bluuebunny/crossref_metadata_2025 huggingface dataset:
Pattern, Process and Function: Elements of a Unified Theory of Hydrology at the Catchment Scale
Authors: Sivapalan Murugesu
Published: April 2006

Catchment hydrology is presently operating under an essentially reductionist paradigm, dominated by small‐scale process theories. Yet, hydrology is full of examples of highly complex behavior, including strong nonlinearities and thresholds, and paradoxes that defy causal explanation through these small‐scale process theories. There are strong interactions and feedbacks between processes, leading to apparent simplicities in the overall catchment response, yet the laws governing these feedbacks are not well understood. Routine measurements and specialized field experiments have been valuable for observing catchment responses and understanding the underlying process controls, but there has been little progress in extrapolating the local knowledge and understanding gained from these well studied (or gauged) catchments to ungauged catchments. Efforts at generalization are hampered by the lack of an appropriate quantitative framework, for example, a classification system, to help identify interesting and useful patterns in the observations. There are many theories governing different elements of catchment hydrology, but not a unified theory that connects these seemingly disparate elements. This article presents the broad outlines of an emerging new, unified theory of hydrology at the catchment scale, and the approaches being used to develop it. The new theory embraces multiscale heterogeneities as a natural and intrinsic part of catchment hydrology. Instead of relying solely on current process theories, it seeks to discover new catchment‐scale process theories that embed within them the effects of natural heterogeneities. Instead of attempting to prescribe in detail the actual patterns of heterogeneity in every catchment, it will seek to incorporate the geomorphic or landforming processes that may have generated them in the first place, and their ecological, pedological, and geomorphological functions. Instead of using our rather meagre observations to calibrate complex models that are based on small‐scale theories, the theory will emphasize the use of patterns in the observations to formulate and test alternative hypotheses about the underlying process controls. Instead of using field measurements to learn more and more about individual catchments, it will seek to find connections between observations in different catchments, to identify broad‐scale or general patterns. The defining feature of the new theory of catchment hydrology will be a sharp focus on the interconnection and feedbacks between pattern and process, over a range of scales, and their interpretation in terms of their “function”, that is, the reason that these connections arise. The renewed focus on pattern, process, and function will revolutionize hydrology, elevate its place within the earth system sciences, and strengthen the scientific foundations of its practice.
Data‐Driven Modeling and Computational Intelligence Methods in Hydrology
Authors: Solomatine Dimitri P
Published: April 2006

Along with the physically based (process) models based on mathematical descriptions of hydrological processes, the so‐called data‐driven models (DDM) are becoming popular. They are based on the use of methods of computational intelligence and machine learning and assume the presence of considerable amount of data describing the modeled phenomenon. The article covers various aspects of DDM, including the data preparation and a brief overview of the used techniques – neural networks, regression and model trees, instance‐based learning, and nonlinear dynamics. A number of references to the successful application of data‐driven methods are provided along with the links to the relevant web sites and software packages. An example where several data‐driven methods were used in a hydrologic forecasting problem is provided.
Storm Systems
Authors: Doswell Charles A
Published: April 2006

The concept of a storm is defined as a weather disturbance that, in the context of hydrological science, produces precipitation or affects the formation and distribution of precipitation. Storm systems are reviewed in terms of their spatial and temporal scale, with the dominant storm system on large scales in midlatitudes being the so‐called extratropical cyclones. Not only do such storms produce considerable weather in their own right, but they provide a setting in which the so‐called mesoscale storm systems can develop. Mesoscale weather is not dominated by any particular form of disturbance, but can take many different forms. The intensity of the weather, including precipitation rates, in mesoscale systems can be much greater than in extratropical cyclones although the duration of such weather is typically less than in large‐scale systems. Small‐scale storms are dominated by convective storms, which can produce the most violent weather of all, including large hail, strong winds, tornadoes, and torrential rainfalls.
Clouds and Precipitation
Authors: Isaac George A , Hallett John
Published: April 2006

Clouds and the precipitation that comes from them are important elements of the hydrological cycle. Clouds provide a blanket for our Earth, both shielding it from radiation from the Sun and trapping heat escaping from the surface. They also generate precipitation through the condensation‐coalescence mechanism, which involves only liquid cloud drops, or through ice initiation leading to large ice particles and eventually snow or rain. Mean annual precipitation amounts reach a maximum near the equator, near 8 mm day
−1
, and decrease poleward to about 1 mm day
−1
. A good understanding of both cloud and precipitation processes is very important for climate and weather predictions. This paper outlines some of the most important processes and provides reference material where more detailed information can be obtained.
Topographic Effects on Precipitation
Authors: Austin Geoffrey L , Dirks Kim N
Published: April 2006

Topography and orography can drastically alter the wind patterns in the atmosphere that in turn can have a major effect on the geographical distribution of precipitation. The processes involved include the upslope enhancement of existing rainfall or the triggering of new rainfall in mountainous regions, and the effect of temperature or humidity contrasts between boundary layer air masses over land and oceanic areas. These processes can result in rainfall patterns ranging from a single triggered thunderstorm to widespread torrential rainfall, as occur when tropical cyclone air masses are pushed up steep coastal mountain ranges. Since the magnitude and distribution of the resulting precipitation pattern can depend critically on the wind speed and direction, as well as the terrain itself, the topographical precipitation processes often determine not only the climatic rainfall patterns of a region but also the interannual variability of this distribution.
Human Impacts on Weather and Climate
Authors: Schneider Stephen H , Mastrandrea Michael D
Published: April 2006

Climate change is a worldwide environmental, social, and economic challenge. It includes issues of air pollution, land use, toxic waste, transportation, industry, energy, government policies, development strategies, and individual freedoms and responsibilities. Human use of the atmosphere as an unpriced dumping space has led to the buildup of gases and particles such as carbon dioxide and methane that can alter the energy exchange between the Earth's surface and space. There is a great deal of confusion among policy makers and the public about just how much is known about our climate, and what makes it change. However, although many aspects of the science of climate change are laced with major uncertainties, many others are actually scientifically well established. Human modification of the climate system has far‐reaching implications for human welfare and the health of our planet. Are our actions causing the climate to change in ways or at rates that will threaten natural systems or make human adaptation difficult? This chapter outlines the current state of scientific knowledge regarding human impacts on the climate system.
Climate Change – Past, Present and Future
Authors: Hengeveld Henry
Published: April 2006

Local climate is a major determinant of both the composition and behavior of a region's ecosystems and of the infrastructure and culture of human society residing within the region. Hence, climatic statistics are an important factor in ecosystem management and planning for socioeconomic development. However, although decision makers often treat these statistics as a constant, there is clear evidence that climate has changed, is changing, and will change. Large natural changes have occurred in the geologic past. However, these have been relatively modest during the past few millennia. On the other hand, human interference with the climate system, primarily through land use change and changes in atmospheric composition, is now adding an unprecedented and increasingly dominant force for change. There is convincing evidence that this interference has already caused a substantial increase in global mean temperatures over the past 50 years. Projected changes for the next century will likely exceed anything yet experienced in human history and could rival the magnitude of very large changes during the past million years, but at a much more rapid rate. Such rapid change will have dramatic implications for, inter alia, the global hydrological cycle and the frequency and severity of extreme weather events.
Rainfall Measurement: Gauges
Authors: Sevruk Boris
Published: April 2006

It is difficult to measure precipitation without introducing systematic errors or biases. The quality of any measurement will depend on the type of gauge used and the details of the installation, as well as the characteristics of the gauge site including exposure and the prevailing weather. All precipitation measurements using can‐type gauges are subject to systematic errors, mostly due to deformation of the wind field above the precipitation gauge orifice, wetting and evaporation losses, splash‐out and splash‐in, blowing snow, and so on. Because these measurement errors are systematic, precipitation gauges frequently show different precipitation figures. This is the case when the gauges are of different construction, or the same construction, but from different manufacturers, or are installed at different heights at the same site near each other, or under different gauge site exposure. With the exception of measurements of blowing and drifting snow, they show less precipitation than the “true” precipitation falling on the ground. Correction procedures are different for rain and snow. They were developed for different types of precipitation gauges and various time intervals according to the availability of necessary input data. Corrections are based either on empirical field methods using the World Meteorological Organization reference standard, laboratory experiments, or numerical simulation.
Surface Radiation Balance
Authors: Unsworth Michael H
Published: April 2006

The surface radiation balance is the algebraic sum of the net solar radiation and the net long‐wave radiation received by a surface. It may be measured directly with a net radiometer, deduced from measurements of its upwelling and downwelling components, or modeled from climatological, geographical, and atmospheric data. Methods of modeling and measuring each of the components are discussed. Downwelling solar radiation from cloudless skies may be modeled from knowledge of atmospheric properties, but effects of clouds require more empirical approaches most suitable for monthly means. Downwelling long‐wave radiation below cloudless skies is well related to temperature and humidity near the ground. An empirical method to account for clouds is given. Reflection coefficients and emissivities of natural surfaces are tabulated for use in calculating upwelling short and long‐wave radiation. Net radiometer designs are reviewed, and examples are given of the surface radiation balance and its components over forest and grass. Issues of spatial variability of the surface radiation balance are discussed.
Transpiration
Authors: Roberts John
Published: April 2006

The transpiration process is the uptake of water by plant roots, transport through the plant and evaporation from the leaf through pores called stomata. Evaporation of water from the leaf is determined by atmospheric conditions such as radiation, temperature and humidity deficit but the plant can limit transpiration by partial or complete stomatal closure. Generally stomata open in response to increasing radiation but tend to close with increasing air humidity deficit and reduced availability of soil moisture. Because the stomata have to be open in daylight for the entry of carbon dioxide into the leaf for the photosynthesis process, water loss is an inevitable consequence. Nevertheless transpiration itself has important roles. Nutrients are brought into the plant when water is taken up from the soil and evaporation of transpired water prevents leaf temperature reaching supra‐optimal levels. There are numerous ways that transpiration might be measured. These include measurements of soil water changes below vegetation or changes in atmospheric humidity above vegetation. Alternatively measurements can be made on individual plants or leaves. In most circumstances the source of water for transpiration is the soil and principally the surface soil layers where most roots are found. In the future, increased levels of atmospheric CO
2
are expected to reduce transpiration through reduction in stomatal aperture.
Principles of Radiative Transfer
Authors: Drusch Matthias , Crewell Susanne
Published: April 2006

The article gives an introduction to classical vector radiative transfer theory (RTT). It comprises a brief summary of the fundamental quantities in RTT and the corresponding definitions. Based on these quantities, the transfer equation for radiation will be introduced in its basic form for a plane‐parallel, horizontally homogeneous atmosphere. Polarization and the Stokes vector are introduced, which extend the scalar radiative transfer equation to the more general form of the vector radiative transfer equation. Scattering processes and interactions with the surface are discussed within the mathematical framework given in the first part of the paper. As an example, the theoretical concept presented is then applied to passive microwave remote sensing. A solution for the radiative transfer equation, which is commonly used for the retrieval of atmospheric quantities (e.g. water vapor) and land surface properties (e.g. soil moisture), is derived based on approximations and simplifications. More general approaches to solve the radiative transfer equation including multiple scattering are described and discussed in a separate section.
Sensor Principles and Remote Sensing Techniques
Authors: England Anthony W
Published: April 2006

Model‐derived estimates of water stored in the upper few meters of soil, in vegetation, and in snow packs are the objectives of a new satellite microwave remote sensing technology. The most mature of the alternatives, this technology will use a combination of longer wavelength satellite microwave brightness observations and Soil Vegetation Atmosphere Transfer (SVAT) models, forced by solar and thermal radiation and by weather, to produce a near‐daily global field of stored water. While the near‐daily temporal resolution is a unique strength for hydrometeorology, its likely spatial resolution of tens of kilometers is a serious weakness for land‐surface hydrology. This article is intended as a review of the concept and the investigations that have contributed to it. It also includes brief surveys of some technologies, like aperture synthesis radiometry and combined active passive sensing, that are being explored to improve spatial resolution.
Estimation of Precipitation Using Ground‐Based, Active Microwave Sensors
Authors: Steiner Matthias
Published: April 2006

Ground‐based radar measurements have provided a wealth of information about precipitation systems, how they evolve and organize themselves. Advances in Doppler and polarimetric radar technology, in particular, have yielded unprecedented insight into the dynamics and microphysics of precipitation formation. This paper summarizes the basic components and principals of radar measurements, and discusses how radar can be used to identify different types of precipitation particles as well as recognize nonmeteorological radar echoes, and obtain quantitative precipitation estimates.
Estimation of Surface Insolation
Authors: Pinker Rachel T
Published: April 2006

The incoming solar radiation from the sun (insolation) that reaches the Earth's surface (about 50% of that emitted from the sun) determines the exchange of energy between the land and the atmosphere and consequently, controls climate and climate change. Environmental satellites are considered useful tools for providing information on surface radiative fluxes at various temporal and spatial scales, improving estimation of terrestrial water and energy storage. For several years now, observations from satellites have been used to obtain information on the various components of atmospheric and surface radiative fluxes, initially, retrospectively, and more recently, operationally. In this paper, discussed are the needs for information on radiative fluxes in hydrological applications, methodologies for obtaining such information by methods of remote sensing, accuracy of such estimates, and current status of data availability. Examples of applications in hydrological studies and climate research are provided, and links to international activities and future prospects are indicated.
Estimation of Water Vapor and Clouds Using Microwave Sensors
Authors: Crewell Susanne
Published: April 2006

Both water vapor and clouds play important roles in the energy and water cycle, and their accurate observation is required. Several techniques to measure water vapor distributions and cloud properties exist with each of them having their specific limitations. Measurements in the infrared and visible spectral range are strongly influenced by clouds and cannot be used to observe water vapor under cloudy conditions. Information about cloud properties is limited to the cloud top and base depending on the use of a space‐ or ground‐based system. Microwave techniques can provide information about water vapor and liquid water also under cloudy conditions, but satellite observations are limited to retrievals above ocean surfaces. Therefore, the combination of different measurements needs to be further pursued in order to gain insight into the three‐dimensional distribution of water vapor and cloud properties as well as their temporal variability. Recently, information about the cloud vertical structure including the occurrence of multiple cloud layers and cloud overlap has become available at a few ground‐reference sites through sensor synergy including active and passive instruments. In the near future, the first global measurements of this kind will be carried out from satellites.
Estimation of Surface Temperature and Surface Emissivity
Authors: Schmugge Thomas J
Published: April 2006

Measurements of thermal radiation at infrared wavelengths (7 to 14 µm) yield much information about the land surface. The primary use of these observations is for surface temperature determination as the emissivity is usually close to one. For this purpose, it is fortuitous that the peak in the thermal emission occurs in an atmospheric transmission window. In addition, there are variations in the emissivity of minerals and soils in the 7–14 µm region, which can be interpreted for identification purposes. These also produce significant emissivity variations for arid or desert areas, where there are considerable exposed soils and rocks. The emissivity for vegetative canopies has been found to be close to one, with little spectral variation. Applications of the derived surface temperature to study the surface energy balance and to estimate the energy fluxes from the land surface are discussed.
Estimation of Surface Freeze–Thaw States Using Microwave Sensors
Authors: McDonald Kyle C , Kimball John S
Published: April 2006

The transition of the landscape between predominantly frozen and nonfrozen conditions in seasonally frozen environments impacts climate, hydrological, ecological, and biogeochemical processes profoundly. Satellite microwave remote sensing is uniquely capable of detecting and monitoring a range of related biophysical processes associated with the measurement of landscape freeze/thaw status. This chapter provides an overview of the development, physical basis, current techniques, and selected hydrological applications of satellite‐borne microwave remote sensing of landscape freeze/thaw states for the terrestrial cryosphere. Major landscape hydrological processes embracing the remotely sensed freeze/thaw signal include timing and spatial dynamics of seasonal snowmelt and associated soil thaw, runoff generation, and flooding, ice breakup in large rivers and lakes, and timing and length of vegetation growing seasons and associated productivity and trace gas exchange. This chapter also summarizes the physical principles of microwave sensitivity to landscape freeze/thaw state, recent progress in applying these principles towards satellite remote sensing of freeze/thaw processes over broad regions, and potential for future global monitoring of this significant phenomenon of the global cryosphere.
Estimation of Surface Soil Moisture Using Microwave Sensors
Authors: Jackson Thomas J
Published: April 2006

Soil moisture has been particularly difficult to measure and map using conventional ground‐based point sampling. Microwave remote sensing–based soil moisture retrieval has been demonstrated using tower and aircraft instruments. The translation of this approach to satellites and the implementation in hydrologic applications has been limited by both the technology and the satellite systems that were available. Recent developments in both science and associated technologies now make the exploitation of the microwave region for soil moisture mapping feasible. A number of new and potential satellite missions are reviewed and evaluated for soil moisture applications.
Estimation of Soil Properties Using HyperspectralVIS/IRSensors
Authors: Huete Alfredo R
Published: April 2006

Knowledge of soil properties and processes are crucial to the understanding of the terrestrial hydrologic cycle and the functioning of terrestrial ecosystems. In this paper, we present the current state and potential of hyperspectral remote sensing techniques for quantitative retrieval of soil properties. Remote sensing is used to detect chemical and physical soil properties either (i) directly from the bare soil pixels, (ii) through advanced spectroscopy methods in mixed “soil‐vegetation‐litter” pixels, and (iii) by measurements of the overlying vegetated canopy to infer soil properties and moisture status. Optical‐geometric properties of soil surfaces reveal information on soil physical features, such as soil structure, crusting, and erosion. We also investigate the use of vegetation water indices to infer soil drying and wetting in the soil root zone. We conclude with a discussion on future needs and directions for remote sensing of soil properties.
Estimation of Suspended Sediment and Algae in Water Bodies
Authors: Ritchie Jerry C , Zimba Paul V
Published: April 2006

Remote sensing techniques can be use to monitor water quality parameters (i.e., suspended sediments‐turbidity, chlorophyll, temperature). Optical and thermal sensors on shoreline booms, boats, aircraft, and satellites provide both spatial and temporal information needed to monitor changes in water quality parameters for developing management practices to improve water quality. Recent and planned launches of platforms equipped with improved spectral and spatial resolution sensors should lead to greater use of remote sensing techniques to assess and monitor water quality parameters. Integration of remotely sensed data, GPS, and GIS technologies provides a valuable tool for monitoring and assessing waterways. Remotely sensed data can be used to create a permanent geographically located database to provide a baseline for future comparisons. The integrated use of remotely sensed data, GPS, and GIS will enable consultants and natural resource managers to develop management plans for a variety of natural resource management applications. This review paper identifies current knowledge regarding the assessment of water quality, and identifies promising techniques for high resolution assessment.
Estimation of the Surface Energy Balance
Authors: Su Zhongbo
Published: April 2006

Radiation and latent and sensible heating are among the most important processes in land–atmosphere exchanges. Hence, quantitative understanding and accurate estimation of these fluxes at large scales are imperative for research and applications in areas ranging from numerical weather forecast, climate research, water cycle study, to water resources management, sustainable agricultural production, and ecological conservation. Quantitative remote sensing is probably the only efficient and economically viable technology to provide regional to global radiometric observations of several physical quantities that are relevant to the estimation of these fluxes. This article examines the physical principles underlying the radiation and turbulent heating, along with exploring the possibilities and difficulties for estimation of these fluxes using remote sensing measurements. Past efforts, recent progresses are reviewed, and future research needs are identified. In particular, the Surface Energy Balance System(SEBS), recently developed in Wageningen, is introduced and its strengths and weakness are analyzed in view of future space‐borne sensor systems. Several applications derived on the basis of SEBS, including estimation of turbulent heat fluxes, evaporative fraction and actual evaporation, estimation of relative soil moisture, and drought monitoring are discussed.
Estimation of River Discharge
Authors: Dingman S Lawrence , Bjerklie David M
Published: April 2006

River discharge, the volume flow rate through a river cross section, is perhaps the most important single hydrologic quantity. It is a major link in the global hydrologic and geologic cycles and a critical component of climate, and represents the rate at which nature makes water available for human use and management. In the form of floods, it constitutes one of the most destructive natural hazards. Measurement of river discharge thus provides vital information for science and society. Measurement by conventional ground‐based methods is expensive and is declining globally. Using various combinations of active and passive imagery and sensors mounted on satellites or aircraft, it is possible to obtain direct quantitative information on several hydraulic variables, including channel configuration and the area, width, elevation, and velocity of the water surface. Computations using conventionally obtained data indicate that this information can be used in various combinations with statistical models and topographic information to generate quantitative time‐ and location‐specific estimates of discharge. Thus, it appears feasible to produce useful river‐discharge information via remote sensing, especially for locations that are remote or otherwise difficult to observe conventionally. However, it is likely that this capability will be useful only for relatively large rivers and that, even for these, measurement accuracy will be considerably lower than is possible with ground‐based measurements. Given the scientific and societal importance of river‐discharge observations, and the present discharge‐measurement network and unlikely prospects for its expansion, further development and testing of these approaches, development of concurrent sets of pertinent remotely sensed variables, and a more complete evaluation of the spacing and timing of observations possible with satellite systems is warranted.
Soil Water Flow at Different Spatial Scales
Authors: Hopmans Jan W , Schoups Gerrit
Published: April 2006

A major challenge in hydrological sciences is the modeling of flow and transport processes and their measurement across a range of spatial or temporal scales. Such needs arise, for example, when watershed processes must be determined from soil hydrological data collected from a limited number of in situ field measurements or analysis of small soil cores in the laboratory. The scaling problem cannot be solved by simple consideration of the differences in space or timescale, for several reasons. First, spatial and temporal variability in soil hydrological properties create uncertainties when changing between scales. Second, flow and transport processes in vadose zone hydrology are highly nonlinear. As a result, vadose zone properties are nonunique and scale‐dependent, resulting in effective properties that vary across spatial scales and merely serve as calibration parameters in hydrologic models. Therefore, their estimation for heterogeneous materials can only be accomplished using scale‐appropriate measurements and models. We present examples of soil water flow at the pore, local, and regional scales. The inherent complexity of flow in heterogeneous soils, and the need to integrate theory with experiment, requires innovative and multidisciplinary research efforts to overcome limitations imposed by current understanding of scale‐dependent soil water flow and transport processes.
Solute Transport in Soil at the Core and Field Scale
Authors: Vanclooster Marnik , Javaux Mathieu , Vanderborght J
Published: April 2006

A detailed understanding of the flow and transport processes of chemicals in soils is needed to implement effective and efficient soil and water management strategies and to meet the current challenges of sustainable development. The solute transport process is a key process determining the mass flow of chemical substances once they are released in the soil solution. The transport process is a highly nonlinear and space‐time dynamic process for which models are still poorly validated at the larger scale. In this article, we review some recent advances in describing solute transport in soil at the macroscopic core scale, corresponding to the scale of most soil measurement devices, and the pedon scale, corresponding to the scale of a small field. After introducing basic concepts, major problems using the classical Convection Dispersion Equation (CDE) model for describing field‐scale solute transport is presented and alternative modeling concepts are introduced.
Determining Soil Hydraulic Properties
Authors: Durner Wolfgang , Lipsius Kai
Published: April 2006

Hydraulic properties are required for modeling water and solute transport in unsaturated soils. The bottleneck for the successful application of numerical simulation models lays usually in their parameter estimation requirements. Methods to determine hydraulic properties can be classified into indirect and direct approaches. Indirect methods encompass the estimation of hydraulic properties by pedotransfer functions from more easily measured soil properties, and the prediction of the unsaturated hydraulic conductivity function from thewater retention curve(WRC). In direct methods, observations of flow attributes from laboratory or field experiments are evaluated. This article reviews common methods to estimate the hydraulic conductivity function from the water retention characteristic and various direct measurement techniques in the laboratory and the field. We conclude with an outlook on contemporary developments in measurement techniques, stressing the key role of inverse modeling of experiments to derive optimum hydraulic properties and the importance of a future combination of noninvasive measurement techniques with inverse modeling by stochastic data fusion.
Inverse Modeling of Soil Hydraulic Properties
Authors: Vrugt Jasper A , Dane Jacob H
Published: April 2006

Numerical‐simulation models of water flow and solute transport in the vadose zone are important tools in environmental research. The accuracy of predictions with these models heavily relies on accurate estimates of the unsaturated soil hydraulic parameters. Initial attempts on the estimation of the soil hydraulic parameters mainly focused on the use of relatively simple static or steady‐state flow experiments. While these laboratory methods have the advantage of being relatively simple to implement, they are typically time consuming and require restrictive initial and boundary conditions to satisfy the assumptions of the corresponding analytical solutions. Significant advances in computational capabilities in the 1980s have stimulated research on the use of Inverse Modelling (IM) for the estimation of soil hydraulic properties. Parameter estimation using IM accommodates much more flexible experimental conditions than required for the static or steady‐state flow methods, thereby enabling a rapid characterization of the hydraulic properties of the considered soil domain. This article explores the historical development of current perspectives on the identification of a single “best” set of soil hydraulic parameters using IM (thereby effectively neglecting the influence of possible sources of uncertainty on the final parameter estimates), and discusses alternative (Bayesian and multiple‐criteria) parameter estimation strategies which can be used to quantify the uncertainty (probabilistic and multiobjective) associated with the inversely estimated soil hydraulic properties. Throughout this article, we use a classical transient laboratory outflow experiment to further illustrate the various aspects of the IM procedure for estimating the soil hydraulic parameters.
Assessing Uncertainty Propagation through Physically Based Models of Soil Water Flow and Solute Transport
Authors: Brown James D , Heuvelink Gerard B M
Published: April 2006

Soil hydrological models are inherently imperfect because they abstract and simplify “real” hydrological patterns and processes. Indeed, an important aim of modeling is to establish the simplest description possible for adequately addressing a particular problem. Also, models are frequently based on input data that are known to be inadequate for some practical purpose. Thus, uncertainties in model outputs originate from uncertainties in input data, which include measurement and interpolation errors, and in models that include conceptual, logical, mathematical, and computational errors. Understanding the causes and consequences of uncertainty in soil hydrological modeling is useful for: (i) establishing the utility of data and models as decision‐support tools; (ii) directing resources towards improving data and models, and (iii) seeking alternative ways of managing soils when the opportunities for accurate modeling are limited. This chapter focuses on statistical methods for assessing uncertainty in soil data and models, propagating uncertainties through models, and assessing the contribution of different sources of uncertainty to the overall uncertainties in model predictions. In addition, it explores the impacts of scale, and changes between scales, on the outcomes of an uncertainty analysis. It concentrates on physically based models of soil water flow and solute transport, and provides numerous examples from the literature here
Erosion Prediction and Modeling
Authors: Nearing Mark , Renard Ken , Nichols Mary
Published: April 2006

Models of soil erosion by water historically have been primarily either empirically‐based or process‐based (sometime referred to as physically‐based). The first models of soil erosion were empirically‐based. The prime example of the empirically‐based model is the Universal Soil Loss Equation (USLE). More recent models have been based on equations that describe the physical, biological, and/or chemical processes that cause or affect soil erosion. It is important to understand that both the process‐based and the empirically‐based model possess a major empirical component, in the sense that the constitutive equations use parameters based on experimental data. This chapter details some of the important elements of soil erosion models and factors that influence their use.
Floodplain Sedimentation – Methods, Patterns, and Processes: A Review with Examples from the LowerRhine, theNetherlands
Authors: Middelkoop Hans
Published: April 2006

Floodplain sedimentation plays a keyrole in the sediment budget of river basins, but it also relates to river management, flood protection, civil engineering, and ecological rehabilitation of rivers. Understanding of the process, therefore, is essential both from a geomorphologic and river management points of view. This paper provides a review of recent research on floodplain sedimentation, focusing on field‐based methods to reconstruct and document spatial patterns of overbank sedimentation, as well on mathematical modeling approaches. Examples of floodplain sedimentation over various timescales are given for the lower Rhine River in the Netherlands. The final section synthesizes the processes, their controls, and the resulting patterns and amounts of floodplain deposition.
Reservoir Sedimentation
Authors: Annandale George W
Published: April 2006

The storage loss due to reservoir sedimentation has been found to exceed the storage added worldwide by the mid 1990s. Recovery of this lost storage is estimated to range between US$ 10 billion and US$ 20 billion per year, not accounting for the growth in world population. Reservoir sedimentation does not only impact water supply and power supply reliability, but also has significant impacts on the environment, including downstream river degradation, and impacts on recreation, flood management, infrastructure, and the economy. The article illustrates how sediment is trapped in reservoirs, provides empirical techniques for calculating the volume of sediment that can be trapped by reservoirs, outlines sediment management techniques, which includes flushing, dredging, dry excavation, hydrosuction, and bypassing, and introduces the life‐cycle management approach. This approach to reservoir sedimentation management is in contrast to the conventional design life approach, which is inappropriate for the design, construction, and management of dams and their reservoirs. Numerous references for further reading are listed.
Sediment Yields and Sediment Budgets
Authors: Walling Desmond E
Published: April 2006

Measurements of sediment transport have been undertaken on many rivers throughout the world. Most of the available data relate to suspended sediment loads and the results provide a wealth of information on the variation of suspended sediment yields in both space and time. This contribution reviews current knowledge regarding land–ocean sediment transfer and sediment fluxes to the oceans, global patterns of sediment yield and their controls, and temporal variability of sediment yields in response to both natural controls and human activity and environmental change. In order to understand the sediment response of a drainage basin, it is important to take account of the complex linkages between sediment mobilisation and sediment output, and particularly the role of both short‐ and long‐term storage. The sediment budget provides a useful conceptual framework for this purpose and current understanding of the structure of catchment sediment budgets is reviewed.
Water Quality
Authors: Meybeck Michel , Peters Norman E , Chapman Deborah V
Published: April 2006

Water quality is the term used to describe the chemical, physical, and biological characteristics of water. Furthermore, the physical, chemical, and biological characteristics of a water body, “its water quality”, determine the suitability of that water for a particular value, for example, potability, ecosystem status, agriculture, industry, recreation. Water‐quality issues have become a rapidly evolving component of the environmental sciences primarily due to the increasing demand on water resources and amenity value and the intricate linkage between water‐quality use and ecosystem health. Water quality varies markedly in time and space. Episodic temporal water‐quality variations can occur in minutes whereas, periodic variations can occur on short timescales from diurnal, associated with variations in light and temperature, to seasonal and longer associated with climatic variations. Spatial variations also vary markedly from millimeters and centimeters in soil profiles to very large‐scale variations (>1000 km), for example, associated with water residence times, lithology, landscape position, land use, and bioclimatic zone. In the absence of the effects of human activities, water quality is primarily controlled by climate (precipitation and temperature) and geology (lithology, geomorphology, soil). The water quality of a given volume of water is derived from the sum of effects of mixing and interactions from all upstream sources including atmospheric deposition (precipitation and dry deposition), soil water (matrix and macropore), groundwater, springs, wetlands, irrigation ditches and canals, streams, ponds, lakes, reservoirs, rivers, and estuaries. In most parts of the world, human activities have now caused multiple and complex changes in background water quality.
Point and NonPoint Source Pollution
Authors: Loague Keith , Corwin Dennis L
Published: April 2006

The information age has ushered in a global awareness of complex environmental problems that do not respect political or physical boundaries: climatic change, ozone layer depletion, deforestation, desertification, and pollution from point and nonpoint sources. Among these global environmental problems, point and nonpoint source pollution represent a perfect example of a complex multidisciplinary problem that exists over multiple scales with tremendous spatial and temporal complexity. A point source of pollution discharges to the environment from an identifiable location, whereas a nonpoint source of pollution enters the environment from a widespread area. The ability to accurately assess present and future point and nonpoint source pollution impacts on ecosystems ranging from local to global scales provides a powerful tool for environmental stewardship and guiding future human activities.
Salinization
Authors: Ruprecht John , Dogramaci Shawan
Published: April 2006

Salinization of land and water resources is a major environmental and economic problem facing many parts of the world. Soil salinization and the consequent degradation of agricultural land are proceeding so fast that many countries may not be able to achieve sustainable agriculture to feed their population in the foreseeable future. Furthermore, many important ecosystems and internationally recognized wetlands are under serious threat from the increased salinity.

Salinization is the process by which the concentration of total dissolved solids in water or soil is increased because of natural or human‐induced processes.

Many aspects of hydrological science underpin the understanding of salinization. The knowledge and understanding of the movement of water and salt from the atmosphere, through landscapes and rivers are essential to understanding salinization.

The main approaches to assessing the impacts of land and water salinization and to evaluating potential solutions include process studies, catchment studies, modeling, and statistical analysis. Each of these is discussed in more detail in the article. A summary of the models available for salinity studies is also outlined.
Reservoirs
Authors: Marzolf G Richard , Robertson Dale M
Published: April 2006

Reservoirs are surface water impoundments that result from the construction of dams on streams and rivers. Limnological principles are applicable to understanding reservoir functions and the processes that control them because they exhibit lake‐like characters (see Lake Ecosystems –
Lake Ecosystems (Stratification and Seasonal Mixing Processes, Pelagic and Benthic Coupling)
. The basic water qualities of reservoirs are dependent, however, on the river that was impounded in their formation. The river's inflow dominates the upstream reaches of reservoirs. Lacustrine processes become more controlling as advective forces diminish and wind‐mixing and solar heating become influential forcing factors and as the cross section of the basin widens and deepens downstream. This transition from riverine to lacustrine imposes longitudinal gradients in reservoirs that emerge as their central feature. Designs of dams and their operating schedules have considerable influence on these in‐reservoir patterns and on the effects of the dam in the downstream river downstream. Various models have been developed to describe the physical, chemical, and biological processes in reservoirs. Additional studies will lead to further information to improve the models, which in turn will lead to a better understanding and management of reservoirs.
Paleolimnology and Paleohydrology
Authors: Fritz Sherilyn C , Saros Jasmine E
Published: April 2006

The study of the sedimentary record of nonmarine aquatic systems (paleolimnology) provides a tool for reconstructing the history of the basins themselves, as well as the history of the atmospheric and terrestrial systems that influence them. Such records allow us to understand environmental dynamics over periods longer than that of recorded history. Among inland aquatic systems, lakes have been studied more extensively; therefore, this review focuses on insights regarding environmental structure and function gained from studying lake sediments. Studies of the lacustrine paleolimnological record have been used to describe patterns of long‐term climatic variation and the response of lakes to changes in climate and catchment vegetation and soils. Although ecological literature from the early decades of the twentieth century suggested that lakes became enriched over time (eutrophic), eventually filling with sediment and becoming wetlands, it is now clear that there are multiple developmental pathways for lakes. Some lakes indeed become enriched, whereas others show a long‐term decline in nutrient concentrations and pH. Similarly, succession from lake to bog or wetland is not a universal trend. Paleolimnological studies also have been used to reconstruct the recent history of eutrophication, lake acidification, and atmospheric pollution. In these studies, the history archived in lake sediments provides a tool for measuring the magnitude of human impacts on the environment and for evaluating whether twentieth century patterns fall outside the natural range of environmental variation.
Hyporheic Exchange Flows
Authors: Bencala Kenneth E
Published: April 2006

Water having entered a stream channel from the surrounding catchment may continue to have connections with the catchment. In the stream's hyporheic zone, water “in the channel” exchanges with “groundwater” in the bed of the stream. Hyporheic exchange flows typically occur at scales small relative to the length and volumetric transport characteristics of the stream. Nevertheless, it is well documented that hyporheic exchange flows significantly influence nutrient dynamics. Additionally, there is evidence of hyporheic exchange flows similarly influencing the processes establishing the concentrations of major‐ions and metals in stream‐catchment systems. It is within the contexts of (i) solute transport and (ii) the continuing connections of streams to their catchments that the hydrologic aspects of hyporheic exchange flows are studied. The Transient Storage Model (TSM), a pseudo‐two‐dimensional representation of stream and hyporheic zone solute transport, is used to identify characteristics of hyporheic zone, physical size, and solute residence times. The TSM is further extended to simulations of reactive solute transport to understand and interpret the biogeochemical processes of streams influencing the solute dynamics of catchments. Active hydrologic research continues to advance the process basis from which quantitative assessments of the role of hyporheic exchange flows will be made.