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Valley width is a fundamental morphologic property of rivers that plays a key role in drainage networks' hydrology, ecology, and geomorphology. In many cases, defining and measuring valley width is far from trivial. Therefore, similar to channel width, the valley width (W) is commonly approximated as a power law function of the drainage area (A) and expressed as W = kcAd. Global observations have shown that the exponent (d) can vary widely but is typically ~0.5. However, in fluvial systems that have undergone drainage reorganization, gradual or abrupt changes in drainage areas along the valley could produce widths that are disproportionate to their drainage areas. As a result, the valley width - drainage area relationship in reorganized systems is expected to differ from undisturbed drainages that have not undergone reorganization.To test this prediction, we studied 12 valleys in the Negev desert, Israel, and classified them into three categories, based on field evidence and remote sensing data: (i) undisturbed valleys, which are minimally affected by reorganization; (ii) beheaded valleys, whose headwaters were beheaded; and (iii) reversed valleys, which have reversed their flow direction by 180 degrees while exploiting their antecedent valleys. Using a new semi-automatic tool to measure valley width on high-resolution DEMs, we calibrated the best-fit power law for each valley to explore the relationships between drainage area and valley width for each valley category.Our results show that the valley width-drainage area scaling in reorganized valleys deviated significantly from those in undisturbed valleys in our field area and global observations. The drainage area exponents (d) were lower in beheaded valleys compared to undisturbed valleys but remained positive. In contrast, reversed valleys were characterized by negative d exponents, indicating valley width decrease with increasing drainage area. For the reversed category, we also explored the independent effect of channel slope (S), where the valley width is W = kb AbSc, which resulted in negative and overall similar values of b and c.In one reversed valley section, we compared the scaling of valley versus channel width as a function of drainage area. We found that in contrast to the downstream narrowing valley, the channel width shows an opposite trend and widens downstream, suggesting that the channel has mostly adjusted to the post-reorganization drainage area distribution. The narrow reversed channel shapes the width of the formative flows, which contrasts significantly with the wide flows of the beheaded valley across the divide. This difference results in a step-change in the unit stream power between the reversed and beheaded channels, potentially leading to a "width feedback" that promotes further divide migration.Our findings can be used to identify landscapes that have been affected by recent drainage reorganization and should be taken into consideration in studies that use the relationship between valley width and drainage area for valley width predictions, stream power calculations, and landscape evolution models.

Current dust storms, originating from afar, are common in Israel and the eastern Mediterranean, and thus most dust sources are considered to be distal. However, recent studies suggest that the latest Quaternary loess accreted in the Northern Negev can also serve as a proximal source of dust. These sources were mostly neglected in past discussions as contributors of dust. Here, we demonstrate that such proximal dust sources, mostly the Negev loess, currently contribute relatively large amounts of recycled dust to the regional dust cycle. We conducted a sampling campaign of deposited dust during individual dust storms and identified high content of coarse silt grains and quartzo-feldspathic minerals within and adjacent to the Negev loess that gradually decreases toward the north. These grains, characteristics of the Negev loess, indicate a short transport distance. In addition, our data reveal that local wind speed is the limiting factor for emitting proximal dust, regardless of the synoptic system. We determined that proximal sources in Israel emit dust during either local events or as a part of regional dust storms originating from afar. We evaluate the minimal contribution of this proximal dust to the total mass of deposited dust as 58–74%, 54–70%, 52–64%, and 26–34% for the northern Negev, central Negev, central mountainous region, and northern Israel, respectively. These estimates indicate that at the desert fringe, both proximal and distal sources of dust should be considered when inferring dust sources from dust geochemistry that can sometimes be similar due to the long dust history.

Drainage reversals occur when a channel reverses its flow direction by 180o while exploiting its antecedent valley. This reorganization mode can critically impact landscapes' hydrologic and geomorphic functionality, but the processes inducing reversals and the related landscape dynamics have not been studied in detail. Reversals are commonly attributed to tectonic tilting. However, in many cases, independent evidence for tilting is missing. Furthermore, when reversals occur toward great escarpments, as was documented in many terrains around the globe, isostatic tilting is expected to occur away and not toward the escarpments.The current study explores a natural laboratory for drainage reversals in the southeastern Negev Desert, Israel. We identified in this field area tens of highland channels that reversed their flow direction eastward and toward the Arava Valley Escarpment.Reversals are established by observations of (1) barbed tributaries that join the main trunk with junction angles > 90o and preserve the antecedent pre-reversal drainage topology; (2) a valley confined drainage divide (windgap) that separates the reversed channel from the antecedent, beheaded channel; and (3) series of terraces that grade west, toward the windgap and opposite to the active flow direction.Based on field observations and morphometric analysis, we propose a new, tilting-independent, mechanism for inducing flow reversals. According to this mechanism, reversal is linked to localized windgap migration within the antecedent channel and away from the escarpment. Migration is driven by slope imbalance across the windgap with steeper slopes at the escarpment side, and by erodibility differences between the hard rocky interfluves and the more erodible valley fill.Using a new algorithm for quantifying valley width, we find that the scaling between drainage area (A) and width (W) differs between reversed, beheaded, and non-reorganized valleys. In addition to providing markers for reorganization, the unique A-W scaling leads to feedback that promotes further windgap migration and generates longer reversals.The oppositely grading terraces that accompany some reversed channels present an outstanding opportunity for quantifying the dynamic and rate of windgap migration. We hypothesize that the abandonment age of each terrace reflects the timing at which the paleo-windgap migrated past the location of the terrace, promoting the incision of the reversed channel that generated the terrace. Accordingly, the abandonment ages of the terraces can inform us about the timing and dynamics of windgap migration.Absolute ages of terrace abandonment were constrained by luminescence dating, with complementary relative ages inferred from chronosequence of reg soils, which develop on abandoned terraces in hyper-arid environments. In agreement with the reversal model, we found that the degree of soil development and the abandonment ages of terraces increase with distance from the windgap eastwards. The average windgap migration rate has been ~1 mm/yr since 200 Kyr, an order of magnitude greater than the vertical incision rate of the reversed channel. The age-distance relations of the terraces indicate episodic windgap migration with a recent stalling. A similar age for other dated windages in the region hints at a regional, possibly climatic control on windgap migration.

Hyperarid (< 80 mm yr-1) soils in hot deserts are characterized by accumulations of soluble salts (gypsum and halite) in diagnostic horizons as a result of limited moisture availability. In most desert terrains, the source for pedogenic gypsum and halite is atmospheric dust and rainwater. The interplay between climatic properties such as frequency and intensity of rain events, rainfall composition, dust flux, and evaporation rates, govern the depth and concentration of these salts. Better understanding of these relationships can improve our estimation of regional paleoenvironmental and paleoclimate conditions. Up to date, only empirical correlations between annual rainfall and pedogenic salt horizons are available.The goals of this study are to: 1) quantify rates of pedogenic gypsum accumulation with time and the role of controlling climatic conditions that govern its accumulation, 2) estimate the most likely climatic scenarios that led to the formation of the diagnostic gypsic horizon developed in late Pleistocene (~ 60 ka) abandoned alluvial fan surfaces in the hyperarid Negev desert, southern Israel. To achieve these goals, we constructed a compartment model that simulates gypsum accumulation in soil and tests its sensitivity to various changes in the long-term climate properties. The model predicts gypsum content and depth of accumulation in the soil profile over thousands of years and more. The input parameters are stochastically simulated rainstorms, evaporation, dust flux, and sulfate concentration in rainwater, at daily time steps. The model was tested and calibrated using data of Holocene (< 11 ka) soil profiles developed on stable alluvial fans in the hyperarid Negev. With the assumption that the climate during the Holocene was not much different than today (i.e., mean annual rainfall < 50 mm). Sensitivity analyses indicate that gypsum accumulation is highly sensitive to mean annual rainfall and sulfate concentration in rainwater. Synthetic gypsum profiles were calculated using different climate scenarios and compared to late Pleistocene soils. Our results suggest that: (a) gypsum accumulation in late Pleistocene soils cannot occur simply by extending current climate conditions for a much longer duration. (b) The plausible climate scenarios for the late Pleistocene must include additional rain input (1.5 – 2.0 times than mean annual rainfall today) and increased sulfate concentration in rainwater (2.0 – 2.5 times than today) to successfully reconstruct the observed accumulated gypsum in mature (60 – 12 Ka) soil profiles.

Quaternary loess covers desert margins and vast areas of the Negev, southern Israel. The Negev loess is among the best-studied desert loess, with research going back to the early 20th century. The contrast between carbonate rocks of the Negev and its silicate-rich coarse-silt loess allows determining the loess sources, learning the synoptic-scale paleoclimatology, and exploring processes of coarse silt formation. Here, we present an overview of new perspectives on the origins and climatic significance of the Negev loess, expand on how (a) coarse silts affected soils farther downwind, and (b) how the loess has now turned into an active dust source.The sources of the Negev loess are the (a) distal Sahara and Arabia delivering fine silts and clays, transported over thousands of kilometers, and (b) proximal sand dunes in Sinai and Negev, advancing and concurrently supplying the coarse silts to the loess accretion through eolian abrasion of sand grains. It was found that the coarse silts which compose the majority of the loess, commenced during the late middle Pleistocene – early late Pleistocene, coeval with the appearance of the advancing Sinai/Negev sand dunes and the first coarse silt accretion in regional soils; The main loess formation episode is ~95-10 ka, when the dunes appeared in the Negev. Within the loess, the dust mass accumulations rates (MAR), and consequently, soil formation rates, spatiotemporally vary according to specific site location and distance relative to the proximal sources. With increasing distance beyond the loess zone, both dust MARs and grain size gradually decrease; thus, whereas Mediterranean mountains located in central Israel, tens of kilometers downwind the loess, exhibit thick soils on top of the carbonate bedrock, the even wetter regions in northern Israel, located hundreds kilometers away from the loess, exhibit only thin soils. Thus, in Mediterranean regions located at the desert fringe, coarse silt influx is one of the main factors in determining the environmental sustainability, rather than only the precipitation amount.During the Holocene, dust MARs in the Negev were much lower than late Pleistocene ones, and loess was not formed. Recently, the Negev loess became a prime source of dust mainly due to anthropogenic interferences, contributing to the regional dust cycle, and thus, at present the loess zone is a dust source rather than a dust sink. Today, the Negev loess is a non-replenishable natural resource that is slowly eroding and disappearing from the landscape.

Soil hydrophobicity was extensively investigated in the context of fire-induced, naturally-occurring and wastewater related hydrophobicity. Oil-induced soil hydrophobicity, however, received much less attention and was not yet investigated under regions characterized by hyper-arid climate. In this study, we investigate the hydrological effects and the persistence of oil-induced soil hydrophobicity in Evrona nature reserve located in the hyper-arid region of southern Israel. The Evrona nature reserve has experienced two oil spills that occurred in 1975 and 2014, providing a unique opportunity to study the hydrophobicity of oil-polluted soils over time. In this study, we 1) apply field monitoring to investigate how the severe hydrophobicity affects water flow in the polluted soils, and 2) conduct laboratory incubation experiments to assess the natural attenuation of hydrophobicity and its relation with the content and composition of hydrocarbons.We set up two monitoring stations in two adjacent streams, of which one is polluted and the other is clean. In each section, an array of water content sensors was installed. Analyses of data measured during two years reveal that during rain and runoff events infiltration in the oil-contaminated soil was much lower relative to the clean soil. Furthermore, infiltration in the oil-contaminated stream showed highly preferential patterns, typical of hydrophobic soils. The reduced infiltration and the establishment of preferential flow paths may lead to negative consequences including increased runoff and erosion, reduction in the water available to native plants, and the generation of fast conduits for contaminant transport.In the second part of the research, incubation experiments were conducted with contaminated soils from the 2014 and 1975 sites. Soils were treated with the addition of either water alone or different combinations of water, nutrients and surfactant. Treated soils were sampled periodically to assess soil hydrophobicity and hydrocarbon content. The results show a concomitant decrease in the hydrophobicity and hydrocarbon content in soils to which water or water and nutrient and/ or surfactants were added. The fastest hydrophobicity reduction was observed in soil to which both nutrients and surfactants were added, while when only water was added to 50% saturation, degradation and hydrophobicity reduction were very slow. Overall, during the one year and a half incubation, the total petroleum hydrocarbon of the treated soils decreased by 40% in the 2014 soil and by up to 80% in the 1975 soil. However, although hydrophobicity was reduced during incubation, the soils still remained severely hydrophobic. This suggests that considering the concentration of hydrocarbons as the sole criterion for the endpoint of soil remediation is not sufficient and the degree of soil hydrophobicity should be evaluated as well.

Convex soil-covered hillslopes are ubiquitous in various tectonic and climatic settings and are often modeled based on a mass balance relating hillslope convexity to regolith transport and soil production. In order to account for chemical weathering of carbonate rocks and dust input to the regolith, two fluxes that are commonly neglected in settings with silicate-dominated bedrock, we modify this mass balance.We studied 7 study sites in carbonate rocks across an Eastern Mediterranean gradient in the mean annual rainfall (250 to 900 mm yr-1) and dust flux (150 to 40 g m-2 yr-1). Combining cosmogenic 36Cl-derived hilltops denudation rates with an estimate of the regolith chemical depletion and dust fraction based on immobile elements, we predict the hillslope curvature and compare our predictions with observations based on high-resolution airborne LiDAR.Our results demonstrate that soft carbonates (chalk) experience faster denudation rates relative to resistant dolo-limestone. However, the harder carbonates are more prone to chemical weathering, which systematically constitutes around half of their total denudation. Soil production rates exhibit a humped dependency on soil thickness, with an apparent maximum at a depth of 8-16 cm.The observed hillslope curvature vary as function of rainfall and dust flux with a minimum at sub-humid sites with intermediate rainfall of 500-600 mm/yr. The predicted curvature based on our new mass balance is not far from the observed curvature, illustrating the prominent effects of dust flux and chemical weathering on hillslope morphology. Our model also implies that drier sites in the south probably experienced a more complex history of regolith production due dust flux fluctuations.By incorporating dust flux and chemical weathering to the classic hillslope evolution model we identify a complex relation between hillslope curvature, soil production, and climate. These two fluxes are not unique to carbonate bedrock and should be incorporated in hillslope evolution models.

This study estimates the relative magnitude of decadal-scale soil redistribution (i.e., soil loss or gain) by slope wash using 137Cs inventories measured in 46 soil profiles at four study sites in the Ft. Irwin area of the Mojave Desert of California, USA. The variability in 137Cs inventories on a

This study estimates the rates of soil erosion by slope wash in an arid region and the various factors that control these rates. Decadal-scale erosion rates were estimated on hillslope scales using inventories of 137Cs that were sampled from 46 soil profiles in four different study sites in the Mojave Desert. Calculated mean soil erosion rates per site range from −3.6 to −24.3 t ha−1 yr−1. Higher mean rates were associated with gently sloping sites that exhibit low percentage of rock and vegetation coverage, whereas lower mean rates corresponded to steep and rocky sites. Individual erosion rates were not correlated to slope gradient or curvature but were negatively correlated with the volume fraction of rocks in the upper soil profile (i.e., upslope rock coverage). Since the slopes get rockier as they get steeper, any increase in erosion rates with increasing slope is outweighed by the inhibiting effect of greater rock cover. This, together with sandy-loam soil texture on the steep slopes hinders runoff and erosion. Our findings are supported by soil data that show greater heterogeneity in the degree of calcic soil development and higher soluble salt contents in more gently sloping sites that are characterized by high erosion rates. The erosion rates reported here for the gently sloping sites are higher than rates calculated for semi-arid regions, probably due to the lower rock and vegetation coverage in these sites compared to wetter areas. These rates are also higher than millennial-scale rates estimated for the Mojave Desert on watershed scales, and suggest that at least part of the eroded sediments are stored in the adjacent streams and do not reach the piedmonts.