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Water volume estimates of shallow desert lakes are the basis for water balance calculations, important both for water resource management and paleohydrology/climatology. Water volumes are typically...

Direct measurements of boulder entrainment in desert wadis are not available. The 2004 flood (peak discharge – 470 m s; recurrence interval – 120 years) in the hyperarid, ungauged Nahal Hatzera ephemeral stream (45 km), transported and deposited 0.85–2.1 m concrete boulders and slabs detached from infrastructure upstream and natural boulders. EDM and drone air-photographic surveys documented the geometry of the study reach and the location of boulders. Analyses of flood slackwater deposits established a paleoflood record of 23 floods with peak discharges of 200–760 m s, during the last 600 years. 1-D HEC-RAS hydraulic analysis provided water surface profiles, discharges and hydraulics, along the study reach and velocity, shear stress and stream power for each boulder. MAX program and Pearson 3 distribution were used for flood frequency analysis. Most of the concrete boulders were deposited in the sub-critical backwater of channel constrictions where velocities were 1.5–2.1 m s. The largest boulders were deposited in super-critical flow where velocity was 8–9.2 m s. The alluvial channel enabled to transport these concrete boulders, reflecting the unstable, active sandy layer of the channel bed over which the boulders moved. The maximum flood shear stress and stream power characterize medium-large floods with return period of 20–120 years and not for the largest floods, as expected. Boulders about 2.1 m and weighing about 15 t can be transported at least once in 120 years. The shear stress and stream power indicate that the moderate-large floods are the most geomorphically effective floods rather than the largest floods in Nahal Hatzera basin. Nevertheless, the ‘geomorphic effectiveness’ of the 2004 flood – a typical desert flash flood with high peak and short duration, was small based on the minor changes along the channel and banks indicating that their resistance thresholds were not exceeded and energy expenditure was mainly on boulders entrainment and transport., The authors wish to thank the Royal Geographical Society Thesiger-Oman International Fellowship UK, for fund no. THES03/16, to P.A.

Validation of emissivity (&epsilon, ) retrievals from spaceborne thermal infrared (TIR) sensors typically requires spatial extrapolations over several orders of magnitude for a comparison between centimeter-scale laboratory &epsilon, measurements and the common decameter and lower resolution of spaceborne TIR data. In the case of NASA&rsquo, s Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) temperature and &epsilon, separation algorithm (TES), this extrapolation becomes especially challenging because TES was originally designed for the geologic surface of Earth, which is typically heterogeneous even at centimeter and decameter scales. Here, we used the airborne TIR hyperspectral Mako sensor with its 2.2 m/pixel resolution, to bridge this scaling issue and robustly link between ASTER TES 90 m/pixel emissivity retrievals and laboratory &epsilon, measurements from the Algodones dune field in southern California, USA. The experimental setup included: (i) Laboratory XRD, grain size, and TIR spectral measurements, (ii) radiosonde launches at the time of the two Mako overpasses for atmospheric corrections, (iii) ground-based thermal measurements for calibration, and (iv) analyses of ASTER day and night &epsilon, retrievals from 21 different acquisitions. We show that while cavity radiation leads to a 2% to 4% decrease in the effective emissivity contrast of fully resolved scene elements (e.g., slipface slopes and interdune flats), spectral variability of the site when imaged at 90 m/pixel is below 1%, because at this scale the dune field becomes an effectively homogeneous mixture of the different dune elements. We also found that adsorption of atmospheric moisture to grain surfaces during the predawn hours increased the effective &epsilon, of the dune surface by up to 0.04. The accuracy of ASTER&rsquo, s daytime emissivity retrievals using each of the three available atmospheric correction protocols was better than 0.01 and within the target performance of ASTER&rsquo, s standard emissivity product. Nighttime emissivity retrievals had lower precision (&lt, 0.03) likely due to residual atmospheric effects. The water vapor scaling (WVS) atmospheric correction protocol was required to obtain accurate (&lt, 0.01) nighttime ASTER emissivity retrievals.

Quantitative geomorphic field studies and modeling efforts have focused on the margins of southwestern Africa as an example for landscape evolution in prolonged aridity conditions and tectonic quiescence of passive margins. These efforts concluded that this region is a prime example of a steady state landscape, in which relief changes extremely slowly. Using cosmogenic isotopes, these studies suggested overall landscape exhumation rates of 5–10 m Ma−1 over the past 105–106 yrs. Slightly slower rates on flat-lying exposed bedrock surfaces and faster exhumation rates along the Namibian Great Escarpment as well as on steep slopes of granitic inselbergs, such as the Gross Spitzkoppe are also documented. Here we explore the state of “steady state” in central Namibia. Concentrations of 10Be were measured in bedrock and sediment samples collected throughout the watershed of the Ugab River (~29,000 km2), which drains the highlands of central Namibia and flows to the Atlantic Ocean. Samples were collected from the main stem of the ephemeral Ugab River, from the slopes and streams draining the Brandberg, which is the largest inselberg in the Namib, and from smaller inselbergs around it. We also sampled several other formerly large, but currently subdued, inselbergs such as the Messum Crater. 10Be concentrations in sediment transported along the axial Ugab River indicate that its drainage basin erodes uniformly at 5–6 m Ma−1 and sediment transport from its headwaters source to the ocean is rapid. 10Be concentrations measured in sediment transported in ephemeral streams draining the Brandberg indicate its erosion at ~4 m Ma−1. However, slower rates of 1–3 m Ma−1 were measured for bedrock samples collected from (a) flat lying bedrock surfaces within the Brandberg, (b) top of small tors that rise only a few meters above their surroundings, and (c) exhumed and denuded large magmatic complexes such as the Messum Crater. Furthermore, we found that bedrock buried under grus in the hyperarid zone of Namib ( We therefore suggest that the “steady state” landscape along the Namibian passive margin be viewed as follows: The entire landscape erodes slowly, generally at ~5 m Ma−1 and this maintains the view of steady state. Small differences in erosion rates between the landscape elements result in very slow and only small changes in relief over time scales ≥106 yrs. We find that the large inselbergs and the Great Escarpment erode primarily by retreat of steep slopes and cliffs within the drainage basins while preserving relief over considerable timescales. In the wetter upper reaches of the Namibian drainage systems, erosion of buried rock is most likely increased by the vegetation-covered soil.

We quantify the volumetric erosion of Israel's actively retreating Mediterranean coastal cliffs between 2006 and 2015 with airborne LiDAR. Our results reveal annual sand contribution of ~ 45,000 m3 from the coastal cliffs into the Nile's littoral cell (NLC), which amounts to ~ 50% of the annual long-shore sand transport previously measured at the northern termination of the NLC. Accordingly, we find that implementation of cliff-retreat mitigation engineering efforts planned for Israel's coastline can lead to a sand deficit in the NLC that may result in increased beach erosion.

Sequences of abandoned alluvial surfaces are ubiquitous desert landscapes considered as responding to Quaternary-timescale climatic fluctuations and tectonic activity. The post-depositional evolution surficial properties of these alluvial surfaces change with time: (a) surface clast breakdown and development of desert pavement, (b) dust and salts accumulation (c) Reg (gravelly) desert soil evolution. In the present study, we focus on the hydrological response and the resulting erosional processes, to these changes in surface properties over a sequence of late-Pleistocene-Holocene, abandoned alluvial surfaces in Nahal Evrona catchment at the hyperarid southern Arava Valley north of the towns of Eilat (Israel) and Aqaba (Jordan). The methods used in the study are: (a) simulated rainfall experiments to determine hydrological characteristics, (b) Terrestrial Laser Scanning (TLS) to determine surface roughness, (c) OSL dating for the chronology, (d) surface and soil descriptions and measurements and grain-size analyses to determine texture of the uppermost Av horizon of the Reg soil (e) GIS analysis of the new drainage network developing over the older abandoned alluvial surfaces. The results show that with time, roughness decreases, infiltration decreases to about 8–9% relative to present rates, point runoff generation increases >5 fold, and as a result, rill and gully erosion initiated, finally leading to the destruction of the alluvial surface. Under the current hyper-arid climate in the southern Negev desert, between ~30 and 50 ka of surface weathering and soil development are required before significant runoff is generated and >100 ka until effective runoff initiates rill and gully erosion over these abandoned alluvial surfaces. This local-source runoff and gully erosion essentially destroys the alluvial surfaces in the southern Arava valley within a period of 400–500 ka. Roughness, extracted from spaceborne Lidar scanning, which is inversely related to surface age, directly reduces friction and resistance to runoff, can provide an estimate for the hydrological characteristics of the surfaces. This can support applications such as rainfall-runoff models, risk assessment, etc.

The evolution of roughness as a function of surface age was used to quantify weathering rates on rocky desert surfaces. Surface topography on eight late Quaternary alluvial terraces, which record the weathering of Holocene (5 ± 1 ka) boulder-strewn deposits into mature (87 ± 2 ka) desert pavements in the Negev desert of Israel, was measured with ground-based lidar. Roughness on each terrace was characterized with power spectral density (PSD) analysis, and changes in PSD as a function of length scale (λ ∼ 0.04–1.50 m) and surface age were used to estimate diminution/weathering rates of the surface rocks. We found PSD values that systematically increase as a power-law function of λ (roughness exponent of ∼2.0) and decrease as an inverse power-law function of surface age. This PSD evolution indicates a fragmentation rock weathering process driven by salt shattering throughout the 87 k.y. period examined. PSD analysis of the lidar data also revealed weathering rates that increase with rock size and decrease as an inverse power-law function of time, from initial values >20 mm/k.y. to

The Chinese Loess Plateau (CLP) is a large, spatially well defined and persistent zone of loess accumulation developed near the fluctuating northwest margin of the East Asian monsoon. Many studies have analyzed its loess sediments to provide insights into paleoclimatic conditions. Although spatial and temporal variations in the grain sizes of CLP sediments are fundamental to this effort, controversy over the origin of the dominant coarse quartz silt has limited interpretations. Reexamination of the spatial pattern of grain-size distribution across the CLP and a field-scale experiment conducted in the Gobi Desert revealed a genetic association between the coarse silt fraction of the loess and primary production of coarse silt through eolian abrasion of sand in the proximal Mu-Us, Tengger, and Badain Jaran sandy deserts. Our results demonstrate the effectiveness of eolian abrasion of quartz sand in primary coarse silt production in Central Asia and identify this process as the most consistent with the wellrecognized systematic northwest-southeast depositional pattern of the CLP. We suggest that only abraded coarse quartz grains transported short distances by long-term persistent eolian activity can build up thick loess sequences to form a massive and spatially well defined loess plateau. These results decouple the production and transport of coarse silt and finer silt and clay particles, which have a more distant and wider provenance, changing the constraints on previous paleoclimatic reconstructions. © 2013 Geological Society of America.

Namibia, with its passive margin setting, long-term tectonic stability, and long-lasting arid climate is a typical granitic landscape characterized by numerous dominating inselbergs. The Namib has been the focus of quantitative studies on the overall rates of bedrock exhumation, escarpment retreat, and sediment generation, transport, and deposition. Results from these studies indicate steady bedrock erosion rates ranging between 1 and 5 mm kyr− 1 over the last 105–106 yr. This rate was determined from samples collected mostly from small inselbergs with low relief. How fast the large and “classic” inselbergs, which dominate the Namib Desert landscape, erode, generate sediment, and contribute to the overall sediment supply has yet to be systematically examined. We document erosion rates of the Gross Spitzkoppe (GS), one of the largest inselbergs in the Namib Desert, Namibia, based on measured concentrations of 10Be in samples collected from bedrock, boulders, and surface grus. The results suggest a slow lowering rate (1–2 mm kyr− 1), while the overall slope processes deliver sediment to the base of the cliff with 5–6 × 105 atoms g− 1 quartz. This concentration is equivalent to an average erosion rate of ~ 8 mm kyr− 1, 2–3 times faster than the bedrock lowering rate. Thus, the inselberg has been eroding by cliff retreat. The concentration of cosmogenic isotopes in slope sediment is controlled by exposure time on the slope, weathering of exposed bedrock, and weathering of subsurface bedrock. Sediment continues to accumulate cosmogenic isotopes while being transported on the flat pediments surrounding the inselberg. Within 1–2 km from the base of the GS, the concentration doubles to ~ 10 × 105 atoms g− 1 quartz. The increase is achieved by dosing due to exposure and mixing with highly dosed sediment eroded from near subsurface bedrock. From this area, grus is transported through a network of low gradient channels to the Atlantic Ocean, ~ 100 km away. Cosmogenic isotope concentration in sediment increases to ~ 30 × 105 atoms of 10Be g− 1 quartz, which is 3–4 times higher than that measured in the grus close to the GS and is controlled by the erosion rate of stable bedrock throughout the entire drainage basin. Dosing due to exposure during transport obviously contributes to the overall measured concentration in the sediment as it approaches the ocean.

An hyperspectral imaging spectrometer measuring in the longwave thermal infrared (7.6-11.6 mum), with a spatial resolution less than 5 mm at a range of 10 m, was used in the field to observe the variability of emissivity spectra of individual rock surfaces. The rocks were obtained commercially, were on the order of 20 cm in size, and were selected to have distinct spectral features: they include alabaster (gypsum), soapstone (steatite with talc), obsidian (volcanic glass), norite (plagioclase and orthopyroxene), and ldquojasperrdquo (silica with iron oxides). The advantages of using an imaging spectrometer to characterize these rocks spectrally are apparent. Large spectral variations were observed within individual rocks that may be attributed to roughness, surface geometry, and compositional variation. Nonimaging spectrometers would normally miss these variations as would small samples used in laboratory measurements, spatially averaged spectra can miss the optimum spectra for identification of materials, and spatially localized components of the rock can be obscured.

The (550‐530Ma) alkaline magmatic suite of the Amram Massif, southern Israel, was emplaced during the transition from an orogenic to an intra-plate tectonic setting in the northeastern Arabian‐Nubian Shield (ANS). The suite ranges from 45 6t o 788 wt % SiO2, and consists of rhyolites, alkali quartz syenites, quartz syenites, monzonites, and co-magmatic mafic to felsic alkaline dikes. These rocks define a continuous chemical evolutionary trend and reveal a correlation between decreasing stratigraphic age and increasing silica content. The felsic members of the suite display A-type characteristics and are genetically linked through fractionation to the more mafic ones. Moderately positive initial eNd values (a2 05), low initial 87 Sr/ 86 Sr values (07036 2), high MgO and Fe2O3 concentrations (410‐895 and 100‐125 wt %, respectively) and relatively flat rare earth element patterns [(La/Yb)na 64 09] in the Amram mafic dikes (456‐495 wt % SiO2), suggest their derivation from the sub-continental lithospheric mantle, above the garnet stability zone. The MELTS program was used to quantitatively model the chemical evolution of the suite. Extensive anhydrous fractionation (490%), of plagioclase, alkali-feldspar, clinopyroxene, olivine, and minor Timagnetite and apatite from parental mafic magmas, represented by the Amram mafic dikes, produced the rhyolitic compositions as well as the intermediate members of the suite. This suggests the presence of a large unexposed body of cumulate rocks at depth, as well as fusion of a large source-region (equivalent to an 5km layer) in the lithospheric mantle. Regarded as a representative example for similar A-type outcrops in this region, this petrogenetic model further suggests that Neoproterozoic‐Early Cambrian A-type magmatism in the northeastern ANS represents a significant post-orogenic addition of mantlederived material to the juvenile crust. This magmatic episode was of a similar magnitude to that of the Cenozoic, extensionrelated, alkaline volcanism of the Arabian plate.

[1] The ratio between co-registered pixels in stereo or repeat image pairs is used to constrain the deviation of the Martian surface from Lambertian reflection due to unresolved shadows at the pixel scale of orbiting imagers. Relative differences between the ratios primarily reflect differences in the effective amount of shadows ‘seen’ by the sensor in the two measurements, and are thus used as a relative proxy for sub-pixel surface roughness (SR). In-scene atmospheric corrections enable simple application of this ‘two-look’ approach, which facilitates robust SR mapping at spatial resolutions comparable to the image data. Relative ‘two-look’ SR estimates can be translated to physical SR parameters through empirical calibration using ground-based measurements. SR variation maps in Gusev crater were derived from THEMIS and HRSC images at 18- and 30- m/pixel, respectively. The latter were calibrated using measurements from Spirit to produce a quantitative clast-coverage map of Spirit's initial traverse in Gusev crater.

Head et al. interpret spectacular images from the Mars Express high-resolution stereo camera as evidence of geologically recent rock glaciers in Tharsis and of a piedmont ('hourglass') glacier at the base of a 3-km-high massif east of Hellas. They attribute growth of the low-latitude glaciers to snowfall during periods of increased spin-axis obliquity. The age of the hourglass glacier, considered to be inactive and slowly shrinking beneath a debris cover in the absence of modern snowfall, is estimated to be more than 40 Myr. Although we agree that the maximum glacier extent was climatically controlled, we find evidence in the images to support local augmentation of accumulation from snowfall through a mechanism that does not require climate change on Mars.

Spectacular images obtained by Mars Express were interpreted as evidence of recently active glaciers, and therefore of snowfall. In an online Brief Communication Arising, Alan Gillespie and colleagues offer an alternative explanation for these structures. Ice, yes. But it could have been frozen groundwater rather than frozen snow. Arising from: J. W. Head et al. Nature 434, 346–351 (2005); Head et al. reply . Head et al.1 interpret spectacular images from the Mars Express high-resolution stereo camera as evidence of geologically recent rock glaciers in Tharsis and of a piedmont (‘hourglass’) glacier at the base of a 3-km-high massif east of Hellas. They attribute growth of the low-latitude glaciers to snowfall during periods of increased spin-axis obliquity. The age of the hourglass glacier, considered to be inactive and slowly shrinking beneath a debris cover in the absence of modern snowfall, is estimated to be more than 40 Myr. Although we agree that the maximum glacier extent was climatically controlled, we find evidence in the images to support local augmentation of accumulation from snowfall through a mechanism that does not require climate change on Mars.