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Arid
Terms modified by Arid Selected AbstractsThe factors influencing the abrasion efficiency of saltating grains on a clay-crusted playaEARTH SURFACE PROCESSES AND LANDFORMS, Issue 5 2001Christopher A. Houser Abstract The entrainment and subsequent transport of PM10 (particulate matter <10,µm) has become an important and challenging focus of research for both scientific and practical applications. Arid and semi-arid environments are important sources for the atmospheric loading of PM10, although the emission of this material is often limited by surface crusts. It has been suggested that the primary mechanisms through which PM10 is released from a crusted surface are abrasion by saltating grains or disturbance by agricultural and recreational activities. To examine the importance of saltation abrasion in the emission of PM10, a series of field wind tunnel tests were conducted on a clay-crusted surface near Desert Wells, Arizona. In a previous part of this study it was found that the emission rate varies linearly with the saltation transport rate, although there can be considerable variation in this relationship. This paper more closely examines the source of the variability in the abrasion efficiency, the amount of PM10 emitted by a given quantity of saltating grains. The abrasion efficiency was found to vary with the susceptibility of the surface to abrasion, the ability of the sand to abrade that surface and the availability of material with a caliper size <10,µm within the crust. Specifically, the results of the study show that the abrasion efficiency is related to the crust strength, the amount of surface disturbance and the velocity of the saltating grains. It is concluded that the spatial and temporal variability of these controls on the abrasion efficiency imposes severe contextual limitations on experimentally derived models, and can make theoretical models too complex and impractical to be of use. Copyright© 2001 John Wiley & Sons, Ltd. [source] Detection of process-related changes in plant patterns at extended spatial scales during early dryland desertificationGLOBAL CHANGE BIOLOGY, Issue 11 2003Jorge Ares Abstract Arid and semiarid shrublands occupy extensive land areas over the world, are susceptible to desertification by anthropic use and can contribute to regional climate change. These prompt the interest to monitor and evaluate these lands adequately in order to detect early stages of degradation. Evaluation topics must refer to biology-relevant characteristics of these systems, while simultaneously satisfying sampling consistency over extended landscape areas. We present an analysis of process-relevant parameters related to changes in the spatial arrangement of the plant canopy of shrublands inferred from high-resolution panchromatic aerial photos and Interferometric Synthetic Aperture Radar imagery. We obtained low-altitude images systematically located along several gradients of land-use intensity in a Patagonian Monte shrubland in Argentina. Images were digitized to spatial resolutions ranging from 0.09 to 0.72 m (pixel size) and the average values and an-isotropic characteristics of the plant canopy patterns were quantified by means of a Fourier metric. We used radar-derived imagery to overlay the panchromatic images on a digital elevation model in order to study the correspondence of potential runoff patterns and the spatial arrangement of plants. We related an-isotropic features of the plant canopy images to the prevailing wind regime. Observed trends were further interpreted on the basis of a spatial-explicit simulation model describing the dynamics of the main functional groups in the plant community. We conclude that early stages of anthropic-driven dryland degradation in the Patagonian Monte can be characterized by the incipient un-coupling of spatial vegetation patterns from those of runoff at a landscape scale, and a progressive coupling to the spatial pattern of the wind regime. The method and metrics we present can be used to quantify early desertification changes in other similar drylands at extended spatial scales. [source] Global perspective on hydrology, water balance, and water resources management in arid basinsHYDROLOGICAL PROCESSES, Issue 2 2010Yanjun Shen Abstract Arid and semiarid regions comprise a large part of the world's terrestrial area and are home to hundreds of millions of people. Water resources in arid regions are rare and critical to society and to ecosystems. The hydrologic cycle in arid and semiarid regions has been greatly altered due to long-term human exploitation. Under conditions of global warming, water resources in these regions are expected to be more unstable and ecosystems likely will suffer from severe water stress. In the current special issue contributed to understanding ecohydrologic processes and water-related problems in arid regions of western China, this paper provides a global perspective on the hydrology and water balance of six major arid basins of the world. A number of global datasets, including the state-of-the-art ensemble simulation of land surface models by GSWP2 (Global Soil Wetness Project II, a project by GEWEX), were used to address the water balance terms in the world's major hydroclimatic regions. The common characteristics of hydrologic cycles and water balance in arid basins are as follows: strong evapotranspiration characterizes the hydrological cycle in arid basins; and in water use sectors irrigation consumes a large amount of water, resulting in degradation of native vegetation. From the ecohydrology viewpoint, a comprehensive study of hydrological and ecological processes of water utilization in arid basins is urgently needed. Copyright © 2009 John Wiley & Sons, Ltd. [source] Hydrograph and unit hydrograph derivation in arid regionsHYDROLOGICAL PROCESSES, Issue 8 2007Zekai Abstract Arid and semi-arid regions expose special hydrological features that are distinctive from humid areas. Unfortunately, humid-region hydrological empirical formulations are used directly in the arid and semi-arid regions without care about the basic assumptions. During any storm rainfall in arid regions, rainfall, infiltration and runoff components of the hydrological cycle have impacts on water resources. The basis of the methodology presented in this paper is the ratio of runoff increment to rainfall increment during an infinitesimally small time duration. This is the definition of runoff coefficient for the same infinitesimal time duration. The ratio is obtained through rational, physical and mathematical combination of hydrological thinking and then integrated with the classical infiltration equation for the hydrograph determination. The parameters of the methodology are explained and their empirical estimations are presented. The methodology works for rainfall and runoff from ungauged watersheds where infiltration measurement can be performed. The comparison of the new approach with different classical approaches, such as the rational formula and Soil Conservation Service method, are presented in detail. Its application is performed for two wadis within the Kingdom of Saudi Arabia. Copyright © 2006 John Wiley & Sons, Ltd. [source] Hydrological regime analysis of the Selenge River basin, MongoliaHYDROLOGICAL PROCESSES, Issue 14 2003X. Ma Abstract Arid and semi-arid regions are very vulnerable to environmental changes. Climate change studies indicate that the environment in such areas will steadily deteriorate with global warming; inland lakes will shrink and desert areas will expand. Mongolia is a landlocked country in north-central Asia that contains a unique ecological system consisting of taiga, steppe, and desert from north to south. The Selenge River basin (280 000 km2) in northern Mongolia is a semi-arid region underlain by permafrost, between latitudes 46 and 52°N, and longitudes 96 and 109°E. The issue of sustainable development of the basin is very important owing to its limited natural resources, including fresh water, forest, and rangeland. To examine the water cycle processes in the basin, a hydrological analysis was carried out using a simple scheme for the interaction between the land surface and atmosphere (big-leaf model) coupled to a hydrological model for the period 1988,92 to estimate the hydrological regime of the basin. Annual precipitation in this period averaged 298 mm, ranging from 212 to 352 mm at a 1 ° × 1 ° resolution based on data from 10 gauges, and the estimated annual evapotranspiration averaged 241 mm, ranging between 153 and 300 mm. This indicates that evapotranspiration accounts for the overwhelming majority of the annual precipitation, averaging 81% and ranging between 64 and 96%. The annual potential evapotranspiration in the basin averaged 2009 mm; the ratio of evapotranspiration (actual to potential evapotranspiration) was 0·12 and the wetness index (annual precipitation to potential evapotranspiration) was 0·15. Copyright © 2003 John Wiley & Sons, Ltd. [source] Structure of biological soil crust communities in Callitris glaucophylla woodlands of New South Wales, AustraliaJOURNAL OF VEGETATION SCIENCE, Issue 3 2006Wendy A. Thompson Abstract: Question: What is the nature of the relationships between cover, diversity and abundance of biological soil crusts, cover and diversity of vascular plants, and annual rainfall, soil texture and forestry practices in Callitris glaucophylla woodlands? Location: Arid and semi-arid Callitris glaucophylla -domi-nated woodlands of eastern Australia. Methods: We documented soil crust-forming mosses, lichens and liverworts at 83 woodland sites along a gradient of declining rainfall. Linear and non-linear regression were used to examine relationships between soil crust species and attributes of vascular plant communities, and a similarity matrix (species abundance X sites) was subjected to Non-metric Multi-Dimensional Scaling (MDS), and Analysis of Similarities (ANOSIM) to show the degree of association between groups of taxa, and soil texture, rainfall classes and forestry practices. Results: We collected 86 taxa. Mosses were dominated by the family Pottiaceae, and lichens were dominated by squamulose forms. Average annual rainfall was highly correlated with soil crust community composition, and loamy soils supported a greater cover and diversity of taxa compared with sandy soils. Increases in tree cover were associated with significant, though weak, increases in abundance, but not diversity, of crusts. Crusts tended to be more diverse in areas that (1) had a sparse cover of ground-storey plants; (2) were relatively stable - as indicated by the proportion of perennial and/or native plants; (3) had more stable soil surfaces; and (4) were unlogged. Litter cover, overstorey thinning, and livestock grazing had no appreciable effect on crust diversity or cover. Conclusions: Callitris glaucophylla woodlands provide substantial habitat for soil crust organisms, and the dense tree cover and closed canopies of Callitris do not appear to have a major influence on the structure of biological crust communities. Unlike other woodland systems, relatively few patches would be required to reserve a high diversity of crust species. [source] Microbiotic crusts as biomarkers for surface stability and wetness duration in the Negev DesertEARTH SURFACE PROCESSES AND LANDFORMS, Issue 12 2009Giora J. Kidron Abstract 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. [source] Techniques to measure the dry aeolian deposition of dust in arid and semi-arid landscapes: a comparative study in West NigerEARTH SURFACE PROCESSES AND LANDFORMS, Issue 2 2008Dirk Goossens Abstract Seven techniques designed to measure the dry aeolian deposition of dust on a desert surface were tested during field experiments in Niger, central-west Africa. Deposition fluxes were measured during eight periods of 3,4 days each. Experimental techniques tested were the MDCO (marble dust collector) method, the Frisbee method, the glass plate method (optical analysis of dust deposited on glass surfaces using particle imaging software), the soil surface method (deposition on a simulated desert floor) and the CAPYR (capteur pyramidal) method. Theoretical techniques tested were the inferential method and the combination method (gradient method extended with a deposition term for coarse dust particles). The results obtained by the MDCO, Frisbee, inferential and combination methods could be directly compared by converting the data to identical standard conditions (deposition on a water surface producing no resuspension). The results obtained by the other methods (glass plate, soil surface, CAPYR) were compared relatively. The study shows that the crude (unconverted) deposition fluxes of the five experimental techniques were similar, while the crude deposition fluxes calculated by the two theoretical techniques were substantially higher, of the order of four to five times as high as for the experimental techniques. Recalculation of the data to identical environmental conditions (the standard water surface) resulted in nearly identical deposition fluxes for the MDCO, Frisbee, inferential and combination techniques, although the latter two still had slightly higher values (but the differences remained small). The measurements illustrate the need to include a grain shape factor in theoretical dust deposition models. Without such a factor, theoretical models overestimate the deposition. The paper also discusses the advantages and disadvantages of the techniques tested. Copyright © 2007 John Wiley & Sons, Ltd. [source] Effects of spatially structured vegetation patterns on hillslope erosion in a semiarid Mediterranean environment: a simulation studyEARTH SURFACE PROCESSES AND LANDFORMS, Issue 2 2005Matthias Boer Abstract A general trend of decreasing soil loss rates with increasing vegetation cover fraction is widely accepted. Field observations and experimental work, however, show that the form of the cover-erosion function can vary considerably, in particular for low cover conditions that prevail on arid and semiarid hillslopes. In this paper the structured spatial distribution of the vegetation cover and associated soil attributes is proposed as one of the possible causes of variation in cover,erosion relationships, in particular in dryland environments where patchy vegetation covers are common. A simulation approach was used to test the hypothesis that hillslope discharge and soil loss could be affected by variation in the spatial correlation structure of coupled vegetation cover and soil patterns alone. The Limburg Soil Erosion Model (LISEM) was parameterized and verified for a small catchment with discontinuous vegetation cover at Rambla Honda, SE Spain. Using the same parameter sets LISEM was subsequently used to simulate water and sediment fluxes on 1 ha hypothetical hillslopes with simulated spatial distributions of vegetation and soil parameters. Storms of constant rainfall intensity in the range of 30,70 mm h,1 and 10,30 min duration were applied. To quantify the effect of the spatial correlation structure of the vegetation and soil patterns, predicted discharge and soil loss rates from hillslopes with spatially structured distributions of vegetation and soil parameters were compared with those from hillslopes with spatially uniform distributions. The results showed that the spatial organization of bare and vegetated surfaces alone can have a substantial impact on predicted storm discharge and erosion. In general, water and sediment yields from hillslopes with spatially structured distributions of vegetation and soil parameters were greater than from identical hillslopes with spatially uniform distributions. Within a storm the effect of spatially structured vegetation and soil patterns was observed to be highly dynamic, and to depend on rainfall intensity and slope gradient. Copyright © 2005 John Wiley & Sons, Ltd. [source] Late Quaternary vadose carbonate diagenesis in coastal and desert dune and beach sands: is there a palaeoclimatic signal?EARTH SURFACE PROCESSES AND LANDFORMS, Issue 12 2004Sue McLaren Abstract A study of the variation in cement amounts in aeolianite and sandy beach calcarenites that range in age from Holocene to Last Interglacial is used to assess whether a palaeoclimatic control on vadose diagenesis can be identi,ed. Examples are taken from modern-day arid to subhumid settings and represent a geographical distribution ranging from Libya and Oman, to the Mediterranean, the Caribbean and Mexico. The results indicate that a palaeoclimatic signal can be identi,ed in some deposits. However, in other sediments there is substantial variability in mean cement abundance within deposits as well as between these sand bodies. Moreover, many of the better cemented sediments are located in areas where there is accelerated diagenesis, such as in the sea spray zone, close to the groundwater table or near to a palaeosurface, rather than in climatically wet regions. The inference is that palaeoclimatic interpretations are substantially complicated by other factors that affect diagenetic processes and change. Therefore caution is needed when studying the role of climate in vadose diagenesis in the light of the effects of other intrinsic and extrinsic controls. This paper does not aim to provide a de,nitive comparison of sites from different climatic zones. The approach taken here is: (a) to see if there is an overall palaeoclimatic signal in the samples studied; (b) to use examples to illustrate how explanations/controls other than those relating to climate can account for the variabilities observed; and (c) if there is a climatic effect, to see if it is the main over-riding control on vadose diagenesis. The conclusions drawn from this research highlight that it is possible to misinterpret evidence if the study is approached with preconceived notions of simplistic relationships between diagenesis and climate. Copyright © 2004 John Wiley & Sons, Ltd. [source] Woody plants modulate the temporal dynamics of soil moisture in a semi-arid mesquite savanna,ECOHYDROLOGY, Issue 1 2010Daniel L. Potts Abstract Climate variability and human activities interact to increase the abundance of woody plants in arid and semi-arid ecosystems worldwide. How woody plants interact with rainfall to influence patterns of soil moisture through time, at different depths in the soil profile and between neighboring landscape patches is poorly known. In a semi-arid mesquite savanna, we deployed a paired array of sensors beneath a mesquite canopy and in an adjacent open area to measure volumetric soil water content (,) every 30 min at several depths between 2004 and 2007. In addition, to quantify temporally dynamic variation in soil moisture between the two microsites and across soil depths we analysed , time-series using fast Fourier transforms (FFT). FFT analyses were consistent with the prediction that by reducing evaporative losses through shade and reducing rainfall inputs through canopy interception of small rainfall events, the mesquite canopy was associated with a decline in high-frequency (hour-to-hour and day-to-day) variation in shallow ,. Finally, we found that, in both microsites, high-frequency , variation declined with increasing soil depth as the influence of evaporative losses and inputs associated with smaller rainfall events declined. In this case, we argue that the buffering of shallow soil moisture against high-frequency variations can enhance nutrient cycling and alter the carbon cycle in dryland ecosystems. Copyright © 2009 John Wiley & Sons, Ltd. [source] Distribution, zoogeography and biology of the Murchison River hardyhead (Craterocephalus cuneiceps Whitley, 1944), an atherinid endemic to the Indian Ocean (Pilbara) Drainage Division of Western AustraliaECOLOGY OF FRESHWATER FISH, Issue 3 2005M. G. Allen Abstract , The Murchison River hardyhead (Craterocephalus cuneiceps) is endemic to the extremely arid Indian Ocean (Pilbara) Drainage Division of Western Australia, where it is found in the Greenough, Hutt, Murchison, Wooramel, Gascoyne and DeGrey rivers, but is absent from numerous rivers within its range. The most likely explanation for the disjunct contemporary distribution is that C. cuneiceps has simply never inhabited the rivers from which it is conspicuously absent (e.g. Ashburton and Fortescue). Biogeographical, geological and palaeoclimatic evidence is presented to support this hypothesis. In the Murchison River, breeding was extremely protracted with recruitment occurring throughout the year. The largest female and male specimens captured were 96 mm total length (TL; 7.73 g) and 86 mm TL (5.57 g), respectively. Sex ratio was 1.09 females:1 male. Batch fecundity ranged from 46 to 454 (mean 167.5 ± 25.7 SE). Estimates for the length at which 50 and 95% of females first spawned were 36.4 and 44.3 mm TL, respectively. Craterocephalus cuneiceps is essentially a detritivore, but also feeds on aquatic invertebrates. Rainfall in the Murchison River catchment is unpredictable and pH, salinity and temperature are variable. A specialised diet, small size and young age at maturity and protracted spawning period, coupled with serial spawning and high fecundity, allows the numerical dominance of this species in competitive, harsh, arid and unpredictable desert environments. Resumen 1. Craterocephalus cuneiceps es una especie endémica de las cuencas del Océano Indico (i.e., Pilbara) de Australia Occidental. Se encuentra en los ríos Greenough, Hutt, Murchison, Wooramel, Gascoyne y DeGrey pero está ausente en numerosos ríos dentro de su área de distribución. La explicación más probable para esta distribución separada en la actualidad es que C. cuneiceps no ha habitado nunca los ríos en los que está ausente tales como los ríos Ashburton y Fortescue. Presentamos evidencia bio-geográfica, geológica y paleo-climática para soportar esta hipótesis. 2. En el río Murchison, la reproducción es extremadamente prolongada con reclutamiento a lo largo de todo el año. Los mayores machos y hembras capturados alcanzaron 96 mm LT (7.73 g) y 86 mm LT (5.57 g), respectivamente. La proporción de sexos fue 1.09 hembras: 1 macho. La fecundidad varió entre 46 y 454 (media 167.5 ± 25.7 SE) y la longitudes a la que el 50 y el 95% de las hembras se reproducen por primera vez alcanzaron 36.4 y 44.3 mm LT, respectivamente. 3. C. cuneiceps es esencialmente detritívoro pero también se alimenta de invertebrados acuáticos. La lluvia sobre la cuenca del río Murchison es impredecible y el pH, la salinidad y la temperatura son variables. Una dieta especializada, pequeño tamaño, una edad joven en la madurez, y un período reproductivo prolongado, ademos de una freza seriada y alta fecundidad, permiten la dominancia numérica de la especie en ambientes competitivos, duros, áridos e impredecibles. [source] Limits of life in hostile environments: no barriers to biosphere function?ENVIRONMENTAL MICROBIOLOGY, Issue 12 2009Jim P. Williams Summary Environments that are hostile to life are characterized by reduced microbial activity which results in poor soil- and plant-health, low biomass and biodiversity, and feeble ecosystem development. Whereas the functional biosphere may primarily be constrained by water activity (aw) the mechanism(s) by which this occurs have not been fully elucidated. Remarkably we found that, for diverse species of xerophilic fungi at aw values of , 0.72, water activity per se did not limit cellular function. We provide evidence that chaotropic activity determined their biotic window, and obtained mycelial growth at water activities as low as 0.647 (below that recorded for any microbial species) by addition of compounds that reduced the net chaotropicity. Unexpectedly we found that some fungi grew optimally under chaotropic conditions, providing evidence for a previously uncharacterized class of extremophilic microbes. Further studies to elucidate the way in which solute activities interact to determine the limits of life may lead to enhanced biotechnological processes, and increased productivity of agricultural and natural ecosystems in arid and semiarid regions. [source] In situ measurement of methane fluxes and analysis of transcribed particulate methane monooxygenase in desert soilsENVIRONMENTAL MICROBIOLOGY, Issue 10 2009Roey Angel Summary Aerated soils are a biological sink for atmospheric methane. However, the activity of desert soils and the presence of methanotrophs in these soils have hardly been studied. We studied on-site atmospheric methane consumption rates as well as the diversity and expression of the pmoA gene, coding for a subunit of the particulate methane monooxygenase, in arid and hyperarid soils in the Negev Desert, Israel. Methane uptake was only detected in undisturbed soils in the arid region (,90 mm year,1) and vertical methane profiles in soil showed the active layer to be at 0,20 cm depth. No methane uptake was detected in the hyperarid soils (,20 mm year,1) as well as in disturbed soils in the arid region (i.e. agricultural field and a mini-catchment). Molecular analysis of the methanotrophic community using terminal restriction fragment length polymorphism (T-RFLP) and cloning/sequencing of the pmoA gene detected methanotrophs in the active soils, whereas the inactive ones were dominated by sequences of the homologous gene amoA, coding for a subunit of the ammonia monooxygenase. Even in the active soils, methanotrophs (as well as in situ activity) could not be detected in the soil crust, which is the biologically most important layer in desert soils. All pmoA sequences belonged to yet uncultured strains. Transcript analysis showed dominance of sequences clustering within the JR3, formerly identified in Californian grassland soils. Our results show that although active methanotrophs are prevalent in arid soils they seem to be absent or inactive in hyperarid and disturbed arid soils. Furthermore, we postulate that methanotrophs of the yet uncultured JR3 cluster are the dominant atmospheric methane oxidizers in this ecosystem. [source] Chlorophyll fluorescence, predawn water potential and photosynthesis in precipitation pulse-driven ecosystems , implications for ecological studiesFUNCTIONAL ECOLOGY, Issue 3 2008V. Resco Summary 1A major research focus in population and community ecology is to establish a mechanistic understanding of plant interactions and demographic responses. The first step towards this mechanistic approach relies on understanding the differences in stress caused by different environmental conditions. Leaf-level photosynthetic rate (A) within and among plant populations provides important insight into population and community processes, but is difficult to acquire with sufficient replication under field conditions. Instead, chlorophyll fluorescence (Fv/Fm) and predawn water potential (,pd) are often used in arid and semi-arid ecosystems. 2Fv/Fm reflects the photoactivation status of photosystem II (PSII), whereas ,pd indicates water availability in the rhizosphere. Here we compare these indices with A in two perennial C4 grasses (native Heteropogon contortus and invasive Eragrostis lehmanniana) and in seedlings of the C3 shrub Prosopis velutina growing on highly contrasting sandy loam and loamy clay soils in experimental plots. Measurements were made the day prior to and up to 7 days following a 39-mm rainfall pulse after 2 months of drought. 3A was more sensitive across a broad range of environmental conditions, whereas Fv/Fm and ,pd only responded to periods of protracted drought. The use of these measures was further complicated because their values varied daily and we observed different time-lags in their response to precipitation pulses. 4We suggest sampling schemes and a priori measurements to capture the value that is representative for the question of interest, and that match the pulsed biological activity in these ecosystems. Finally, we suggest the use of these measures in combination with measurements providing integration over longer time periods, such as ,13C, ,18O and N concentration in bulk leaf tissue. [source] Freshwater availability as the constraining factor in the Middle Paleoindian occupation of North-Central FloridaGEOARCHAEOLOGY: AN INTERNATIONAL JOURNAL, Issue 3 2009David K. Thulman The locations of reliable surface water exposures during the Middle Paleoindian period (10,800,10,200 14C yr B.P.) in north-central Florida are reconstructed and compared to the concentrations of Middle Paleoindian projectile points. Estimates of water table levels and surface water flow in Florida's karst geology confirm prior climate reconstructions for that time indicating the area was arid and supported a xeric ecology in most upland locales. Surface water flow data from recent extreme droughts and water table estimations are used to identify the areas of highest probability for surface water availability. The distribution of the highest concentrations of Middle Paleoindian points correlates with the areas of highest probability, indicating that scarce surface water sources were the strongest constraint on occupation location during that time. © 2009 Wiley Periodicals, Inc. [source] Understanding Dryland Landscape Dynamics: Do Biological Crusts Hold the Key?GEOGRAPHY COMPASS (ELECTRONIC), Issue 3 2008Heather A. Viles Understanding landscape dynamics in arid and semi-arid areas is becoming increasingly important, as global change threatens to upset linked ecological and geomorphological systems with potentially serous impacts on livelihoods and environments. Biological crusts (composed of lichens, algae, fungi and bacteria) cover many rock, soil and sediment surfaces in arid and semi-arid areas and provide a key to understanding future dryland landscape dynamics. Such crusts have been found to play a number of key geomorphic and ecological roles, and are identified as important ecosystem engineers and biogeomorphological agents that could be used in environmental restoration. However, they have also been reported to be highly fragile and susceptible to disturbance, and damage to them may result in non-linear consequences for linked dryland geomorphological and ecological systems. This paper outlines the current state of knowledge on biological crusts in arid and semi-arid areas and calls for increased collaboration between geomorphologists and ecologists and better links between studies of biological crusts on rock and soil surfaces. [source] Desert environments: landscapes and stratigraphyGEOLOGY TODAY, Issue 5 2009Peter G. Fookes It is common to think of hot deserts, i.e. hot arid or dry lands, as areas of little rain situated in the middle parts of the world, that are simply ,just there'. However, most of the world's deserts have a long geological history, sometimes of 50 million years or more and ways have been developing for some time now, particularly from geomorphological studies, of not only erecting the law of superposition of strata for the desert but also ,absolute' dating. The authors have often worked commercially in deserts world-wide but their recent experiences in the Oman have brought home to them the excellent work that has been going on in the last two or three decades in evaluating the geological history of deserts. The Oman experience is described in a feature in the next issue. [source] Soil inorganic carbon storage pattern in ChinaGLOBAL CHANGE BIOLOGY, Issue 10 2008NA MI Abstract Soils with pedogenic carbonate cover about 30% (3.44 × 106 km2) of China, mainly across its arid and semiarid regions in the Northwest. Based on the second national soil survey (1979,1992), total soil inorganic carbon (SIC) storage in China was estimated to be 53.3±6.3 PgC (1 Pg=1015 g) to the depth investigated to 2 m. Soil inorganic carbon storages were 4.6, 10.6, 11.1, and 20.8 Pg for the depth ranges of 0,0.1, 0.1,0.3, 0.3,0.5, and 0.5,1 m, respectively. Stocks for 0.1, 0.3, 0.5, and 1 m of depth accounted for 8.7%, 28.7%, 49.6%, and 88.9% of total SIC, respectively. In contrast with soil organic carbon (SOC) storage, which is highest under 500,800 mm yr,1 of mean precipitation, SIC storage peaks where mean precipitation is <400 mm yr,1. The amount and vertical distribution of SIC was related to climate and land cover type. Content of SIC in each incremental horizon was positively related with mean annual temperature and negatively related with mean annual precipitation, with the magnitude of SIC content across land cover types showing the following order: desert, grassland >shrubland, cropland >marsh, forest, meadow. Densities of SIC increased generally with depth in all ecosystem types with the exception of deserts and marshes where it peaked in intermediate layers (0.1,0.3 m for first and 0.3,0.5 m for latter). Being an abundant component of soil carbon stocks in China, SIC dynamics and the process involved in its accumulation or loss from soils require a better understanding. [source] Large annual net ecosystem CO2 uptake of a Mojave Desert ecosystemGLOBAL CHANGE BIOLOGY, Issue 7 2008GEORG WOHLFAHRT Abstract The net ecosystem CO2 exchange (NEE) between a Mojave Desert ecosystem and the atmosphere was measured over the course of 2 years at the Mojave Global Change Facility (MGCF, Nevada, USA) using the eddy covariance method. The investigated desert ecosystem was a sink for CO2, taking up 102±67 and 110±70 g C m,2 during 2005 and 2006, respectively. A comprehensive uncertainty analysis showed that most of the uncertainty of the inferred sink strength was due to the need to account for the effects of air density fluctuations on CO2 densities measured with an open-path infrared gas analyser. In order to keep this uncertainty within acceptable bounds, highest standards with regard to maintenance of instrumentation and flux measurement postprocessing have to be met. Most of the variability in half-hourly NEE was explained by the amount of incident photosynthetically active radiation (PAR). On a seasonal scale, PAR and soil water content were the most important determinants of NEE. Precipitation events resulted in an initial pulse of CO2 to the atmosphere, temporarily reducing NEE or even causing it to switch sign. During summer, when soil moisture was low, a lag of 3,4 days was observed before the correlation between NEE and precipitation switched from positive to negative, as opposed to conditions of high soil water availability in spring, when this transition occurred within the same day the rain took place. Our results indicate that desert ecosystem CO2 exchange may be playing a much larger role in global carbon cycling and in modulating atmospheric CO2 levels than previously assumed , especially since arid and semiarid biomes make up >30% of Earth's land surface. [source] Herbivory and plant growth rate determine the success of El Niño Southern Oscillation-driven tree establishment in semiarid South AmericaGLOBAL CHANGE BIOLOGY, Issue 12 2006MILENA HOLMGREN Abstract While climatic extremes are predicted to increase with global warming, we know little about the effect of climatic variability on biome distribution. Here, we show that rainy El Niño Southern Oscillation (ENSO) events can enhance tree recruitment in the arid and semiarid ecosystems of north-central Chile and northwest Peru. Tree-ring studies in natural populations revealed that rainy El Niño episodes have triggered forest regeneration in Peru. Field experiments indicate that tree seedling recruitment in Chile is much less successful than in Peru due mostly to larger mortality caused by herbivores. The dramatic impact of herbivores in Chile was derived from the combined result of slower plant growth and the presence of exotic herbivores (European rabbits and hares). The interplay of herbivory and climatic effects we demonstrated implies that rainy ENSO events may represent ,windows of opportunity' for forest recovery if herbivore pressure is minimized at the right moment. [source] Climatic signals in growth and its relation to ENSO events of two Prosopis species following a latitudinal gradient in South AmericaGLOBAL CHANGE BIOLOGY, Issue 5 2006BERNAT C. LÓPEZ Abstract Semiarid environments throughout the world have lost a major part of their woody vegetation and biodiversity due to the effects of wood cutting, cattle grazing and subsistence agriculture. The resulting state is typically used for cattle production, but the productivity of these systems is often very low, and erosion of the unprotected soil is a common problem. Such dry-land degradation is of great international concern, not only because the resulting state is hardly productive but also because it paves the way to desertification. The natural distribution of the genus Prosopis includes arid and semiarid zones of the Americas, Africa and Asia, but the majority of the Prosopis species are, however, native to the Americas. In order to assess a likely gradient in the response of tree species to precipitation, temperature and their connection to El Niño southern oscillation (ENSO) events, two Prosopis species were chosen along a latitudinal gradient in Latin America, from northern Peru to central Chile: Prosopis pallida from a semi-arid land in northern and southern Peru and P. chilensis from a semiarid land in central Chile. Growth rings of each species were crossdated at each sampling site using classical dendrochronological techniques. Chronologies were related with instrumental climatic records in each site, as well as with SOI and N34 series. Cross-correlation, spectral and wavelet analysis techniques were used to assess the relation of growth with precipitation and temperature. Despite the long distance among sites, the two Prosopis species presented similar responses. Thus, the two species' growth is positively correlated to precipitation, while with temperature it is not. In northern Peru, precipitation and growth of P. pallida present a similar cyclic pattern, with a period of around 3 years. On the other hand, P. pallida in southern Peru, and P. chilensis also present this cyclic pattern, but also another one with lower frequency, coinciding with the pattern of precipitation. Both cycles are within the range of the ENSO band. [source] Net changes in regional woody vegetation cover and carbon storage in Texas Drylands, 1937,1999GLOBAL CHANGE BIOLOGY, Issue 3 2003GREGORY P. ASNER Abstract Although local increases in woody plant cover have been documented in arid and semiarid ecosystems worldwide, there have been few long-term, large-scale analyses of changes in woody plant cover and aboveground carbon (C) stocks. We used historical aerial photography, contemporary Landsat satellite data, field observations, and image analysis techniques to assess spatially specific changes in woody vegetation cover and aboveground C stocks between 1937 and 1999 in a 400-km2 region of northern Texas, USA. Changes in land cover were then related to topo-edaphic setting and historical land-use practices. Mechanical or chemical brush management occurred over much of the region in the 1940,1950s. Rangelands not targeted for brush management experienced woody cover increases of up to 500% in 63 years. Areas managed with herbicides, mechanical treatments or fire exhibited a wide range of woody cover changes relative to 1937 (,75% to + 280%), depending on soil type and time since last management action. At the integrated regional scale, there was a net 30% increase in woody plant cover over the 63-year period. Regional increases were greatest in riparian corridors (33%) and shallow clay uplands (26%) and least on upland clay loams (15%). Allometric relationships between canopy cover and aboveground biomass were used to estimate net aboveground C storage changes in upland (nonriparian) portions of regional landscapes. Carbon stocks increased from 380 g C m,2 in 1937 to 500 g C m,2 in 1999, a 32% net increase across the 400 km2 region over the 63-year period. These plant C storage change estimates are highly conservative in that they did not include the substantial increases in woody plant cover observed within riparian landscape elements. Results are discussed in terms of implications for ,carbon accounting' and the global C cycle. [source] Large-scale pattern of biomass partitioning across China's grasslandsGLOBAL ECOLOGY, Issue 2 2010Yuanhe Yang ABSTRACT Aim, To investigate large-scale patterns of above-ground and below-ground biomass partitioning in grassland ecosystems and to test the isometric theory at the community level. Location, Northern China, in diverse grassland types spanning temperate grasslands in arid and semi-arid regions to alpine grasslands on the Tibetan Plateau. Methods, We investigated above-ground and below-ground biomass in China's grasslands by conducting five consecutive sampling campaigns across the northern part of the country during 2001,05. We then documented the root : shoot ratio (R/S) and its relationship with climatic factors for China's grasslands. We further explored relationships between above-ground and below-ground biomass across different grassland types. Results, Our results indicated that the overall R/S of China's grasslands was larger than the global average (6.3 vs. 3.7). The R/S for China's grasslands did not show any significant trend with either mean annual temperature or mean annual precipitation. Above-ground biomass was nearly proportional to below-ground biomass with a scaling exponent (the slope of log,log linear relationship between above-ground and below-ground biomass) of 1.02 across various grassland types. The slope did not differ significantly between temperate and alpine grasslands or between steppe and meadow. Main conclusions, Our findings support the isometric theory of above-ground and below-ground biomass partitioning, and suggest that above-ground biomass scales isometrically with below-ground biomass at the community level. [source] Geochemical Tracers to Evaluate Hydrogeologic Controls on River SalinizationGROUND WATER, Issue 3 2008Stephanie J. Moore The salinization of rivers, as indicated by salinity increases in the downstream direction, is characteristic of arid and semiarid regions throughout the world. Historically, salinity increases have been attributed to various mechanisms, including (1) evaporation and concentration during reservoir storage, irrigation, and subsequent reuse; (2) displacement of shallow saline ground water during irrigation; (3) erosion and dissolution of natural deposits; and/or (4) inflow of deep saline and/or geothermal ground water (ground water with elevated water temperature). In this study, investigation of salinity issues focused on identification of relative salinity contributions from anthropogenic and natural sources in the Lower Rio Grande in the New Mexico-Texas border region. Based on the conceptual model of the system, the various sources of water and, therefore, salinity to the Lower Rio Grande were identified, and a sampling plan was designed to characterize these sources. Analysis results for boron (,11B), sulfur (,34S), oxygen (,18O), hydrogen (,2H), and strontium (87Sr/86Sr) isotopes, as well as basic chemical data, confirmed the hypothesis that the dominant salinity contributions are from deep ground water inflow to the Rio Grande. The stable isotopic ratios identified the deep ground water inflow as distinctive, with characteristic isotopic signatures. These analyses indicate that it is not possible to reproduce the observed salinization by evapotranspiration and agricultural processes alone. This investigation further confirms that proper application of multiple isotopic and geochemical tracers can be used to identify and constrain multiple sources of solutes in complex river systems. [source] Salinization of a Fresh Palaeo-Ground Water Resource by Enhanced RechargeGROUND WATER, Issue 1 2003F.W. Leaney Deterioration of fresh ground water resources caused by salinization is a growing issue in many arid and semi-arid parts of the world. We discuss here the incipient salinization of a 104 km2 area of fresh ground water (<3000 mg/L) in the semiarid Murray Basin of Australia caused by widespread changes in land use. Ground water 14C concentrations and unsaturated zone Cl soil water inventories indicate that the low salinity ground water originated mainly from palaeo-recharge during wet climatic periods more than 20,000 years ago. However, much of the soil water in the 20 to 60 m thick unsaturated zone throughout the area is generally saline (> 15,000 mg/L) because of relatively high evapotranspiration during the predominantly semiarid climate of the last 20,000 years. Widespread clearing of native vegetation over the last 100 years and replacement with crops and pastures leads to enhancement of recharge rates that progressively displace the saline soil-water from the unsaturated zone into the ground water. To quantify the impact of this new hydrologic regime, a one-dimensional model that simulates projected ground water salinities as a function of depth to ground water, recharge rates, and soil water salt inventory was developed. Results from the model suggest that, in some areas, the ground water salinity within the top 10 m of the water table is likely to increase by a factor of 2 to 6 during the next 100 years. Ground water quality will therefore potentially degrade beyond the point of usefulness well before extraction of the ground water exhausts the resource. [source] Global perspective on hydrology, water balance, and water resources management in arid basinsHYDROLOGICAL PROCESSES, Issue 2 2010Yanjun Shen Abstract Arid and semiarid regions comprise a large part of the world's terrestrial area and are home to hundreds of millions of people. Water resources in arid regions are rare and critical to society and to ecosystems. The hydrologic cycle in arid and semiarid regions has been greatly altered due to long-term human exploitation. Under conditions of global warming, water resources in these regions are expected to be more unstable and ecosystems likely will suffer from severe water stress. In the current special issue contributed to understanding ecohydrologic processes and water-related problems in arid regions of western China, this paper provides a global perspective on the hydrology and water balance of six major arid basins of the world. A number of global datasets, including the state-of-the-art ensemble simulation of land surface models by GSWP2 (Global Soil Wetness Project II, a project by GEWEX), were used to address the water balance terms in the world's major hydroclimatic regions. The common characteristics of hydrologic cycles and water balance in arid basins are as follows: strong evapotranspiration characterizes the hydrological cycle in arid basins; and in water use sectors irrigation consumes a large amount of water, resulting in degradation of native vegetation. From the ecohydrology viewpoint, a comprehensive study of hydrological and ecological processes of water utilization in arid basins is urgently needed. Copyright © 2009 John Wiley & Sons, Ltd. [source] Hydrograph and unit hydrograph derivation in arid regionsHYDROLOGICAL PROCESSES, Issue 8 2007Zekai Abstract Arid and semi-arid regions expose special hydrological features that are distinctive from humid areas. Unfortunately, humid-region hydrological empirical formulations are used directly in the arid and semi-arid regions without care about the basic assumptions. During any storm rainfall in arid regions, rainfall, infiltration and runoff components of the hydrological cycle have impacts on water resources. The basis of the methodology presented in this paper is the ratio of runoff increment to rainfall increment during an infinitesimally small time duration. This is the definition of runoff coefficient for the same infinitesimal time duration. The ratio is obtained through rational, physical and mathematical combination of hydrological thinking and then integrated with the classical infiltration equation for the hydrograph determination. The parameters of the methodology are explained and their empirical estimations are presented. The methodology works for rainfall and runoff from ungauged watersheds where infiltration measurement can be performed. The comparison of the new approach with different classical approaches, such as the rational formula and Soil Conservation Service method, are presented in detail. Its application is performed for two wadis within the Kingdom of Saudi Arabia. Copyright © 2006 John Wiley & Sons, Ltd. [source] Three-parameter discontinuous distributions for hydrological samples with zero valuesHYDROLOGICAL PROCESSES, Issue 15 2005Stanislaw Weglarczyk Abstract A consistent approach to the frequency analysis of hydrologic data in arid and semiarid regions, i.e. the data series containing several zero values (e.g. monthly precipitation in dry seasons, annual peak flow discharges, etc.), requires using discontinuous probability distribution functions. Such an approach has received relatively limited attention. Along the lines of physically based models, the extensions of the Muskingum-based models to three parameter forms are considered. Using 44 peak flow series from the USGS data bank, the fitting ability of four three-parameter models was investigated: (1) the Dirac delta combined with Gamma distribution; (2) the Dirac delta combined with two-parameter generalized Pareto distribution; (3) the Dirac delta combined with two-parameter Weibull (DWe) distribution; (4) the kinematic diffusion with one additional parameter that controls the probability of the zero event (KD3). The goodness of fit of the models was assessed and compared both by evaluation of discrepancies between the results of both estimation methods (i.e. the method of moments (MOM) and the maximum likelihood method (MLM)) and using the log of likelihood function as a criterion. In most cases, the DWe distribution with MLM-estimated parameters showed the best fit of all the three-parameter models. Copyright © 2005 John Wiley & Sons, Ltd. [source] The geochemical characteristics of the Paraná River suspended sediment load: an initial assessmentHYDROLOGICAL PROCESSES, Issue 7 2003Pedro J. Depetris Abstract Most water in the Paraná River drainage basin is supplied by the tropical Upper Paraná (over 60% of the total annual water discharge, 550 km3). The total suspended solids (TSS) load (c. 80 × 106 t year,1), however, is essentially furnished (50,70%) by the mountainous, arid and mostly sediment-mantled upper Bermejo River drainage basin. This characteristic suggests that the Paraná River solid load (TSS, 600 km upstream from the mouth) is largely recycled sedimentary material, whose discharge-weighted mean chemical index of alteration is c. 71. The extended UCC-normalized multi-elemental diagrams are similar to those of other world rivers. Nevertheless, the detailed inspection of UCC-normalized rare earth element (REE) ,spidergrams' reveals a lithological source for the Paraná River TSS that might be compatible with either tholeiitic flood basalts (widespread in the upper drainage) or with young Andean intermediate volcanic rocks. In view of the Bermejo River's dominant role as a sediment contributor, we feel that the signature preserved in the Paraná's TSS is the latter. Conversely, the Uruguay River TSS REE signature is certainly determined by the extensive weathering products of Jurassic,Cretaceous tholeiitic basalts. Copyright © 2003 John Wiley & Sons, Ltd. [source] | |||||||||||||||||||||||||||||||