position of the cliffs, allowing for the calculations of cliff retreat. Exposure ages systematically increase with the distance from the present-day cliff line and cover the last 100 kyr, implying an average retreat rate of 4.18 mm/yr. However, the dataset splits into several sub-populations, each with its own trend, meaning that the rates of retreat varied with time. A maximum was reached during the Late Pleistocene and transition to the Holocene, when the rates exceeded 10 mm/yr and may have reached 100 mm/yr. The rates seemed distinctly lower between MIS5 and LGM and during the Holocene. We propose that the transition towards a warmer climate at the Pleistocene/Holocene boundary, with an increased frequency of freezing-thawing cycles, the degradation of permafrost and unblocking of groundwater drainage enhanced caprock disintegration and accelerated the break-off rates of caprock. During periods of most efficient cliff disintegration and boulder production the rates of escarpment retreat at the field site could have been two orders of magnitude higher than in the hyperarid areas. However, despite changing rates of retreat major processes involved in escarpment evolution seem to have remained the same. [ABSTRACT FROM AUTHOR]

Over the last decade, much attention has focused on wetland resilience to disturbances such as extreme weather events, longer climate change, and human activities. In geomorphology and cognate disciplines, resilience is defined in various ways and has physical and socioeconomic dimensions but commonly is taken to mean the ability of a system to (A) withstand disturbance, (B) recover from disturbance, or (C) adapt and evolve in response to disturbance to a more desirable (e.g., stable) configuration. Most studies of wetland resilience have tended to focus on the more-or-less permanently saturated humid region wetlands, but whether the findings can be readily transferred to wetlands in drylands remains unclear. Given the natural climatic variability and overall strong moisture deficit characteristic of drylands, are such wetlands likely to be more resilient or less resilient? Focusing on wetlands in the South African drylands, this paper uses existing geomorphological, sedimentological, and geochronological data sets to provide the spatial (up to 50 km 2 ) and temporal (late Quaternary) framework for an assessment of geomorphological resilience. Some wetlands have been highly resilient to environmental (especially climate) change, but others have been nonresilient with marked transformations in channel–floodplain structure and process connectivity having been driven by natural factors (e.g., local base–level fall, drought) or human activities (e.g., channel excavation, floodplain drainage). Key issues related to the assessment of wetland resilience include channel–floodplain dynamics in relation to geomorphological thresholds, wetland geomorphological ‘life cycles’, and the relative roles of natural and human activities. These issues raise challenges for the involvement of geomorphologists in the practical application of the resilience concept in wetland management. A key consideration is how geomorphological resilience interfaces with other dimensions of resilience, especially ecological resilience and socioeconomic resilience, the latter commonly being defined in terms of ecosystem service delivery. [ABSTRACT FROM AUTHOR]

Coastal dunes have received growing attention in recent years because of the ecosystem services they provide and the high anthropic pressure that they have historically been subjected to and continue to endure. Such pressure especially affects arid dune fields where any changes are more noticeable due to their natural dynamics. The aim of this paper is to analyze the relationship between the degradation of an arid aeolian sedimentary system (El Médano, Tenerife, Spain) due to historical long-term land uses and its subsequent biogeomorphological evolution. The methodology combines historical sources (historical documents, aerial and common photographs and oral sources) and current ones (orthophotos, LiDAR data and field work). In order to analyze the response of the system to these uses, 8 plots were chosen in which 3 different historical land uses had resulted in the total or partial elimination of the vegetation and landforms. Biogeomorphological variables were measured on 461 nebkhas in these plots. The main results show that the historical land uses studied in this paper (aerodrome , aggregate extraction and crop cultivation) modified the entire system, with changes observed in the topography, vegetation cover and sediment availability for transport. Changes in topography and distance from shore were important factors in its recovery capacity. This research contributes to our understanding of the different aeolian sedimentary dynamics that can be detected in the same dune field (especially with nebkha landforms) and is of particular importance for making appropriate environmental management decisions to facilitate the recovery of the ecosystem. • Aggregate extractions on the foredune have an irreversible impact. • Different environmental patterns in a dunefield are responses to historical land uses. • Distance to the coast acts as a main factor in system recovery. • Historical land uses modify the natural gradients of the system. • Biogeomorphological patterns in 461 nebkhas were analyzed. [ABSTRACT FROM AUTHOR]

Gullies that terminate at a vertical-wall are ubiquitous throughout arid and semiarid regions. Multi-year assessments of gully evolution and headcut advance are typically accomplished using traditional ground surveys and aerial photographs, with much recent research focused on integrating data collected at very high spatial resolutions using new techniques such as aerial surveys with blimps or kites and ground surveys with LiDar scanners. However, knowledge of specific processes that drive headcut advance is limited due to inadequate observation and documentation of flash floods and subsequent erosion that can occur at temporal resolutions not captured through repeat surveys. This paper presents a method for using very-high temporal resolution ground-based time-lapse photography to capture short-duration flash floods and gully head evolution in response. In 2004, a base level controlling concrete weir was removed from the outlet of a 1.29 ha semiarid headwater drainage on the Walnut Gulch Experimental Watershed in southeastern Arizona, USA. During the ten year period from 2004 to 2014 the headcut migrated upchannel a total of 14.5 m reducing the contributing area at the headwall by 9.5%. Beginning in July 2012, time-lapse photography was employed to observe event scale channel evolution dynamics. The most frequent erosion processes observed during three seasons of time-lapse photography were plunge pool erosion and mass wasting through sidewall or channel headwall slumping that occurred during summer months. Geomorphic change during the ten year period was dominated by a single piping event in August 2014 that advanced the channel head 7.4 m (51% of the overall advance) and removed 11.3 m 3 of sediment. High temporal resolution time-lapse photography was critical for identifying subsurface erosion processes, in the absence of time-lapse images piping would not have been identified as an erosion mechanism responsible for advancing the gully headwall at this site. [ABSTRACT FROM AUTHOR]

This paper explores the relationship between vegetation dynamics and dune mobility in an arid transgressive coastal dune system, specifically the dune field of Maspalomas (Gran Canaria, Canary Islands). The aim is to understand the strategies of colonization and survival that plant communities have developed in slacks that face dune advance. The relationship between plant colonization and dune migration was performed by following Tamarix canariensis and Traganum moquinii plants for several years. Morphological data about each individual as well as the distance of each plant to the dune were measured. A study of the colonization patterns developed by T. moquinii , T. canariensis , Cyperus laevigatus and Launaea arborescens communities was performed by analyzing the evolution of consolidated plant patches and adult plants in relation to the dune advance. This was achieved using digital orthophotos and spatial analysis from geographic information systems. Initiation of plant colonization over transgressive dunes occurs on both wet and dry slacks. The results show that both plant colonization and development of adult plants are largely related to dune mobility. Thus, survival of T. moquinii and T. canariensis plants under dune migration conditions is related to both distance to the dune front and plant height at the moment of burial. Distance from the dune front and plant height increases chance of survival. The dynamics of adult plants is also related to dune displacement rates. Thus, each community has different thresholds of resistance to mobility rates. The T. canariensis community withstands average rates higher than 3 m/year. Its arboreal structure allows this species to grow high enough to resist the advance of the dunes and burial. For the T. moquinii community, the population decreases gradually to eventually disappear when dune mobility rates exceed 4 m/year. The C. laevigatus community develops at dune mobility rates lower than 3 m/year, decreasing its surface area at higher rates. The L. arborescens community endures dune migration rates of at least 1.8 m/year. However, different distances between the dune front and the vegetated area also significant factor, because these can compensate for the effects of displacement rates. Thus, the closer a vegetated area is to a dune front, the lower the rates of displacement must be to produce a greater reduction in the surface vegetation. Plant communities present two patterns of plant colonization to resist burial by sand, one vertical and the other horizontal. The horizontal pattern is employed by C. laevigatus and L. arborescens communities and consists of locating new generations of plants in progressive alignment with the dune front migration. The vertical pattern is employed by the T. canariensis community, and consists of increasing the heights of the plants. The T. moquinii community can utilize both patterns because it reacts positively to some degree of burial since it is located in areas where the dunes reach different heights. [ABSTRACT FROM AUTHOR]

Abstract: There is a growing demand for digital soil maps for environmental planning, modeling and management. If mapped soil classes are taken from a hierarchical taxonomic system, a question arises: which taxonomic level is most appropriate to be depicted on the map with a given sample size, available environmental covariates and the strength of predictive relations between covariates and the soil classes? Pedodiversity, the study and measurement of soil diversity, can be considered as a framework to analyze spatial patterns depicted on soil maps. This paper discusses the selection of the taxonomic level for soil mapping in an arid region in southeast Iran on the basis of (1) the purity of a digital soil class map derived from an artificial neural network (ANN) prediction method using environmental covariates and (2) pedodiversity indices of these soil maps. The prediction of soil classes and the calculation of diversity indices were carried out for taxonomic categories of order, suborder, great group, and subgroup. Using the feed forward back-propagation algorithm, three-layer ANNs with input, hidden and output layers were trained for soil class prediction at each category level. In most predictions, the combined use of terrain attributes and geomorphic surfaces provided the best results. When the taxonomic level changed from order to subgroup, the purity decreased, whereas the values of the diversity indices increased. The highest purity and lowest diversity are observed at the order level, indicating a good quality map in terms of its purity, but reflecting only little soil diversity, thus with a low usage potential. On the other hand, soil maps at the level of subgroup illustrate high diversity and low purity, so that the predicted map units are highly uncertain. This map is also inappropriate for users. We introduced an index combining the diversity and purity which indicated that the best taxonomic level for soil mapping in the study area is the great group, with both high diversity and purity. [Copyright &y& Elsevier]

Abstract: Gyprock is a common rock in evaporitic formations, which have a wide distribution across all continents. However, outcrops of gyprock are rather limited, occurring mainly in low rainfall areas (i.e. southwest of USA, Italy, Spain). Gyprock is one of the most common lithology in the Ebro Tertiary Basin. Here, semiarid conditions propitiate the development of specific weathering processes and landforms with significant implications for understanding the genesis of soils and the geomorphological evolution of these areas. Gypsum crystallization within the pores and fissures of gyprock close to the surface leads to volume increase and pressures, with the consequent weathering of this rock. As a result, considerable porosity develops producing peculiar morphologies on gyprock outcrops (domes and decimetre-scale blisters), specific microscopic features (voids with plain walls, horse-shoe pores and serrated pores), as well as microscopic fabrics (grating fabric, skeletal fabric, fan fabric and botryoidal fabric). This paper presents a morphological characterization of the material forming the various types of weathering features, from microscopic to mesoscopic scale. The macromorphology and micromorphology of those features were used to interpret their genesis. [Copyright &y& Elsevier]

Abstract: Due to overgrazing and agricultural intensification, gully erosion severely affects sub-Saharan countries; however, insufficient quantitative studies exist for this part of the world. This paper presents data on gully head retreat rates in Northern Ethiopia and relates these rates to gully and environmental characteristics. The monitoring of headcuts over one rainy season (2010) revealed that present-day retreat rates are low, with average annual linear (R l), areal (R a) and volumetric (V e) retreat rates of 0.34my−1, 1.70m2 y−1 and 5.2m3 y−1, respectively. These results express the positive effects of recent soil and water conservation practices on gully stabilization. Significantly higher values of R l (up to 1.93my−1) occurred in the Vertisol areas affected by soil piping. When considering the medium- to long time scale (1–47years) using archival terrestrial (and aerial) photographs, headcut retreat rates proved to be significantly higher than those in the short term. The averages for R l, R a and V e are 3.8my−1, 31.5m2 y−1 and 47.7m3 y−1, respectively. Retreat rates are up to 10 times higher after road construction. For the medium to long term, headcut retreat rates were positively related to the catchment area (A). A power relationship that best describes the relation between V e and A is V e =0.53 A 0.31 (r 2 =0.27, n =18). Compared to other areas worldwide, regressive erosion has been rapid in Ethiopia as a result of the degraded and steep landscape combined with erosive rains and the occurrence of Vertisols. In Vertisols, headcut retreat is controlled by soil piping. Because no adequate techniques exist to control gully initiation and development in Vertisols, alternative techniques should be developed that deactivate soil pipes. [Copyright &y& Elsevier]

Abstract: This paper describes the construction and application of a new field instrument, a Gauged Sediment Trap (GaST). Our GaST measures and captures aeolian-transported sediment, can be deployed for long periods of time (weeks), is easy to construct, and is extremely durable. The instrument consists of a water-level logger contained within an underground, water-filled sleeve, into which sediment is funneled from a surface trap. As sediment enters the sleeve water is displaced proportionally to the volume of sediment allowing for continuous monitoring of sediment accumulation. The rate of sediment transport and flux, for a given time period, is estimated through laboratory calibration using sediment from the study area and the GaST. The instrument can be affixed to almost any surface-sediment trap, whether saltation or bedload transport is the target. Data from laboratory calibration and several field tests staged in the summer and winter on Bogue Banks, NC, are presented. Using the HOBO® U20 Water-Level Logger, the GaST accurately captured sediment transport at a resolution of 5.71g (2.15cm3) of sediment per unit time, depending on sampling interval and overall sediment flux. The GaST predicted total sediment mass and volumes with less than 5% error. With an adjustable sampling interval, the GaST can be deployed for various lengths of time but may be limited by the rate of sediment accumulation (they can overflow) and the evaporation rate inside the sleeve (the water could completely evaporate in arid settings). [Copyright &y& Elsevier]

Abstract: Gully erosion is responsible for a substantial amount of soil loss and is generally considered an indicator of desertification. Hence, mapping these gully features provides essential information needed on sediment production, identification of vulnerable areas for gully formation, land degradation, and environmental and socio-economical effects. This paper investigates the use of object-oriented image analysis (OOA) to extract gully erosion features from satellite imagery, using a combination of topographic, spectral, shape (geometric) and contextual information obtained from IKONOS and GEOEYE-1 data. A rule-set was developed and tested for a semi-arid to sub-humid region in Morocco. The percentage of gully system area indicated negligible overestimations between the reference area and the OOA area in two sub-watersheds (0.03% and 1.77%). We also observed that finer gully-related edges within the complex gully systems were better identified semi-automatically than was possible by manual digitization, suggesting higher detection accuracy. OOA-based gully mapping is quicker and more objective than traditional methods, and is thus better suited to provide essential information for land managers to support their decision making processes, and for the erosion research community. [Copyright &y& Elsevier]

Abstract: Based on samples taken from four large sand seas of northern China, this paper first provides basic data about the concentrations and chemistries of soluble salts in deserts of northern China and then discusses the origins and parameters triggering geographical variations. The total concentration of soluble salts in the aeolian sands of four large sand seas in northern China ranges between 0.14‰ and 1.32‰, with the pH of the soluble salts solution (mixing ratio of sand and water 1:5) changing between 8.4 and 9.6, confirming alkaline soil conditions in these regions. Sodium chloride and bicarbonate are the dominant salts occurring as soluble salts in the aeolian deposits of these sand seas. The geographical changes of soluble salts'' concentration display a clear correlation with regional climatic parameters, i.e., precipitation and temperature. The domination of sedimentation of soluble salts in the aeolian sands deposited via atmospheric processes is discussed, which are heavily associated with dry deposition. The mean percentages of Na and Cl, derived from dry depositions, are estimated to be >90% in both Badain Jaran and Taklamakan deserts. [Copyright &y& Elsevier]

Abstract: This paper examines the relationship between land surface properties (e.g. soil, vegetation, and lithology) and landscape morphology quantified by the catchment descriptors: the slope–area (S–A) relation, curvature–area (C–A) relation, and the cumulative area distribution (CAD), in two semi-arid basins in central New Mexico. The first site is composed of several basins located in today''s desert elevations with mesic north-facing and xeric south-facing hillslopes underlain by different lithological formations. The second site is a mountainous basin exhibiting vegetation gradients from shrublands in the lower elevations to grasslands and forests at higher elevations. All three land surface properties were found to have significant influences on the S–A and C– A relations, while the power-law exponents of the CADs for these properties did not show any significant deviations from the narrow range of universal scaling exponents reported in the literature. Among the three different surface properties we investigated, vegetation had the most profound impact on the catchment descriptors. In the S–A diagrams of the aspect-controlled ecosystems, we found steeper slopes in north-facing aspects than south-facing aspects for a given drainage area. In elevation-controlled ecosystems, forested landscapes exhibited the steepest slopes for the range of drainage areas examined, followed by shrublands and grasslands in all soil textures and lithologies. In the C–A diagrams, steeper slopes led to a higher degree of divergence on hillslopes and a higher degree of convergence in the valleys than shallower slopes. The influence of functional types of vegetation detected on observed topography provided some initial understanding of the potential impacts of life on the organization of topography. This finding also emphasizes the critical role of climate in catchment development. We suggest that climatic fluctuations that are capable of replacing vegetation communities could lead to highly amplified hydrological and geomorphic responses. [Copyright &y& Elsevier]

Abstract: Swarms of barchan dunes are common in arid environments. Unlike immobile or slowly moving dunes whose stratigraphy can be used to reconstruct paleoenvironmental changes, the high migration rate of barchans seems to prohibit their use as geoarchives. In this paper, a possible use of barchan swarms for paleoenvironmental reconstructions is presented. The approach is based on dune migration. In a case study based on 500 dunes in the Pampa de Jaguay aeolian transport corridor in coastal southern Peru, middle to late Holocene changes in the supply of aeolian material are inferred from calculating the time of dune initiation at the coastline. Limitations posed by the assumptions underlying this approach are discussed. [Copyright &y& Elsevier]

Abstract: The importance of fluvial processes to the development of late Quaternary landforms in arid regions is well established, particularly for the American Southwest. In contrast, much less information exists about past fluvial processes for other arid regions, such as Arabic countries in the Middle East. This paper reviews the characteristics of late Quaternary fluvial deposits and landforms in Syria, an arid country with little previous Quaternary geomorphic research. Relatively detailed information has been obtained from areas adjacent to archaeological sites because of collaborative activities with geomorphologists and other geoscientists. Such activities have resulted in intensive field surveys, and associated results of complementary archaeological studies of lithic artifacts are useful for establishing the chronology of fluvial deposits. Although the number of existing studies is limited, the available information points to marked changes in the mode of fluvial processes in response to late Quaternary climatic change. Fluvial sedimentation was enhanced during the wet periods of MIS 3–4, the Pleistocene–Holocene transition, and the mid-Holocene. In contrast, the LGM was characterized by limited fluvial sedimentation under a drier climate as well as enhanced carbonate precipitation near the land surface to form calcrete and oncoids. [Copyright &y& Elsevier]

Abstract: Slope change at alluvial-fan toes provides a clue to understanding differences between alluvial fans and other depositional landforms. This paper analyses topographic data for 430 alluvial fans and their adjacent lowlands in Japan and the American Southwest (ASW), typical humid and arid regions with abundant alluvial fans. Arcs showing fan toes were identified and representative areas above and below each arc were then delineated based on a GIS buffer analysis. The mean slope of the area above (S A) and below the fan toe (S B), and the ratio of S A to S B (RS) were computed. Overall fan slope (S O), total fan length (L) and angle of the fan apex (AF) were also measured. Relationships among these morphometric parameters reflect differences in climate and sedimentary processes. Fans in the ASW tend to have straight profiles with similar values for S A and S O, indicating that fan gradients from deposition resulted from low-frequency/high-magnitude storms have been preserved well under an arid climate. Higher RS values result from good preservation of both steep fan surfaces and gentle adjacent lowlands, which is also ascribable to infrequent sediment redistribution. In Japan, S A tends to be smaller than S O, and RS is often close to unity reflecting frequent sediment redistribution on fans and across fan toes under a humid climate. RS in both the regions tends to converge into unity with increasing L. Longer alluvial fans are more strongly dominated by fluvial processes than by mass-flows, especially at their toes. Longer fans also tend to have small AF, leading to fluvial processes similar to those on floodplains. Sediment re-mobilisation at the toes of such fans is more frequent, resulting in smaller differences between S A and S B. [Copyright &y& Elsevier]

Abstract: A single shrub in a widely spaced shrubby area acts as a roughness element and, therefore, it can affect the distribution of the rainfall received on the ground surface surrounding it. This paper focuses on such rainfall distribution on arid hillslopes and its eco-geomorphic implications. Relatively simple methods were used to measure the rainfall around shrubs growing in a small basin that is associated with a prevailing wind direction during rain events. There was a trend toward significantly reduced rainfall onto bare soil areas located on the leeward side, compared with that on other bare soil areas along the hillslope. This may alert to another potential cause for patchiness in the hydrological response of arid hillslope systems. [Copyright &y& Elsevier]

Sand transport by wind in arid regions is a geomorphic process affected by the grain size distribution. Knowledge of the transport behavior of each size group in mixed sand transport is important for understanding the saltation dynamics in dune migration. This paper focuses on the characteristics of mass flux, mass concentration, horizontal velocity and transport rate of different particle size groups and the relation to their mass proportion in air. Experiments were carried out to analyze these relations in a wind tunnel by particle tracking velocimetry. The mass flux of different size particles usually decays exponentially with height for larger size groups and deviates from exponential decay for smaller size groups. The trend of mass concentration of different size groups with height is similar to the mass flux. The mean horizontal velocity of different size groups generally increases with increasing height and decreases with the increase of particle size at the same height. With increasing height, the ratio of mass flux of each size group to the total mass flux increases for the smaller size group but decreases for the larger size group, and the transition diameter of particles is about 0.14 mm for the 0.16 mm sand sample and about 0.21–0.27 mm for the 0.24 mm sand sample. For the smaller size group, the ratio of the mass-weighted average horizontal velocity of each size group to that of all particle sizes is >1.0, and the ratio of the transport rate of each size group to the total transport rate is usually larger than its mass proportion in air; this is the opposite for the larger size groups. The maximum deviation of mass flux proportion of each size group from its mass proportion in air is about ±15%, which is determined by the mass-weighted average horizontal velocity of each size group. • Transport behavior of each size group in mixed sand transport is analyzed. • Proportions of mass flux and transport rate differ from mass proportion in air. • Deviation from mass proportion is determined by horizontal speed of each group. [ABSTRACT FROM AUTHOR]

Aeolian sand movement contributes to evolution of surface landforms on Earth and Mars, as well as on Venus and Titan. Most natural landforms in arid regions are molded by wind with a general feature of unsteadiness. Many researches have improved our cognition of saltation in unsteady flow, but it is probably true that this issue is not yet fully analysed. In this paper, we conducted three-dimensional numerical simulations of wind-blown sand under unsteady wind conditions. The trajectories of sand particles are simulated by using a Lagrangian model. Particle motion, particle-bed interaction and wind modification have been taken into account. Results indicate that the characteristics of sand transport in unsteady and steady wind conditions profoundly differ; the sediment transport in unsteady conditions is significantly enhanced and increases with the amplitude of the sinusoidal wind fluctuations; the time series of sediment transport reveals a sine-like variation which lags behind the variation of the wind with a time delay of about 1 s (response time of saltation to wind variation). In unsteady conditions, sand transport shows an 'overshoot' which is embodied in the initial wave of the sediment transport. The impact of unsteady sediment transport should be paid more attention in future Aeolian research. Our work undertakes the key point by conducting three-dimensional numerical simulation of solid particles over unsteady wind conditions. Corresponding to different amplitudes and frequencies, the sediment transport rate reveals a sine-like variation, and the average sediment transport rate of unsteady flow is greater than that of uniform one. Unlabelled Image • 3D numerical simulations of wind-blown sand under unsteady wind are conducted in terms of LES. • Characteristics of unsteady wind have an impact on sediment transport and particle velocity. • The first wave crest of sediment transport is affected by wind period. [ABSTRACT FROM AUTHOR]