共查询到20条相似文献，搜索用时 15 毫秒
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The word geoid appeared for the first time in 1873 in a work by Listing, at a time when an increasing number of measurements of meridian arcs were made, in order to determine the figure of the Earth.From that time, the mathematical methods (least squares, inverse methods ...) and the observation and modelling methods (collecting data, precision ...) have made concurrently great progress. Owing to these advances, the geometric method first, and the dynamic method next were developed by geodesy for the computation of the geoid. In particular, the increasing of data brought by satellites since the 60's led to a detailed knowledge of the geoid.The determination of the geoid is, for geodesy, a goal in itself. The geophysical viewpoint is to study the inner structure and the internal dynamics of the Earth, by the interpretation of the shape of the geoid and its anomalies. The two approaches are complementary to one another and react one on the other: from geodesy to geophysics and from geophysics to geodesy.
The geoid has three advantages that can be stated in a few words each, although the third advantage brings in a complex subject. They are: first, the advantage of thinking in terms of small numbers; second, the fact that we know or can know directly the form of the geoid as a physical reality over much of the earth's surface; third, the geophysical implications of a knowledge of the geoid.
Walter D. Lambert, 1961 相似文献
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Stuart Hamilton 《水文研究》2007,21(22):3105-3106
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Song Chunlei Cao Xiuyun Li Jianqiu Li Qingman Chen Guoyuan Zhou Yiyong 《中国科学D辑(英文版)》2006,49(1):102-113
Phosphatase may accelerate the process of lake eutrophication through improving phosphorus bioavailability. This mechanism was studied in three Chinese eutrophic shallow lakes (Lake Taihu, Lake Longyang and Lake Lianhua). Phosphatase activity was related to the concentration of soluble reactive phosphorus (SRP) and chlorophyll a. Stability of dissolved phosphatase in reverse micelles may be attributed to molecular size, conformation and active residues of the enzyme. At the site with Microcystis bloomed in Lake Taihu, dissolved phosphatase activity was higher and more stable in micelles, SRP concentrations were lower in interstitial water, the contents of different forms of phosphorus and the amounts of aerobic bacteria were lower while respiration efficiency was higher in sediments. Phosphobacteria, both inorganic and organic and other microorganisms were abundant in surface water but rare in sediments. Therefore, internal phosphorus may substantially flux into water column by enzymatic hydrolysis and anaerobic release, together with mobility of bacteria, thereby initiating the bloom. In short, biological mechanism may act in concert with physical and chemical factors to drive the internal phosphorus release and accelerate lake eutrophication. 相似文献
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Contributions of phosphatase and microbial activity to internal phosphorus loading and their relation to lake eutrophication 总被引：3，自引：2，他引：3
Phosphatase may accelerate the process of lake eutrophication through improving phosphorus bioavailability. This mechanism was studied in three Chinese eutrophic shallow lakes (Lake Taihu, Lake Longyang and Lake Lianhua). Phosphatase activity was related to the concentration of soluble reactive phosphorus (SRP) and chlorophyll a. Stability of dissolved phosphatase in reverse micelles may be attributed to molecular size, conformation and active residues of the enzyme. At the site with Microcystis bloomed in Lake Taihu, dissolved phosphatase activity was higher and more stable in micelles, SRP concentrations were lower in interstitial water, the contents of different forms of phosphorus and the amounts of aerobic bacteria were lower while respiration efficiency was higher in sediments. Phosphobacteria, both inorganic and organic and other microorganisms were abundant in surface water but rare in sediments. Therefore, internal phosphorus may substantially flux into water column by enzymatic hydrolysis and anaerobic release, together with mobility of bacteria, thereby initiating the bloom. In short, biological mechanism may act in concert with physical and chemical factors to drive the internal phosphorus release and accelerate lake eutrophication. 相似文献
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In this paper we study the rooted tree model applying the concepts of probability to obtain results of importance in understanding power-law distributions in pure populations and also in an ensemble of pure populations. The well-known Gutenberg-Richter relation, which is an empirical relation providing the number of earthquakes whose magnitude exceeds a given value, is shown to be an asymptotic form of survivor function of earthquake magnitudes. The implications of this model are briefly discussed in relation to other branches of sciences where power-law distributions are encountered. 相似文献
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ABSTRACTWe thank van Emmerik et al. for their discussion of our opinion paper. We do not fully agree that publication of negative results will be very effective in promoting experimental hydrology. Instead, we think that experimental hydrology is considered to be more risky than modelling studies because of difficulties in publishing inconclusive results and the potential for failure. 相似文献
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Variational data assimilation methods optimize the match between an observed and a predicted field. These methods normally
require information on error variances of both the analysis and the observations, which are sometimes difficult to obtain
for transport and dispersion problems. Here, the variational problem is set up as a minimization problem that directly minimizes
the root mean squared error of the difference between the observations and the prediction. In the context of atmospheric transport
and dispersion, the solution of this optimization problem requires a robust technique. A genetic algorithm (GA) is used here
for that solution, forming the GA-Variational (GA-Var) technique. The philosophy and formulation of the technique is described
here. An advantage of the technique includes that it does not require observation or analysis error covariances nor information
about any variables that are not directly assimilated. It can be employed in the context of either a forward assimilation
problem or used to retrieve unknown source or meteorological information by solving the inverse problem. The details of the
method are reviewed. As an example application, GA-Var is demonstrated for predicting the plume from a volcanic eruption.
First the technique is employed to retrieve the unknown emission rate and the steering winds of the volcanic plume. Then that
information is assimilated into a forward prediction of its transport and dispersion. Concentration data are derived from
satellite data to determine the observed ash concentrations. A case study is made of the March 2009 eruption of Mount Redoubt
in Alaska. The GA-Var technique is able to determine a wind speed and direction that matches the observations well and a reasonable
emission rate. 相似文献
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The quantification of diffuse input loads of nutrients to rivers is a challenge due to limited observed data. This study aimed to develop a simple model that can relate in-stream nutrient concentrations due to diffuse sources with land cover categories within a catchment affecting a river reach. A previously developed point-diffuse model was used to distinguish the diffuse nutrient signature within South African Department of Water Affairs historical monitoring flow and water quality data for selected river gauges. The diffuse signature was related to land cover categories within respective catchments using Principal Component Analysis (PCA), and influential land cover categories were used to construct land cover models relating land cover categories with in-stream nutrient concentrations. Generally, the land cover categories affecting diffuse signatures of nutrients as indicated by PCA were expected. Using land cover information, the developed land cover models performed well in re-creating the diffuse in-stream nutrient signature as determined by the point-diffuse model. 相似文献
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We propose a new method to constrain lateral variations of temperature and composition in the lower mantle from global tomographic models of shear- and compressional-wave speed. We assume that the mantle consists of a mixture of perovskite and magnesio-wüstite. In a first stage, we directly invert VP and VS anomalies for variations of temperature and composition, using the appropriate partial derivatives (or sensitivities) of velocities to temperature and composition. However, uncertainties in the tomographic models and in the sensitivities are such that variations in composition are completely unconstrained. Inferring deterministic distributions of temperature and composition being currently not possible, we turn to a statistical approach, which allows to infer several robust features. Comparison between synthetic and predicted ratios of the relative shear- to compressional-velocity anomalies indicates that the origin of seismic anomalies cannot be purely thermal, but do not constrain the amplitude of the variations of temperature and composition. We show that we can estimate these variations using histograms of the relative VP and VS anomalies at a given depth. We computed histograms for a large variety of cases and found that at the bottom of the mantle, variations in the volumic fraction of perovskite from −14 to 10% are essential to explain seismic tomography. In the mid-mantle, anomalies of perovskite are not required, but moderate variations (up to 6%) can explain the observed distributions equally well. These trade-offs between anomalies of temperature and composition cannot be resolved by relative velocity anomalies alone. An accurate determination of temperature and composition requires the knowledge of density variations as well. We show that anomalies of iron can then also be resolved. 相似文献
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Vegetation dynamics and hydrological processes are major components of terrestrial ecosystems, and they interact strongly with each other. Studies of hydrological responses to vegetation dynamics are usually conducted on a long-term scale, whereas the hydrological responses within a single year have rarely been studied. In the present study, Poyang Lake runoff (PYL-R) model, a new hydrological model coupled with leaf area index (LAI) remote sensing products, was established and applied to simulate the runoff process in the Poyang Lake Watershed. The simulation results obtained in three sub-watersheds of the Poyang Lake Watershed (Ganjiang Watershed, Xinjiang Watershed, and Fuhe Watershed) agreed well with the observations (Nash efficiency coefficient values and R values exceeded 0.6 and 0.9, respectively). The PYL-R experiment (PYL-R-E) model was designed as a contrast model without considering the impact of LAI. The simulated monthly runoff results obtained using the PYL-R and PYL-R-E models were compared, and the within-year changes in the differences between the two results were analysed to evaluate and quantify the impact of vegetation dynamic on runoff. From January to July, when LAI values increased by around 2.6 m2 m−2, monthly runoff depth differences between PYL-R and PYL-R-E results increased by 35.25, 27.98, and 29.14 mm in the Ganjiang, Xinjiang, and Fuhe watersheds, respectively. Dense vegetation caused high interception and evapotranspiration during summer, which largely reduced runoff. By contrast, during winter, the effect of vegetation was weaker on runoff process whereas the impacts of other factors (e.g., precipitation) were higher. The sensitivity of monthly runoff to vegetation dynamics varied greatly throughout the whole year. In particular, during August and September, the LAI-caused runoff changes were very high, accounting for 28–42% of monthly runoff in the sub-watersheds. Our findings clarify the effects of changes in vegetation on hydrological processes over short time scales, thereby providing insights into the effects of scale on eco-hydrological processes. 相似文献
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《Water Policy》2000,1(6):623-636
This article invites policy makers to reframe some familiar policy debates on water through using history. While violence has and will continue around water, water is far more humanity’s learning ground for building community than it is a cause of war. Increased interdependence through water sharing plans and infrastructure networks can be seen as increases of our flexibility and capacity to respond to exigencies of nature and reduce our vulnerability to events such as droughts and floods and thereby increase security. The history of social organization around river basins and watersheds is humanity’s richest records of our dialogue with nature. It is among the most fertile areas for learning about how the political and technical interact. The spatial and functional characteristics of the river basins influenced human settlement and interaction long before the idea of the river basin started to be formalized into legal and administrative terms. The direction of flow of rivers influenced the movement of civilization. Rivers have been crucial to means of communication leading to the formation of political units. The article concludes with calling for new ethics in water management. It calls for an ethic built on: a sense of purpose and on an active co-designing with nature and not solely on preservation; a balance between humans and technology and among structural and on-structural approaches; and a new balance of the sacred and utilitarian in water. 相似文献
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关于应力触发和应力影概念在地震预报中应用的一些思考 总被引：39，自引：3，他引：36
应力触发和应力影概念及其在地震预报中的应用,是学术界关注的热点,尚有一系列需要研究的问题。例如初始应力场的确定,合理流变模型的选取和计算,孔隙压力的变化和影响,断层强度随时间的变化等。特别在估计应力场变化时,人们现今只注意到大地震对应力场造成的变化。一些新的观测事实表明,大量的小地震效应,慢地震的效应和无震滑动的效应,也不容忽视。定量考虑它们的效应,是今后应力触发和应力影研究中应该注意的课题。 相似文献
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Jacob Bortnik Richard M. Thorne Nigel P. Meredith 《Journal of Atmospheric and Solar》2009,71(16):1636-1646
A brief overview is given of the history of plasmaspheric hiss research, particularly in the context of the recent work by Bortnik et al. (2008) indicating that chorus could be the likely source of plasmaspheric hiss. Previous suggestions given in the literature for this theory are reviewed and then the mechanism itself is outlined, focusing on the characteristic cyclical trajectories executed by typical ray paths that enter into the plasmasphere. A number of directional propagation studies performed in the past are then discussed as well as other work which bears relevance to the present mechanism. 相似文献
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提出了直接找渍气的必要性和可能性,回顾了以往失误的原因,推荐了百灵软件,并期望各石油公司经理层能使用该软件,化较小的代价对地震资料进一步解释利用而提高钻井成功率,并最终降低油气成本。 相似文献