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1.
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.  相似文献   

2.
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.  相似文献   

3.
Giora J. Kidron 《水文研究》2015,29(7):1783-1792
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.  相似文献   

4.
Water is a major limiting factor in arid and semi‐arid agriculture. In the Sahelian zone of Africa, it is not always the limited amount of annual rainfall that constrains crop production, but rather the proportion of rainfall that enters the root zone and becomes plant‐available soil moisture. Maximizing the rain‐use efficiency and therefore limiting overland flow is an important issue for farmers. The objectives of this research were to model the processes of infiltration, runoff and subsequent erosion in a Sahelian environment and to study the spatial distribution of overland flow and soil erosion. The wide variety of existing water erosion models are not developed for the Sahel and so do not include the unique Sahelian processes. The topography of the Sahelian agricultural lands in northern Burkina Faso is such that field slopes are generally low (0–5°) and overland flow mostly occurs in the form of sheet flow, which may transport large amounts of fine, nutrient‐rich particles despite its low sediment transport capacity. Furthermore, pool formation in a field limits overland flow and causes resettlement of sediment resulting in the development of a surface crust. The EUROSEM model was rewritten in the dynamic modelling code of PCRaster and extended to account for the pool formation and crust development. The modelling results were calibrated with field data from the 2001 rainy season in the Katacheri catchment in northern Burkina Faso. It is concluded that the modified version of EUROSEM for the Sahel is a fully dynamic erosion model, able to simulate infiltration, runoff routing, pool formation, sediment transport, and erosion and deposition by inter‐rill processes over the land surface in individual storms at the scale of both runoff plots and fields. A good agreement is obtained between simulated and measured amounts of runoff and sediment discharge. Incorporating crust development during the event may enhance model performance, since the process has a large influence on infiltration capacity and sediment detachment in the Sahel. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

5.
There has been limited success in determining critical thresholds of ground cover or soil characteristics that relate to significant changes in runoff or sediment production at the microscale (<1 m2), particularly in semi‐arid systems where management of ground cover is critical. Despite this lack of quantified thresholds, there is an increasing research focus on the two‐phase mosaic of vegetation patches and inter‐patches in semi‐arid systems. In order to quantify ground cover and soil related thresholds for runoff and sediment production, we used a data mining technique known as conditional inference tree analysis to determine statistically significant values of a range of measured variables that predicted average runoff, peak runoff, sediment concentration and sediment production at the microscale. On Chromic Luvisols across a range of vegetation states in semi‐arid south‐eastern Australia, large changes in runoff and sediment production were related to a hierarchy of different variables and thresholds, but the percentage of bare soil played a primary role in predicting runoff and sediment production in most instances. The identified thresholds match well with previous thresholds found in semi‐arid and temperate regions (including the approximate values of 30%, 50% and 70% total ground cover). The analysis presented here identified the critical role of soil surface roughness, particularly where total ground cover is sparse. The analysis also provided evidence that a two‐phase mosaic of patches and inter‐patches identified via rapid visual assessment could be further delineated into distinct groups of hydrological response, or a multi‐phase rather than a two‐phase system. The approach used here may aid in assessing scale‐dependent responses and address data non‐linearity in studies of semi‐arid hydrology. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

6.
The term glaciokarst describes a landscape where both glacial and karstic processes have contributed to geomorphological evolution and has been applied to a range of environments from the high arctic to the alpine Mediterranean. Nevertheless, glaciokarstic environments globally often exhibit significant variation in geomorphological processes and landforms due to these geographical differences. The Burren, County Clare, Republic of Ireland, is often quoted as a quintessential glaciokarstic landscape. However, the Burren and other similar environments would appear to lean towards one end of the glaciokarst spectrum, where solutional and biological processes have been dominant throughout the Holocene, in contrast to ice‐dominated glaciokarsts where karstic and biokarstic processes are temperature‐limited and cryospheric processes remain the principal geomorphological agents. Holocene landscape evolution and the development of a range of meso‐, micro‐ and nano‐scale karren features on limestone surfaces appears to be largely biokarstic in origin. Karstification of many glacially scoured limestone pavements would have begun under acidic soil cover, with biological soil processes contributing to smooth, rounded cryptokarstic surface forms. Holocene soil erosion is attributed to anthropogenic activity, climatic fluctuations and the evolution of the karstic groundwater system leading to vertical soil loss through widening grikes. Exposed limestone pavements subject to subaerial conditions often exhibit extensive lichen colonization which has been shown to influence the overall rate of karstification and contribute to the development of micro‐ and meso‐scale bioweathering features. Where cryptokarstic features have been exhumed from beneath soil cover, their evolution under subaerial conditions leads to intermediate, polygenetic karren features. In light of our current understanding of the Burren landscape, it is proposed that the term glaciobiokarstic may be a better expression to encompass the biological processes that have played a fundamental role in the evolution of the Burren and similar landscapes, without neglecting the contribution of glacial and inorganic karstic processes. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

7.
The processes involved in the development of high‐altitude, low‐relief areas (HLAs) are still poorly understood. Although cosmogenic nuclides have provided insights into the evolution of HLAs interpreted as paleo‐surfaces, most studies focus on estimating how slowly they erode and thereby their relative stability. To understand actual development processes of HLAs, we applied several techniques of cosmogenic nuclides in the Daegwanryeong Plateau, a well‐known HLA in the Korean Peninsula. Our denudation data from strath terraces, riverine sediments, soils, and tors provide the following conclusions: (1) bedrock incision rate in the plateau (~127 m Myr?1) is controlled by the incision rate of the western part of the Korean Peninsula, and is similar to the catchment‐wide denudation rate of the plateau (~93 m Myr?1); (2) the soil production function we observed shows weak depth dependency that may result from highly weathered bedrock coupled with frequent frost action driven by alpine climate; (3) a discrepancy between the soil production and catchment‐wide denudation rates implies morphological disequilibrium in the plateau; (4) the tors once regarded as fossil landforms of the Tertiary do not reflect Tertiary processes; and (5) when compared with those of global paleo‐surfaces (<20 m Myr?1), our rapid denudation rates suggest that the plateau cannot have maintained its probable initial paleo landscape, and thus is not a paleo‐surface. Our data contribute to understanding the surface processes of actively eroding upland landscapes as well as call into question conventional interpretations of supposed paleo‐surfaces around the world. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

8.
Talus flatirons are debris‐covered relict slopes, disconnected from the source area, which are relatively common in arid and semi‐arid areas. Talus flatiron sequences record the alternation of accumulation and incision phases. These chronosequences may be used for infer temporal changes in the morphogenetic processes acting on the slopes as well as information on the local paleoclimatic history. Talus flatiron sequences developed in the Tremp Depression, eastern Spanish Pyrenees, are analysed from the geomorphological, chronological and paleoenvironmental perspective. The two groups of relict slopes differentiated by means of detailed geomorphological mapping have been dated by optically stimulated luminescence (OSL) and radiocarbon dating at 25–20 kyr (S3) and 5·4–1·7 cal kyr (S2). The talus flatiron group S3 is correlated with a fluvial terrace of the Noguera Pallaresa River (c. 23 kyr bp ). The comparison of the ages obtained in the Tremp Depression with chronologies published for talus flatiron sequences in semi‐arid areas and other paleoclimatic proxies suggests that the aggradation phases in the slopes occurred during periods with higher humidity and vegetation cover. The chronological differences observed between semi‐arid Spain and the Tremp Depression may be partly related to the more humid climate of the latter mountain area. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

9.
In semiarid ecosystems, the transfer of water, sediments, and nutrients from bare to vegetated areas is known to be crucial to ecosystem functioning. Rainfall simulation experiments were performed on bare‐soil and vegetated surfaces, on both wet and dry soils, in semiarid shrub‐steppe landscapes of SE Spain to investigate the spatial and temporal factors and interactions that control the fine‐scale variation in water infiltration, runoff and soil loss, and hence the water and sediment flows in these areas. Three types of shrub‐steppe landscapes varying in plant community and physiography, and four types of plant patches (oak shrub, subshrub, tussock grass, and short grass mixed with chamaephytes) were studied. Higher infiltration and lower runoff and soil loss were measured on vegetation patches than on bare soils, for both dry and wet conditions. The oak‐shrub patches produced no runoff, while the subshrub patches showed the highest runoff and soil loss. Despite these differences among patch types, the influence of vegetation patch type on the variables analysed was not significant. The response of bare soil surfaces clearly varied between landscape types, yet the differences were only relevant under dry soil conditions. Stone cover, particularly the cover of embedded stones, and crust cover, were the key explanatory variables for the hydrological behaviour of bare soils. The study documents quantitatively how bare soils and vegetation patches function as runoff sources and runoff sinks, respectively, for a wide range of soil moisture conditions, and illustrates that landscape‐type effects on bare‐soil runoff sources may also exert an important control on the site hydrology, while the role of the vegetation patch type is less important. The effects of the control factors are modulated by antecedent soil moisture, with dry soils showing the most contrasting soil water infiltration between landscapes and surface types. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

10.
A. Montenegro  R. Ragab 《水文研究》2010,24(19):2705-2723
Brazilian semi‐arid regions are characterized by water scarcity, vulnerability to desertification, and climate variability. The investigation of hydrological processes in this region is of major interest not only for water planning strategies but also to address the possible impact of future climate and land‐use changes on water resources. A hydrological distributed catchment‐scale model (DiCaSM) has been applied to simulate hydrological processes in a small representative catchment of the Brazilian northeast semi‐arid region, and also to investigate the impact of climate and land‐use changes, as well as changes associated with biofuel/energy crops production. The catchment is part of the Brazilian network for semi‐arid hydrology, established by the Brazilian Federal Government. Estimating and modelling streamflow (STF) and recharge in semi‐arid areas is a challenging task, mainly because of limitation in in situ measurements, and also due to the local nature of some processes. Direct recharge measurements are very difficult in semi‐arid catchments and contain a high level of uncertainty. The latter is usually addressed by short‐ and long‐time‐scale calibration and validation at catchment scale, as well as by examining the model sensitivity to the physical parameters responsible for the recharge. The DiCaSM model was run from 2000 to 2008, and streamflow was successfully simulated, with a Nash–Sutcliffe (NS) efficiency coefficient of 0·73, and R2 of 0·79. On the basis of a range of climate change scenarios for the region, the DiCaSM model forecasted a reduction by 35%, 68%, and 77%, in groundwater recharge (GWR), and by 34%, 65%, and 72%, in streamflow, for the time spans 2010–2039, 2040–2069, and 2070–2099, respectively, could take place for a dry future climate scenario. These reductions would produce severe impact on water availability in the region. Introducing castor beans to the catchment would increase the GWR and streamflow, mainly if the caatinga areas would be converted into castor beans production. Changing an area of 1000 ha from caatinga to castor beans would increase the GWR by 46% and streamflow by 3%. If the same area of pasture is converted into castor beans, there would be an increase in GWR and streamflow by 24% and 5%, respectively. Such results are expected to contribute towards environmental policies for north‐east Brazil (NEB), and to biofuel production perspectives in the region. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

11.
Precipitation is often the sole source of water replenishment in arid and semi‐arid areas and, thus, plays a pertinent role in sustaining desert ecosystems. Revegetation over 40 years using mainly Artemisia ordosica and Caragana korshinskii at Shapotou Desert Experimental Research Station near Lanzhou, China, has established a dwarf‐shrub and microbiotic soil crust cover on the stabilized sand dunes. The redistribution of infiltrated moisture through percolation, root extraction, and evapotranspiration pathways was investigated. Three sets of time‐domain reflectometry (TDR) probes were inserted horizontally at 5, 10, 15, 20, 30 and 40 cm depths below the ground surface in a soil pit. The three sets of TDR probes were installed in dwarf‐shrub sites of A. ordosica and C. korshinskii community with and without a microbiotic soil crust cover, and an additional set was placed in a bare sand dune area that had neither vegetation nor a microbiotic soil crust present. Volumetric soil moisture content was recorded at hourly intervals and used in the assessment of infiltration for the different surface covers. Infiltration varied greatly, from 7·5 cm to more than 45 cm, depending upon rainfall quantity and soil surface conditions. In the shrub community area without microbiotic soil crust cover, infiltration increased due to preferential flow associated with root tunnels. The microbiotic soil crust cover had a significant negative influence on the infiltration for small rainfall events (～10 mm), restricting the infiltration depth to less than 20 cm and increasing soil moisture content just beneath the soil profile of 10 cm, whereas it was not as strong or clear for larger rainfall events (～60 mm). For small rainfall events, the wetting front depth for the three kinds of surface cover was as follows: shrub community without microbiotic soil crust > bare area > shrub community with microbiotic soil crust. In contrast, for large rainfall events, infiltration was similar in shrub communities with and without microbiotic soil crust cover, but significantly higher than measured in the bare area. Soil water extraction by roots associated with evapotranspiration restricted the wetting front penetration after 1 to 3 h of rainfall. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

12.
Erosion rates and processes define how mountainous landscapes evolve. This study determines the range of erosion rates in a semi‐arid landscape over decadal time spans and defines the dominant processes controlling variability in erosion rates. The varying topography and climatic regimes of the Xiying Basin (Qilian Shan Mountains, China) enables us to examine the relative roles of sheet wash versus rainsplash and the influence of vegetation on soil erosion and deposition. Soil erosion rates since 1954 were determined using 137Cs along 21 transects at four sites with varying gradient, rainfall, and vegetation cover. The mean 137Cs derived soil erosion rate ~0.42 mm/a was consistent with the catchment level erosion rate derived from total sediment yield for a 44 year record. However, there is considerable variability in 137Cs erosion rates both between transects and along transects, perhaps reflecting variation not only in the effectiveness of individual processes but also in their relative roles. We compare the 137Cs‐derived erosion rates with 1‐D models for sediment flux that incorporate sheet wash and rainsplash processes, testing them over a previously untested 60 year timescale. The variability in 137Cs erosion rates along transects is best replicated by sheet wash dominated simulations, suggesting that this is the dominant erosion process in this semi‐arid landscape. The functional form of the sheetwash model can also explain our observations that 137Cs erosion rates decrease with upslope length (i.e. distance down slope) while its variability increases. However, sparsely vegetated sites, located in slightly drier locations, have higher erosion rates, and are not as accurately modeled as densely vegetated sites, suggesting that patchiness of vegetation introduces fine scale variability in erosion rates on these slopes. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

13.
The response of runoff and erosion to soil crusts has been extensively investigated in recent decades. However, there have been few attempts to look at the effects of spatial configuration of different soil crusts on erosion processes. Here we investigated the effects of different spatial distributions of physical soil crusts on runoff and erosion in the semi‐arid Loess Plateau region. Soil boxes (1.5 m long × 0.2 m wide) were set to a slope of 17.6% (10°) and simulated rainfall of 120 mm h?1 (60 minutes). The runoff generation and erosion rates were determined for three crust area ratios (depositional crust for 20%, 33%, and 50% of the total slope) and five spatial distribution patterns (depositional crust on the lower, lower‐middle, middle, mid‐upper, and upper slope) of soil crusts. The reduction in sediment loss (‘sediment reduction’) was calculated to evaluate the effects of different spatial distributions of soil crusts on erosion. Sediment yield was influenced by the area ratio and spatial position of different soil crusts. The runoff rate reached a steady state after an initial trend of unsteadily increasing with increasing rainfall duration. Sediment yield was controlled by detachment limitation and then transport limitation under rainfall. The shifting time of erosion from a transport to detachment‐limiting regime decreased with increasing area of depositional crust. No significant differences were observed in the total runoff among treatments, while the total sediment yield varied under different spatial distributions. At the same area ratio, total sediment yield was the largest when the depositional crust was on the upper slope, and it was smallest when the crust was deposited on the lower slope. The sediment reduction of structural crust (42.5–66.5%) was greater than that of depositional crust (16.7–34.3%). These results provide a mechanistic understanding of how different spatial distributions of soil crusts affect runoff and sediment production. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

14.
A three‐dimensional numerical modelling system is developed to study transformation processes of water resources in alluvial fan and river basin along the middle reaches of the Heihe River Basin, Northwest China, an arid and semi‐arid region. Integrating land utilization, remote sensing and geographic information systems, we have developed a numerical modelling system that can be used to quantify the effects of land use and anthropogenic activities on the groundwater system as well as to investigate the interaction between surface water and groundwater. Various hydraulic measurements are used to identify and calibrate the hydraulic boundary conditions and spatial distributions of hydraulic parameters. In the modelling study, various water exchanges and human effects on the watershed system are considered. These include water exchange between surface water and groundwater, groundwater pumping, lateral water recharges from mountain areas, land utilization, and infiltration and evaporation in the irrigation and non‐irrigation areas. The modelling system provides a quantitative method to describe spatial and temporal distributions and transformations between various water resources, and it has application to other watersheds in arid and semi‐arid areas. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

15.
Arid alluvial fan and fluvial dry wash surfaces in Stonewall Flat, Nevada, USA, are characterized using surface geomorphic surveys, soil pits, botanical line surveys, and varnish microlamination dating techniques. Active and abandoned washes, and active fan surfaces are dominated by primary geomorphic processes of high‐energy sedimentation from flash floods. These surfaces are characterized by bar and swale topography, a lack of stone pavements, soil horizons, and rock varnish. Younger terraces and slightly older intermediate fan surfaces are in transition from primary sedimentation processes to lower energy secondary surface‐modifying processes of sheet wash and eolian transport and deposition. These surfaces are characterized by faint to no bar and swale topography, incipient to moderately well‐developed pavements and soil horizons, and abundant coppices. Old and stable fan surfaces are dominated by lower energy secondary processes and manifest well‐developed pavements, soils, and sparse coppices around widely distributed shrubs. Varnish microlamination dating yields ages of 13·15 ka for intermediate fan surfaces and 25·55 to 86·75 ka for stable fan surfaces. Plant communities co‐developing with these surfaces affect and are affected by both primary and secondary geomorphic fan processes. Relatively active surfaces contain few woody species. Co‐dominance of shrubs and annuals with abundant annuals between the shrubs is characteristic of surfaces transitional from primary processes to secondary processes. Stable surfaces dominated by secondary processes are characterized by woody perennials, with long‐lived woody species inhabiting the oldest surfaces. Feedback mechanisms between early botanical communities and eolian deposition affect coppice and pavement development. In turn, these surface features control both the composition and distribution of botanical communities on older, more stable surfaces. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.  相似文献   

16.
Micromorphology has important application in earth surface process and landform studies particularly in alluvial settings such as the Indo‐Gangetic Plains (IGP) with different geomorphic surfaces to identify climatic changes and neotectonic events and their influence on pedogenesis. The soils of the IGP extending from arid upland in the west to per humid deltaic plains in the east developed on five geomorphic surfaces namely QIG1 to QIG5 originating during the last 13.5 ka. Four soil‐geomorphic systems across the entire IGP are identified as: (i) the western Yamuna Plains/Uplands, (ii) the Yamuna‐Ganga Interfluve, (iii) the Ganga‐Ghaghara Interfluve, and (iv) the Deltaic Plains. Thin section analysis of the soils across the four soil‐geomorphic systems provides a record of provenance, mineral weathering, pedogenic processes and polygenesis in IGP. The soils over major parts of the IGP dominantly contain muscovite and quartz and small fraction of highly altered feldspar derived from the Himalayas. However, soils in the western and eastern parts of the IGP contain large volumes of fresh to weakly altered plagioclase and smectitic clay derived from the Indian craton. The soils in western Yamuna Plains/Uplands dominated by QIG2–QIG3 geomorphic surfaces and pedogenic carbonate developed in semi‐arid climate prior to 5 ka. However, soils of the central part of the IGP in the Yamuna‐Ganga Interfluve and Ganga‐Ghaghara Interfluve regions with dominance of QIG4–QIG5 surfaces are polygenetic due to climate change over the last 13.5 ka. The clay pedofeatures formed during earlier wet phase (13.5–11 ka) show degradation, loss of preferred orientation, speckled appearance in contrast with the later phase of wet climate (6.5–4 ka). The soils over the deltaic plains with dominance of vertic features along with clay pedofeatures suggest that illuviation of fine clay is an important pedogenic process even in soils with shrink‐swell characteristics. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

17.
Connectivity has recently emerged as a key concept for understanding hydrological response to vegetation change in semi‐arid environments, providing an explanatory link between abiotic and biotic, structure and function. Reduced vegetation cover following woody encroachment, generally promotes longer, more connected overland flow pathways, which has the potential to result in an accentuated rainfall‐runoff response and fluxes of both soil erosion and carbon. This paper investigates changing hydrological connectivity as an emergent property of changing ecosystem structure over two contrasting semi‐arid grass to woody vegetation transitions in New Mexico, USA. Vegetation structure is quantified to evaluate if it can be used to explain observed variations in water, sediment and carbon fluxes. Hydrological connectivity is quantified using a flow length metric, combining topographic and vegetation cover data. Results demonstrate that the two woody‐dominated sites have significantly longer mean flowpath lengths (4 · 3 m), than the grass‐dominated sites (2 · 4 m). Mean flowpath lengths illustrate a significant positive relationship with the functional response. The woody‐dominated sites lost more water, soil and carbon than their grassland counterparts. Woody sites erode more, with mean event‐based sediment yields of 1203 g, compared to 295 g from grasslands. In addition, the woody sites lost more organic carbon, with mean event yields of 39 g compared to 5 g from grassland sites. Finally, hydrological connectivity (expressed as mean flowpath length) is discussed as a meaningful measure of the interaction between structure and function and how this manifests under the extreme rainfall that occurs in semi‐arid deserts. In combination with rainfall characteristics, connectivity emerges as a useful tool to explain the impact of vegetation change on water, soil and carbon losses across semi‐arid environments. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

18.
The headwaters of mountainous, discontinuous permafrost regions in north‐eastern Mongolia are important water resources for the semi‐arid country, but little is known about hydrological processes there. Run‐off generation on south‐facing slopes, which are devoid of permafrost, has so far been neglected and is totally unknown for areas that have been affected by recent forest fires. To fill this knowledge gap, the present study applied artificial tracers on a steppe‐vegetated south‐facing and on two north‐facing slopes, burned and unburned. Combined sprinkling and dye tracer experiments were used to visualize processes of infiltration and water fluxes in the unsaturated zone. On the unburned north‐facing slope, rapid and widespread infiltration through a wet organic layer was observed down to the permafrost. On the burned profile, rapid infiltration occurred through a combusted organic and underlying mineral layer. Stained water seeped out at the bottom of both profiles suggesting a general tendency to subsurface stormflow (SSF). Ongoing SSF could directly be studied 24 h after a high‐intensity rainfall event on a 55‐m hillslope section in the burned forest. Measurements of water temperature proved the role of the permafrost layer as a base horizon for SSF. Repeated tracer injections allowed direct insights into SSF dynamics: A first injection suggested rather slow dispersive subsurface flow paths; whereas 18 h later, a second injection traced a more preferential flow system with 20 times quicker flow velocities. We speculate that these pronounced SSF dynamics are limited to burned slopes where a thermally insulating organic layer is absent. On three south‐facing soil profiles, the applied tracer remained in the uppermost 5 cm of a silt‐rich mineral soil horizon. No signs of preferential infiltration could be found, which suggested reduced biological activity under a harsh, dry and cold climate. Instead, direct observations, distributed tracers and charcoal samples provided evidence for the occurrence of overland flow. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

19.
Soil erosion is one of the most severe land degradation processes in the Mediterranean region. Although badlands occupy a relatively small fraction of the Mediterranean area, their erosion rates are very high. Many studies have investigated to what extent vegetation controls soil erosion rates. This study, however, deals with the impact of erosion on vegetation establishment. In semi‐arid badlands of the Mediterranean, soil water availability constitutes the main limiting factor for vegetation development. As a consequence, south‐facing slopes are typically less vegetated due to a very large water stress. However, these findings do not necessarily apply to humid badlands. The main objective of this paper is to determine the topographic thresholds for plant colonization in relation to slope aspect and to assess the spatial patterns of vegetation cover and species richness. We surveyed 179 plots on highly eroded badland slopes in the Central Pyrenees. We defined four aspect classes subdivided into slope angle classes. Colonization success was expressed in terms of vegetation cover and species richness. Slope angle thresholds for plant colonization were identified for each slope aspect class by means of binary logistic regressions. The results show that a critical slope angle exists below which plants colonize the badland slopes. Below this critical slope angle, plant cover and species richness increase with a decreasing slope angle. The largest critical slope angles in humid badlands are observed on south‐facing slopes, which contrasts with the results obtained in semi‐arid badlands. North‐facing slopes however are characterized by a reduced overall vegetation cover and species richness, and lower topographic threshold values. The possible underlying processes responsible for this slope‐aspect discrepancy in vegetation characteristics are discussed in terms of environmental variables that control regolith development, weathering and erosion processes. Moreover, possible restoration strategies through the use of vegetation in highly degraded environments are highlighted. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

20.
Decoupling the impacts of climate and tectonics on hillslope erosion rates is a challenging problem. Hillslope erosion rates are well known to respond to changes in hillslope boundary conditions (e.g. channel incision rates) through their dependence on soil thickness, and precipitation is an important control on soil formation. Surprisingly though, compilations of hillslope denudation rates suggest little precipitation sensitivity. To isolate the effects of precipitation and boundary condition, we measured rates of soil production from bedrock and described soils on hillslopes along a semi‐arid to hyperarid precipitation gradient in northern Chile. In each climate zone, hillslopes with contrasting boundary conditions (actively incising channels versus non‐eroding landforms) were studied. Channel incision rates, which ultimately drive hillslope erosion, varied with precipitation rather than tectonic setting throughout the study area. These precipitation‐dependent incision rates are mirrored on the hillslopes, where erosion shifts from relatively fast and biologically‐driven to extremely slow and salt‐driven as precipitation decreases. Contrary to studies in humid regions, bedrock erosion rates increase with precipitation following a power law, from ～1 m Ma?1 in the hyperarid region to ～40 m Ma?1 in the semi‐arid region. The effect of boundary condition on soil thickness was observed in all climate zones (thicker soils on hillslopes with stable boundaries compared to hillslopes bounded by active channels), but the difference in bedrock erosion rates between the hillslopes within a climate region (slower erosion rates on hillslopes with stable boundaries) decreased as precipitation decreased. The biotic‐abiotic threshold also marks the precipitation rate below which bedrock erosion rates are no longer a function of soil thickness. Our work shows that hillslope processes become sensitive to precipitation as life disappears and the ability of the landscape to respond to tectonics decreases. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

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