bluuebunny/crossref_metadata_2025 huggingface dataset:
Soil Water Potential Measurement
Authors: Durner Wolfgang , Or Dani
Published: April 2006

Soil water potential controls the dynamics of water in soils. This article describes methods to measure soil water potential. Available methods differ with respect to the moisture range where they are applicable. In the moist range, the pore water pressure of the soil is directly measured with tensiometers. In the intermediate moisture range, reference porous media are suitable, where the water saturation changes with water potential. Gypsum blocks, granular matrix sensors, heat dissipation matric potential sensors, and filter paper are used. If embedded in the soil and in hydraulic equilibrium with it, soil water potential is obtained by a calibration relationship from a water content dependent measurement in the reference porous medium. In relatively dry soils, water potential is derived from vapor pressure measurements by thermocouple psychrometry. This article presents the theoretical background of the various measurement techniques and illustrates the construction principles of measuring instruments. Problems and pitfalls of water potential measurements, in particular with tensiometers, are discussed. A list of manufacturers of water potential measurement instruments is given at the end of the article and a reference list provides material for further background information.
Measuring Sediment Loads, Yields, and Source Tracing
Authors: Leeks Graham
Published: April 2006

The sediment load of rivers is, for both practical purposes and field research, usually divided into the finer suspended load and coarser bed load. A range of instruments can be used, depending on the mode of sediment transport. Measurements of the bed load can be made using fixed and portable traps, samplers and tagging methods, and electronic detection systems. Suspended sediment load is usually measured with both manual and automatic samplers. Continuous measurements are made possible by using turbidity as a surrogate measure of suspended sediment concentrations. An advantage in the use of the manual and automatic samplers is to provide samples which can be subjected to further analysis (e.g. of particle size distributions or chemistry). One of the opportunities this can provide is to identify up‐catchment sediment sources, including channel, gully, sheet/rill, and surface erosion under different land uses. Source “fingerprints” are identified from combinations of sediment properties. Multivariate analysis and mixing models can then be used to quantify the relative contributions from individual sources.
On the Worldwide Riverine Transport of Sediment – Associated Contaminants to the Ocean
Authors: Swarzenski Peter W , Campbell Pamela L
Published: April 2006

The continuous and persistent weathering of the continents and the ensuing transport of both eroded and anthropogenic products by rivers account for almost all the dissolved solutes, particulates, and sediment‐associated contaminants delivered to the ocean. Transported within this riverine load are essential nutrients (e.g. P, N, C), (oxy)hydroxides of Fe/Mn that act as highly efficient riverine scavengers, an ever‐changing suite of man‐made organic and inorganic constituents, as well as a wide variety of natural solutes (e.g. Si, Ca, Mg, Sr). The environmental behavior of these river‐borne constituents is controlled by dynamic phase partitioning between operationally defined dissolved, colloidal, and particulate pools control. As most trace elements are bound onto river particulates, their fate is defined foremost by the nonconservative nature of such solids as they move along the particle‐size continuum or are exchanged and remobilized in river bed and floodplain sediments. This paper describes the global riverine transport of sediment‐associated contaminants to the ocean.
Acidic Deposition: Sources and Effects
Authors: Driscoll Charles T , Fallon‐Lambert Kathy , Chen Limin
Published: April 2006

Acidic deposition delivers acids and acidifying compounds to the Earth's surface, which are then transported through soil, vegetation, and surface waters and, in turn, set off a cascade of adverse ecological effects. Acidic deposition has altered forest soil by accelerating the leaching of available base cations, enhancing the accumulation of sulfur and nitrogen, and increasing the concentration of dissolved inorganic aluminum in soil waters. Soils that are compromised by acidic deposition are less able to neutralize additional amounts of acidic deposition, and provide poorer growing conditions for plants. Acidic deposition has impaired the surface water quality by lowering pH, decreasing
acid‐neutralizing capacity
(
ANC
), and increasing concentrations of dissolved inorganic aluminum. These changes have reduced the species diversity and abundance of aquatic life. Regulatory controls initiated in Europe and North America over the last three decades have decreased emissions of sulfur dioxide and to a lesser extent nitrogen oxides. Emission reductions have resulted in widespread decreases in concentrations of sulfate in surface waters, with some waters showing an increase in ANC. Given the loss of acid‐neutralizing base cations and the accumulation of sulfur and nitrogen in soil, many ecosystems have become more sensitive to additional acidic deposition and recovery will likely be delayed. Long‐term research shows that deeper emissions cuts will lead to greater and faster recovery from acidic deposition in impacted regions.
Nutrient Cycling
Authors: Edwards Anthony C , Wetzel Robert G
Published: April 2006

Nutrient concentrations within aquatic ecosystems have increased markedly during the last fifty years due to a variety of causes. Significant changes to the rates of nutrient cycling, particularly of nitrogen and phosphorus, have influenced their biological availability on a global scale. Three broad groupings of processes that directly influence nutrient cycling within freshwaters are described. Retention modifies the availability of nutrients for transport, while selectivity between individual chemical species results in preferential uptake and/or transport, and finally transformation processes alter the physical state or reactivity of particular nutrients. Differences in spatial and temporal dynamics of delivery and transport mechanisms among individual nutrients have caused variable relative changes in nitrogen and phosphorus concentrations and fluxes. These composite dynamic factors make it difficult to couple causal relationships between nutrient sources and their impacts.
Urban Water Quality
Authors: Ellis J Bryan , Marsalek Jiri , Chocat Bernard
Published: April 2006

Steady growth of population, due to overall population increases and continuing migration from rural to urban areas, creates enormous demands and stresses on urban waters with respect to water supply, drainage, flood protection, wastewater management, and beneficial uses of receiving waters and groundwater. Urban water issues are therefore in the forefront of water management priorities in practically all regions of the world, though often for broadly varying reasons. Key issues of urban water management are discussed in this article, which focuses on the evolution of urban drainage infrastructure, characterization of urban drainage, urban runoff impacts on receiving waters, urban drainage management, water, and wastewater reuse and future perspectives and priorities. The discussion focuses on the collection and transport of urban effluents (sewer systems), characterization of urban drainage provided by combined or storm sewers (flows and their quality), impacts of urban drainage effluents on receiving waters, and groundwater and impact mitigation by integrated urban drainage management with the emphasis placed on the management of surface runoff and water/wastewater reuse. While the progress in integrated engineering science, watershed‐based management and new water technologies is impressive, the challenges of maintaining and improving urban water services, particularly in low‐income countries, are formidable and may be further exacerbated by demographic, social, and climate change.
Pathogens
Authors: Gerba Charles P
Published: April 2006

Water plays a major role in human disease by serving as the vehicle for the transmission of disease‐causing microorganisms (pathogens), and as a habit for some of their insect hosts. Worldwide, it is estimated that a child dies every 8 s from water‐related diseases. New pathogens, which are transmitted by water, are recognized on a continuing basis providing challenges to both watershed management and water treatment technologies. Microorganisms transmitted by the fecal‐oral route or waterborne pathogens are the most widespread since many may occur in the feces of both infected humans and animals. All surface waters can be expected to contain waterborne pathogens at one time or another. Feces from animals and sewage discharges contaminate surface waters. Groundwater may be contaminated from infiltration from land disposed human and animal wastes or water contaminated with these wastes, septic tanks, and unlined landfills. Contamination of surface and groundwater is always greatest after rainfall events due to the flushing of accumulated animal feces and infiltration of soils. Waterborne pathogens may survive for days to months in the environment. Survival of pathogens in the water environment is largely dependent on temperature and sunlight exposure.
Trophic Dynamics
Authors: Cleveland Cory C , Townsend Alan R , McKnight Diane M
Published: April 2006

The carbon (C) and water cycles are intimately linked in terrestrial ecosystems. Thus, an understanding of the processes regulating transfers of water in terrestrial ecosystems requires an understanding of the carbon cycle, and in particular, the factors constraining carbon movement in the soil‐plant‐atmosphere continuum and through trophic levels in ecosystems. The linkages between the C and the water cycles are mediated primarily through biological processes, and are bidirectional in nature. For example, precipitation (and hence ecosystem water availability) strongly regulates plant growth and biogeochemical cycling in soils. Subsequently, plant growth and soil biogeochemistry strongly influence evaporation and atmospheric water vapor (and hence precipitation). Plant growth and soil processes are also cyclically linked. Thus, plant and soil interactions can have important implications for water cycling. However, while major climatic variables (including precipitation) may drive biological patterns and processes at large scales, other ecological interactions also regulate both plant and soil processes. An appreciation of these ecological factors is important to understanding the relationship between C and water, and to predicting how global environmental change is likely to affect the interactions between the C and water cycles.
Satellite‐Based Analysis of Ecological Controls for Land‐Surface Evaporation Resistance
Authors: Running Steven W , Kimball John S
Published: April 2006

Lack of available water constrains ecological processes for two‐thirds of the earth's biosphere. These water limitations are manifested as either physical water deficiency or as a chemical unavailability of water as a result of being frozen. This article summarizes global principles of water limitations on the biosphere, and physiological limitations on plants. It presents hydrometeorological principles of evapotranspiration and organizing logic of the soil–vegetation–atmosphere transfer models commonly used to compute evapotranspiration. We then introduce remote sensing from both optical/thermal and active/passive microwave sensors for calculating landscape scale evapotranspiration. Finally, we offer a multisensor‐based integrated surface resistance to define landscape water availability under all conditions.
Groundwater Microbial Communities
Authors: McKinley James P , Fredrickson James K , Colwell Frederick S
Published: April 2006

Microbial communities in aquifers consist of diverse interactive individuals which break down complex organic matter for metabolic energy. Microbes are adapted to function over environmental conditions ranging from freezing to boiling, acidic to alkaline. They can use oxygen as a reducible metabolite during organic carbon oxidation, but, since oxygen is rapidly depleted in subsurface environments, different groups of organisms may also rely upon other compounds such as reducible metals or upon fermentation. Community members are interdependent, but compete for resources, and communities often have predominant groups that rely on recognizable chemical pathways such as sulfate reduction. The predominant group varies spatially and temporally, as the available nutrients change or are depleted.
Rainfall Excess Overland Flow
Authors: Smith Roger E , Goodrich David C
Published: April 2006

One of the processes that can generate surface runoff is rainfall excess, which is a process controlled at the surface of the soil. This occurs when rainfall reaches the soil at a rate in excess of the soil's ability to absorb, which is called infiltrability. This dynamic property can in uniform soil be described by a rather well‐developed infiltration theory. Surface water flow toward a receiving channel may in geometrically simple conditions be described by the kinematic or diffusive wave equations. The surface water is in continuous interaction with the soil's changing infiltrability. Both infiltration theory and surface flow equations are introduced here, and the interactions and complexities arising from spatial variations are discussed. These processes are incorporated in modern hydrologic response models, using numerical solutions as well as analytic solutions. The application of this theory in hydrology, however, must be informed by scale considerations and the appropriate treatment of natural complexities. Some scale limitations and some approximate methods for treating spatial soil variations are illustrated in this article, with reference to relevant literature.
Isotope Hydrograph Separation of Runoff Sources
Authors: Buttle Jim M
Published: April 2006

Use of isotopic tracers to characterize water ages, origins, and flow pathways in drainage basins is reviewed, along with the assumptions that underlie isotopic hydrograph separations (IHSs). Points of agreement regarding the use of the IHS approach in small drainage basins are examined, as are some of the key challenges associated with application of the approach in future research. Despite a growing awareness of the limitations of IHS, the method offers a powerful tool to examine a number of critical issues in hydrology, particularly when employed in conjunction with hydrometric techniques and the use of other tracers. These issues include the estimation of water residence times, the identification of process thresholds, the assessment of hydrological response to land use change, and the use of isotopic data to calibrate and test hydrological models.
Land Use and Land Cover Effects on Runoff Processes: Urban and Suburban Development
Authors: Endreny Theodore A
Published: April 2006

Urban and suburban development alters watershed imperviousness and connectivity, resulting in alterations to the hydrologic and hydraulic runoff response. Stormwater conveyance devices have been incorporated into most development to address the goal of floodwater removal for protection of life and property. Urban and suburban development laws generally consider the stormwater drainage system as part of the basic infrastructure and leave right of way and space to locate the component parts. Drainage systems are designed to protect life, enable vehicular access, extend pavement life cycle, control the volume and velocity of stormwater runoff along curbs and gutters, and convey stormwater runoff to natural or human constructed drainage ways and receiving waters. Engineering equations have been developed to design construction for optimal runoff behavior, such as timing, volume, and rate, into, through, and from conveyance devices. Design equations are also utilized for assessing the performance of constructed environments, to ascertain whether stormwater devices need rehabilitation or expansion. New frontiers in stormwater include model spatially distributed development that incorporates high‐density elevation maps, radar rainfall, and details of remotely sensed infrastructure as well as ecological restoration simulation, including groundwater and river remediation for wildlife and recreation.
Land Use and Land Cover Effects on Runoff Processes: Fire
Authors: Luce Charles H
Published: April 2006

Fire dramatically alters hydrologic processes in many regions of the world. Individual fires reduce vegetation and change soil characteristics, sometimes producing dramatic runoff events in the years shortly after a fire. The greatest determinant of the effect of fire on runoff generation is the severity of the fire, which relates to the frequency of fires and other climatic and vegetation characteristics. Severe fires can produce hydrophobic soils or increase risk of soil surface sealing, reducing infiltration rates. Measurements of the spatial pattern of water repellent soils are useful for estimating potential runoff from postfire storms. The most severe events occur during convective storms, so the spatial extent of individual postfire floods is generally limited in extent. Recovery of water repellent soils is relatively rapid, with significant reductions occurring within a few years. Longer‐term changes to hydrology are related to the reduced evapotranspiration caused by loss of vegetation biomass. In forests, changes to annual water balances may last decades.
Rainfall‐Runoff Models for Real‐Time Forecasting
Authors: Todini Ezio
Published: April 2006

This article describes the use of rainfall‐runoff models within the frame of real‐time flood forecasting and warning. After assessing the requirements of real‐time flood forecasting systems, some of the more well known ones available for operational purposes are briefly mentioned. The different types of rainfall‐runoff models used in practice are then presented, within the framework set on the one hand by the operational system requirements, and on the other hand by the historical evolution of research and development in the fields of hydrological and meteorological forecasting. After discussing the problems of real‐time updating and of the assessment of the predictive uncertainty, two example applications are illustrated. The article concludes with a description of current and future research themes for extending the forecasting lead time, assessing forecasting uncertainty, and appropriately communicating it to the end users.
Rainfall‐Runoff Modeling for Flood Frequency Estimation
Authors: Lamb Rob
Published: April 2006

Rainfall‐runoff models are amongst the most important tools for the practical solution of flood estimation problems, as well as for theoretical investigations of controls on the flood frequency curve or for analysis of catchment and climate change. This article aims to provide a discussion of the use of rainfall‐runoff modeling for flood estimation, with an emphasis on overall methodology rather than on specific hydrological models. After a brief introduction to some basic concepts of flood analysis, the article describes the two main approaches for applying rainfall‐runoff models in flood estimation, namely, event‐based modeling and continuous simulation. The theoretical basis for each approach is described along with typical model structures. Topics covered include rainfall models, hydrological model calibration and the estimation of parameters for modeling at ungauged or data‐poor sites. The article concentrates on the development of the continuous simulation method, and includes a number of illustrations of its application, both from research studies and applied hydrology.
Rainfall‐Runoff Modeling for Integrated Basin Management
Authors: Leavesley George
Published: April 2006

Integrated basin management is concerned with the interactions of physical, ecological, economic, and social systems as they affect the operation, planning, and policy making processes inherent in the management of land and water resources. Systems of integrated hydrological, chemical, biological, ecological, and socioeconomic models are typically used to assess the effects of proposed management alternatives on basin resources, or to manage basin resources in real time. Water is a common thread linking many of the components among these models. The ability to adequately simulate rainfall‐runoff processes and their interactions with processes related to other system components significantly affects the integrated system results. Model complexity and compatibility, data availability, and a number of error sources that include data, model parameters, and model structure, are major concerns in the use of integrated modeling systems. The effects of these error sources on the uncertainty in simulation results become even more complex in integrated system application in which output from one model is typically used as input to other models. The number and complexity of available integrated modeling systems represent a wide variety of approaches with different models, operational features, user interfaces, data formats, and analytical tools. For this review, integrated modeling systems have been characterized as using either a (i) dedicated‐system approach with a predefined set of models or (ii) modeling‐framework approach that typically enables the application of a user‐selected set of models. A number of available integrated systems within these two classifications are described and an example of application of a modular‐framework approach is presented.
Rainfall‐Runoff Modeling: Transfer Function Models
Authors: Young Peter C
Published: April 2006

This article discusses continuous and discrete‐time transfer function (TF) models within the context of the Data‐Based Mechanistic (DBM) modeling of hydrological systems. Although, at a superficial level, TF models simply provide a convenient and concise way of presenting differential or difference equations in an input‐output form, they are much more than this. In particular, they allow for the consideration of linear dynamic systems in simple algebraic terms and for their analysis within a dynamic systems and control context. This is useful in various ways, particularly where a higher order TF model is identified and estimated directly from hydrological data. Often, this data‐based model can then be decomposed into serial, parallel, and feedback connections of first‐order systems that can be interpreted in hydrologically meaningful terms, thereby facilitating the model's use in hydrological systems analysis. The article also shows that TF models can be considered in time‐variable and state‐dependent parameter (SDP) form, so allowing for them to describe nonstationary and nonlinear systems. And, since it is straightforward to consider all TF models in stochastic terms, they provide a powerful vehicle for uncertainty, forecasting, and risk analysis. The practical utility and power of DBM TF models is illustrated by two real hydrological examples.
Uniform Flow
Authors: Graf Walter H , Altinakar Mustafa S
Published: April 2006

The equation of continuity and of motion will be developed. The different relationships for the determination of the coefficients of friction for fixed and mobile channel beds are subsequently presented. The calculation of discharge is elaborated. Elementary knowledge about flow in curves as well as instabilities at the free‐water surface are exposed.
Unsteady Flow
Authors: Graf Walter H , Altinakar Mustafa S
Published: April 2006

Flow is unsteady, if the flow depth as well as other hydraulic parameters vary with time. An unsteady flow is usually also nonuniform. The one‐dimensional hydrodynamic equations, known are the equations of Saint‐Venant, are developed. The simplified equations of Saint‐Venant are used to treat the kinematic wave as well as the diffusive wave. The flood wave is also briefly discussed. Numerical solutions to unsteady flow will be treated in a subsequent chapter (seeNumerical Modeling of Unsteady Flows in Rivers).
Numerical Modeling of Unsteady Flows in Rivers
Authors: Krishnappan Bommanna G , Altinakar Mustafa S
Published: April 2006

Solution methods to solve the unsteady flow equations are reviewed in this article. The basis of the method of characteristics is outlined. Some of the commonly used finite difference schemes are reviewed. A detailed description of an unsteady flow model called MOBED is given to highlight the various assumptions and simplifications that are involved in the development of an unsteady flow model. Testing of the model using a laboratory data set measured by Treske is described. Such a data set can serve as a benchmark data for the testing of unsteady flow models. Integral form of governing equations and their properties are introduced. High‐resolution conservative finite‐volume schemes for modeling flows with discontinuities are briefly discussed. Examples of numerical solutions using a robust one‐dimensional upwind finite‐volume code are presented.
Debris Flow
Authors: Armanini Aronne , Fraccarollo Luigi , Larcher Michele
Published: April 2006

The principal features of debris flow are described. One section is devoted to debris‐flow triggering, accounting for the geomechanical criteria by Takahashi (1978) and for most recent hydrological distributed models. In a second section, the rheology of debris flows is described: the theory of dispersive stresses by Bagnold, the kinetic theories of dense gases, and some recent experimental observations about debris flows in equilibrium. A section is devoted to mathematical models and the final section discusses debris flows countermeasures.
Groundwater as an Element in the Hydrological Cycle
Authors: Alley William M , La Baugh James W , Reilly Thomas E
Published: April 2006

Groundwater commonly is portrayed in diagrams of the hydrological cycle as a single pool of water with simple transfers of water to and from the land surface, oceans, and atmosphere. In reality, groundwater is an integral part of a complex hydrological cycle that involves the continuous movement of water on Earth. We begin with a global perspective on the importance of groundwater in the hydrological cycle and then address the importance of understanding the dynamics of groundwater‐flow systems. We next turn our attention to groundwater budgets and the effects of humans and climate on these budgets. The final section is devoted to a discussion of interactions between groundwater and surface water, and we highlight the importance of these interactions in the hydrological cycle.
Characterization of Porous and Fractured Media
Authors: Renard Philippe , Gómez‐Hernández Jaime , Ezzedine Souheil
Published: April 2006

The characterization of porous or fractured media is a site, scale, and project‐specific process aiming at a quantitative description of the geometry and properties of the geological structures controlling groundwater flow and solute transport. The characterization process involves four main steps (i) the definition of the domain and the goals of the characterization; (ii) the collection and analysis of field observations allowing the construction of a geometrical model; (iii) the collection and analysis of field measurements allowing to construct a property model; and (iv) the collection and analysis of field data relative to the state of the system and their integration within the geometrical and property models (inverse problem). When data are sufficient and structures are relatively well known, deterministic techniques of interpolation can be successfully applied to construct the geometric or the parameter models. However, because of the lack of sufficient data, stochastic models are often employed to characterize the heterogeneity that usually exists; such models also facilitate the quantification of the uncertainty in model predictions. Without describing the details of every technique, this article provides an overview of the tools most often used for the characterization of porous or fractured aquifers.
Inverse Methods for Parameter Estimations
Authors: Sun Ne‐Zheng , Sun Alexander Y
Published: April 2006

Estimation of various flow and mass transport parameters can be seen as the inverse problem of groundwater modeling. In this article, current methodologies for parameter estimation are classified according to whether they consider the errors in observation data, in parameter structure, and in model applications. The structures of hydraulic parameters are usually very complex and unknown. This article gives an overview on adaptive parameterization methods for simultaneously identifying the parameter values, the complexity level, and the pattern of parameter structure. Methods for model reliability analysis are described. Finally, a generalized inverse problem is introduced that can find a representative parameter for a given model application.
Hydrodynamics of Groundwater
Authors: Stauffer Fritz
Published: April 2006

The mathematical formulation of problems of flow in aquifers is usually based on a formulation of the fluid mass balance, which has to be fulfilled at any location in the domain under consideration. A knowledge of the fluid mass flux through the saturated aquifer material, or the motion equation, is required. The resulting partial differential equation can be solved for a chosen flow domain, given parameters, and initial and boundary conditions. The solution results in the steady state or transient field of the piezometric head, from which the flux and the velocity fields may be expressed.
Modeling Solute Transport Phenomena
Authors: Bear Jacob
Published: April 2006

The article presents the conceptual and well‐posed mathematical models that describe the transport of solutes, or chemical species, in porous media, as encountered when considering the transport of dissolved salts and contaminants in aquifers. The various modes of transport: advection dispersion and diffusion are discussed and incorporated in the models. Solute sources are also incorporated in the models, including adsorption and an introduction to reactive transport. We shall emphasize that the fluid's velocity, and in unsaturated flow, also the fluid's saturation is required as input information to any solute transport problem. However, the model of flow problem that provides this information is discussed in
Hydrodynamics of Groundwater
. Unsaturated flow models are discussed in
Unsaturated Zone Flow Processes
. The case of solute transport with variable density is not included, as it is presented in
Sea Water Intrusion Into Coastal Aquifers
. More about chemical and biological reactions is presented in
Groundwater Pollution and Remediation
.
Groundwater Pollution and Remediation
Authors: McNab Walt , Hoffman Fred , Dooher Brendan
Published: April 2006

Contamination of groundwater resources by a variety of anthropogenic pollutants from both point and non‐point sources represents a key global environmental problem. Contaminant species of concern include solvents, fuel hydrocarbons, heavy metals, pesticides, nitrate, and radionuclides. Groundwater contamination reflects local physical hydrogeological considerations but also the inherent properties of the contaminants. An understanding of the factors controlling contaminant behavior in the subsurface is a necessary prerequisite to exploring possible remedial solutions. These factors include the partitioning processes that govern the distribution of contaminant species between the aqueous phase, the solid phase (including adsorption onto soil particle surfaces), the gas phase, and, in some cases, nonaqueous phase liquids. Chemical and biological transformation processes also impact the behavior of different types of subsurface contaminants. The designs of passive and active groundwater remediation approaches often reflect consideration of one or more of these factors; however, the efficacy of most methods is often most closely dependent upon the physical characteristics of the subsurface environment, such as the distribution of hydraulic conductivity.
Aquifer Recharge
Authors: Nimmo John R , Healy Richard W , Stonestrom David A
Published: April 2006

Aquifer recharge is important both for hydrologic understanding and for effective water resource management. Temporal and spatial patterns of unsaturated‐zone processes such as infiltration largely determine its magnitude. Many techniques of recharge estimation exist. Water budget methods estimate all terms in the continuity equation except recharge, which is calculated as the residual. Detailed hydrologic models based on water‐budget principles can produce recharge estimates at various scales. Empirical methods relate recharge to meteorologic and geographic parameters for a specific location. Surface‐water methods include stream‐hydrograph analyses to estimate baseflow (groundwater discharge) at lower elevations in a watershed, which is taken to equal the recharge that has occurred at higher elevations. Subsurface methods include analysis of water‐table fluctuations following transient recharge events, as well as diverse unsaturated‐zone methods. The zero‐flux plane method determines the recharge rate from the change in water storage beneath the zero‐flux depth, a boundary between water moving upwards due to evapotranspiration and water moving downward due to gravity. Lysimeter methods use buried containers filled with vegetated soil to mimic natural conditions. Water exiting the bottom is considered to be recharge. Darcian methods for estimating flux densities use unsaturated hydraulic conductivities and potential gradients, indicating recharge rates under appropriate conditions. Chemical mass‐balance methods use conservative tracers that move with recharging water. Tracer concentrations in deep unsaturated‐zone water, together with tracer input rates, indicate recharge rates. Distinct chemical “markers” can indicate travel times, hence, recharge rates. Thermal methods use heat as a tracer. Moving water perturbs temperature profiles, allowing recharge estimation. Geophysical methods estimate recharge based on water‐content dependence of gravitational, seismic, and electromagnetic properties of earth materials.
Snow Cover
Authors: Brown Ross D , Goodison Barry E
Published: April 2006

Snow cover is encountered over most of the Northern Hemisphere (NH) mid‐ and high‐latitudes during the winter season and over many mountainous regions of the world for extended periods. Snow cover is an important component of the climate system through its role in modifying energy and moisture fluxes between the surface and the atmosphere, and through its role as a water store in hydrological systems. Snow also plays critical roles in ecological and biological systems, and in nutrient and carbon cycling. This article provides an introductory overview of snow cover including: definition of terms (snowfall, solid precipitation, snow depth, snow density, snow water equivalent); a review of methods for measuring snow cover properties; a discussion of the processes and concepts involved in the spatial and temporal variability of snow cover; a look at avalanches and recent efforts to model these; and finally, a discussion of how snow cover is changing in response to global warming.
Water Flow through Snow and Firn
Authors: Marsh Philip
Published: April 2006

Water flux through snow and firn has important implications for the timing and magnitude of runoff from seasonal snowpacks, glaciers, and ice caps. Studies conducted over the last few decades have greatly improved our understanding of water flux through snow and firn, including knowledge of the development of wetting fronts as meltwater enters dry snow, preferential flow paths, grain growth of wet snow, and the growth of ice layers and ice columns within the snowpack. Such studies have resulted in the development of a variety of analytical and numerical models for calculating runoff from melting snow and firn. Although our understanding of key processes continues to increase, there are major gaps in our knowledge that limit our ability to formulate comprehensive physically based models of snow metamorphism and water flux through snow and firn. Part of the problem is a lack of data, which is due to the great difficulty of conducting experiments on snow near 0°C and our inability to measure the properties of snow required for estimating the value of parameters needed for numerical simulations. In order to overcome these problems, there is a need for the development of new measurement techniques and carefully designed field and laboratory experiments. Comprehensive models are required for predicting not only melt runoff, but also the release of pollutants from the snowcover, the large‐scale properties of the snowcover necessary for understanding large‐scale energy‐balance and global‐change problems, and the release of meltwater from cold glaciers and icecaps under future climate scenarios.
Hydrochemical Processes in Snow‐Covered Basins
Authors: Pomeroy John W , Jones H Gerald , Tranter Martyn , Lilbæk Gro
Published: April 2006

This article reviews several aspects of snow hydrochemistry: the chemistry of snowfall including chemical incorporation in snowfall and snowfall chemistry variability, the chemistry of cold, dry snowcovers including snow redistribution, snow–atmosphere chemical exchange and in‐pack chemical transformations, the chemistry of wet and melting snowcovers including solute leaching, particulate interactions and microbial activity, and snow‐covered basin hydrochemistry with an emphasis on nutrient chemistry. The emphasis is on the processes of chemical transformation in seasonal snowpacks and meltwaters with strong attention to the broad ecosystem view of snow chemistry rather than solely focusing on acidification effects from snowmelt. The seasonal snowcover is shown to be a dynamic hydrochemical system with strong ecological interactions. Besides wet deposition by snowfall and rain, the processes of wind redistribution, dry deposition, volatilization, crystal metamorphism, photolysis, microbial uptake and release, solute elution, and meltwater movement strongly affect the chemistry of both the snowpack and meltwaters. Snowmelt chemistry alone is rarely directly responsible for major chemical fluctuations in water bodies, but meltwater has an important role in transporting ions from soils and organic material to water bodies.
Subglacial Drainage
Authors: Sharp Martin
Published: April 2006

Subglacial drainage can occur wherever ice at a glacier bed reaches the pressure melting point. The subglacial drainage system is fed from a mixture of surface, englacial, subglacial, and groundwater sources that differ in terms of their spatial distribution and characteristic patterns of temporal variability. Subglacial drainage systems are not readily accessible, and knowledge of their characteristics is derived from a range of indirect methods including radio‐echo sounding, the use of artificial tracers, monitoring, and manipulation of subglacial conditions via boreholes, and monitoring of glacial runoff properties. Subglacial water flow is driven by gradients in hydraulic potential, and occurs through either fast/channelized or slow/distributed systems located at the ice‐bed interface, or through subglacial aquifers. Water can be stored subglacially in cavities, in the pore space of subglacial sediments, or in subglacial lakes. Drainage system structure evolves continually on various timescales in response to changing water inputs, evolving glacier geometry, and changes in glacier flow dynamics. Major hydrological events in such systems include the spring and fall transitions, outburst floods, and structural changes related to glacier advances and glacier surging. In the absence of variable water inputs from the glacier surface, subglacial drainage systems may be sensitive to forcing by earth, ocean, and atmospheric tides.
River‐Ice Hydrology
Authors: Prowse Terry D
Published: April 2006

The hydrology of almost 60% of rivers in the Northern Hemisphere is affected by ice for at least a portion of the winter. At the higher latitudes, river ice is the major controller of most fluvial processes. Although traditionally the pursuit of hydraulic engineers, hydrologic studies of river ice have broadened greatly over the last decade. Initially, this was driven by recognition that many hydrologic extremes produced by river ice, such as floods and low flows, exceed those of the more intensively studied, open‐water period. Added recognition that river ice strongly influenced other fluvial disciplines, such as geomorphology and aquatic ecology, increased its scientific visibility. This manuscript reviews the basic physics of river ice; evaluates its effect on various hydrologic processes and events from freeze‐up to breakup; and discusses its extended role in affecting sediment transport/erosion, river morphology and a number of key ecological processes including geochemical mixing and habitat modification. Arguments are made for increased focus on river‐ice hydrology given its significance to physical and ecological processes, and the growing need to understand the effects of climate change and flow regulation on cold regions rivers.
Global Water Cycle (Fundamental, Theory, Mechanisms)
Authors: Pagano Thomas C , Sorooshian Soroosh
Published: April 2006

Water cycle is the never‐ending movement of water on Earth. Water continuously cycles through various reservoirs in the ocean, sky, and soil. A variety of measurement systems are necessary to quantify the fluxes between reservoirs, ranging from simple buckets to measure rainfall to sophisticated satellites orbiting the Earth. The hydrologic cycle is mainly characterized by its variability in space and time. The Earth contains an incredible diversity in climates, including rainforests, parched deserts and frozen tundra. Water cycle at any given location changes both rapidly and slowly, with the passing of flash floods to the decadal shifts in ocean patterns. Anticipated impacts of human‐induced climate change on the hydrologic cycle are as yet unknown, but the consequences are potentially severe. Nonetheless, humans have a long history of altering the hydrologic cycle including the diversion and impoundment of streamflow, pumping of groundwater, irrigation of fields and management of forests. While the water cycle has been recognized for ages, many interesting fundamental research questions remain.
The Role of Large‐Scale Field Experiments in Water and Energy Balance Studies
Authors: Shuttleworth W James , Gash John HC
Published: April 2006

Over the last two decades, increased awareness of the potential importance of anthropogenic drivers of “global change” and growing interest in predicting climate variability at seasonal to interannual timescales has stimulated the development of new experimental techniques and new modeling methods to upscale conventional observations of land‐surface exchanges to area scales consistent with the grid scales used in General Circulation Models, and large‐scale experiments in important global biomes that had previously been neglected. The nature, purpose, and scale of such large‐scale field experiments, many fostered by international programs such as the World Climate Research Programme (WCRP) and the International Geosphere‐Biosphere Programme (IGBP), has evolved with time. Initial studies focused on exploring the value of remotely sensed data when upscaling or provided data at several sites that sampled the most important vegetation covers across large biomes. One important class of experiments, the Mesoscale Field Experiments, laid emphasis on gathering data over an area comparable with that of a Global Circulation Model (GCM) grid square and explored the nature and significance of mesoscale coupling between the land surface and the overlying atmosphere and, in particular, the response of the atmosphere to heterogeneity in vegetation cover. Subsequently, the spatial scale of studies increased in a suite of Continental‐Scale Experiments (CSEs) which explored the extent to which the atmospheric component of energy and water budgets can be reconciled with the equivalent surface budgets, fostered improvement in the performance of the regional scale–coupled hydrometeorological models, and investigated “land memory” processes that operate at regional scale and at the seasonal timescales. Recently the emphasis has been on seeking to link several CSEs together in a Coordinated Enhanced Observing Period (CEOP) using satellite systems to provide observations at the global scale. This article describes the evolving purpose and developing nature of large‐scale field experiments over the period 1984–2004 and their role in providing improved knowledge and modeling of land‐surface energy exchanges.
Modeling of the Global Water Cycle: Numerical Models (General Circulation Models)
Authors: Koster Randal D
Published: April 2006

Atmospheric general circulation models (AGCMs) are powerful tools for exploring climate sensitivity and the global water cycle. The wealth of data provided by an AGCM allows a complete statistical characterization of its water cycle, a far more complete picture than would be allowed from real‐world observations. AGCM numerical experiments have been used to isolate the mechanisms that control water cycle variability, illustrate the potential for water cycle prediction, and quantify water cycle changes resulting from human activities. The AGCM fields can also be merged with observations (via data assimilation) to provide the best estimates possible of the true water cycle. Unfortunately, biases in simulated fields resulting from coarse resolution and inadequate physical parameterizations continue to plague AGCMs, forcing climate modelers to temper their faith in the models' results.
Long‐Term Predictions (Climate Simulation and Analysis)
Authors: Betts Richard A
Published: April 2006

The global hydrological cycle is expected to change over the next century as a consequence of human perturbations to the Earth System. Human activities are modifying the composition of the atmosphere, in particular the concentrations of greenhouse gases such as CO2and the concentrations of aerosols. This exerts a radiative forcing of the climate system, which already appears to be warming the global climate. These changes are predicted to increase in the future, and are expected to modify precipitation and evaporation regimes, and hence impact on hydrology, with streamflow increasing in some regions and decreasing in others. Furthermore, changes in atmospheric composition may also affect ecosystems directly, with plant physiological responses to CO2increasing carbon storage, modifying biogeography, and decreasing evapotranspiration. This may have further effects on hydrology in addition to climate change. Human‐induced changes in land cover, such as deforestation and afforestation, may also directly modify hydrology and climate by perturbing the surface water and energy budgets as well as by contributing to radiatively‐forced climate change through changes in atmospheric composition. This article compares and contrasts the climatic, ecological, and hydrological changes predicted as a result of these three human perturbations to the Earth System, and discusses the key interactions between them.