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321.
This study deals with the variability of mixing height during daylight hours in the summer months for weak wind regimes. A two-dimensional model was employed using simulated input variables which are quite representative of conditions found over the midwestern United States in late summer and early fall. With the aid of this model and various analytical techniques, the dependence of the urban mixing height on such factors as horizontal advection, downward heat flux across the stable mixing-layer interface, lapse rate in the stable layer, etc., was delineated and compared with actual mixing height variations observed in St. Louis, Missouri during selected days for August, 1972.The experiment indicated the following: (1) A spatially symmetric surface heating profile over a city is accompanied by a similarly symmetric mixing-height profile in the absence of vertical wind shear; (2) When the same heating assumption is invoked and vertically variable wind profiles are introduced, the model-generated mixing-height contours become increasingly asymmetric with vertical wind shear; (3) The modelled mixing heights are more sensitive to temperature fluctuations than to those of wind over the range of speeds studied (wind speeds 4ms–1); (4) Present operational methods of predicting the time of erosion of an inversion (based upon forecast surface temperature ranges and adiabatic diagram considerations) underestimate breakup time by a factor which is proportional to the amount of available downward heat flux from the stable layer into the mixed layer below. 相似文献
322.
Disequilibrium response of permafrost in boreal continental western Canada to climate change 总被引:6,自引:0,他引:6
In the boreal forest of continental western Canada, permafrost is restricted toSphagnum-dominated peatlands on which air photo interpretation reveals the occurrence of five types of surface physiography. Concentrated in the northern part of the boreal forest, permafrost is present in peat plateaus with and without collapse scars. In the southern part of the boreal forest, continental bogs dominate, representing ombrotrophic peatlands that have never contained permafrost. In the midboreal zone, internal lawns are present in bogs and in fens. These internal lawns do not presently contain permafrost but did in the recent past, representing degradation of permafrost since the Little Ice Age. Evaluation of the distribution of these peat landforms indicates that today 30% of bogs contain permafrost at the –0.4 °C isotherm and 50% of bogs contain permafrost at the –1.2 °C isotherm, whereas in the past, 30% of bogs contained permafrost at the –1.4 °C isotherm and 50% of bogs contained permafrost at the –2.3 °C isotherm. Although spatial degradation has occurred with a shifting of permafrost northwards in response to warming since the Little Ice Age, permafrost cover has increased in any given area where present-day temperatures are between 0.5 and –3.5 °C. 相似文献
323.
Models that address the impacts of climate change on forests are reviewed at four levels of biological organization: global, regional or landscape, community, and tree. The models are compared for their ability to assess changes in fluxes of biogenic greenhouse gases, land use, patterns of forest type or species composition, forest resource productivity, forest health, biodiversity, and wildlife habitat. No one model can address all of these impacts, but landscape transition models and regional vegetation and land-use models have been used to consider more impacts than the other models. The development of landscape vegetation dynamics models of functional groups is suggested as a means to integrate the theory of both landscape ecology and individual tree responses to climate change. Risk assessment methodologies can be adapted to deal with the impacts of climate change at various spatial and temporal scales. Four areas of research needing additional effort are identified: (1) linking socioeconomic and ecologic models; (2) interfacing forest models at different scales; (3) obtaining data on susceptibility of trees and forest to changes in climate and disturbance regimes; and (4) relating information from different scales.The U.S. Government right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.Managed by Martin Marietta Energy Systems, Inc., for the U.S. Department of Energy under contract DE-AC05-84OR21400. 相似文献
324.
325.
Data from wellfields in the Scioto River valley aquifer near Piketon, Ohio, USA, were used to evaluate groundwater quantity
and quality and to formulate wellfield-management concepts for future use of the alluvial aquifer. The percentage of induced
river infiltration pumped from wells next to the river was estimated using a numerical groundwater-flow model and chemical
mass-balance techniques. These analyses indicate that 50 to 88 percent of the wellfield water is derived from induced infiltration
of river water. Model simulations correlate the percentage of well water from induced infiltration to wellfield pumpage, river
stage, proximity of wells to the river, riverbed hydraulic conductivity, aquifer hydraulic conductivity, and river-stage level.
Induced infiltration of river water has influenced the quality of groundwater, making the wellfield water more similar to
the chemical character of the river than to that of the alluvial aquifer.
Resumen: Los datos procedentes de un conjunto de pozos en el acuífero de valle del Río Scioto, cerca de Pinkerton, Ohio; se usaron para evaluar la cantidad y calidad de las aguas subterráneas, así como para planificar la gestión de los pozos cercanos al mismo, fue estimado mediante un modelo numérico de flujo y técnicas de balance químico de masa. Los análisis indican que entre el 50 y 88 porciento del agua bombeada procede de infiltración inducida. Simulaciones realizadas con el modelo permitieron correlacionar el porcentaje del agua bombeada que proviene de infiltración inducida, con distintas variables: caudal de bombeo, tramo del río, proximidad de los pozos al río, conductividad hidráulica del lecho del río, conductividad hidráulica del acuífero y nivel del tramo de río. La infiltración inducida ha afectado a la calidad del agua de los pozos, cuyo carácter químico es más parecido al del agua del río que al del acuífero aluvial.
Résumé: Les données provenant de champs captants dans l'aquifère de la valée de la Scioto, près de Piketon (Ohio), ont été utilisées pour évaluer la quantité et la qualité de l'eau souterraine et pour définir les conditions de gestion de l'exploitation future de cet aquifère alluvial. La part de la réalimentation par la rivière, sous l'effet de pompages, dans les eaux des puits voisins du cours d'eau, a été estimée au moyen d'un modèle numérique d'écoulement souterrain et par des méthodes chimiques du bilan de matière. Ces analyses indiquent que 50 à 88 pourcent de l'eau pompée provient de la réalimentation par la rivière. Les simultations corrèlent la part pompée provenant de la réalimentation avec les pompages, le niveau de la rivière. La réalimentation provoquée à partir de la rivière a influencé la qualité de l'eau souterraine en conférant à l'eau des puits des caractères plus proches de ceux de la rivière que de ceux de l'aquifère alluvial.相似文献
326.
327.
Allison L. Dugdale Christopher J. L. Wilson Lawrence D. Leader Jamie A. Robinson L. Jonathon Dugdale 《Mineralium Deposita》2009,44(2):205-219
Historically, carbonate spots have been identified as an indicator of gold mineralization throughout central Victoria, Australia.
However, the exact timing relationships between the growth of carbonates, development of deformation fabrics, and the introduction
of gold has only been determined in more recent times through isolated studies on individual gold deposits. Detailed examination
of the evolution of hydrothermal alteration associated with the Magdala gold deposit at Stawell recognized the fact that there
were at least two generations of carbonate growth, an early rounded ankerite phase that predated gold mineralization and a
later euhedral siderite phase coincident with gold mineralization. This pattern of carbonate growth is repeated in the majority
of significant gold deposits, including Bendigo and Ballarat, throughout central Victoria. Timing relationships within the
carbonates suggest that a fluid was introduced along bedding planes and early deformation fabrics prior to the main upright
folding events that significantly modified the original sedimentary basin. It is suggested that the early rounded carbonates
may have formed as a result of anaerobic oxidation of methane, derived from the sediments and advected along normal growth
faults within the sedimentary basin, through interaction with downward diffusing seawater sulfate. Although the growth of
the early carbonates is not related to gold mineralization, the change in the speciation of the carbonate during the later
carbonate event is critical and can be tracked using a simple geochemical index that can be used not only in areas of outcrop
but also in conjunction with exploration undercover. 相似文献
328.
329.
Frances Rivera‐Hernandez Dawn Y. Sumner Tyler J. Mackey Ian Hawes Dale T. Andersen 《Sedimentology》2019,66(3):917-939
Perennially ice‐covered lakes can have significantly different facies than open‐water lakes because sediment is transported onto the ice, where it accumulates, and sand grains preferentially melt through to be deposited on the lake floor. To characterize the facies in these lakes, sedimentary deposits from five Antarctic perennially ice‐covered lakes were described using lake‐bottom observations, underwater video and images, and sediment cores. One lake was dominated by laminated microbial mats and mud (derived from an abutting glacier), with disseminated sand and rare gravel. The other four lakes were dominated by laminated microbial mats and moderately well to moderately sorted medium to very coarse sand with sparse granules and pebbles; they contained minor interstitial or laminated mud (derived from streams and abutting glaciers). The sand was disseminated or localized in mounds and 1 m to more than 10 m long elongate ridges. Mounds were centimetres to metres in diameter; conical, elongate or round in shape; and isolated or deposited near or on top of one another. Sand layers in the mounds had normal, inverse, or no grading. Nine mixed mud and sand facies were defined for perennially ice‐covered lakes based on the relative proportion of mud to sand and the style of sand deposition. While perennially ice‐covered lake facies overlap with other ice‐influenced lakes and glaciomarine facies, they are characterized by a paucity of grains coarser than granules, a narrow range in sand grain sizes, and inverse grading in the sand mounds. These facies can be used to infer changes in ice cover through time and to identify perennially ice‐covered lakes in the rock record. Ancient perennially ice‐covered lakes are expected on Earth and Mars, and their characterization will provide new insights into past climatic conditions and habitability. 相似文献
330.
A.W. Dale P. Van Cappellen D.R. Aguilera P. Regnier 《Earth and Planetary Science Letters》2008,265(3-4):329-344
A simplified version of a kinetic–bioenergetic reaction model for anaerobic oxidation of methane (AOM) in marine sediments [Dale, A.W., Regnier, P., Van Cappellen, P., 2006. Bioenergetic controls on anaerobic oxidation of methane (AOM) in coastal marine sediments: a theoretical analysis. Am. J. Sci. 306, 246–294.] is used to assess the impact of transport processes on biomass distributions, AOM rates and methane release fluxes from the sea floor. The model explicitly represents the functional microbial groups and the kinetic and bioenergetic limitations of the microbial metabolic pathways involved in AOM. Model simulations illustrate the dominant control exerted by the transport regime on the activity and abundance of AOM communities. Upward fluid flow at active seep systems restricts AOM to a narrow subsurface reaction zone and sustains high rates of methane oxidation. In contrast, pore-water transport dominated by molecular diffusion leads to deeper and broader zones of AOM, characterized by much lower rates and biomasses. Under steady-state conditions, less than 1% of the upward dissolved methane flux reaches the water column, irrespective of the transport regime. However, a sudden increase in the advective flux of dissolved methane, for example as a result of the destabilization of methane hydrates, causes a transient efflux of methane from the sediment. The benthic efflux of dissolved methane is due to the slow growth kinetics of the AOM community and lasts on the order of 60 years. This time window is likely too short to allow for a significant escape of pore-water methane following a large scale gas hydrate dissolution event such as the one that may have accompanied the Paleocene/Eocene Thermal Maximum (PETM). 相似文献