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Several Metres (several + metre)
Selected AbstractsOblique rainfall and contemporary geomorphological dynamics (Serra da Estrela, Portugal)HYDROLOGICAL PROCESSES, Issue 4 2004Gonçalo Vieira Abstract Coarse sand accumulations are polygenic microforms that attain a width of several metres, a height up to 30,40 cm, a gradient of 8,12° and a slope length up to 1 m. These accumulations are frequent in the gruss-covered plateaus of the granite mountains of central and northern Portugal, but they have been described in other mountain areas (i.e. Cairngorms, Scotland). Though these microforms are frequent features, studies on them are rare. They have been attributed to complex genesis controlled primarily by aeolian processes, but also by wash and cryogenic dynamics. Results presented here add new insights into the origin of the sand accumulations and emphasize the importance of rainsplash-saltation induced by oblique rainfall as the main transportation mechanism. The study was conducted in the Serra da Estrela, a granite mountain in central Portugal (1993 m above sea level) and is supported by a detailed mapping of the orientation of the accumulations, monitoring of the surface material and analysis of meteorological data. The results are particularly significant since they indicate that the coarse sand accumulations are very active features that show a clear climatic and ecological signal. Copyright © 2004 John Wiley & Sons, Ltd. [source] Determining long time-scale hyporheic zone flow paths in Antarctic streamsHYDROLOGICAL PROCESSES, Issue 9 2003Michael N. Gooseff Abstract In the McMurdo Dry Valleys of Antarctica, glaciers are the source of meltwater during the austral summer, and the streams and adjacent hyporheic zones constitute the entire physical watershed; there are no hillslope processes in these systems. Hyporheic zones can extend several metres from each side of the stream, and are up to 70 cm deep, corresponding to a lateral cross-section as large as 12 m2, and water resides in the subsurface year around. In this study, we differentiate between the near-stream hyporheic zone, which can be characterized with stream tracer experiments, and the extended hyporheic zone, which has a longer time-scale of exchange. We sampled stream water from Green Creek and from the adjacent saturated alluvium for stable isotopes of D and 18O to assess the significance and extent of stream-water exchange between the streams and extended hyporheic zones over long time-scales (days to weeks). Our results show that water residing in the extended hyporheic zone is much more isotopically enriched (up to 11, D and 2·2, 18O) than stream water. This result suggests a long residence time within the extended hyporheic zone, during which fractionation has occurred owing to summer evaporation and winter sublimation of hyporheic water. We found less enriched water in the extended hyporheic zone later in the flow season, suggesting that stream water may be exchanged into and out of this zone, on the time-scale of weeks to months. The transient storage model OTIS was used to characterize the exchange of stream water with the extended hyporheic zone. Model results yield exchange rates (,) generally an order magnitude lower (10,5 s,1) than those determined using stream-tracer techniques on the same stream. In light of previous studies in these streams, these results suggest that the hyporheic zones in Antarctic streams have near-stream zones of rapid stream-water exchange, where ,fast' biogeochemical reactions may influence water chemistry, and extended hyporheic zones, in which slower biogeochemical reaction rates may affect stream-water chemistry at longer time-scales. Copyright © 2003 John Wiley & Sons, Ltd. [source] Comparison of plume structures of carbon dioxide emitted from different mosquito trapsMEDICAL AND VETERINARY ENTOMOLOGY, Issue 1 2006Miriam F. Cooperband Abstract., A large field wind tunnel was used to compare four types of CO2 -baited mosquito traps. This study compared the plume structure and concentration of CO2 emitted by each trap, flow of suction into the trapping systems, flow of CO2 being released, trap shape and configuration, differences in visual appearance, and differences in temperature and humidity of emissions at the source of CO2 compared to ambient air. The structure of the CO2 plumes emitted by each trap differed considerably. All four plumes were turbulent, causing the concentration of CO2 within several metres of the source to attenuate to between 375 and 875 p.p.m. The Encephalitis Virus Surveillance (EVS) trap emitted concentrations of CO2 exceeding 20 000 p.p.m., the detection limits of our equipment, whereas the Mosquito Magnet® Freedom (MMF), Mosquito Magnet® Liberty (MML) and Mosquito Magnet® X (MMX) traps released CO2 at peaks of about 3500, 7200 and 8700 p.p.m., respectively. The MMX trap produced the greatest air velocity at both the suction inlet and CO2 outlet, followed by the MMF, MML and the EVS traps, respectively. [source] Near-ground solar radiation along the grassland,forest continuum: Tall-tree canopy architecture imposes only muted trends and heterogeneityAUSTRAL ECOLOGY, Issue 1 2010DAVID D. BRESHEARS Abstract Solar radiation directly and indirectly drives a variety of ecosystem processes. Our aim was to evaluate how tree canopy architecture affects near-ground, incoming solar radiation along gradients of increasing tree cover, referred to as the grassland,forest continuum. We evaluated a common type of canopy architecture: tall trees that generally have their lowest level of foliage high above, rather than close to the ground as is often the case for shorter trees. We used hemispherical photographs to estimate near-ground solar radiation using the metric of Direct Site Factor (DSF) on four sites in north Queensland, Australia that formed a grassland,forest continuum with tree canopy cover ranging from 0% to 71%. Three of the four sites had tall Eucalyptus trees with foliage several metres above the ground. We found that: (i) mean DSF exceeded >70% of the potential maximum for all sites, including the site with highest canopy cover; (ii) DSF variance was not highly sensitive to canopy coverage; and (iii) mean DSF for canopy locations beneath trees was not significantly lower than for adjacent intercanopy locations. Simulations that hypothetically placed Australian sites with tall tree canopies at other latitude,longitude locations demonstrated that differences in DSF were mostly due to canopy architecture, not specific site location effects. Our findings suggest that tall trees that have their lowest foliage many metres above the ground and have lower foliar density only weakly affect patterns of near-ground solar radiation along the grassland,forest continuum. This markedly contrasts with the strong effect that shorter trees with foliage near the ground have on near-ground solar radiation patterns along the continuum. This consequence of differential tree canopy architecture will fundamentally affect other ecosystem properties and may explain differential emphases that have been placed on canopy,intercanopy heterogeneity in diverse global ecosystem types that lie within the grassland,forest continuum. [source] The role of glacitectonic rafting and comminution in the production of subglacial tills: examples from southwest Ireland and AntarcticaBOREAS, Issue 4 2007John F. Hiemstra Sedimentological and structural geological data from two sites in southwest Ireland and Antarctica provide evidence for the formation of subglacial till by the brecciation and crushing of bedrock rafts. Up-sequence transitions, from undisturbed bedrock, to deformed bedrock, to crushed and brecciated bedrock, to massive matrix-supported till with far-travelled erratics, represent a process-form continuum of till production. Initially, bedrock fragments and rafts up to several metres in length are liberated from the substrate by glacitectonic thrusting and plucking. These rafts are then crushed to produce the matrix of a till. Such products are commonly referred to as comminution tills, although the original definition focused primarily on the second phase of the process (crushing of bedrock rafts and fragments) as well as abrasion of bedrock. Data from Ireland and Antarctica indicate that rafting of bedrock is an essential part of the process of till formation. This process is facilitated by weak sedimentary bedrock, which can be displaced along joints and bedding planes to form rafts that are then incorporated into the ,proto-till' prior to being crushed subglacially. Our field data suggest that bedrock failure and displacement of such rafts can occur to depths of 3 m. The occurrence of erratics in the uppermost part of the till demonstrates that the glacier effectively mixes far-travelled and local materials. [source] | ||||||||||