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61.
The rainfall received by a small plot of tropical deciduous forest on sand dunes in Veracruz, Mexico, was partitioned into stemflow and throughfall components to determine whether funnelling by stemflow could reduce soil leaching by transmitting large volumes of water through vertical soil pathways beneath each stem. Although soil infiltration capacities were high, only a very small proportion of incoming rainfall was funnelled by canopy stems. This is attributed to the widely-branched morphology of mature trees. Smaller trees and shrubs were more effective funnellers of rainfall, and a crude estimate of the magnitude of stemflow in the understorey stratum in one rain event suggested a contribution approximately ten times that of canopy stemflow. However, even if augmented by the understorey stratum in this way, total stemflow is unlikely to have exceeded 10 per cent of gross precipitation, implying that it does not represent an important leaching-avoidance mechanism in this forest. 相似文献
62.
Interpreting complex fluvial channel and barform architecture: Carboniferous Central Pennine Province,northern England 下载免费PDF全文
The Bashkirian Lower Brimham Grit of North Yorkshire, England, is a fluvio‐deltaic sandstone succession that crops out as a complex series of pinnacles, the three‐dimensional arrangement of which allows high‐resolution architectural analysis of genetically‐related lithofacies assemblages. Combined analysis of sedimentary graphic log profiles, architectural panels and palaeocurrent data have enabled three‐dimensional geometrical relationships to be established for a suite of architectural elements so as to develop a comprehensive depositional model. Small‐scale observations of facies have been related to larger‐scale architectural elements to facilitate interpretation of the palaeoenvironment of deposition to a level of detail that has rarely been attempted previously, thereby allowing interpretation of formative processes. Detailed architectural panels form the basis of a semi‐quantitative technique for recording the variety and complexity of the sedimentary lithofacies present, their association within recognizable architectural elements and, thus, the inferred spatio‐temporal relationship of neighbouring elements. Fluvial channel‐fill elements bounded by erosional surfaces are characterized internally by a hierarchy of sets and cosets with subtly varying compositions, textures and structures. Simple, cross‐bedded sets represent in‐channel migration of isolated mesoforms (dunes); cosets of both trough and planar‐tabular cross‐bedded facies represent lateral‐accreting and downstream‐accreting macroforms (bars) characterized by highly variable, yet predictable, patterns of palaeocurrent indicators. Relationships between sandstone‐dominated strata bounded by third‐order and fifth‐order surfaces, which represent in‐channel bar deposits and incised channel bases, respectively, chronicle the origin of the preserved succession in response to autocyclic barform development and abandonment, major episodes of incision probably influenced by episodic tectonic subsidence, differential tilting and fluvial incision associated with slip on the nearby North Craven Fault system. Overall, the succession represents the preserved product of an upper‐delta plain system that was traversed by a migratory fluvial braid‐belt system comprising a poorly‐confined network of fluvial channels developed between major sandy barforms that evolved via combined lateral‐accretion and downstream‐accretion. 相似文献
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Waterborne carbon (C) export from terrestrial ecosystems is a potentially important flux for the net catchment C balance and links the biogeochemical C cycling of terrestrial ecosystems to their downstream aquatic ecosystems. We have monitored hydrology and stream chemistry over 3 years in ten nested catchments (0.6–15.1 km2) with variable peatland cover (0%–22%) and groundwater influence in subarctic Sweden. Total waterborne C export, including dissolved and particulate organic carbon (DOC and POC) and dissolved inorganic carbon (DIC), ranged between 2.8 and 7.3 g m–2 year–1, representing ~10%–30% of catchment net ecosystem exchange of CO2. Several characteristics of catchment waterborne C export were affected by interacting effects of peatland cover and groundwater influence, including magnitude and timing, partitioning into DOC, POC, and DIC and chemical composition of the exported DOC. Waterborne C export was greater during the wetter years, equivalent to an average change in export of ~2 g m–2 year–1 per 100 mm of precipitation. Wetter years led to a greater relative increase in DIC export than DOC export due to an inferred relative shift in dominance from shallow organic flow pathways to groundwater sources. Indices of DOC composition (SUVA254 and a250/a365) indicated that DOC aromaticity and average molecular weight increased with catchment peatland cover and decreased with increased groundwater influence. Our results provide examples on how waterborne C export and DOC composition might be affected by climate change. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
66.
Braided rivers exhibit highly variable morphologies, morphodynamic behaviours and resulting depositional records. To evaluate relationships between characteristics of braided-river channel belts and river depth, water discharge and streambed gradient, 39 numerical modelling experiments were conducted with the software Delft3D to simulate braided-river evolution under a broad range of boundary conditions. Data from model outputs were integrated with observations from 63 natural braided rivers differing with respect to river depth and streambed gradient. The modelled rivers each underwent similar evolutions, yet each culminated in markedly different final river morphologies, dependent on discharge and riverbed gradient. The rivers underwent evolutionary stages of: (i) formation of transverse unit bars with limited relief from an initially featureless bed; (ii) channel development around bars and in some cases dissecting transverse unit bars; (iii) formation of relatively simpler compound bars; and (iv) amalgamation of these simpler compound bars into more complex compound bars. Quantitative relationships relating to braided-river channel-belt morphology and organization are established, and the following results are noted: (i) bar elongation (length-to-width ratio) is correlated positively with riverbed gradient; (ii) bar height and area are correlated positively with discharge, and negatively with riverbed gradient; (iii) the river depth is the main predictor of mean braid-bar area; and (iv) the degree of braiding is primarily associated with river width-to-depth ratio and riverbed gradient. Results arising from this research improve our understanding of controls on the morphology and architectures of braided fluvial channel belts; they provide a novel empirical characterization that can be applied for predicting channel depth, bar morphology, streambed gradient, and degree of braiding of modern fluvial systems and of the formative rivers of ancient preserved successions. 相似文献
67.
Jessica L. Gary Steven M. Colman Nigel J. Wattrus C. F. M. Lewis 《Journal of Paleolimnology》2012,47(3):299-311
Glacial Lake Agassiz, the largest of the North American glacial lakes, discharged through several different outlets during
its history, although the timing and location of discharge remain controversial. However, one discharge event is well established
based on extensive onshore observations: drainage through the Nipigon Lake area into the Superior basin about 10,700 years
ago, following retreat of ice of the Marquette advance from the basin. High-resolution, single-channel seismic-reflection
data collected with a small airgun were acquired to test the hypothesis that the Post-Marquette drainage event left diagnostic
stratigraphic and geomorphic signatures beneath Lake Superior. The unique bathymetry of northwestern Lake Superior, where
water depth plunges off Nipigon and Black Bays, makes this location ideal for the characterization of the post-Marquette depositional
and erosional features. According to our hypothesis, the initial, sudden discharge of high-velocity water would have eroded
channels through the bays. The steep and sudden drop-off into the Superior basin would have caused the flow to slow and drop
much of the sediment it was carrying. Our results confirm the existence of both erosional features and depositional sediment
packages related to Lake Agassiz discharge at this time. The erosional features include deeply incised bedrock channels in
the bays. The depositional features comprise subaqueous fans that are thickest in the deep water areas adjacent to the outlets
and thin lakeward and laterally away from the channels. The seismic character of the basal units of the fans, proximal to
the channels, is chaotic and only very weakly stratified, suggesting that these deposits represent coarse sediment laid down
during the initial, high-energy stages of the flood. These sediments are overlain by a stratified package which is interpreted
as the fine grained sediment associated with the later, low-energy stages of the flood. The combination of channels and subaqueous
fans is inferred to be diagnostic of high-energy Lake Agassiz discharge into the Superior basin, and they serve as analogs
for hypothesized discharge at other times. 相似文献
68.
Patterns of development in tourist accommodation enterprises on farms in England and Wales 总被引:2,自引:0,他引:2
The spatial distribution of UK farm-based tourist accommodation is not known in detail, although this type of facility has been an important element in the growth of rural tourism and the most common option for farm diversification. A database of farms in England and Wales offering tourist accommodation has been prepared and the individual entries georeferenced to enable their locations to be determined with respect to selected Scenic Areas (Areas of Outstanding Natural Beauty, Heritage Coasts and National Parks). Exploratory analysis of this database indicates a possible ‘neighbourhood effect’ in which the provision of farm-based accommodation within buffer zones immediately surrounding these designated areas has a higher level of penetration in the farming community than in more distant areas. Further research is outlined to examine the interaction between actors in farming and planning communities using the geographical framework presented here. 相似文献
69.
The majority of coastal Antarctic stations release untreated sewage into the near-shore marine environment. This study examined bacterial reproduction within the temporary sewage-holding tanks of Rothera Research Station (Adelaide Island, Antarctic Peninsula) and monitored sewage pollution in the local marine environment. By continuously flushing the sewage-holding tanks with cold seawater we inhibited microbial reproduction and decreased the numbers of bacteria subsequently released into the sea by >90%. The widespread use of this simple method could significantly reduce the numbers of faecal coliform and other non-native microorganisms introduced into the Antarctic marine environment. 相似文献
70.
David Hughes, Robert Rosner and Nigel Weiss describe what was achieved during a programme on stellar magnetic fields at the Isaac Newton Institute in Cambridge. Over a four-month period more than 90 participants visited the Institute for a mixture of structured workshops and informal collaboration. 相似文献