Development of the saltation system under controlled environmental conditions     

Samantha Arnold 《地球表面变化过程与地形》2002,27(8):817-829

The transport of sand by the wind occurs predominantly by the process of saltation. Following the entrainment of sand by an above threshold wind, the saltation system is regulated by the mutual interaction of the atmospheric boundary‐layer, the sand cloud and the sand bed. Despite existing data on the spatial and temporal development of the sand transport system, very little is known about the development of the saltation system towards equilibrium. Results are presented from wind‐tunnel experiments that were designed to address the simultaneous spatial and temporal development of the saltation system, with and without artificial sand feed. The development of the saltation system was monitored over a streamwise length of 8 m during a period of 3600 s. Mass flux data were measured simultaneously at 1 m intervals by the downwind deployment of seven Aarhus sand traps. Wind velocity data were collected throughout the experiments. The downwind spatial development of the saltation system is manifested by an overshoot in mass flux and friction velocity prior to declining towards a quasi‐equilibrium. Mass flux overshoots at approximately 4 m downwind, in remarkable agreement with existing data of a comparable scale. Friction velocity overshoots at approximately 6 m downwind, a result not previously witnessed in saltation studies. The overshoot of mass flux prior to the overshoot in friction velocity is a spatial manifestation of the time lag between the entrainment of grains and the deceleration of the wind by the grains in transport. Temporally, the development of the saltation system is controlled by the availability of entrainable grains from the sand bed. Through time the saltation system develops from a transport‐limited to a supply‐limited system. The depletion of the sand bed through time limits the appropriateness of the assumption of ‘equilibrium’ for the universal prediction of mass flux. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

Estimating the timescale of fluvial response to anthropogenic disturbance using two generations of dams on the South River,Massachusetts, USA     

Samantha Dow  Noah P. Snyder  William B. Ouimet  Anna M. Martini  Brian Yellen  Jonathan D. Woodruff  Robert M. Newton  Dorothy J. Merritts  Robert C. Walter 《地球表面变化过程与地形》2020,45(10):2380-2393

Centuries-long intensive land-use change in the north-eastern United States provides the opportunity to study the timescale of geomorphic response to anthropogenic disturbances. In this region, forest-clearing and agricultural practices following EuroAmerican settlement led to deposition of legacy sediment along valley bottoms, including behind mill dams. The South River in western Massachusetts experienced two generations of damming, beginning with mill dams up to 6-m high in the eighteenth–nineteenth century, and followed by construction of the Conway Electric Dam (CED), a 17-m-tall hydroelectric dam near the watershed outlet in 1906. We use the mercury (Hg) concentration in upstream deposits along the South River to constrain the magnitude, source, and timing of inputs to the CED impoundment. Based on cesium-137 (¹³⁷Cs) chronology and results from a sediment mixing model, remobilized legacy sediment comprised % of the sediment load in the South River prior to 1954; thereafter, from 1954 to 1980s, erosion from glacial deposits likely dominated (63 ± 14%), but with legacy sediments still a substantial source (37 ± 14%). We also use the CED reservoir deposits to estimate sediment yield through time, and find it decreased after 1952. These results are consistent with high rates of mobilization of legacy sediment as historic dams breached in the early twentieth century, and suggest rapid initial response to channel incision, followed by a long decay in the second half of the century, that is likely dependent on large flood events to access legacy sediment stored in banks. Identifying sources of sediment in a watershed and quantifying erosion rates can help to guide river restoration practices. Our findings suggest a short fluvial recovery time from the eighteenth–nineteenth century to perturbation during the first half of the twentieth century, with subsequent return to a dominant long-term signal from erosion of glacial deposits, with anthropogenic sediment persisting as a secondary source. © 2020 John Wiley & Sons, Ltd.  相似文献   

Oxidation of arsenite to arsenate on birnessite in the presence of light     

Samantha L. Shumlas  Soujanya Singireddy  Akila C. Thenuwara  Nuwan H. Attanayake  Richard J. Reeder  Daniel R. Strongin 《Geochemical transactions》2016,17(1):5

The effect of simulated solar radiation on the oxidation of arsenite [As(III)] to arsenate [As(V)] on the layered manganese oxide, birnessite, was investigated. Experiments were conducted where birnessite suspensions, under both anoxic and oxic conditions, were irradiated with simulated solar radiation in the presence of As(III) at pH 5, 7, and 9. X-ray absorption spectroscopy (XAS) was used to determine the nature of the adsorbed product on the surface of the birnessite. The oxidation of As(III) in the presence of birnessite under simulated solar light irradiation occurred at a rate that was faster than in the absence of light at pH 5. At pH 7 and 9, As(V) production was significantly less than at pH 5 and the amount of As(V) production for a given reaction time was the same under dark and light conditions. The first order rate constant (k_obs) for As(III) oxidation in the presence of light and in the dark at pH 5 were determined to be 0.07 and 0.04 h^?1, respectively. The As(V) product was released into solution along with Mn(II), with the latter product resulting from the reduction of Mn(IV) and/or Mn(III) during the As(III) oxidation process. Post-reaction XAS analysis of As(III) exposed birnessite showed that arsenic was present on the surface as As(V). Experimental results also showed no evidence that reactive oxygen species played a role in the As(III) oxidation process.  相似文献   

Spatio-temporal analysis of flowering using LiDAR topography     

Samantha Hart  Elena Mikhailova  Christopher Post  Patrick McMillan  Julia Sharp  William Bridges 《地理学报(英文版)》2017,27(1):62-78

Spatio-temporal patterns of flowering in forest ecosystems are hard to quantify and monitor. The objectives of this study were to investigate spatio-temporal patterns(e.g. soilssimple slope classesslope aspectand flow accumulation) of flowering around Lake IssaqueenaSouth Carolina(SCUSA) using plant-flowering database collected with GPS- enabled camera(stored in Picasa 3 web albums and project website) on a monthly basis in 2012 and Li DAR-based topography. Pacolet fine sandy loam had the most flowering plantsfollowed by Madison sandy loamboth dominant soil types around the lake. Most flowering plants were on moderately steep(17%–30%) and gently sloping(4%–8%) slopes. Most flowering plants were on west(247.5°–292.5°)southwest(202.5°–247.5°)and northwest(292.5°–337.5°) aspects. Most flowering plants were associated with minimum and maximum flows within the landscape. Chi-square tests indicated differences in the distributions of the proportions of flowering plants were significant by soil typeslopeaspectand flow accumulation for each month(February-November)for all months(overall)and across months. The Chi-square test on area-normalized data indicated significant differences for all months and individual differences by each month with some months not statistically significant. Cluster analysis on flowering counts for nine plant families with the most flowering counts indicated no unique separation by clusterbut implied that the majority of these families were flowering on strongly sloping(9%–16%) slopeson southwest(202.5°–247.5°) aspectsand low flow accumulation(0–200). Presented methodology can serve as a template for future efforts to quantify spatio-temporal patterns of flowering and other phenological events.  相似文献   

Indigenous Payment for Environmental Service (PES) Opportunities in the Northern Territory: negotiating with customs     

Samantha Muller 《The Australian geographer》2008,39(2):149-170

Recent government initiatives have acknowledged Indigenous ranger groups in the ‘top end’ (northern region) of the Northern Territory (NT), Australia, for their holistic role in environmental management and community development. The services provided by these groups are of national significance with respect to biodiversity protection, ongoing cultural maintenance, employment for remote communities, border protection, and biosecurity. However, the acknowledgement of these services has received only limited translation into meaningful financial and technical support for Indigenous ranger groups. The present paper highlights the value of environmental services provided by Indigenous ranger groups and contrasts this with the inadequate recognition of these services by government agencies. Although government budget initiatives and policy directions are moving towards greater recognition of Indigenous environmental service provision, border protection negotiations between key NT Indigenous sea ranger groups and Australian Customs Service illustrate some of the limiting factors to progression of this recognition. The present paper highlights the necessity of ensuring accountability of processes to Indigenous rangers themselves so that payment for environmental services does not become an imposition, but rather a mechanism for realising Indigenous community aspirations, as well as addressing issues of regional, state/territory, and national significance.  相似文献   

Remote sensing analysis of physical complexity of North Pacific Rim rivers to assist wild salmon conservation     

Matthew Luck  Niels Maumenee  Diane Whited  John Lucotch  Samantha Chilcote  Mark Lorang  Daniel Goodman  Kyle McDonald  John Kimball  Jack Stanford 《地球表面变化过程与地形》2010,35(11):1330-1343

Salmon populations are highly variable in both space and time. Accurate forecasting of the productivity of salmon stocks makes effective management and conservation of the resource extremely challenging. Furthermore, widespread and consistent data on the productivity of species‐specific and total salmon stocks in a river are almost nonexistent. Ranking rivers based on physical complexity derived from remote sensing allows rivers to be objectively compared. Our approach considered rivers with great geomorphic complexity (e.g. having expansive, multichanneled floodplains and/or on‐channel lakes) as likely to have greater productivity of salmon than rivers flowing in constrained or canyon‐bound channels. Our objective was to develop a database of landscape metrics that could be used to rank the rivers in relation to potential salmon productivity. We then examined the rankings in relation to existing empirical (monitoring) data describing productivity of salmon stocks. To extract the metrics for each river basin we used a digital elevation model and multispectral satellite imagery. We developed procedures to extract channel networks, floodplains, on‐channel lakes and other catchment features; variables such as catchment area, channel elevation, main channel length, floodplain area, and density of hydrojunctions (nodes) were measured. We processed 1509 catchments in the North Pacific Rim including the Kamchatka Peninsula in Russia and western North America. Overall, catchments were most physically complex in western Kamchatka and western Alaska, and particularly on the Arctic North Slope of Alaska. We could not directly examine coherence between potential and measured productivity except for a few rivers, but the expected relationship generally held. The resulting database and systematic ranking are objective tools that can be used to address questions about landscape structure and biological productivity at regional to continental extents, and provide a way to begin to efficiently prioritize the allocation of funding and resources towards salmon management and conservation. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Intensification, Degradation and Soil Improvement: Utilising Structuration Theory for a Differentiated Analysis of Population Pressure Outcomes in Highland Tanzania     

Samantha Jones 《Singapore journal of tropical geography》2000,21(2):131-148

This paper reviews the evidence for and consequences of population pressure in an area of highland Tanzania. While intensification is found to be occurring, both soil improvement and soil productivity decline are evident. A household-level, spatially disaggregated analysis is therefore adopted to understand the variegated pattern of population pressure outcomes. The approach is guided by structuration theory to gain a deeper understanding of the contexts and processes shaping different outcomes. Gender, wealth and land types are found to be influential variables affecting responses to population pressure in this case study.  相似文献   

An evaluation of the ecohydrological separation hypothesis in a semiarid catchment   总被引：1，自引：0，他引：1       下载免费PDF全文

Ryan J. McCutcheon  James P. McNamara  Matthew J. Kohn  Samantha L. Evans 《水文研究》2017,31(4):783-799

The ecohydrological separation hypothesis states that transpiration through plants and drainage to streams and groundwater are sourced from separate soil water pools, which possess distinct isotopic signatures. Evidence for ecohydrological separation has relied on the globally ubiquitous observation that plant water and draining water are isotopically distinct. We evaluated the ecohydrological separation hypothesis in the Dry Creek Experimental Watershed in the semiarid, snow‐dominated landscape of southwest Idaho, USA. We found that plant water is indeed isotopically distinct from streams and groundwater. However, we were unable to track those waters to subsurface soil waters, nor were we able to relate soil water mobility to isotopic composition. Soil waters of different mobility can be isotopically similar, and isotopic distinction in soil water can occur for reasons not related to mobility. We suggest that isotopic distinction between root‐absorbed and draining waters may not be an appropriate indicator of ecohydrological separation of soil waters, and that hydrologic explanations for such isotopic distinction may not be sufficient.  相似文献   

Legitimacy of Different Knowledge Types in Natural Resource Governance and Their Functions in Inter-Institutional Gaps     

H. M. Tuihedur Rahman  June Y. T. Po  Arlette S. Saint Ville  Nicolas D. Brunet  Stephen M. Clare  Samantha Darling 《社会与自然资源》2019,32(12):1344-1363

Abstract

This study expands the Inter-Institutional Gaps (IIGs) framework to conceptualize the legitimacy associated with different types of ecological knowledge (e.g., scientific, traditional and local) used in natural resource governance. We draw on primary qualitative data, and document analysis to examine a case of inland fisheries management in the north-eastern floodplain of Bangladesh. We posit that the pragmatic, moral, cognitive, and regulative legitimacy for different types of ecological knowledge are repeatedly reevaluated by rule-makers and resource users in the process of rule-devising. Results show that inter-institutional gaps may be perpetuated when formal rules do not sufficiently consider traditional and local ecological knowledge. While it is widely proposed that systematically incorporating different knowledge types can better address local-national policy problems, this study underscores that the source of legitimacies for different knowledge types often differs across formal and informal institutional actors. Recognizing the differences is critical to fishers’ resource management.  相似文献   

A spectral response approach for detecting dominant phytoplankton size class from satellite remote sensing   总被引：1，自引：0，他引：1  

Robert J W Brewin  Samantha J Lavender  Nick J Hardman-Mountfor  Takafumi Hirata 《海洋学报(英文版)》2010,29(2):14-32

An important goal in ocean colour remote sensing is to accurately detect different phytoplank- ton groups with the potential uses including the validation of multi-phytoplankton carbon cycle models; synoptically monitoring the health of our oceans, and improving our understanding of the bio-geochemical interactions between phytoplankton and their environment. In this paper a new algorithm is developed for detecting three dominant phytoplankton size classes based on distinct differences in their optical signatures. The technique is validated against an independent cou- pled satellite reflectance and in situ pigment dataset and run on the 10-year NASA Sea viewing Wide Field of view Sensor (SeaWiFS) data series. Results indicate that on average 3.6% of the global oceanic surface layer is dominated by microplankton, 18.0% by nanoplankton and 78.4% by picoplankton. Results, however, are seen to vary depending on season and ocean basin.  相似文献