Wind erosion on the north‐eastern Tibetan Plateau: constraints from OSL and U‐Th dating of playa salt crust in the Qaidam Basin

The arid Qaidam Basin is the largest (~3.88 × 10⁴ km²) basin on the north‐eastern Tibetan Plateau. Wind erosion in the area has been regarded as an important trigger for intra‐basin tectonic balance upheaval, geomorphologic development and as a major supplier of dust to the Chinese Loess Plateau downwind. An initial estimate of the rate of wind erosion (Kapp et al., 2011) based on geological cross‐sections has suggested up to 3.2 × 10⁴ km³ of sediments has been deflated over the past 2.8 Ma, lowering the landscape by an average of 0.29 mm/yr. In this paper we re‐evaluate this estimate by dating surface crusts present on three playas within the basin. Understanding the development of these playas is crucial to assessing the overall role of the wind in shaping the regional landscape because they are typically capped with a thick salt crust which effectively protects them from wind erosion. Optically stimulated luminescence (OSL) and U‐series dating from a pit section and from the top of a deep drill core, together with results from magnetostratigraphy and a climate proxy record correlated to the marine oxygen isotope record, are used here to determine the age of the playa plains and suggest that the salt crusts have an age of c. 0.1 Ma. This young age and the wide distribution of resistant thick salt crusts of the playa plains indicate a much lower degree of wind erosion than previously suggested. The crusts protect the surface from significant surface erosion (including sediment exhumation and unloading) and whilst some wind erosion does occur, it is unlikely to be sufficient to trigger tectonic uplift of the basin or to be a major dust source for the Loess Plateau as previously suggested. Copyright © 2013 John Wiley & Sons, Ltd.     

Dust emission from wet and dry playas in the Mojave Desert,USA

The interactions between playa hydrology and playa‐surface sediments are important factors that control the type and amount of dust emitted from playas as a result of wind erosion. The production of evaporite minerals during evaporative loss of near‐surface ground water results in both the creation and maintenance of several centimeters or more of loose sediment on and near the surfaces of wet playas. Observations that characterize the texture, mineralogic composition and hardness of playa – surfaces at Franklin Lake, Soda Lake and West Cronese Lake playas in the Mojave Desert (California), along with imaging of dust emission using automated digital photography, indicate that these kinds of surface sediment are highly susceptible to dust emission. The surfaces of wet playas are dynamic surface texture and sediment availability to wind erosion change rapidly, primarily in response to fluctuations in water‐table depth, rainfall and rates of evaporation. In contrast, dry playas are characterized by ground water at depth. Consequently, dry playas commonly have hard surfaces that produce little or no dust if undisturbed except for transient silt and clay deposited on surfaces by wind and water. Although not the dominant type of global dust, salt‐rich dusts from wet playas may be important with respect to radiative properties of dust plumes, atmospheric chemistry, windborne nutrients and human health. Published in 2007 by John Wiley & Sons, Ltd.     

Salt ramps: Wind-induced depositional features on Tunisian playas

Semicircular and crescent-shaped accumulations of salt crystals developed on salt crusts were measured on three Tunisian playas in September 1990. These features have been termed salt ramps. Their morphology and chemistry suggest that they are formed in the late stages of shallow ephemeral lake desiccation in playa basins. They form by salt precipitation from shallow brine lakes that are blown across salt-encrusted playa surfaces by the wind. Moreover, they appear to be short-lived features and their degradation is related to the flooding of playas with less saline water, and possibly rainfall and deflation.     

Evaporation and land surface energy budget at the Salar de Atacama, Northern Chile

Playa systems are driven by evaporation processes, yet the mechanisms by which evaporation occurs through playa salt crusts are still poorly understood. In this study we examine playa evaporation as it relates to land surface energy fluxes, salt crust characteristics, groundwater and climate at the Salar de Atacama, a 3000 km² playa in northern Chile containing a uniquely broad range of salt crust types. Land surface energy budget measurements were taken at eight representative sites on this playa during winter (August 2001) and summer (January 2002) seasons. Measured values of net all-wave radiation were highest at vegetated and rough halite crust sites and lowest over smooth, highly reflective salt crusts. Over most of the Salar de Atacama, net radiation was dissipated by means of soil and sensible heat fluxes. Dry salt crusts tended to heat and cool very quickly, whereas soil heating and cooling occurred more gradually at wetter vegetated sites. Sensible heating was strongly linked to wind patterns, with highest sensible heat fluxes occurring on summer days with strong afternoon winds. Very little energy available at the land surface was used to evaporate water. Eddy covariance measurements could only constrain evaporation rates to within 0.1 mm d⁻¹, and some measured evaporation rates were less than this margin of uncertainty. Evaporation rates ranged from 0.1 to 1.1 mm d⁻¹ in smooth salt crusts around the margin of the salar and from 0.4 to 2.8 mm d⁻¹ in vegetated areas. No evaporation was detected from the rugged halite salt crust that covers the interior of the salar, though the depth to groundwater is less than 1 m in this area. These crusts therefore represent a previously unrecorded end member condition in which the salt crusts form a practically impermeable barrier to evaporation.     

The role of playas in pedogenic gypsum crust formation in the Central Namib Desert: a theoretical model

The formation of Namibia's extensive pedogenic gypsum crusts (CaSO₄·2H₂O) is interpreted in a new light. It is suggested that gypsum primarily precipitates at isolated points of evaporitic concentration, such as inland playas, and that deflation of evaporitic‐rich gypsum dust from these playas contributes to the formation of pedogenic gypsum duricrusts on the coastal gravel plains of the Namib Desert surrounding these playas. This study establishes the nature, extent and distribution of playas in the Central Namib Desert and provides evidence for playa gypsum deflation and gravel plain deposition. Remote sensing shows the distribution of playas, captures ongoing deflation and provides evidence of gypsum deflation. It is proposed that, following primary marine aerosol deposition, both inland playas and coastal sabkhas generate gypsum which through the process of playa deflation and gravel plain redeposition contributes to the extensive pedogenic crusts found in the Namib Desert region. Copyright © 2001 John Wiley & Sons, Ltd.     

Runoff‐induced sediment yield over dune slopes in the Negev Desert. 2: Texture,carbonate and organic matter

Runoff‐induced sediments were collected in the Hallamish dune field for four years (1990–1994). Runoff and consequently water‐transported sediments were generated on the dunes owing to the presence of a thin microbiotic crust. These sediments were analysed for their particle‐size distribution and carbonate content. In addition, the organic matter content was calculated by measuring the chlorophyll content within the runoff. The results were compared to the slope parent material, i.e. the crust and the underlying sand, as well as to playa sediments, which are scattered within the Hallamish interdunal areas, and which were previously hypothesized to originate from runoff‐induced sediments. Higher amounts of fines (silt and clay) and carbonate characterize the footslopes in comparison to the midslopes. Intermediate contents of fines (17 per cent) and carbonate (8 per cent) characterized the sediments in comparison to the fines (27 per cent) and carbonate (15 per cent) of the crust and to the fines (4 per cent) and carbonate (4 per cent) of the underlying sand. The runoff‐induced fines and carbonate contents were significantly different from those of the playas, suggesting that the playa flats do not originate from runoff‐induced sediments. The sediments were enriched with organic matter. Organic matter which originates from the crust amounted to 0·3–0·4 per cent as compared to ≤0·1 per cent in the bare sand. Nevertheless, the crust was found to be relatively resilient to water flow. Only 0·1–0·5 per cent of the crust was annually eroded off the slope by water, with south‐facing crusts showing higher resilience than north‐facing crusts. The data may thus assist in the evaluation of the crust's residence time. Copyright © 2001 John Wiley & Sons, Ltd.     

Erodibility of some crust forming soils/sediments from the Southern Aral Sea Basin as determined in a wind tunnel

Severe dust storms in the Southern Aral Sea Basin have become common with the desiccation of the sea. The high incidence of dust in the area has had severe ecological consequences. Within the framework of efforts to reduce this phenomenon, deflatability as well as deflatability‐related characteristics of some prominent soils/sediment surfaces in the Southern Aral Sea Basin were examined. The materials included a salt crust from a developed Solonchak, a Takyr crust and a Takyr‐like soil, and salt crusts from undeveloped Solonchaks formed on the exposed bottom of the Aral Sea. Characteristics determined were particle size distribution, dry aggregate size distribution and salt, carbonate and organic carbon contents. Deflatability was examined using a suction type wind tunnel with a SENSIT‐type sensor to detect airborne unconsolidated material, on materials treated to different moisture levels and with a chemical stabilizer, and on restored crusts created from the unconsolidated materials. Fine sand dominates in the materials, and in the Takyr crust and Takyr‐like soils is accompanied by significant amounts of silt and clay. All materials contain moderate amounts of carbonate and are low in organic matter. All soils/sediments contain salts, but in the salt crusts of the Solonchaks the salt fraction dominates. They all have more than 50 per cent PM₈₅₀ (particles with diameter <850 µm), indicating a relatively high deflatability potential. The materials from the Takyr crusts and Takyr‐like soil with a high proportion of fine aggregates had the lowest threshold friction velocities, while the salt crusts of the Solonchaks with a high proportion of coarse aggregates had the highest. This suggests that Takyrs and Takyr‐like soils are the most deflatable and Solonchak soils the least deflatable. These differences are attributed to the presence of salts that create stable, large aggregates in the Solonchak crusts. Wetting of the materials to three moisture levels considerably increased threshold friction velocity. The increase was most prominent in the salt‐rich materials, and was attributed to the rapid formation of surface films by drying in the course of the wind tunnel determinations. Applications of chemical stabilizers at two levels also considerably increased threshold friction velocity. On the restored crusts, threshold friction velocity dramatically increased, occasionally to non‐recordable values. This increase was monitored with both the salt crusts characteristic for the Solonchak soils and the fine‐grained crusts characteristic for the Takyr soils. The stability was attributed to the tightly packed salt particles in the salt crusts, and to the cohesive properties of the fine‐grained materials in the Takyr crusts. Once the crusts were ruptured, however, strong deflation commenced. These results suggest that by maintaining moisture in the soils/sediments (for example, by maintaining a high water table in the Amu‐Darya river flood plain) deflation can be reduced. By the same means, deflation can be reduced by creating new crusts or by preserving existing crusts. Copyright © 2005 John Wiley & Sons, Ltd.     

Evaporative losses from soils covered by physical and different types of biological soil crusts

Evaporation of soil moisture is one of the most important processes affecting water availability in semiarid ecosystems. Biological soil crusts, which are widely distributed ground cover in these ecosystems, play a recognized role on water processes. Where they roughen surfaces, water residence time and thus infiltration can be greatly enhanced, whereas their ability to clog soil pores or cap the soil surface when wetted can greatly decrease infiltration rate, thus affecting evaporative losses. In this work, we compared evaporation in soils covered by physical crusts, biological crusts in different developmental stages and in the soils underlying the different biological crust types. Our results show that during the time of the highest evaporation (Day 1), there was no difference among any of the crust types or the soils underlying them. On Day 2, when soil moisture was moderately low (11%), evaporation was slightly higher in well‐developed biological soil crusts than in physical or poorly developed biological soil crusts. However, crust removal did not cause significant changes in evaporation compared with the respective soil crust type. These results suggest that the small differences we observed in evaporation among crust types could be caused by differences in the properties of the soil underneath the biological crusts. At low soil moisture (<6%), there was no difference in evaporation among crust types or the underlying soils. Water loss for the complete evaporative cycle (from saturation to dry soil) was similar in both crusted and scraped soils. Therefore, we conclude that for the specific crust and soil types tested, the presence or the type of biological soil crust did not greatly modify evaporation with respect to physical crusts or scraped soils. Copyright © 2011 John Wiley & Sons, Ltd.     

Application of AVHRR to monitoring a climatically sensitive playa. case study: Chott El Djerid,Southern Tunisia

The importance of monitoring changes in the levels of lakes within endorheic basins using remotely sensed data as a means of assessing changes in regional aridity is noted. Large salt playas are highlighted as ephemeral lakes that can display extreme sensitivity to changes in regional rainfall patterns, and which commonly do not have extensively managed catchments. To explore the application of high temporal frequency monitoring of salt playas using remote sensing, the Chott el Djerid, a large salt playa situated in southern Tunisia was targeted. A short time series of 39 Advanced Very High Resolution Radiometer (AVHRR; resolution 1.1 km at nadir) images of the Chott el Djerid (spanning 36 months between 1987 and 1990) were compiled along with climate information from a weather station at Tozeur. Using image histogram manipulation, lake areas were extracted from the time series. A good level of agreement was observed between recorded rainfall events and the presence of surface water on the playa, and for a limited sample of large flood events it was found that there were significant relationships between rainfall, evaporation and estimated lake areas (r² = 98.5, p < 0.001). Overall, these data suggest that contemporary lake formation is largely controlled by temporal changes in effective precipitation within the basin. In addition, it was found that the coefficient of variation of the time series, and a combination of temporal reflectance profiles extracted from it, could be used to give a direct indication of which sedimentary surfaces on the playa are affected by large flood events, and the extent to which these events may be preserved within the recent sedimentary record at these sites. Copyright © 1999 John Wiley & Sons, Ltd.     

Evapotranspiration from the margin and moist playa of a closed desert valley

Two long term microclimate measurement stations with Bowen ratio capability have been used to study water cycling in a closed desert basin. Microclimate variables including the temperature and vapor pressure gradients were monitored continuously and were used to estimate the Bowen ratio, sensible and latent heat fluxes during 1986 and 1987. Despite having a water table that varied between the surface and 30 cm below the surface, the playa had little evaporation except after rainfall events. The very high osmotic pressure of the soil and salt crust caused most of the absorbed radiation to be partitioned to sensible heat. In contrast, along the margin the thin grass and brush cover transpired water freely, with the latent heat flux exceeding 60% of available energy for much of the season. The higher air temperatures above the playa raised potential evapotranspiration (ET) significantly higher than along the margin throughout the summer. The annual average actual ET of the playa was only 36% of the margin. During the drier summer period (May–October), this ratio decreased to < 28%. Immediately after a rainfall event, evaporation rates of the two sites were similar, but the playa rate was quickly reduced as the osmotic potential increased. During this study, the playa lost < 229 mm of subsurface water to evaporation annually, while > 638 mm were lost from the margin groundwater supply.

The 24-h solar and net radiation correlations were 0.80 and 0.94 for the playa and margin, respectively. The lower correlation for the playa resulted from the wide variation of albedo with surface moisture changes. The annual average albedo values for the playa and margin were 0.64 and 0.46, respectively.     

Microbiotic crusts as biomarkers for surface stability and wetness duration in the Negev Desert

Microbiotic crusts play an important role in arid and semi‐arid regions. Yet, very little information exists regarding the factors that impact their development. In an attempt to assess the main factors that may determine their growth, measurements of the amount of fines (silt and clay), rain, moisture content, wetness duration and wind erosion and deposition were carried out along a 12 station transect within a partially crusted dune field in the western Negev Desert and compared to the crust cover and chlorophyll content. Surface stability was the only variable that exhibited significant relationship with crust cover while daylight wetness duration exhibited strong positive relationship (r² = 0·92–0·99) with the crust's chlorophyll content. The data point out that microbiotic crusts may serve as a useful biomarker for surface stability. While wetness duration and wind will control crust cover and the crust chlorophyll content in semi‐stable habitats (with absolute annual change in sand level of 2–3 mm), stable habitats (absolute change <1 mm) will be controlled primarily by moisture, while habitats with low surface stability (absolute change of tens and hundreds of millimeters) will be primarily controlled by wind. Furthermore, owing to the strong positive relationship between daylight wetness duration and the crust's chlorophyll content, the crust may serve as a useful biomarker for the quantification of surface wetness duration. Copyright © 2009 John Wiley & Sons, Ltd.     

Effects of microbiotic crusts under cropland in temperate environments on soil erodibility during concentrated flow

Several studies illustrate the wind and water erosion‐reducing potential of semi‐permanent microbiotic soil crusts in arid and semi‐arid desert environments. In contrast, little is hitherto known on these biological crusts on cropland soils in temperate environments where they are annually destroyed by tillage and quickly regenerate thereafter. This study attempts to fill the research gap through (a) a field survey assessing the occurrence of biological soil crusts on loess‐derived soils in central Belgium in space and time and (b) laboratory flume (2 m long) experiments simulating concentrated runoff on undisturbed topsoil samples (0.4 × 0.1 m²) quantifying the microbiotic crust effect on soil erosion rates. Three stages of microbiotic crust development on cropland soils are distinguished: (1) development of a non‐biological surface seal by raindrop impact, (2) colonization of the soil by algae and gradual development of a continuous algal mat and (3) establishment of a well‐developed microbiotic crust with moss plants as the dominant life‐form. As the silt loam soils in the study area seal quickly after tillage, microbiotic soil crusts are more or less present during a large part of the year under maize, sugar beet and wheat, representing the main cropland area. On average, the early‐successional algae‐dominated crusts of stage 2 reduce soil detachment rates by 37%, whereas the well‐developed moss mat of stage 3 causes an average reduction of 79%. Relative soil detachment rates of soil surfaces with microbiotic crusts compared with bare sealed soil surfaces are shown to decrease exponentially with increasing microbiotic cover (b = 0·024 for moss‐dominated and b = 0·006 for algae‐dominated crusts). In addition to ground surface cover by vegetation and crop residues, microbiotic crust occurrence can therefore not be neglected when modelling small‐scale spatial and temporal variations in soil loss by concentrated flow erosion on cropland soils in temperate environments. Copyright © 2007 John Wiley & Sons, Ltd.     

Two‐hourly surface change on supra‐tidal rock (Marengo,Victoria, Australia)

A traversing micro‐erosion meter was used to measure rock surface micro‐topography over 40 cm² on a supra‐tidal cliff face from early morning to late evening in late spring. From 06:00 hours to 22:00 hours the relative heights of 188 coordinates were obtained using the meter at 2‐hour intervals, resulting in a data set of 1607 readings. Monitoring shows that rock surfaces are dynamic entities, with significant rise and fall relative to the first measurement at shorter timescales than previously reported. The maximum positive rise between readings was 0·261 mm and lowering was 0·126 mm. The pattern of change did not relate as expected to environmental variables such as temperature or insolation. Rather, the surface showed greater surface change in the early morning and late afternoon. It is hypothesized that this pattern relates to the expansion and contraction of lichen thalli as moisture is absorbed during higher humidity in the morning and late afternoon. The implications of these results for weathering studies are considered. Copyright © 2006 John Wiley & Sons, Ltd.     

Temporal aspects of the abrasion of microphytic crusts under grain impact

Wind‐tunnel simulations of the response of two moss crusts to grain impact indicate that, given sufficient time, these surfaces will deteriorate under very low wind velocities only slightly above u_*t for the loose, saltating grains. In parallel with these experiments, the frequency distributions of ultimate strength and penetration energy were determined for each of the two crust types via penetrometry. Pohlia was found to be stronger than Tortula; but, even so, both of these crusts had ultimate strengths 20–350 times higher than the force delivered by a single grain impacting each surface at a velocity of 1 ms^?1. In comparison, the modulus of deformation and penetration energy data were very similar for the two surface types, especially for the weakest areas of crust development. This observation is in accord with the wind‐tunnel simulations that also found no consistent difference in the response of these two crust types to impact. In comparison with crusts formed by clay and salt, fibrous microphytic crusts are morphologically complex and typically weak. The notable elasticity of these surfaces does reduce the force of grain impact, and thereby provides some protection against rupture. One of the central conclusions of this study suggests that not only is the particle kinetic energy at impact important in crust breakdown, but also tiny fractures at points of localized stress concentration contribute to a progressive reduction in the integrity of the filament net. In some of the experiments conducted as part of this study, up to 50 or more minutes of constant bombardment was required to produce small abrasion marks on selected areas of the microphytic crust. This study prepares a foundation for future experiments needed to examine the breakdown of complex crusts formed in nature. Copyright © 2002 John Wiley & Sons, Ltd.     

Effective run‐off flow length over biological soil crusts on silty loam soils in drylands

Surface hydrological behaviour is important in drylands because it affects the distribution of soil moisture and vegetation and the hydrological functioning of slopes and catchments. Microplot scale run‐off can be relatively easily measured, i.e. by rainfall simulations. However, slope or catchment run‐off cannot be deduced from microplots, requiring long‐time monitoring, because run‐off coefficients decrease with increasing drainage area. Therefore, to determine the slope length covered by run‐off (run‐off length) is crucial to connect scales. Biological soil crusts (BSCs) are good model systems, and their hydrology at slope scale is insufficiently known. This study provides run‐off lengths from BSCs, by field factorial experiments using rainfall simulation, including two BSC types, three rain types, three antecedent soil moistures and four plot lengths. Data were analysed by generalized linear modelling, including vascular plant cover as covariates. Results were the following: (i) the real contributing area is almost always much smaller than the topographical contributing area; (ii) the BSC type is key to controlling run‐off; run‐off length reached 3 m on cyanobacterial crust, but hardly over 1 m on lichen crust; this pattern remained through rain type or soil moisture; (iii) run‐off decreased with BSC development because soil sealing disappears; porosity, biomass and roughness increase and some changes occur in the uppermost soil layer; and (iv) run‐off flow increased with both rain type and soil moisture but run‐off coefficient only with soil moisture (as larger rains increased both run‐off and infiltration); vascular plant cover had a slight effect on run‐off because it was low and random. Copyright © 2014 John Wiley & Sons, Ltd.     

A multi‐method investigation of temperature,moisture and salt dynamics in tafoni (Tafraoute,Morocco)

Despite numerous investigations and theoretical models, tafoni weathering is still not fully understood largely because of limited data available on temperature, moisture and salt regimes. We investigated tafoni developed in granite in the Tafraoute region, Morocco, through an exploratory, two‐week multi‐method field campaign. Temperatures were measured with iButtons and by means of infrared thermography; moisture distribution and progress were captured with handheld moisture sensors and with drilled‐in iButtons. Salts were analysed in drill dust samples from different positions and rock depths. The results derived from very different techniques mutually support one another. Salts and moisture are concentrated near the base of the investigated tafoni, probably due to a saturated pore water body around the base of rock tors. Salts are accumulated close to the rock surface in tafoni, but not on the surrounding rock surfaces. A clear correlation was found between moisture and salt contents. Within a tafone, areas of higher humidity also display increased salt concentration near the surface. The temperature/humidity records allied with ionic analyses suggest that sodium sulfate dominates and is likely to undergo frequent phase changes from thenardite to mirabilite and vice versa. Two pathways of salt transport in and around tafoni are assumed based on the data: infiltration with rainfall on the top and around tors and boulders, and capillary rise from saturated pore water bodies to the surface. Copyright © 2015 John Wiley & Sons, Ltd.     

Rock surface millimetre‐scale roughness and weathering of supratidal Mallorcan carbonate coasts (Balearic Islands)

Quantitative data on rock surface millimetre‐scale roughness are presented concerning the splash and spray geomorphologic domains of two coastal profiles developed on Mediterranean carbonate rocks. Differences of the roughness characteristics are attributed to rock properties, weathering agents and bioerosion. In the splash zone, roughness is related to sparsely distributed patterns of bioerosion, salt weathering and wave attack. In the spray zone, smooth surfaces seem to be the response to the solution processes that predominate, exerting a more homogenous influence on rock surface evolution. Copyright © 2006 John Wiley & Sons, Ltd.     

Runoff generation in a sandy area—the nizzana sands,Western Negev,Israel

This paper presents the results of sprinkling experiments conducted over the lower part of vegetated and crusted linear dunes as well as over flat playa surfaces that appear in the interdune corridors. Data obtained show that these two surface units respond quickly to rainstorms. Runoff generation can be expected for any storm exceeding 2-3 mm and runoff coefficients are high. When the topsoil algal crust, 1 to 2 mm thick, is removed from the surface of the dune, infiltration increases drastically and eliminates any possibility of runoff generation under present-day rainfall conditions. This data may be of great help in the understanding of the geomorphology and sedimentary sequence of the corridors separting linear dunes.     

The role of crust thickness in runoff generation from microbiotic crusts

Contrary to humid areas where runoff takes place following the saturation of the soil column, runoff in arid and semiarid zones takes place when rain intensities exceed the infiltration capability of the upper soil crust, whether physical crust or microbiotic crust (MC). This type of overland flow, known as Hortonian overland flow (HOF), is not fully understood, especially in the case of MC. In particular, little is known regarding the effect of crust thickness and its fine (silt and clay) content on runoff generation, with some scholars claiming that runoff generation is positively correlated with crust thickness and fine content. In an attempt to determine the effect of crust thickness and to assess the role played by the silt and clay on runoff generation, a set of field and lab experiments were undertaken on MCs inhabiting sand dunes in the Negev Desert (Israel). These included sprinkling experiments coupled with measurements of the physical (thickness, silt and clay) and biological (chlorophyll, protein, total carbohydrates) properties of 0.5–10‐mm‐thick crusts. The data showed that runoff generation took place on surfaces as thin as ~0.5–0.7 mm only, and was not correlated with the fine (silt and clay) content. The implications for HOF and for arid ecosystems are discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Hydrochemical processes in ground water-discharge playas,Central Australia

A large groundwater system in the Amadeus Basin, central Australia, discharges to a chain of playa lakes 500 km long. The playas contain highly concentrated brines; these are sodium-chloride rich waters with appreciable magnesium and sulphate and very low concentrations of calcium and bicarbonate. Gypsum, glauberite, and other evaporite minerals are precipitating in the playas. The groundwaters evolve to brine by concurrent processes of dissolution, evaporative concentration, mineral precipitation, and mineralogical change. Chemical evolution is considered with reference to a concentration factor based on chloride. Ion transfer calculations demonstrate losses of magnesium and bicarbonate throughout, as a result of precipitation. Sodium, potassium, calcium, and sulphate are gained initially as a result of dissolution but lost subsequently as a result of precipitation. Larger playas in the chain, exemplified by Lake Amadeus, have dual shallow and deep groundwater flow paths whereas the smaller playas, exemplified by Spring Lake, have only shallow flow paths. Brines in the larger playas are diluted by deep groundwaters and this is reflected in the degree of saturation attained with respect to particular minerals. Thus, saturation with respect to gypsum and glauberite is attained earlier in Spring Lake than in Lake Amadeus. Saturation with respect to halite is attained in Spring Lake but not in Lake Amadeus. Both playas are undersaturated with respect to hexahydrite and sylvite although these minerals occur in efflorescent crusts in Spring Lake.