bluuebunny/crossref_metadata_2025 huggingface dataset:
Snowmelt Runoff Generation
Authors: Woo Ming‐Ko
Published: April 2006

Snowmelt runoff has major influence on water supply and hazards for many temperate and polar regions. The magnitude and timing of runoff depend on the processes of snow accumulation and melt, which are controlled by factors that include latitude, topography, and land use. The amount of runoff production is affected by meltwater loss to evaporation and infiltration, the latter is often limited by frozen soil conditions. Runoff is delivered as flow in the snow, overland flow on bare ground, channeled flow in rills and interhummock cracks, or as subsurface flow in organic soils. The flow may be modified by the presence of snow or river ice in the channels, which often causes runoff delay and amplification of peak flows. Streamflow hydrographs for small basins typically show diurnal cycles with high discharge during warm days and declines in the cool and terminating periods of snowmelt. Hydrographs of large basins are an integration of the runoff patterns of their subbasins. In terms of snow management, the control of snowmelt floods is a consideration, yet many arid regions look to snowmelt runoff as a source of water supply.
Intersite Comparisons of Rainfall‐Runoff Processes
Authors: Jones Julia
Published: April 2006

This article argues that despite the limitations of rainfall‐runoff data, there are compelling reasons for hydrologists to conduct many more intersite comparisons of rainfall‐runoff data. Inferences about hydrologic processes are drawn from an unnecessarily narrow subset of temporal scales, spatial scales, and geographic conditions given the range of data available. In intersite comparison of rainfall‐runoff data, we face the same challenge as hydrologic modelers, namely: how can we discriminate among alternative mechanistic explanations for any given rainfall‐runoff (or runoff‐runoff) dataset? This article (1) provides a justification for additional intersite comparisons given the history and lessons learned from rainfall‐runoff and related studies, (2) demonstrates how intersite comparison helps discriminate among alternative hydrologic mechanisms, and (3) outlines steps involved in conducting intersite comparative analysis with a particular focus on analysis of primary data. The article argues that the best approach to intersite hydrology is (re)analysis of original data, which allows us to (1) expand the sample size to include data not yet analyzed and replicate hypothesis tests; (2) ask new questions, even of old data, posed by the current context for hydrology; (3) make new comparisons among records not formerly juxtaposed; (4) use novel statistical approaches to reveal hitherto obscure features of the data, and (5) use ancillary data to refine hypothesis tests about hydrologic mechanisms. The article provides suggestions for the steps involved in intersite comparison including (!) identifying a question, (2) developing a study design, (3) selecting, accessing, and merging datasets, and (4) choosing statistics for comparison of rainfall and runoff data. The endeavor of intersite comparison of rainfall and runoff data is analogous and complementary to the parallel quest for methods of parameter identification and model structure selection in hydrologic modeling.
Rainfall‐Runoff Modeling: Introduction
Authors: Beven Keith J
Published: April 2006

This section provides an introduction to the theme of rainfall‐runoff modeling in hydrology. It provides a summary of the purposes of rainfall‐runoff modeling; a classification of rainfall‐runoff models; a brief account of process descriptions in rainfall‐runoff models; and a short history of rainfall‐runoff modeling. It also discusses the problem of model choice from the wide range of possibilities available, and the important question of model calibration using different types of observational data before considering the prediction of the effects of future change (when no observations are possible). Finally, a guide is given to the future of rainfall‐runoff modeling. Reference to the other sections in this article are given throughout, as well as to relevant available texts.
Rainfall‐Runoff Modeling: Distributed Models
Authors: Reggiani Paolo , Schellekens Jaap
Published: April 2006

The present article presents a discussion of distributed modeling used in watershed hydrology from a modern perspective. The need for distributed modeling is explained on the basis of situations encountered in hydrological practice. The basic equations governing water and energy transfer in hydrology are introduced at the point‐ or microscale. It is explained how these equations are used in the different approaches through change in spatial scale from the microscale to an intermediate scale, the macroscale, or the megascale, which corresponds to the spatial dimension of some smaller or larger parts of the catchments. The strengths and limitations that have been recognized for the various approaches will be pointed out. As a possible alternative a flux‐based approach, formulated entirely at the megascale, is highlighted. Its potential scope for practical applications will be discussed in the context of the closure problem for mass momentum and energy fluxes in hydrology.
Model Calibration and Uncertainty Estimation
Authors: Gupta Hoshin V , Beven Keith J , Wagener Thorsten
Published: April 2006

All rainfall‐runoff models are, by definition, simplifications of the real‐world system under investigation. The model components are aggregated descriptions of real‐world hydrologic processes. One consequence of this is that the model parameters often do not represent directly measurable entities, but must be estimated using measurements of the system response through a process known as model calibration. The objective of this calibration process is to obtain a model with the following characteristics: (i) the input‐state‐output behavior of the model is consistent with the measurements of catchment behavior, (ii) the model predictions are accurate (i.e. they have negligible bias) and precise (i.e. the prediction uncertainty is relatively small), and (iii) the model structure and behavior are consistent with current hydrologic understanding of reality. This article describes the historic development leading to current views on model calibration, and the algorithms and techniques that have been developed for estimating parameters, thereby enabling the model to mimic the behavior of the hydrologic system. Manual techniques as well as automatic algorithms are addressed. The automatic approaches range from purely random techniques, to local and global search algorithms. An overview of multiobjective and recursive algorithms is also presented. Although it would be desirable to reduce the total output prediction error to zero (i.e. the difference between observed and simulated system behavior) this is generally impossible owing to the unavoidable uncertainties inherent in any rainfall‐runoff modeling procedure. These uncertainties stem mainly from the inability of calibration procedures to uniquely identify a single optimal parameter set, from measurement errors associated with the system input and output, and from model structural errors arising from the aggregation of real‐world processes into a mathematical model. Some commonly used approaches to estimate these uncertainties and their impacts on the model predictions are discussed. The article ends with a brief discussion about the current status of calibration and how well we are able to represent the effects of uncertainty in the modeling process, and some potential directions.
Rainfall‐Runoff Modeling for Assessing Impacts of Climate and Land Use Change
Authors: Bronstert Axel
Published: April 2006

Rainfall‐runoff models are frequently used as a tool to assess the impacts of climate and land‐use changes on the hydrological cycle. This requires that the applied rainfall‐runoff model is able to represent the dynamics of the hydrological cycle of the catchment under study, in particular, if land‐use change is of concern, that is, the effects of land‐use or land‐cover change on the runoff generating processes. This may be accomplished by applying either a process‐based model or a conceptual model if the latter's parameters can be regionalized with respect to land‐use change. Furthermore, the chosen model must be able to “represent” the influence of different climatic/ meteorological boundary conditions. This implies, of course, that we know the main dynamics of the catchment under consideration.
Rainfall‐Runoff Modeling of Ungauged Catchments
Authors: Blöschl Günter
Published: April 2006

Catchments where no runoff data are available are termed ungauged catchments. For these catchments, the parameters of rainfall‐runoff models cannot be obtained by the calibration on runoff data and hence need to be obtained by other methods. Model parameters that require calibration are usually transposed from similar gauged catchments. This article reviews concepts for identifying hydrologic similarity as well as methods for transposing the parameters of both event models and explicit soil moisture accounting (ESMA) models. Model parameters that are physically based are usually measured or inferred from other data within the ungauged catchment of interest. This article summarizes the most important methods and discusses the issues of using point scale field data in rainfall‐runoff models. Alternatives to runoff data for model calibration are suggested. The value of soft data and qualitative field observations is emphasized.
Downward Approach to Hydrological Model Development
Authors: Sivapalan Murugesu , Young Peter C
Published: April 2006

This article presents the top‐down or downward approach to hydrological model development as an alternative to the bottom‐up, reductionist approach, which is the current, dominant paradigm in the hydrological sciences. It discusses the philosophical underpinnings as well as the pros and cons of each approach, and illustrates the application of the downward approach through several examples. These examples variously emphasize three key elements in the application of the downward approach. First, the analysis of identifiable “signatures” or “features” in the available data including the definition of a generic model form that can accommodate these features; second, model development including model structure identification and parameter estimation; and last, model refinement or “fingering down” (Klemes, 1983; Jarvis, 1993), in which the model is improved by the use of additional information or by adding more causal factors. Two main approaches to downward modeling are identified and discussed: the classical stochastic systems approach and the deterministic, conceptual modeling approach. The advantages of the downward approach are highlighted, particularly as they relate to the development of models that have sufficient causal basis to make predictions in basins other than those on which the model was developed.
Open Channel Flow – Introduction
Authors: Graf Walter H , Altinakar Mustafa S
Published: April 2006

The introduction begins with a presentation of the different types of channels as well as with the corresponding flow regimes. Subsequently, the notions of the distribution of velocity and of pressure are exposed.
Transport of Sediments
Authors: Graf Walter H , Altinakar Mustafa S
Published: April 2006

A channel bed, made up of a granular material, is usually of a form which varies in space and time. The movement of the sediments, which make up the bed, represents a rather complex phenomenon. The hydrodynamic equations of the flow over a mobile bed are developed. The different modes of the transport of (noncohesive) sediments as bed load and as suspended load are presented. The formulae for the calculations of the transport of the total load are exposed, as well as their domain of application.
Mountain Streams
Authors: Armanini Aronne
Published: April 2006

This paper illustrates the main features of the hydraulics of mountain streams. The effects of small submergence and riverbed steepness on sediment motion initiation and transport are discussed together with the effect of big roughness elements and pool‐riffle and step‐pool sequences on velocity distribution and flow resistance. Finally, the most used torrent defense techniques are described.
Regulated Lowland Rivers
Authors: de Vriend Huib J
Published: April 2006

The article describes flow, sediment transport, and morphological processes in regulated lowland rivers and the impact of human activities thereon. It gives some elementary examples of the morphological response to deviations from a prismatic straight channel, in an attempt to bridge part of the gap between textbook knowledge and reality. Moreover, it shows that even a regulated river is still to some extent natural, thus variable and unpredictable.
Hydraulics of Wells and Well Testing
Authors: Renard Philippe
Published: April 2006

Well testing and well hydraulics play a major role in applied hydrogeology. While well hydraulics aims at modeling the groundwater behavior in response to a perturbation – such as pumping – in a well, well testing aims at using these models in an inverse approach to infer the properties of the aquifer or of the well itself. The history of well hydraulics and well testing started in 1863 with Dupuit, who developed the first analytical solution to model radial flow to a well in steady state. In 1935, Theis published the most important analytical solution. The Theis solution assumes that the aquifer is confined, bidimensional, homogeneous, and isotropic. Subsequently, numerous models have been developed with the aim to enlarge the domain of applications. This article presents a brief review of these various models and describes their behavior in terms of drawdown and log derivative of the drawdown. The log derivative is used as a tool to help in the identification of the most appropriate model when analyzing field data.
Numerical Models of Groundwater Flow and Transport
Authors: Holzbecher Ekkehard , Sorek Shaul
Published: April 2006

The article gives an introduction to numerical modeling of flow and transport problems and to software tools that are currently in use for modeling such phenomena. Details are explained on numerical approximations leading to different numerical models. Extensions for reactive transport are mentioned. Basic guidelines and criteria are given that should be taken into account by the modeler in order to improve the accuracy of results. Inverse modeling is presented as an advanced feature. Some examples of pre‐ and postprocessing, as implemented in several codes, are given, in addition to fundamental properties of solution methods. Finally, most common codes are listed with basic features and web‐sites.
Sea Water Intrusion into Coastal Aquifers
Authors: Bear Jacob
Published: April 2006

The objective of this article is to present the problem of seawater intrusion into a coastal aquifer and the conceptual and complete mathematical models that describe it. Three seawater intrusion models are presented: two‐ and three‐dimensional sharp interface models and a three‐dimensional transition zone model.
Anthropogenic Land Subsidence
Authors: Gambolati Giuseppe , Teatini Pietro , Ferronato Massimiliano
Published: April 2006

Fluid removal from subsurface reservoirs, in the form of gas, oil, groundwater, geothermal water, and brine, produces a compaction of the depleted formations which migrates totally or partially to the ground surface thus inducing anthropogenic land subsidence. The paper presents: (i) a list of the major subsiding areas worldwide, (ii) a review of the mechanism which causes a measurable settlement above aquifer systems and gas/oil fields, (iii) a description of the currently available techniques to measure land subsidence and in situ rock compaction, (iv) a brief description of some mathematical models to predict the magnitude of subsidence, and (v) a description of a few remedial options that are available to control the event and mitigate the related environmental impact.
Energy Balance and Thermophysical Processes in Snowpacks
Authors: Lehning Michael
Published: April 2006

This contribution discusses heat and mass fluxes relating to snow. The first process treated is heat flux through snow. Snow can be described as a bulk material or as consisting of different phases. When a separate water vapor phase is considered, vertical mass flux due to vapor pressure differences can be treated. A significant amount of heat is transported along with the vapor fluxes because of the phase changes occurring when water molecules enter the vapor phase at one point and deposit back onto the ice matrix somewhere else. The vapor fluxes in snow also cause snow metamorphism changing the crystals' form and size. Equilibrium metamorphism dominates when weak, large‐scale temperature gradients exist, and water molecules are mainly rearranged locally by surface tension differences. Metamorphism is called
kinetic
when vertical vapor fluxes due to a large‐scale temperature gradient lead to a snow crystal re‐formation.

Mass‐ and energy fluxes in the snow cover are driven by surface exchange. The surface turbulent fluxes of sensible heat and moisture are derived from atmospheric surface layer similarity theory. The long‐wave radiation balance leads to a strong surface cooling especially during cold nights. Short‐wave radiation penetrates the snow cover and deposits energy at greater depths. Finally, the surface mass transport process of snow redistribution is treated with its subprocesses, saltation and suspension.
Role of Glaciers and Ice Sheets in Climate and the Global Water Cycle
Authors: Siegert Martin J
Published: April 2006

Glaciers and ice sheets store nearly 70% of the world's freshwater and would raise sea level by around 70 m if they melted. They are, thus, an important component of the global water cycle. Their role in climate can be assessed through current responses to global warming and their influence on the environment during periods of glaciation. At the Last Glacial Maximum, ∼21 000 years ago, the glacial cover caused sea level to be 115 to 130 m lower than at present. The climate of Ice Age Earth was affected in many ways by ice sheets. First, the expansion of ice sheets caused huge changes to the surface albedo and, hence, the radiation budget of the planet. Second, the orography of North America and Eurasia was altered significantly by large ice sheets, which had implications for atmospheric flow. Third, as the ice sheets grew and decayed, the influx of freshwater to the oceans, and in particular the North Atlantic, affected the ocean circulation, which in turn affected climate. The result of this ice–ocean–atmosphere interaction was large‐scale short‐term oscillations in air temperatures, which have been recorded in Greenland ice cores. The ice and climate processes identified for the glacial cycle are relevant today. For example, the Greenland ice sheet is currently melting at its southern margin, and releasing freshwater to the North Atlantic. Importantly, global temperatures were a few degrees higher in the previous interglacial, and the Greenland ice sheet was about half its current size. As global warming continues, the response of the Greenland ice sheet could be to return to its Eemian configuration. Although far smaller in volume, glaciers respond much quicker to climate change than do ice sheets. In the last century, most have experienced decay, and the runoff generated has been estimated to account for at least 10% of the 15 to 20 cm of sea‐level rise measured (seeMass and Energy Balances of Glaciers and Ice Sheets,). In combination, glacier and ice‐sheet melting may send large quantities of water into the oceans, some of it rapidly. This could have serious implications for the climate of the North Atlantic and, through teleconnection, the world.
Permafrost Hydrology
Authors: Hinzman Larry D , Kane Douglas L , Woo Ming‐ko
Published: April 2006

Permafrost is earth material that has temperature at or below 0°C for at least two consecutive summers. Above the permafrost is the active layer, a zone that freezes in winter and thaws in summer. Even though the principles governing water movement in permafrost areas are the same as those in more temperate regions, interactions of extremes in climate and the land surface characteristics render permafrost hydrology different from the hydrology of temperate latitudes. Ice‐rich permafrost prevents infiltration of rainfall or snowmelt water, often maintaining a moist to saturated active layer where the permafrost table is shallow. Most hydrologic activities are confined above ground or in the thin active layer, which supplies summer moisture to plants and for evaporative flux. Limited storage capacity of the thawed zone does not support extended baseflow in a stream, though the proportion of baseflow increases as the percentage of permafrost extent decreases. In areas where permafrost is discontinuous or where it has thawed substantially near the surface, local hydrology may display a marked different character as there are stronger exchanges between the surface water and the ground water system, or water may drain laterally resulting in drier surface conditions. Runoff paths range from interhummock cracks, soil pipes, water tracks to distinct channels; and permafrost affects the ground water and contaminant migration pathways. Understanding the interdependence of permafrost, hydrology, and ecosystems is critically important to enable accurate projections of future conditions in the high latitudes.
Global Water Budgets – Fundamental Theory and Mechanisms
Authors: Adam Schlosser C
Published: April 2006

The global water cycle represents the transport and transformation of water within the Earth system, and, in doing so, distributes freshwater over the Earth's surface. The cycling of water in the Earth's system employs water in all three of its phases: solid, liquid, and gaseous. In its coarsest and most conventional depiction, the Earth system's cycling of water is viewed as the continual displacement of water taken from the ocean, transported through the atmosphere, deposited over land, and ultimately fed back to the ocean. However, many processes and pathways are responsible for this global transit and cycling. Therefore, to monitor the global water cycle in the context of all of these key interactions with the global environment requires a comprehensive representation of the budgets and cycling of all phases of water storage. An analytic discussion is provided to elucidate the various mechanisms that contribute to the global transport of water within and between the Earth's atmosphere, land, and ocean systems
Short‐Term Predictions (Weather Forecasting Purposes)
Authors: Kallos George , Pytharoulis Ioannis
Published: April 2006

In recent years there is a growing need in hydrology for high‐resolution precipitation predictions. Accurate quantitative precipitation forecasts at various spatiotemporal scales can be made by the atmospheric numerical models that utilize sophisticated parameterization schemes to represent the various physical processes. Two soil parameterization schemes are presented here: a) the bucket model which is a simple single‐layer scheme and was the first one to be used in numerical models, and b) the multilayer Oregon State University scheme which is currently utilized in operational and research mode. A number of successful short‐term predictions of severe precipitation events are also presented. These predictions were produced by various full‐physics numerical models, such as the SKIRON/Eta modeling system, RAMS and the UKMO Unified model, under different forcing mechanisms. The performance of the models in representing the spatiotemporal variability of precipitation is mainly discussed. In general, the atmospheric numerical models exhibited a skill in providing accurate short‐term predictions of the spatiotemporal variability and severity of precipitation.
Water and Forests
Authors: Jewitt Graham
Published: April 2006

Years of research in various parts of the world has highlighted the interaction between forests and water. Forests impact on the hydrological cycle affecting processes such as interception, transpiration, infiltration, groundwater recharge, and runoff. These inevitably manifest themselves as impacts on the water resource, both positive and negative. In this article, the role of forests in the hydrological cycle is described, and the potential impacts for water resources are considered.
Land Use and Water Quality
Authors: Baker Andy
Published: April 2006

The literature investigating the relationship between land use and water quality is synthesized. It is demonstrated that although some general correlations between land use and water quality can be observed, in general the relationship is complex with correlations in individual watersheds likely to be site or regionally specific. Land cover measurements discussed include land use, land cover, ecotone, buffer strip, and indices of landscape metrics, whereas water quality issues include spatial and temporal sampling strategies and the use of nonconservative tracers. Examples of land use–water quality relationships are given for both the impacts of urbanization and the intensification of agriculture. A paucity of studies in developing countries and the need to advance models of land use–water quality relationships are highlighted.
Land Use and Water Resources under a Changing Climate
Authors: Giambelluca Thomas W
Published: April 2006

Water resources are becoming increasingly scarce because of the mounting demand for water associated with population growth and economic development. The availability of and need for water resources are strongly influenced by land use. Land use and land cover change can alter the regional hydrologic cycle by changing evapotranspiration (ET), runoff, soil moisture, and perhaps even precipitation, resulting in changes in water supply and demand. In addition to changes associated with land use, climate warming may significantly influence the global hydrological cycle, leading to a variety of regional hydrological changes. Higher atmospheric CO
2
levels and changing climate will influence land cover in various ways. Conversely, many types of land cover change disrupt the global carbon cycle, contributing to changes in atmospheric greenhouse gases. The drivers of climate and land cover change will be subject to complex feedbacks involving the hydrological cycle and water resources. Hence, it is important to consider the web of interacting factors affecting water resources in the context of climate and land use change. This article reviews the effects of land cover and land use change on water supply and demand, the predicted effects of global warming on water resources, the interaction between climate change and land cover change, and the combined effects of land use and climate change on water resources.
Hydromorphological Quality – A Policy Template for Channel Design in River Restoration
Authors: Large Andrew R G , Newson Malcolm D
Published: April 2006

The European Union's
Water Framework Directive
(
WFD
) is the most substantial piece of European water legislation to date and will have a profound effect on how water is managed in Europe over the next 25 years. The term “hydromorphological” was introduced by the WFD to describe the quality of water bodies in addition to physicochemical and ecological elements, and has rapidly become a prominent descriptor. Reference conditions are described as “having no or only very minor alterations” resulting from human activity yet, at the same time, there is considerable confusion as to how far back we have to go to approximate such conditions. A core uncertainty or ambiguity concerns the possible differences between structural definitions of a reference condition and those that relate to processes, or “functionality”. Whilst ecohydrology and hydroecology are clearly critical in the provision of a science foundation for the WFD, the introduction of hydromorphological quality as a measure of the condition of fresh waters requires new (or, at least, innovative) thinking.

A momentous challenge for those involved with the concept and realization of “good ecological quality” is to assess it, monitor it, and restore it within a basin‐wide strategy, considering also the element of land use and management. The potential role of fluvial geomorphology in understanding, assessing, monitoring, and restoring hydromorphological quality in river systems is stressed.
Land‐Atmosphere Models for Water and Energy Cycle Studies
Authors: Nijssen Bart , Bastidas Luis A
Published: April 2006

Land surface models simulate the temporal evolution of the energy and water balance at and near the land surface. These models can be used coupled to an atmospheric model, in which case feedbacks between the land surface and atmosphere are explicitly represented, or they can be used in a stand‐alone, uncoupled mode, in which case observed or simulated atmospheric conditions act as time‐varying boundary conditions. This article reviews the basic layout of land surface models, their history, and their uses, specifically in uncoupled simulations. In this context, the challenges of model performance evaluation and the role of model intercomparison projects are discussed. The article ends with a look toward the future and the next series of developments that land surface models are likely to undergo.
Use of Climate Information in Water Resources Management
Authors: Hartmann Holly C
Published: April 2006

Failures in water resources management, evolving societal objectives, and advances in the scientific understanding of hydroclimatic systems have led to reassessment of techniques traditionally used in support of water resources management policies and practices. Climate information, encompassing a variety of timescales, is clearly useful for water resources management, yet remains underutilized. This article examines historical climate information provided by instrumental records and paleoclimatological indicators, and prognostications of future climate conditions provided by climate forecasts and assessments of climate change, and issues related to their use in water resources management. Two issues are of central importance: nonstationarity and uncertainty. The water resources community must accept that in many cases one of the fundamental assumptions underlying classical hydrology is simply incorrect. However, the water resources management community need not assess climate information and its use in isolation. Networks of researchers, including hydroclimatic and social scientists, are actively seeking more integrated involvement with the water resources management community, are adapting their research agenda and activities to address the needs of decision makers and stakeholders, and are committed to developing practical products and decision‐support tools.
Biochemical Processes/Kinetics
Authors: Voit Eberhard O.
Published: April 2006

Most biochemical processes are catalyzed by enzymes that facilitate the conversion of one type of metabolite into another. Enzymes are proteins whose activity the cells control rapidly through inhibitors, activators, or other modulators and, on a longer time scale, through altered gene expression. Kinetics addresses questions related to the dynamics of enzyme‐catalyzed reactions and of systems of such reactions. As this dynamic is nonlinear and therefore complex, it is presently difficult to analyze large systems in detail. The stoichiometric approach simplifies this complexity by formulating networks of reactions as linear equations, where all influxes into a metabolite pool are balanced against all effluxes from this pool. At the biologically important steady state, the analysis of stoichiometric systems becomes a matter of linear algebra. For smaller systems, enough kinetic information is available to formulate nonlinear models that account for the cell's various means of regulation. Traditionally, the reactions in such models have been based on representations like the kinetic rate law of Henri, Michaelis, and Menten. As these rate laws become cumbersome for systems of more than a few steps, new kinetic approaches use alternative representations, such as power‐law functions, which are manageable for realistically sized pathway systems.
Echocardiography
Authors: Jacobs Lawrence D.
Published: April 2006

Echocardiography is a powerful diagnostic imaging modality that uses the complex interaction of ultrasound waves with the heart and associated tissues to create images of these structures. Echocardiography's versatility comes mainly from the fact that it is able to simultaneously provide structural and physiologic data in real time, noninvasively, and without deleterious effects on biologic tissues.

A basic understanding of the physics of ultrasound and the manner in which ultrasound interacts with the body is crucial for understanding the strengths and weaknesses of echocardiography as a diagnostic tool. It is these basic principles of cardiac ultrasound that have been refined into the more complex and highly useful echocardiographic imaging modalities, such as Doppler, color Doppler, tissue Doppler, and harmonic imaging, which are commonly used in clinical cardiology today. Newer, cutting‐edge technologies, including contrast echocardiography, myocardial perfusion imaging, and real‐time 3‐D echocardiographic imaging have been developed and are just beginning to make their impact felt in clinical use.

As echocardiography is so versatile, with applications in the entire spectrum of cardiac disease, some consider it an extension of the physical examination. It is this clinical use, combined with a steady flow of new innovations and an excellent safety profile, that ensures that echocardiography will remain one of the most powerful and commonly performed diagnostic tests in medicine for the foreseeable future.
Human Motion Analysis
Authors: Morasso Pietro G. , Sanguineti Vittorio
Published: April 2006

Human motion analysis is addressed under three major headings: biomechanics, measurements, and motor control.

The relevant biomechanical principles are subdivided into three main topics: (a) trajectory formation (including the principle of motor equivalence and the kinematic invariances), (b) kinematics (the Denavit–Hartenberg notation, direct and inverse kinematics), and (c) dynamics.

The range of measurement techniques employed in human motion analysis is quite large. We categorize them in the following manner: (a) kinematic measurements (relative angular rotations, absolute angular orientation, position of body points with optical or magnetic techniques, accelerometry), (b) force measurements (wrist sensors and force platforms), (c) EMG measurements, and (d) stiffness measurements.

Basic motor control principles are anlayzed in regard to three main issues: (a) the role of the mechanical properties of muscles, (b) the need of dynamic compensation with feedforward or feedback control mechanisms, and (c) the spatio‐temporal organization of locomotion patterns.
Lyapunov Exponents
Authors: Dingwell Jonathan B.
Published: April 2006

The analysis of potentially chaotic behavior in biological and biomedical phenomena has attracted increased attention in recent years. Chaotic systems exhibit a trademark “sensitive dependence on initial conditions” (SDIC), meaning that very small perturbations will cause the resulting trajectories to separate exponentially fast. Lyapunov exponents directly measure SDIC by quantifying the exponential rates at which neighboring orbits on an attractor diverge (or converge) as the system evolves in time. Dissipative deterministic systems that exhibit at least one positive Lyapunov exponent are by definition “chaotic.” Hence, Lyapunov exponents are considered one of the most useful diagnostic tools available for analyzing potentially chaotic dynamical systems. However, Lyapunov exponents are also notoriously difficult to estimate reliably from experimental data and one should always be very skeptical about accepting any claims of “chaos” based solely on findings of positive Lyapunov exponents alone. This article reviews the basic definitions of Lyapunov exponents and outlines several of the more widely disseminated algorithms available to estimate them from experimental time series data. Critical issues related to finding spurious exponents, analyzing noisy signals, and validating against surrogate data are reviewed. A brief discussion of how Lyapunov exponents have been applied to study a wide variety of biomedical and biological phenomena is also presented. Finally, some thoughts on appropriate interpretations of such findings and the future directions for using Lyapunov exponents to study potentially chaotic systems are presented.
Medical Devices: Regulations, Codes, and Standards
Authors: Munzner Robert
Published: April 2006

The approach that various countries have taken to regulate the marketing of medical devices differs, greatly as does their use of standards for medical devices. The United States has the most complex system because of its highly detailed legislation and the numerous amendments to that legislation. Whereas the European Community has adopted a method‐based certification of conformance to standards, the U.S. rules do not require conformance to standards. The complexity of the U.S. system is because of its evolution from a law that presumed medical devices being marketed before enactment of the law are reasonably safe and effective unless proven otherwise. Most new devices enter the U.S. market based on “substantial equivalence” to devices that are presently being marketed. Except for devices that have been exempted, the specifications of all devices entering the U.S. market are examined by the U.S. Food and Drug Administration.
Medical Expert Systems
Authors: Hudson Donna L.
Published: April 2006

The first medical expert system was designed over 30 years ago. Since that time, advances in both computer technology and software design have permitted the development of increasingly sophisticated models. Most medical expert systems are designed to function as decision support tools. The original systems used techniques from artificial intelligence as a basis for developing models that were symbolic in nature in which the reasoning structure and knowledge base were separate entities. The hallmark of these systems is the use of natural language, the easy representation of concepts, and the ability to provide explanations. Relevant artificial intelligence methodologies include knowledge representation, knowledge acquisition, and problem solving. In the course of thirty years, many additional techniques have been employed, including causal and deep reasoning, fuzzy logic, approximate reasoning, hybrid systems, consequential reasoning, and intelligent agents. Hybrid systems and intelligent agent models meld symbolic techniques with data‐based models to bring all available information to bear on a problem. Medical applications include all major specialties and range from very specific areas such as antibiotic therapy to entire domains such as internal medicine.
Microsensors and Nanosensors
Authors: Nagel David J.
Published: April 2006

Sensors are systems or devices that provide signals, which are correlated to some physical, chemical, or biological condition in their neighborhood. The signals can be transformed, by use of a calibration curve, into information about the environment of a sensor. There are two classes of sensors, point sensors that provide information only from their immediate surroundings and volume sensors (imagers or cameras) that can give information on conditions in an extended region. Sensors are old, but recent advances in miniaturization technologies have been leading to very small and capable sensors. Micromachining processes, which grew out of the microelectronics industry, have been used for about two decades to make microelectromechanical systems (MEMS) that are relative cheap and extraordinarily reliable. Microsensors for pressure and acceleration are now sold by the tens of millions annually. In the past decade, processes have been developed to grow nano‐materials, which have structures on the scale of molecules. They are proving to be very useful for small sensors, especially for chemical and biological applications. Two‐dimensional arrays of bio‐sensors, based on DNA or antibodies to recognize the target molecules in a sample, are especially important. They, and other small sensors, enable almost immediate diagnostics at the point of care, rather than sending samples to a central laboratory. Small sensors make it possible to put diagnostic equipment inside the body. They are expected to have an increasing impact on biomedical engineering and clinical medicine.
Nanophase Materials
Authors: Ballard Jake D. , Dulgar-Tulloch Aaron J. , Siegel Richard W.
Published: April 2006

Great strides are being made in our ability to synthesize and assemble nanoscale building blocks to create nanophase materials with novel properties and functionalities. The novel properties of nanostructures are derived from their confined sizes and their very large surface‐to‐volume ratios. The former give rise to unique size‐dependent properties in the nanoscale (1–100 nm) regime, while the latter give rise to the ability of nanoscale additions to conventional material matrices to dramatically change the host material's properties. Nanostructured surfaces have also been shown to elicit more favorable and selective biomolecule and cellular responses than surfaces at coarser length scales. The characteristics of nanophase materials are described and what is known about their interactions with proteins and cells is reviewed with extensive references. While many studies have been carried out to probe such interactions, we are still at a very early stage of understanding the fundamental issues controlling these interactions. Much careful work remains before these important interactions are fully understood so that they can be controlled to impact biomedicine to benefit society.
Orthotics
Authors: Fatone Stefania
Published: April 2006

Orthoses are externally applied devices used to modify the structural or functional characteristics of the neuro‐musculo‐skeletal system. They can be fitted to almost any part of the body from the leg to the arm, spine, and head, and are named systematically based on the principle joint/s they encompass. The basic functional goals of orthoses are to provide support, to correct deformity, or to modify motion occurring at a joint. Three general biomechanical principles apply to most orthotic devices: moments and force couples, pressure tolerance of tissues, and alignment of joint axes. When fabricating an orthosis, material selection depends on the function to be performed by the orthosis and with the orthosis. Metals and thermoplastics are the most common materials in modern orthotics, although thermosets are also used. Many different kinds of orthoses are available, not only with respect to the body part they encompass, but also how they are fabricated: Orthoses may be either prefabricated or custom‐fabricated. Three main categories of orthoses exist: Lower limb orthoses, from the foot to the hip, generally play an important role in weight‐bearing and locomotion; spinal orthoses, including cranial orthoses, are used to reduce pain, correct or decrease the progression of deformity, and prevent or minimize further injury; and upper limb orthoses, both static and dynamic, play an important role in aiding function of the upper limb, especially the hand. Within each category, many different types of orthoses are available. The main types of orthoses are described with respect to their treatment objectives, materials from which they are constructed, and mechanisms of action.
Sleep Apnea Syndrome
Authors: Heinzer Raphael , Jordan Amy , Malhotra Atul
Published: April 2006

Diseases affecting the respiratory system are usually present during the day and the night; however, sleep apnea/hypopnea syndrome is a disease manifesting only during sleep. It is defined as a cessation (apnea) or a reduction of breathing (hypopnea) of 10 seconds or more during sleep. Each of these apneas or hypopneas is usually terminated when the person awakens for a few seconds. Typically, patients do not remember these transient awakenings.

There are two general categories of apnea: central and obstructive. These categories are classified according to the presence or the absence of respiratory effort. Central sleep apnea is a result of the absence of nerve impulses from the brain to the respiratory muscles and therefore occurs in absence of respiratory effort. Obstructive sleep apnea is much more common. This condition is caused by repetitive obstruction of the upper airway (pharynx) during sleep. Therefore, respiratory efforts are still present during apnea and usually increase throughout the apnea. The individual makes larger and larger respiratory efforts until they arouse from sleep apnea and reopen their upper airway so breathing can resume. As will be discussed, some overlap exists between obstructive and central apnea in both disease presentation (so‐called mixed apnea) and in pathogenesis.
Human Nervous System, Noninvasive Coupling of Electronically Generated Data into
Authors: Kercel Stephen , Bach-y-Rita Paul
Published: April 2006

The tongue display unit (TDU) is a practical technology for coupling machine‐generated data into the human nervous system. Through the TDU, a human user experiences what the machine captures by direct perception. It is an enabling technology through which the human literally becomes “one with the machine,” as if the machine were another appendage of the body. Unlike conventional human‐machine interface (HMI) analog and digital displays, direct coupling to the nervous system is based on unconscious integration and is far faster and more effective than conscious interpretation of data. TDU‐based human‐machine synergism enables anticipatory behavior derived from themeaningof a data stream, rather than a reactive response to individual data. It provides a means to integrate anticipatory function into manmade processes. Unlike the invasive direct‐coupling HMI strategies being investigated by many researchers, this interface requires no surgical implants.

The TDU, developed at the University of Wisconsin Medical School, uses electrotactile contact with the tongue. Prototype TDU‐based sensory prosthesis have been applied to the restoration of lost senses, such as sight to blind persons and balance to victims of vestibular loss. Earlier vibro‐ and electrotactile interfaces to various skin areas (abdomen, finger, forehead, back) have also been used with artificial sensors for vision substitution and to restore the lost sense of touch in victims of leprosy.

The TDU's potential goes beyond enabling the restoration of lost senses. Decades of experience in sensory substitution suggest that the brain can abstract meaning from many different forms of data passing through the TDU, implying that it can enable subjects to directly experience processes that would otherwise be outside the range of human sensibilities. For example, using this interface to monitor a large computer network, an administrator may be able to unconsciously abstract meaning from data flows, and to sense problems in the network “by feel.” In other words, the TDU provides a practical means of creating artificial senses.

This development has many potential broad impacts. First, it heralds a new and highly significant type of relationship between human and machine; they become one. Second, it enables a level of process monitoring that is unavailable through any other technology, enabling dramatic improvements in the quality, safety, and reliability of practically any industrial or commercial processes. Third, it provides a novel strategy for solving many of the most perplexing problems in the information technology industry.