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Rates of soil development provide critical information about the types and rates of geomorphic and landscape evolutionary processes. Soil development in arid and semiarid regions of the southwestern United States is predominantly controlled by influx of eolian dust, yet our ability to quantify the rates of morphologic development and of dust and carbonate incorporation is limited by available age control. We describe 10 pedons in the Socorro area of central New Mexico and analyze their silt, clay, and carbonate contents. These soils have well-established direct or indirect age control that we used to estimate average rates of dust and carbonate accumulation over the past ~0.5–800 ka. We also computed the profile development index (PDI) for these soils using 10 common morphologic properties and compare our resulting PDI chronofunction to those from northern and southern New Mexico. We find that the net silt-and-clay content increases in progressively older soils at rates similar to the profile-mass carbonate contents, presumably sourced primarily from eolian dust. Our chronofunction comparison indicates that soil development occurs more rapidly in higher latitude regions of New Mexico than in the drier warmer climates of the Socorro area and southern New Mexico. We interpret the N–S regional trend of soil development considering a regional climate gradient. We conclude that greater mean annual precipitation and cooler mean annual temperatures and/or slower rates of eolian dust accumulation into the soil profile at higher latitude northern sites cause the observed differences in regional soil development. This would promote greater mobility of available silt and clay, and also increase rates of soil formation, as indicated by the presence of argillans in late-Pleistocene soils of northern New Mexico.

Recent research on dust emissions from eolian dunes seeks to improve regional and global emissions estimates and knowledge of dust sources, particularly with a changing climate. Dust emissions from dune fields can be more accurately estimated when considering the whole eolian system composed of active to stabilized dunes, interdunes, sand sheets, and playas. Each landform can emit different concentrations of dust depending on the supply of silt and clay, soil surface characteristics, and the degree to which the landforms are dynamic and interact. We used the Portable In Situ Wind Erosion Laboratory (PI-SWERL) to measure PM10 (particulate matter 300 t/km2/yr.

The formation of the Kelso Dunes in the eastern Mojave Desert, California, was a landscape-changing event triggered by an increase in sediment supply that followed the incision of Afton Canyon by the Mojave River ca. 25 ka. Eastward migration of sand dunes occurred along a well-defined eolian transport corridor. Dunes temporarily blocked washes resulting in substantial aggradation of eolian and fluvial sediments. Stratigraphic exposures reveal numerous fining-up sequences with interbedded eolian sands that provide evidence of dune dams and subsequent aggradation. Luminescence ages reveal that dune blocking and aggradation correspond to a regional pulse of alluvial fan sedimentation that occurred ca. 14–9 ka. Meanwhile, relative landscape stability occurred downstream of dune dams, resulting in the formation of a moderately developed soil on abandoned fluvial deposits. The next pulse of alluvial fan activity ca. 6–3 ka likely resulted in the breaching of the dune dams, followed by incision. Eolian system sediment state theory suggests that eolian activity in the Mojave Desert is closely tied to enhanced sediment supply, primarily related to the Mojave River–Lake Mojave system. Our data suggests that Intermittent Lake Mojave I, ca. 26–22 ka, triggered a large dune-building event that impounded massive amounts of sediment derived from alluvial fans deposited during the Pleistocene-Holocene transition. Breaching of dune dams and sediment recycling may have also increased sediment supply that contributed to late Holocene eolian activity. This profound impact on the regional geomorphology highlights the critical importance of eolian-fluvial interactions in desert environments.

Vesicular A (Av) horizons, and associated overlying desert (rock) pavement, are ubiquitous features across desert environments. Extensive research has demonstrated that the Av horizons develop from the incorporation of dust (eolian sediment) during soil development; however, two conflicting models have emerged regarding the age of the Av horizons. Published luminescence (OSL) ages from Av horizons suggest that Av horizons are Holocene, with reported ages commonly ≤5 ka. In addition, other studies have suggested Av horizons and desert pavements are Holocene in age because Late Pleistocene environmental conditions (primarily an increase in vegetation cover) largely destroyed desert pavements and Av horizons prior to the Holocene, especially for surfaces above 300-400 m elevation. In contrast, time-related trends in the morphology of Av horizons suggest that Av horizons and pavements must have existed prior to the Holocene. Geochronology and soil morphology from two soil chronosequences formed on alluvial fans in the Mojave Desert (soils ~0.5 ka to ~100 ka, ~900 m above sea level) and in the Sonoran Desert (soils ~0.5 ka to ~250 ka; ~200 m above sea level) indicate that Av horizons existed prior to the Holocene and that the strength of Av development coincides with increasing age of the surface. In both chronosequences, Av horizon properties of eolian derived silt and clay, development of soil structure, horizon thickness, all systematically increase with surface age on soils with no evidence of past erosion or substantial soil mixing. Soil morphology and depth profile relations further support that soil profiles are intact with no evidence of erosion or mixing just prior to the Holocene. OSL dates of Av horizons are considerably younger than soil profiles dated with cosmogenic nuclides and OSL. Some examples include: Av: 5ka/soil: 10-12ka; Av: 1-3ka/soil: 16-21ka; Av: 2-6ka/soil: 50-60ka; Av: 1ka/soil: 210 ka. Mixing of the Av and episodic addition of Holocene dust cannot alone account for age inconsistencies. Recent research using OSL for thermochronology indicates that closure of electron traps occurs between 35o to 50o C. Measured hourly summer temperatures in Av horizons (Sonoran and Mojave Desert sites) commonly exceed 35 o to 50oC May through September. We suggest that anomalously young ages for Av horizons may be due to high soil temperatures and degradation of the OSL system.

The mechanical feasibility of co-seismic displacement along low-angle normal fault systems remains an outstanding problem in tectonics. In the southwestern Basin and Range of North America, large magnitude extension during Miocene – Pliocene time was accommodated along a regionally extensive system of low-angle detachment faults. Whether these faults remain active today and, if so, whether they rupture during large earthquakes are questions central to understanding the geodynamics of distributed lithospheric deformation and associated seismic hazard. Here we evaluate the geometric and kinematic relationships of fault scarps developed in Pleistocene – Holocene alluvial and lacustrine deposits with low-angle detachment faults observed along the western flank of the Panamint Range, in eastern California. We combine analysis of high-resolution topography generated from airborne LiDAR and photogrammetry with a detailed chronology of alluvial fan surfaces and a calibrated soil chronosequence to characterize the recent activity of the fault system. The range-front fault system is coincident with a low-angle (15-20°), curviplanar detachment fault that is linked to strike-slip faults at its southern and northern ends. Fanglomerate deposits in the hanging wall of the detachment are juxtaposed with brecciated bedrock in the footwall across a narrow fault surface marked by clay-rich gouge. Isochron burial dating of the fanglomerate using the 26Al and 10Be requires displacement in the past ~800 ka. The degree of soil development in younger alluvial deposits in direct fault contact with the footwall block suggest displacement along the main detachment in the past as ~80-100 ka. The geometry of recent fault scarps in Holocene alluvium mimic range-scale variations in strike of the curviplanar detachment fault, suggesting that scarps merge with the detachment at depth. Moreover, fault kinematics inferred from displaced debris-flow levees and from fault striae on the bedrock range front are consistent with slip on a low-angle detachment system beneath the valley. Finally, paleoseismic results from a trench at the southern end of the fault system suggest 3-4 surface ruptures during past ~4-5 ka, the most recent of which (MRE) occurred ~330-485 cal yr BP. Scarps related to the MRE can be traced for at least ~50 km northward along the range front and imply surface displacements of 2-4 meters during this event. Thus, we conclude that ongoing dextral shear along the margin of the Basin and Range is, in part, accommodated by co-seismic slip along low-angle detachment faults in Panamint Valley. Our results have important implications for the interaction of fault networks and seismic hazard in the region.

Interpretations of surface exposure ages derived from the accumulation of cosmogenic isotopes commonly are hampered by a lack of field documentation that is necessary to identify and constrain if any post-depositional surface modifications have occurred (e.g. surface erosion, burial) that will impact age interpretations. Previous authors have discussed these issues, but the community still has not fully adopted the practice of interpreting surface exposure ages in conjunction with detailed soil stratigraphic observations. We employ this “novel” approach by documenting a soil chronosequence from the Gould wash alluvial fan sequence near Cibola, AZ to demonstrate how soil stratigraphy can provide constraints for the relative stability of depositional surfaces and can influence interpretations of TCN ages.The Cibola chronosequence represents a range of alluvial fan ages that extend well beyond those commonly observed in the desert southwestern US (typically 36Cl depth profiles. Fan deposits largely consist of volcaniclastic alluvium derived from the local Trigo Peaks and distal Castle Dome mountain blocks and show both a reduction of bar-and-swale surface topography and an increasing expression of desert pavement with relative surface age. The soil profiles consist of Av-Bk-BCky-Cky-Ck vertical horizon sequences (~125-cm thick) in the youngest fan units to Avk-Btky-Bkym-Bky-BCky-Cky-C (~400-cm thick) in the oldest fan unit that reflect systematic changes in soil thickness, structure, rubification of B horizons, and relative accumulations of eolian derived silt, clay, and salts as a function of relative surface age.Chlorine-36 depth profile analysis yielded variable fan ages that are largely controlled by the magnitude of allowable erosion. Model results for which input data were parameterized to optimize unconstrained erosion rates indicate surface exposure ages of 46 (2A), 114 (2B), 268 (3A), and 386 (4A) ka. These are associated with best-fit erosion rates of 0-6 mm/kyr that indicate 0-136 cm net erosion. By comparison, results for which erosion rates were constrained to ~1 mm/kyr based on soil stratigraphic observations yielded exposure ages of 41 (2A), 114 (2B), 209 (3A), and 287 (4A) ka, resulting in differences of 10-25% of the unconstrained ages. The systematic morphological trends observed in the soil profiles do not support inferences of net erosion exceeding 30 cm and therefore cannot support the results from unconstrained parameter optimization. Although statistical optimization schemes provide better model fits to the data as indicated by chi-shared minimization routines, current models cannot account for field observations or for inferred constraints on surface modifications based on cosmogenic isotope concentrations alone. That task is better suited for and required by the sampling protocol to achieve more reliable surface exposure dates.

A late Pleistocene to Holocene alluvial stratigraphy has been established for the basins of La Paz and San Jose del Cabo, in the southern tip of the Baja California peninsula, Mexico. Six discrete alluvial units (Qt1 through Qt6) were differentiated across the region using a combination of geomorphologic mapping, sedimentological analysis, and soil development. These criteria were supported using radiocarbon, optically stimulated luminescence and cosmogenic depth-profile geochronology. Major aggradation started shortly after ∼70 ka (Qt2), and buildup of the main depositional units ended at ∼10 ka (Qt4). After deposition of Qt4, increasing regional incision of older units and the progressive development of a channelized alluvial landscape coincide with deposition of Qt5 and Qt6 units in a second, incisional phase. All units consist of multiple 1–3 m thick alluvial packages deposited as upper-flow stage beds that represent individual storms. Main aggradational units (Qt2-Qt4) occurred across broad (>2 km) channels in the form of sheetflood deposition while incisional stage deposits are confined to channels of ∼0.5–2 km width. Continuous deposition inside the thicker (>10 m) pre-Qt5 units is demonstrated by closely spaced dates in vertical profiles. In a few places, disconformities between these major units are nevertheless evident and indicated by partly eroded buried soils. The described units feature sedimentological traits similar to historical deposits formed by large tropical cyclone events, but also include characteristics of upper-regime flow sedimentation not shown by historical sediments, like long (>10 m) wavelength antidunes and transverse ribs. We interpret the whole sequence as indicating discrete periods during the late Pleistocene and Holocene when climatic conditions allowed larger and more frequent tropical cyclone events than those observed historically. These discrete periods are associated with times when insolation at the tropics was higher than the present-day conditions, determined by precessional cycles, and modulated by the presence of El Nino-like conditions along the tropical and northeastern Pacific. The southern Baja California alluvial record is the first to document a precession-driven alluvial chronology for the region, and it constitutes a strong benchmark for discrimination of direct tropical influence on any other alluvial record in southwestern North America.

Single grains of K-feldspar from alluvial fan units are dated using a more time-stable signal, the post-infrared infrared stimulated luminescence, or, ‘post-IR IRSL’. A quick measurement protocol is discussed, ‘fast post-IR IRSL,’ that stimulates first with the IR diodes at the lower temperature and then measures grain-by grain at the higher temperature. A criterion is offered for rejecting outlying grains based on hierarchical clustering. Single-grain fading rates are found to diverge from single aliquot fading values, and the fading rates from the brightest subset of grains correspond well with an infinite age cobble and independent age control. Age comparison with a cosmogenic depth-profile age shows agreement at 1σ. The depositional chronology suggests that the climate responsible for regionally-extensive, upper-regime floods which aggraded the older units, transitioned into a climate producing weaker channelized floods around the Late Pleistocene–Holocene transition.

Average geologic slip rates along the central Garlock fault, in eastern California, are thought to have been relatively steady at 5–7 mm/yr since at least the Late Pleistocene, yet present-day rates inferred from geodetic velocity fields are indistinguishable from zero. We evaluate the possibility of non-steady slip over millennial timescales using displaced Late Holocene alluvium along the central Garlock fault in Pilot Knob Valley. Truncation of a Late Holocene alluvial fan deposit against a shutter ridge requires a minimum of 30–37 m of displacement since deposition of the fan; maximum allowable displacement is 43–50 m. The extent of soil development atop the fan surface and optically stimulated luminescence ages bracket fan deposition between 3.5 and 4.5 ka. Together, these data require that slip rates during the Late Holocene were ∼7–14 mm/yr, with a preferred rate of ∼11–13 mm/yr. Our results, in conjunction with previous estimates of displacement over the past ∼15 ka, require significant temporal variations in strain release along the Garlock fault and confirm previous suggestions that interactions among fault systems in eastern California give rise to alternating periods of fault activity and quiescence.

[1] Quaternary alluvial fans have diverse surface morphologies related to both depositional and weathering processes. Numerous studies have demonstrated that the surface expression and morphometry of alluvial fans can be used as an indicator of their relative age of deposition, but only recently has there been an effort to utilize high-resolution topographic data to differentiate alluvial fans by surface age with automated and quantifiable routines. We developed a quantitative model for mapping the relative age of alluvial fan surfaces based on multiparameter surface roughness values computed from 1 m resolution lidar topographic data. Roughness is defined as a function of observational scale and integration of slope, curvature, and aspect topographic parameters. Alluvial fan roughness values were computed across multiple observation scales (3 × 3 m to 150 × 150 m) based on the standard deviation (SD) of slope, curvature (tangential), and aspect topographic parameters. Plots of roughness value versus size of observation scale suggest that the SD of each parameter over a 7 × 7 m observation window best identified the signature of surface roughness elements. Roughness maps derived from slope, curvature, and aspect at this scale were integrated using fuzzy logic. The integrated roughness map was then classified into five relative morphostratigraphic surface age categories (active wash to ~400 ka) and statistically compared with a similar fivefold surface age map of alluvial fans developed using traditional field surveys and aerial photographic interpretation. The model correctly predicted the distribution and relative surface age of ~61% of alluvial fan landforms based on traditional mapping techniques.

Climate effects on landscape evolution during the Late Pleistocene–Holocene transition (∼14.6–8 ka) in southwestern North America traditionally are linked to the activity of the North American Monsoon and to vegetation change related to a decrease in winter precipitation acting in response to orbital cyclicity. We performed an integrated analysis of regional alluvial fan, lacustrine and paleobotanical records for the area comparing them with hemispheric and regional paleoclimate proxies. Our focus was on the potential role the Tropical Pacific has as a synoptic pattern modulator and moisture source for hydrogeomorphic activity in the region. Our analysis indicates that the onset of alluvial fan aggradation in most of the region at ∼13.5 ka could have been a response to semi-permanent El Nino-like conditions in the Tropical Pacific, which enhanced the frequency of winter frontal storms as well as increased penetration of tropical cyclones in the region. The North American Monsoon was restricted in extent and intensity until ∼7 ka and probably was not a major factor in alluvial fan aggradation. A second stage of alluvial fan aggradation from 11.5 to ∼9 ka was dominated by hyper-concentrated flows and sheet-flood sedimentation, along with deposition in fluvial settings. Storms were probably were linked to landfall of enhanced water vapor bands in the leading edge of winter extra-tropical cyclones with moisture advected directly from the Tropical Pacific. At ∼8 ka, favorable conditions for the occurrence of these storms waned and storm tracks shifted northward. Analysis of modern analogs for storm types described above as prevalent during this period indicates that changes in circulation patterns across the Tropical Pacific can affect storm properties enough to explain the observed geomorphic effects, regardless of other factors traditionally considered of large impact like vegetation change. Our results suggest that the Tropical Pacific plays a larger role than currently thought in landscape evolution of the region.

[1] Vesicular A (Av) soil horizons form beneath desert pavements from the accretion of aeolian sediment (dust) commonly thought to be derived primarily from desiccating pluvial lakes and playas, with contributions from ephemeral washes and alluvial fans. Particle size distributions of Av horizons are typically bimodal with primary modes of very fine silt and fine sand, suggesting that the horizon matrix is derived from multiple sources. Here we conduct detailed chemical and physical analysis of both Av horizon soil samples and potential sources of aeolian sediment to better constrain the relative contributions of dust associated with the development of Av horizons. Geochemical data from both sand (125–250 µm) and silt (2–32 µm) fractions in Av horizons and potential dust sources in the eastern Mojave Desert and western Sonora Desert, USA, point to large contributions from nearby sources including distal alluvial fans and washes, and comparably lower contributions from regional sources such as playas. The silt mode is derived from suspension transport of dust, and the fine sand mode is derived from saltating sand. The desiccation of pluvial lakes in the Mojave Desert is commonly believed to have driven episodes of aeolian activity, contributing to sand dunes and Av horizon formation. We propose that alluvial fans and washes are underappreciated as desert dust sources and that pulses of dust from late Pleistocene and Holocene alluvial fans dwarfed pulses of dust from desiccating pluvial lakes in the eastern Mojave Desert.

River incision into bedrock resulting from the combined effects of tectonic uplift and climate governs long-term regional landscape evolution. We determined spatial and temporal patterns of post-orogenic stream incision from a sequence of well-preserved staircase terraces developed over the last 1 Ma in the Central Pyrenees and its southern foreland Ebro basin (NE Spain). Extensive remnants of ten vertically separated terraces (Qt1 to Qt10, from oldest to youngest) were mapped along 170 km of the Cinca River valley, transverse to the Pyrenean mountain belt. Multiple outcrops appear in the upper reach of the valley (Ainsa sector, 50 km from headwaters) as well as in the lower reach (Albalate sector, 125 km from headwaters). Fluvial incision into bedrock was calculated using (i) differentially corrected GPS measurements of the altitude of straths and (ii) numerical dating of alluvial sediments from the lower terraces (Qt5 to Qt9) by Optically Stimulated Luminescence, previously reported by Lewis et al. (2009), and supplemented with new dates for the upper terraces (Qt1, Qt2 and Qt3) based on paleomagnetism and supported by soil development. Considering altitude differences and the elapsed time between successive well preserved terrace couples (Qt3–Qt7, Qt7–Qt9 and Qt9-Active channel), mean bedrock incision rates ranged from 0.76 to 0.38 m ka− 1, at the upper reach of the valley (Ainsa section), and from 0.61 to 0.20 m ka− 1, at the lower reach (Albalate section). River incision along the valley produced vertically separated, near-parallel longitudinal terrace profiles evidencing a rapid near-uniform regional uplift as response to (i) the tectonic lithospheric thickening in NE Iberia and (ii) the erosional download rebound related to the Ebro basin exorheism. Moreover, a subtle upstream divergence of strath profiles may have been a consequence of an increase in uplift rate toward the head of the valley. Additionally, incision rates changed over time as indicate results from the lower reach (Albalate section); the maximum rate was 1.48 m ka− 1 between Qt7 (61 ka) and Qt8 (47 ka), and the minimum rate was 0.11 m ka− 1 between Qt3 (401 ka) and Qt5 (178 ka). The highest incision rates were produced after the Marine Isotope Stage 4 most likely in response to (i) an increased snowmelt discharge during the subsequent deglaciation related to the last maximum advance of glaciers in the southern Pyrenees, and (ii) a limited width of the valley after Qt7 formation, resulting from the deactivation of the westward river migration. Therefore, incision rates over the last 1 Ma in the Cinca River valley were basically controlled by near-uniform bedrock uplift, in the context of climate variability. The results reported in this study represent significant data on fluvial incision in NE Iberia, and provide an assessment of the regional post-tectonic landscape evolution., Earth and Environmental Sciences Division, Los Alamos National Laboratory, Estados Unidos, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, España, Desert Research Institute, Estados Unidos, Departamento de Geografía y Ordenación del Territorio, Universidad de Zaragoza, España, Unidad de Zaragoza, Instituto Geológico y Minero de España, España, Department of Geography, The University of Sheffield, Reino Unido, Museo Nacional de Ciencias Naturales, Consejo Superior de Investigaciones Científicas, España

Simulating soil moisture dynamics in coarse-textured alluvial soils under high evaporative demand and infrequent rainfall challenges both characterization and numerical methods. For this study, three geomorphically young disturbed soils in the central Mojave Desert were characterized using direct, indirect, and inverse parameter estimation methods for numerical simulations using HYDRUS. Observations of soil water content (θ) and matric head ( h ) were used to formulate our multiobjective framework optimized using the AMALGAM algorithm. Both the direct and indirect parameterization methods reproduced θ but proved inadequate for h data. Sensitivity analysis found the parameters related to the dry end of the soil hydraulic function—including residual water content (θ r ) and pore-size distribution ( n) —most sensitive but often conflicting between the two criteria while soil temperature was effectively simulated but insensitive. At the sites, a large dynamic range of h (1–400 m) was accompanied by a relatively small range of θ (0.04–0.10 m3 m−3), limiting our ability to resolve the wet-end of the hydraulic functions. Direct parameterization from infiltrometer data and indirect pedotransfer functions had good agreement with θ but largely underestimated h . Pareto analysis determined the trade-off between fitting both criteria and revealed model structure inadequacies as well as bifurcations into parameter populations, favoring either θ or h error minimization. The Pareto solutions compensated for the underestimation of h by forcing the α and n parameters to uncharacteristically low values for such coarse textures. This compensation kept the water capacity sufficiently high at lower θ to maintain conductivity toward the surface. These results are likely a consequence of either the disturbed soil system or hysteresis between measurements of θ and h.

The Paonia-McClure Pass area of Colorado has been recognized as a region highly susceptible to mass movement. Because of the dynamic nature of this landscape, accurate methods are needed to predict susceptibility to movement of these slopes. The area was evaluated by coupling a geographic information system (GIS) with logistic regression methods to assess susceptibility to landslides. We mapped 735 shallow landslides in the area. Seventeen factors, as predictor variables of landslides, were mapped from aerial photographs, available public data archives, ETM + satellite data, published literature, and frequent field surveys. A logistic regression model was run using landslides as the dependent factor and landslide-causing factors as independent factors (covariates). Landslide data were sampled from the landslide masses, landslide scarps, center of mass of the landslides, and center of scarp of the landslides, and an equal amount of data were collected from areas void of discernible mass movement. Models of susceptibility to landslides for each sampling technique were developed first. Second, landslides were classified as debris flows, debris slides, rock slides, and soil slides and then models of susceptibility to landslides were created for each type of landslide. The prediction accuracies of each model were compared using the Receiver Operating Characteristic (ROC) curve technique. The model, using samples from landslide scarps, has the highest prediction accuracy (85 %), and the model, using samples from landslide mass centers, has the lowest prediction accuracy (83 %) among the models developed from the four techniques of data sampling. Likewise, the model developed for debris slides has the highest prediction accuracy (92 %), and the model developed for soil slides has the lowest prediction accuracy (83 %) among the four types of landslides. Furthermore, prediction from a model developed by combining the four models of the four types of landslides (86 %) is better than the prediction from a model developed by using all landslides together (85 %).

A basic tenet of arid region geomorphology is that a decrease in hillslope vegetation density coupled with increases in monsoonal precipitation after a change to a warmer and drier climate is an essential factor for an increase in sediment yield and fan aggradation. Over the last two decades vegetative cover change is increasingly cited as a prominent factor in triggering the onset of fan aggradation in deserts of southwestern North America, especially in analysis of fan deposition during the late Pleistocene–Holocene (LPH) climatic transition. To assess this assertion, we compiled paleobotanical and alluvial fan stratigraphy histories from 18 published studies broadly representing four different areas in the Mojave and northern Sonoran Deserts. Instead of focusing on chronology aggregation and comparison with global or regional climate change proxies forced by orbital parameters, we discriminated by altitudinal regions, looking for linkages between local vegetation change and alluvial deposition. Results indicate that the onset of extensive alluvial fan deposition (1) began well before changes in catchment vegetative cover, and (2) can occur during several possible combinations of vegetation change. The ambiguous relation between vegetation change and alluvial fan aggradation indicates that vegetation had a reduced role in LPH aggradation. Other factors, such as local storm intensity and water and sediment redistribution pathways on hillslopes, need to be considered in this analysis given the important role that the combined hillslope/alluvial system has on arid region ecosystem functions.

Stratigraphy and age control of late Pleistocene loess, associated glacial outburst flood deposits and flood-cut unconformities in the Channeled Scabland, Washington State, United States, indicate a significant Cordilleran ice sheet advance during marine Oxygen Isotope Stage 4. Glacial outburst flood deposits from stage 2 (classic Bretz flood deposits, ca 21 to 13 ka) and related features in the Channeled Scabland overlie a widespread layer of loess that contains buried soils and the Mount St. Helens set C tephra (ca 46 ka). This loess in turn overlies deposits of the penultimate episode of giant outburst floods and an unconformity cut by those floods. Regional trends in the thickness, texture, and overall composition of the older loess are strikingly similar to those from the youngest loess, known to be derived from stage 2 flood deposits. We conclude that the older loess also is derived from fine-grained flood deposits. Luminescence ages, tephrochronology, and soil development rates indicate that the bulk of deposition of the older loess occurred during stage 3, following glacial outburst flooding marked by a regional flood-cut unconformity. The apparent cyclical pattern of cold-climate buried soils, flood deposits, and thick loess accumulations demonstrate that sediment supply renewed by flood episodes is a major control on accumulation of loess on glacial timescales.

The measurement of natural dust emissions from desert landforms is crucial in environmental hazard assessment and field checking the accuracy of global dust models. More than 500 individual dust measurements from eight common desert landforms in southern California were collected using the PI-SWERL (Portable In Situ Wind Erosion Lab). The largest emitters of dust are dry washes (13.787 to 0.007 mg m − 2 s − 1 ), dunes, playa margins, distal alluvial fans, and lacustrine beaches. Low emitters include salt-crusted playas (0.692 to 0.002 mg m − 2 s − 1 ), silt–clay-crusted playas, and desert pavements. High emissions are a function of saltating sand that bombards the surface, liberating dust-sized particles for entrainment. Low dust emissions are primarily a function of surface crusting, gravel armoring, and vegetation density. PI-SWERL measurements reveal that emission rates can vary by at least three orders of magnitude, reflecting local variability in soil texture and continuity of surface crusts. Shear-stress partitioning models can be applied to dust data measured by the PI-SWERL to account for large surface roughness features, such as vegetation. The results presented here give an approximation of the contributions to atmospheric dust loading by landforms in the Mojave Desert, and can potentially be used to improve atmospheric dust models.

The time at which deserts established their current arid or hyper-arid conditions remains a fundamental question regarding the history of Earth. Cosmogenic isotope exposure ages of desert pavement and welded, calcic–gypsic–salic Reg soils that developed on relatively flat alluvial surfaces ∼2 Ma ago in the Negev Desert indicate long geomorphic stability under extremely dry conditions. Over a short interval during their initial stage of development between 1–2 Ma, these cumulative soils are characterized by calcic soils reaching maximum stage III of carbonate morphology. This interval is the only period when calcic soil horizons formed on stable abandoned alluvial surfaces in the southern Negev Desert. Since ∼1 Ma pedogenesis changed toward more arid soil environment and the formation of gypsic–salic soil horizons that were later followed by dust accumulation. The dichotomy of only moderately-developed calcic soil (stages II–III) during a relatively long time interval (105–106 years) indicates an arid environment that does not support continuous development but only occasional calcic soil formation. The very low δ18O and relatively high δ13C values of these early pedogenic carbonates support soil formation under arid climatic conditions. Such an environment was probably characterized by rare and relatively longer duration rainstorms which occasionally allowed deeper infiltration of rainwater and longer retention of soil moisture. This, in turn enabled the growth of sparse vegetation that enhanced deposition of pedogenic carbonate. At ∼1 Ma these rare events of slightly wetter conditions ceased and less atmospheric moisture reached the southern Negev Desert leading to deposition of soluble salts and dust deposited in the soils. The combination of long-term hyperaridity, scarcity of vegetation and lack of bioturbation, salts cementation, dust accumulation and tight desert pavement cover, has protected the surfaces from erosion forming one of the most remarkably stable landscapes on Earth, a landscape that essentially has not eroded, but accumulated salt and dust for more than 106 yr.

The late middle Pleistocene initiation of loess accretion in the Negev Desert, Israel, resulted from a chain of events starting with the exposure of shallow offshore Nile delta sands beginning ∼180 ka, during a period of glacially lowered eustatic sea level. This exposure allowed the formation of the dunes of the Sinai-Negev erg and the eastward movement of the dunes under the gusty glacial-time winds in northern Sinai. This eastward dune advance occurred by eolian saltation that generated the coarse silts that accumulated downwind as loess. We postulate that the absence of earlier Negev loess is not a result of interglacial erosion as previously proposed but is due to the relatively recent convergence of the above conditions necessary for loess formation. We based our interpretation on analyses of two long-term natural dust traps and the association of coarse quartz silt production with the advancing quartz-rich dunes over this carbonate terrain. Although the Nile delta is considered an early to middle Pleistocene feature, and therefore could have been the source of sand throughout the Quaternary, it attained the shallow depth range of glacial-interglacial sea-level changes only during late marine oxygen isotope stage 7 to early stage 6, allowing its wide exposure. In Quaternary times prior to erg formation, Negev dust was finer, composed of silt and clays derived from distal sources in the Sahara and Arabian deserts.

Flights of rotorcraft over the desert floor can result in significant entrainment of particulate matter into the atmospheric boundary layer. Continuous or widespread operation can lead to local and regional impacts on visibility and air quality. To account for this pollutant source in air quality models, a parameterization scheme is needed that addresses the complex vertical distribution of dust ejected from the rotorcraft wake into the atmospheric surface layer. A method to parameterize the wind and turbulence fields and shear stress at the ground is proposed here utilizing computational fluid dynamics and a parameterized rotor model. Measurements taken from a full scale experiment of rotorcraft flight near the surface are compared to the simulation results in a qualitative manner. The simulation is shown to adequately predict the forward detachment length of the induced ground jet compared to the measured detachment lengths. However, the simulated ground vortex widths and vorticity deviate substantially from the measured values under a range of flight speeds. Results show that the method may be applicable for air quality modeling assuming slow airspeeds of the rotorcraft, with advance ratios of 0.005–0.02.

The integration of geomorphic mapping, soil stratigraphy, and radiocarbon dating of alluvial deposits offers insight to the timing, magnitude, and paleoclimatic context of Holocene fan sedimentation near Yuma, Arizona. Mapping of 3400 km2indicates about 10% of the area aggraded in the late Holocene and formed regionally extensive alluvial fan and alluvial plain cut-and-fill terraces. Fan deposits have weakly developed gravelly soils and yielded a date of 3200–2950 cal yr BP from carbonized wood. Alluvial plain deposits have weakly developed buried sandy soils and provided a date of 2460–2300 cal yr BP from a terrestrial snail shell. Precipitation records were analyzed to form historical analogues to the late Holocene aggradation and to consider the role of climatic variability and extreme hydrologic events as drivers of the sedimentation. The historical precipitation record indicates numerous above-average events correlated to the Southern Oscillation Index (SOI) in the region, but lacks any significant reactivation of alluvial fan surfaces. The timing of aggradation from 3200 to 2300 cal yr BP correlates well with other paleoclimatic proxy records in the southwestern U.S. and eastern Pacific region, which indicate an intensification of the El Niño-Southern Oscillation (ENSO) climatic pattern and rapid climate change during this period.

Oxygen isotope ratios of pedogenic carbonates were analyzed along a climatic gradient in the Palouse region of eastern Washington State to evaluate changes in their isotopic composition with changing climate. A modern analog approach was used to assess the relationship between the istotope ratios in the pedogenic carbonates with those in soil water (measured in June when we anticipated carbonate formation might occur because of low soil water content), and meteoric water from two National Atmospheric Deposition Program (NADP) sites adjacent to our sequence. Mean annual meteoric water δ18O values (wrt VSMOW) from the two NADP sites varied slightly with elevation, —13.9‰ at the Starkey site (elevation 1253 m) and —13.0‰ at the Palouse site, (elevation 766 m) but showed distinct seasonal trends with weighted means ranging from —11.9‰ during autumn to —15.0‰ during the winter. Carbonate δ18O (wrt VPDB) varied significantly with MAP, ranging from —14.1‰ at the most arid site to —11.4‰ at the wettest site but displayed a trend opposite to that of soil water δ18O (—4 to —12‰ VSMOW for the 70 cm depth). Neither measured soil water δ18O values nor mean meteoric water δ18O values generated realistic temperatures of formation for the observed trend in carbonate δ18O values. When mean annual temperatures across the gradient were used for temperature of formation, calculated soil water δ18O in equilibrium with carbonate δ18O was similar to winter precipitation δ18O at the most arid sites but became increasingly similar to mean annual meteoric δ18O as MAP increased. We suggest that the unexpected trend in carbonate δ18O is largely due to the diminishing influence of winter precipitation/snowmelt on soil moisture as MAP increases across the climatic gradient and that seasonal effects of meteoric water may need to be considered in pedogenic mineral isotopic equilibrium in temperate regions.

The Rio Diamante is located in a segment of the southern Central Andes that is transitional in terms of morphology and foreland tectonics (33–37°S). The piedmont reach of the river flows eastwards between the main mountain front and the San Rafael Block (the southernmost foreland uplift to the east of the southern Central Andes). Unlike adjacent rivers, the Rio Diamante has incised into pre-Quaternary bedrock to form a deep canyon across the piedmont. Five fill and strath terraces were mapped, correlated, and surveyed along the ∼35 km piedmont reach to determine their paleo-long profiles. Chronological data for the terraces were provided by geochemical correlation of interbedded tephras to a previously dated ignimbrite, as well as by eight cosmogenic 10Be ages, and suggest a synchronous relationship between fill terrace deposition and glaciation upstream. Terraces are tentatively correlated with oxygen isotope stages 16 (Qt1), 12 (Qt2), 4 (Qt3) and 2 (Qt4 and Qt5). Minor spatial variation in incision along the reach is apparent from the long profile of the Qt2 strath (∼450 ka), which shows low-amplitude folding. The long-term reach average rate of bedrock incision is estimated to be ∼0.1–0.5 m/kyr and the recent shorter term (post-OIS 2) rate is estimated at ∼2 m/kyr. Mid and late Quaternary uplift of the piedmont area is a likely cause for the incision; however, other possible explanations include a delayed response to pre-mid Quaternary uplift, or a response to changes in climate and sediment supply on a tectonically stable portion of the piedmont flanked by subsiding basins. Base-level fall due to subsidence on the eastern side of the San Rafael Block creating a westward migrating knickzone may have contributed to the incision, but the depth of incision is greater than the observed subsidence and no major knickzone is present in the modern river profile of the study reach. Studies such as this on piedmont geomorphic processes are important for understanding the topographic and tectonic transitions that characterise the southern Central Andes.

We correlate Upper Pleistocene glacial and fluvial deposits of the Cinca and Gallego River valleys (south central Pyrenees and Ebro basin, Spain) using geomorphic position, luminescence dates, and time-related trends in soil development. The ages obtained from glacial deposits indicate glacial periods at 85 ± 5 ka, 64 ± 11 ka, and 36 ± 3 ka (from glacial till) and 20 ± 3 ka (from loess). The fluvial drainage system, fed by glaciers in the headwaters, developed extensive terrace systems in the Cinca River valley at 178 ± 21 ka, 97 ± 16 ka, 61 ± 4 ka, 47 ± 4 ka, and 11 ± 1 ka, and in the Gallego River valley at 151 ± 11 ka, 68 ± 7 ka, and 45 ± 3 ka. The times of maximum geomorphic activity related to cold phases coincide with Late Pleistocene marine isotope stages and Heinrich events. The maximum extent of glaciers during the last glacial occurred at 64 ± 11 ka, and the terraces correlated with this glacial phase are the most extensive in both the Cinca (61 ± 4 ka) and Gallego (68 ± 7 ka) valleys, indicating a strong increase in fluvial discharge and availability of sediments related to the transition to deglaciation. The global Last Glacial Maximum is scarcely represented in the south central Pyrenees owing to dominantly dry conditions at that time. Precipitation must be controlled by the position of the Iberian Peninsula with respect to the North Atlantic atmospheric circulation system. The glacial systems and the associated fluvial dynamic seem sensitive to 1) global climate changes controlled by insolation, 2) North Atlantic thermohaline circulation influenced by freshwater pulses into the North Atlantic, and 3) anomalies in atmospheric circulation in the North Atlantic controlling precipitation on the Iberian Peninsula. Our scenario of glacial and fluvial evolution during the Late Pleistocene in northern Spain could be extrapolated to other glaciated mountainous areas in southern Europe.

All exposed rocks on Earth’s surface experience erosion; the fastest rates are documented in rapidly uplifted monsoonal mountain ranges, and the slowest occur in extreme cold or warm deserts—millennial submeterscale erosion may be approached only in the latter. The oldest previously reported exposure ages are from boulders and clasts of resistant lithologies lying at the surface, and the slowest reported erosion rates are derived from bedrock outcrops or boulders that erode more slowly than their surroundings; thus, these oldest reported ages and slowest erosion rates relate to outstanding features in the landscape, while the surrounding landscape may erode faster and be younger. We present erosion rate and exposure age data from the Paran Plains, a typical environment in the Near East where vast abandoned alluvial surfaces (10 2 –10 4 km 2 ) are covered by well-developed desert pavements. These surfaces may experience erosion rates that are slower than those documented elsewhere on our planet and can retain their original geometry for more than 2 m.y. Major factors that reduce erosion converge in these regions: extreme hyperaridity, tectonic stability, fl at and horizontal surfaces (i.e., no relief), and effective surface armoring by a clast mosaic of highly resistant lithology. The 10 Be concentrations in amalgamated desert pavement chert clasts collected from abandoned alluvial surfaces in the southern Negev, Israel (representing the Sahara-Arabia Deserts), indicate simple exposure ages of 1.5–1.8 Ma or correspond to maximum erosion rates of 0.25–0.3 m m.y. –1 . The 36

One of the greatest challenges to the U.S. Department of Defense is maintaining the sustainability of military lands while maximizing their use for training activities. Restoration of disturbed lands in the Mojave Desert has proven to be a challenge, particularly revegetation from seed. This study presents soil moisture dynamics from three restoration sites at the National Training Center, Ft. Irwin, CA. The soil microclimate (matric potential, water balance, and temperature) was monitored beneath various surface treatments to better assess their effects on seed germination and establishment. Results show that under the typically hot and dry climatic conditions that prevail, the soil microclimate is a hostile environment. Irrigation had the most positive effect on the seedbed. Surface mulches including gravel, straw, emulsion, and bark had limited benefits often detrimental. Although seedlings germinated, survival after three months was negligible. Results presented here suggest that successful seeding with broadcast sprinkler irrigation in the central Mojave Desert is not possible unless the seedbed area is limited or advantageous environmental conditions prevail. Improved knowledge of the germination requirements and applied numerical modeling would result in more successful seedbed management.

Dust has long been recognized as an important parameter for testing military vehicles and equipment due to the potential for adversely affecting performance and durability. Vehicle dust test courses were originally established in areas of heavy underlying fine particulate layers (silt and clay) and used for cyclic tests. These soils, known as desert pavements, are geomorphically old and slowly develop through the trapping of atmospheric dust over tens of thousands of years. As such, these soils are both a sink and source for eolian material. However, this dust is a finite resource for military testing at the Yuma Proving Ground (YPG), the Army’s primary environmental hot weather test center. There is concern that the continuous utilization of YPG dust test courses over the past half century has eroded through the overlying dust-rich soil to the extent that the true natural environmental conditions are no longer adequately represented. The objectives of this study were (1) to characterize current dust courses at YPG in order to (2) gain a better understanding of the current state of dust availability and (3) assess the overall sustainability of high dust-potential soils needed for proper military testing. Soil samples collected from dust test courses and adjacent undisturbed soils, as well as dust collected on tactical test vehicles, were analyzed for mineralogy, geochemistry, and physical properties. Results indicate adjacent undisturbed soils exhibit a higher dust potential (higher silt and clay) and elevated reactive salts and carbonates indicating a significant loss of fines from the dust test courses. Routine vehicle traffic and surface preparation has eroded and mixed the dust layer with the underlying sand and gravel-rich horizons, thus depleting the dust test courses of their dust producing potential. Desert pavements are stable geomorphic landforms but the soils have finite dust content when disturbed and therefore are unsustainable for long-term repeated dust testing. Disturbance of these soils may potentially affect ecologically sensitive areas. As such, more geomorphically active areas such as floodplains may provide greater sustainability for long-term testing.

The U.S. Army Yuma Proving Ground is the Department of Defense desert environment test center within the Sonoran Desert of Arizona. The Yuma Proving Ground has ∼320 km of unpaved vehicle test courses that cross a variety of landforms of diverse geologic age and characteristics. The surface materials of the courses ranges from bedrock to silt and their topography varies from steep and rolling to flat. Research presented here aims to provide a systematic characterization of the terrain of eight vehicle endurance and three dust courses so that their comparability with other desert areas of the World may be assessed. Landform and surface cover (upper 1 m) characterization was accomplished by geomorphic mapping based on 1-m resolution IKONOS satellite imagery, 10-m digital elevation models, field verification, and by assimilating pre-existing soil surveys and geologic maps, as well as site-specific investigations. Results provide an assessment of each test course, including information on the landform, geology, surface materials, soil type, degree of desert pavement development, dust content, and percent slope. Data is presented both on individual terrain property maps for each course and in the form of tabulated data for each official milepost marker along the courses. The results for one course area and an example of how they may be used to assess comparability with another desert of interest are presented here with the objective to improve the fidelity of desert testing during material research, development, testing and evaluation prior to deployment in the field.

Salt flux through soils can significantly influence local and global processes. For example, desert soils can atypically concentrate NO 3 - at depth in soil profiles. CaCO 3 precipitation/dissolution can play significant roles as either sinks or sources of global carbon. The objectives of this work were to develop a salt-flux model for long-term (>1000 years) simulations of desert soils and examine the consequences of climate, soils, system inputs, and land-use change on salt movement in arid soils. The field study was conducted at the Nevada Test Site in the northern Mojave Desert. New additions to the CALGYP model allowing for site-specific parameterization included stochastic rainfall model, salt inputs, soil water-holding capacities, and soil CO 2 profiles. New ions added to the model included Na + , K + , Mg 2+ , Cl − , and NO 3 - . About 81% of Ca 2+ input remained within the surface 1.0 m of soil as CaCO 3 , which argues in favor of soil CaCO 3 serving as a recalcitrant sink for global carbon. In contrast, ≈99.96% of Na + , K + , Mg 2+ , Cl − , NO 3 - , and SO 4 2 - ions leached to soil depths >1.0 m and 94.3% leached to soil depths >2.0 m. This is true despite only 1.64% of the rainfall leached beyond 1.0 m and 0.020% of the rainfall leached beyond 2.0 m. The leachability of NO 3 - and Cl − to soil depths > 2.0 m agrees with NO 3 - and Cl − accumulations at depth in Mojave Desert soils (1.3–2.7 m). Simulation of extreme events and years with a stochastic rainfall model and accurate soil water-holding capacities are critical for modeling water and salt flux through soils.

A Holocene alluvial morphostratigraphy, based on geomorphological mapping, sedimentological analysis, soil development and radiocarbon chronology is presented for the semiarid Blanca basin in the Bardenas Reales Natural Park (NE, Spain). Four morphopedosedimentary alluvial units (MU1, MU2, MU3 and MU4) were differentiated, which may correspond to two palaeogeographical stages. The first one includes the morphopedosedimentary unit MU1 and is the most extensive in the area. It is made up of a complex aggradation (fill up) sequence comprising a braided channel system, a sand–mud flat (9200–7900 cal BP) and a playa-lake environment related to a nearly closed depression. An intensive fluvial entrenchment period is recognized, leading the opening of the Blanca basin, before the second stage that includes the units MU2, MU3 and MU4. This set of units forms a sequence of cut-and-fill and nested fill terraces recording the alluvial activity in the Blanca basin during the last 6 ka. The MU2 unit, 5900–3300 cal BP in age, represents a period of high geomorphic instability with rapid alluvial sedimentation/channel incision oscillating changes. On the other hand, the MU3 unit has an age of 1050–500 cal BP and appears as a thin extensive alluvial cover overlying previous units. Finally, the MU4 unit comprises superposition of numerous flood depositional sequences, centimetric in scale, dated from 450–10 cal BP. The established soil chronosequence supports the numerical chronology of the four units. These accumulation periods (MU1, MU2, MU3, MU4) are associated with dry phases and/or high hydrological variability (floods and droughts) at a regional scale. Changes on the North Atlantic atmospheric circulation mode over Southern Europe may provide the key to explain the nature of alluvial activity during Holocene in Bardenas Reales., Projects BTE2003-01296 and CGL2006-08973/BTE of the Spanish Government and the European Regional Development Fund have supported this work. We thank Alejandro Urmeneta (Junta de Bardenas Reales de Navarra) for collaboration with permissions and technical support in the field and Sophie Baker for the assistance with editing this manuscript. This is a contribution by PALEOQ and Cuencas Sedimentarias Continentales groups (Aragón Regional Government).

The impact of large twentieth century floods on the riparian vegetation and channel morphology of the relatively wide anabranching and braided Nahal Arava, southern Israel, was documented as part of developing tools to (a) identify recent large floods, (b) determine these flood's respective magnitudes in alluvial ungauged streams, and (c) determine long-term upper bounds to flood stages and magnitudes. Along most of its course Nahal Paran, a major tributary that impacts the morphology, floods and sediments of Nahal Arava at the study reach, is a coarse-gravel, braided ephemeral stream. Downstream of the Arava–Paran confluence, aeolian and fluvial sand delivered from eastern Arava valley alters the channel morphology. The sand has accreted up to 2.5 m above the distinct current channels, facilitating the recording of large floods. This sand enhances the establishment of denser riparian vegetation (mainly Tamarix nilotica and Haloxylon persicum ) that interacts with floods and affects stream morphology. A temporal association was found between specific floods recorded upstream and tree-ring ages of re-growth of flood-damaged tamarix trees (‘Sigafoos trees’) in the past 30 years. This association can be utilized for developing a twentieth century flood chronology in hyperarid ungauged basins in the region. The minimum magnitude of the largest flood that covered the entire channel width, estimated from flood deposits, is approximately 1700–1800 m 3 s − 1 . This is a larger magnitude than the largest gauged flood of 1150 m 3 s − 1 that occurred in 1970 about 30 km upstream in Nahal Paran. Our estimation agrees with flood magnitude estimated from the regional envelope curve of the largest floods. Based on Holocene alluvial stratigraphy and OSL dating in the study reach we also conclude that flood stages did not reach the late Holocene (∼ 2.2 ka) surface and therefore we estimate a non-exceedance upper bound of ∼ 2000 m 3 s − 1 flood magnitudes for Nahal Arava during that interval. This study indicates that in unfavorable areas the combination of hydrology, fluvial morphology and botanic evidence can increase our understanding of ungauged basins and give information crucial for hydrology planning.

Av horizons found in desert pavement environments are known to evolve pedogenically over geologic time. This study was conducted to determine whether increased pedogenic development of the Av (vesicular) horizon over relative time impacts the hydraulic properties of individual soil peds and the mechanism of infiltration as inferred by dye patterns. We examined peds from the Av horizons associated with desert pavements that mantled three different alluvial deposits with different relative surface ages (Qf5 (∼ 10 ka), Qf3 (∼ 50–100 ka), and Qf2 (∼ 10–50 ka)) and included an additional surface (Qf6 (∼ 4 ka)) for the dye studies. We hypothesized that increases in the development of the Av over time would lead to a more structured soil surface with greater potential flow between soil peds and lower hydraulic conductivity of the soil peds themselves. Results showed that average K s and α of the Qf5 peds were significantly greater than estimated for the Qf2 and Qf3 peds. Although K s was greater for the Qf5 peds, the steady-state infiltration rate was equal for the Qf3, Qf2, and Qf5 surfaces, perhaps indicating a reduction in matrix flow through soil peds and an increase in interped flow between soil peds. Studies were also conducted using dyed water and a tension disc infiltrometer, set to saturation. Following the tests the soil was excavated in 2-cm increments and photographed. The dye patterns for the Qf6 indicated that the water moved rapidly through the soil matrix into deeper soil, and without any preferential flow around peds. The Qf5 exhibited more dye-stained soil at depth than seen in the Qf2 and Qf3, perhaps due to higher ped conductivity. Qualitative observations on the Qf2 and Qf3 suggest that water flowed primarily along the ped faces and then toward the ped interiors. The results suggest that infiltration through Av horizons evolves from a matrix-dominated process on the younger soils to a preferential flow-dominated process on older surfaces.