A | |
Abiotic | Abiotic factors are the non-living components of an organism's environment. The term abiotic is also used to denote a process, which is not facilitated by living organisms. |
Absorption |
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Acid Rain | Rain which in the course of its history has combined with chemical elements or pollutants in the atmosphere and reaches the Earth's surface as a weak acid solution. IGH |
Adjustment test | see module 1a, 10.1 |
Adsorption | Describe the interaction of the dissolved species with the surface or interface of the sorbent. |
Advection | Describes the downstream transport of solute mass at the mean flow velocity. |
Aerodynamics resistance | see module 1a, 4.2.2 |
Aerosol | System of solid or liquid particles suspended in a gaseous medium, having a negligible falling velocity. UNSD |
Agricultural runoff | Water that flows from agricultural fields. Agricultural runoff is a major source of pesticides in water. UNSD |
Alluvium | see module 1a, 6.6 |
Amino Acid | Organic nitrogen containing acids which are used to construct proteins. FPG |
Analytical methods | Mathematical methods based on algebraic or differential calculus principles. |
Anisotropy | Condition of a medium having physical properties varying with the direction. |
Annual flow | see module 1a, 6.1.3 |
Anthropogenic activities | are diversity of human affairs done for getting some profits, more secure or a larger confort in the life of people. |
Aquifer | A porous water-bearing of permeable rock, sand or gravel capable of yielding economically significant quantities of useful groundwater. |
Archive | see module 1a, 10.1 |
Atmometer | see module 1a, 9.2.1 |
Atmosphere | The gaseous layer covering the Earth. |
Autocorrelation test | see module 1a, 10.1 |
Autotroph | An organism that produces food molecules inorganically by using a light or chemical based sources of external energy. This organism does not require outside sources of organic food energy for survival. FPG |
C | |
Calibration | A mathematical procedure for assessing the model parameters. |
Carbon Cycle |
|
(CO2) Carbon Dioxide | Colourless, odourless and non-poisonous gas that results from fossil fuel combustion and is normally a part of ambient air. It is also produced in the respiration of living organisms (plants and animals), and considered to be the main greenhouse gas, contributing to climate change. UNSD. |
Catchment | The area of land from which rainfall or snowmelt flows into a stream, lake, reservoir etc. Syn: catchment area, drainage basin, watershed, river basin, basin area. |
Chemoautotrophs | Chemoautotrophs are bacteria that obtain energy from oxidation of inorganic compounds such as ammonia, nitrites, and sulphides; they synthesise carbohydrates and are found in cave communities and ocean depths. |
Chemical oxygen demand (COD) | Mass concentration of oxygen equivalent to the amount of a specified oxidant consumed by dissolved or suspended matter when a water sample is treated with that oxidant under defined conditions. (ISO/6107) |
Cloud | The colloidal system of condensing products into the liquid, solid or mixed phase, the products being in suspension within the atmosphere. When the cloudy elements grow and become heavy, they drop out of clouds forming the precipitations. |
Coefficient of uniformity Cu | is defined as: where D10 is the size such that 10% of the particles are smaller than that size and D60 is similarly defined for 60%. |
Composition of the flood waves | Addition of the corresponding discharges of the main river and a tributary downstream a confluence. | Concentration time |
|
Conceptual model |
|
Conceptual hydrologic model | The simplified mathematical representation of some or all of the processes in the hydrologic cycle by a set of hydrologic concepts espressed in mathematical notations and linked together in a time and space sequence corresponding to that occuring in nature. | Condensation | The change in water phase, from a vapour state into a liquid state. The condensation is produced by way of air cooling below the dew point and by the deposition of the water vapours on the nucleus of condensation. | Conformity test | see module 1a, 10.1 | Confined aquifer | An aquifer overlain and underlain by relatively impermeable beds in which groundwater is confined under pressure sufficiently to be raised above the top of the impermeable overlying bed. | Constraints | Limits imposed to variables or to relations between variables. | Containment | Refers to systems that prevent the further spread of contamination; systems that control the groundwater flow direction around the contaminated sites. | Contamination | Introduction into water of any undesirable substance not normally present in water, e.g. micro-organisms, chemicals, waste or sewage, which renders the water unfit for its intended use. | Conversion of mass | Conversion of mass simply states that the accumulation of mass in a unit volume of water is equal to the difference between mass entering the volume and mass leaving the volume. | Convolution equation | An equation used to derive a storm hydrograph from a unit hydrograph (add and lag method):
where: |
Convolution integral | The fundamental equation of the linear systems, using the impulse time function and a unit response function (kernel function, weighting function) to compute the output of the system. Syn: Duhamel convolution |
Cybernetics | Science analysing the basic control principles of systems. |
Cybernetics systems | Systems which regulate their activity through feed-back connections. |
D | |
Daily mean discharge | see module 1a, 10.1 |
Darcy law | A law expressing the proportionality of the specific discharge of a liquid flowing through a porous medium to the hydraulic gradient in laminar flow (low Reynolds number). |
Decision variables | Unknowns of a mathematical model describing the system functionality. |
Degree of saturation (Sr) | or saturation ratio is commonly expressed in three equivalent forms:
|
Denitrification | Natural source of dinitrogenoxide (N2O) from the bacterial or chemical reduction of nitrates in water or soil, producing first nitrites, then nitrogen. UNSD |
Design storm | The estimate of rainfall amount and distribution over a particular drainage area, accepted or used in determining the design flood |
Design flood | The flood hydrograph of instantaneous peak discharge , either observed or synthetic, which is chosen for the design of a hydraulic structure or river control, such as the outlet works of a dam, the height of a dam or levee, or the maximum water level in a reservoir. The design flood is not necessary the largest flood possible and the choice is usually made after a study of the river, the damage liable to be cused, and the economic factors to be involved. The design flood shoud be the maximum flood that the hydraulic structure can safely pass. |
Deterministic model | Model using only deterministic variables. |
Deterministic variables | A variable taking as the result of an experiment only a unique well-defined value. |
Dispersion | Thereby increasing the volume and decreasing the concentration of the day - containing water. |
Distributed | Function of space coordinates. Examples:
|
Distributed hydrologic model | A hydrologic model which divides the catchment into a large number of small subareas , simulate each separately, and combine them to obtain catchment response. |
Distributed parameter model | A model that attempts to describe physical processes and mechanisms in space and time (Syn: Distributed hydrologic model). |
Diurnal fluctuations | Diurnal fluctuations is permanent shift of temperature from a day to the other. |
DNA | deoxyribonucleic acid, chief constituent of chromosomes UNSD |
Domain of stability | Domain of validity of the Muskingum parameters: |
Drainage area | see module 1a, 2.1.1 |
E | |
Ecosystem | System in which, by the interaction between the different organisms present and their environment, there is a cyclic interchange of materials and energy. |
Effective porosity |
|
Engineering models | Models using simplified approaches, adapted to a limited information in order to obtain a satisfactory description of the system. |
Epure | see module 1a, 2.2.1.1 |
Equilibrium approach | Equilibrium approach is implemented to determine the steady - state composition of waters. |
Error | Deviation from the necessary output. |
Error function | The sum of the squares deviations between the measured and the computed values according to a chosen model. |
Eutrophication | Enrichment of water by nutrients, especially compounds of nitrogen and phosphorus, that will accelerate the growth of algae and higher forms of plant life. syn. enrichment (ISO/6107) |
evaporation | Emission of water vapour by a free surface at a temperature below the boiling point. Through evaporation water leaves the land and oceans, rising into atmosphere. |
G | |
Geographical zoning | Geographical zoning is a process of separation of an area according to the prevailing feature of a characteristic parameter. |
Geomorphic instantaneous unit hydrograph | An instantaneous unit hydrograph the shape of which is related to the stream pattern of the watershed. |
Gradient | A vector perpendicular on the contour lines, defined by the relation: |
Gradient methods | Numerical methods for searching the optimum based on the computation of the derivative of the objective function. |
Gradient of change | Gradient of change is the rat, at which a parameter or a state of process moves from one level to the other. |
Gray box | The conceptual approach to modeling a physical system which is to develop a relationship between the input and the output with introducing some physical relevance in the equations and parameters used in the model. |
Greenhouse effect | warming of the Earth's atmosphere caused by a build-up of carbon dioxide and other greenhouse or trace gases that act like a pane of glass in a greenhouse, allowing sunlight to pass through and heat the Earth but preventing a counterbalancing loss of heat radiation. UNSD. |
Groundwater | Subsurface water occupying the saturation zone, from which wells, springs, or streams are fed. |
Groundwater flow | Movement of water in an aquifer. It is considerably slower than surface runoff. Groundwater flows until reaches lakes, rivers or oceans. It can also reach the surface through artesian wells. see module 1a, 6.4 |
H | |
Heat exchange | Heat exchange is the flux of thermal energy between objects. |
Hessian matrix | The matrix of the second partial derivatives. |
Heterotrophic Bacteria | Bacteria that depend upon the decomposition of organic substances for their food . UNSD |
Hydraulic conductivity | A measure of the ease with wich a fluid will pass through a porous Earth material, determined by the size and shape of the pore space in the material and their degree of interconnection as well as by the viscosity of the fluid. |
Hydraulic conductivity at saturation | see module 1a, 5.1.2 |
Hydraulic gradient | The rate of change of total head per unit of distance of flow at a given point and in a given direction; if not specified, the direction is generally understood to be that of the maximum rate of decrease in hydraulic head. |
Hydraulic head | The height of the column of water above a referrence plane (such as sea level) that can be supported by the hydraulic pressure at a given point. |
Hydrical profile | see module 1a, 5.2.3 |
Concentration of hydrogen ions in milliequivalents per litre of solution, generally expressed in pH units. | |
see module 1a, 2.2.1.3 | |
see module 1a, 8.1 | |
see module 1a, 1.3 | |
Hydrologic system | A structure or volume in space, surrounded by a boundary, that accepts water and other inputs, operates on them internally, and produces them as outputs. |
Hydrologic unit |
|
Hydrology | see module 1a, 1.1 |
Hydrometric data gathering agencies | see module 1a, 10.1 |
Hydrosphere | That part of the Earth covered by water and ice. |
Hyetogram | A graphic representation of rain intensity variation within a given period. |
I | |
Infiltration |
|
Injection well | A well through which fluids are injected into subsurface. |
Instability of the Muskingum model | Situation in which the downstream discharges at the first step of the computation are negative, due to the violation of the constraints imposed to the parameters. |
Instantaneous unit hydrograph |
|
Interception | The process by which precipitation is caught and held by vegetation (canopy and litter structures) then may be lost by evaporation without reaching the ground. |
Intrinsec permeability | A measure of the relative ease with wich a porous medium can transmit a liquid under a potential gradient; it is a property of the medium alone and it is independent of the nature of the liquid and of the force field causing movement. |
Isochrones | see module 1a, 2.2.2.2 |
Isohyet | A line connecting the points of equal same precipitation quantities. |
J | |
K | |
L | |
Lagrange multipliers | Weighting factors of the constraints in a constrained optimisation problem. |
Least square method | The most used method for formulating parameters calibration, consisting in the minimization of an error function. |
Limnology | see module 1a, 2.2.2.2 |
Linear models | The model variables are at the first power. |
Linear systems | Systems for which the proportionality between cause and effect as well as the additively of the effects can be accepted. |
Lumped | Global. Examples:
|
Lumped hydrologic model | A hydrologic model which treats a whole catchment, or a significant portion of it, without considering spatial variations in parameters throughout the entire catchment as if it were homogeneous in character and suject to uniform rainfall. |
Lumped parameter model | A model that aggregates spatially distributed parameters. Lumped parameter models transforms actual rainfall input into runoff output by conceptualizing that all watershed processess occur at one spatial point (as in a black box). Model parameters may or may not have a direst physical definition in the system. |
Lysimeter | see module 1a, 9.2.1 |
M | |
Macroporosity | represents that porosity which allows great water flow through porous media. |
Mass humidity | see water content |
Mathematical model |
|
Matrix potential | The work per unit quantity of pure water that has to be done to overcome the attractive forces of water molecules and the attraction of water to solid surfaces. The matrix potential is negative above the water table and zero below a free water table (Syn: matric potential). |
Maximal evapotranspiration | see module 1a, 4.2.3 |
Methane (CH4) | colourless, non-poisonous and flammable gaseous hydrocarbon created by anaerobic decomposition of organic compounds. Methane is a potent greenhouse gas. UNSD |
Method of sedimentation | to determine the particle size distribution of a fine-grained soil, cohesive and low cemented, consists in a sedimentation process based on different velocity of sedimentation of solid particles of a soil suspension prepared in distilled water and a deflocculating agent. |
Method of sieving | to determine the particle size distribution of a coarse-grained soil, uncemented and cohesiveless, consists in passing of whole quantity of soil through several standard test sieves ordered successively from greater to smaller mesh sizes. |
Microporosity | represents that porosity which allows only capillary water movement through porous media. |
Migration | Movement of organisms in an intentional way between two points in space. Many migrations are seasonal. (FPG) |
Mixed analysis | to determine the particle size distribution of a soil available for both fine and coarse soils is a combination of both sedimentation and sieving method. |
Model | A simplified representation of a complex system. It describes some but not all the characteristics of the system. It involves similarity without identity. |
Model calibration | A process of parameter estimation using known data. |
Modeling |
|
Model validation | A compulsory step in the modelling process in order to prove the model capacity to provide credible and verifiable results. |
Model verification | A process of comparing parameter estimates against a new set of data once the model has been calibrated. |
Monthly flow coefficient | see module 1a, 6.1.1 |
Muskingum model | A linear relation used for the hydrologic flood routing |
Muskingum parameters | K and X, where K is approximately the propagation time and X is a weighting factor |
N | |
Natural Resources | Natural assets (raw materials) occurring in nature that can be used for economic production or consumption. See also renewable natural resources and non-renewable natural resources bustion of fuels) with light. It causes eye irritation. UNSD |
NAPL | A liquid consisting of organic compounds that are not completely miscible with water. |
Net rain | see module 1a, 5.1.2 |
Nitrate |
nitrogen-containing compound that can exist in the atmosphere or as a dissolved gas in water. It may produce harmful effects on humans and animals. UNSD |
Nitrification | biological process involving the conversion of nitrogen-containing organic compounds into nitrates and nitrites. It is part of the nitrogen cycle and considered to be beneficial since it converts organic nitrogen compounds into nitrates that can be absorbed by green plants.UNSD |
Nitrites | nitrous oxide salts used in food preservation. UNSD |
Numerical methods | Mathematical procedure using an iterative process, convergent in a limited number of steps to a limit value. |
Numeric model | Mathematical procedure used for finding the solution of a mathematical model. |
Numeric simulation | Description of the system’s behaviour using a mathematical model and the corresponding computer code. |
P | |
Parameter |
|
Parameter optimization | Selection of the best value for a parameter. |
Parameters identification | Graphical of mathematical procedure leading to the best evaluation of the model parameters, so that the computed downstream discharge to be as close as possible to the measured discharges in the same section. |
Parameters validation | The confirmation of the calibrated parameters values by performing simulations for other situations than those used for calibration, for which one still has measured values. |
Particle size distribution curve | of a soil represents the weight percentage of the total weight, of all particles smaller than any given diameter vs. that diameter. |
Percolation | see module 1a, 5.1.2 |
Percolation gauge | see module 1a, 9.2.1 |
pH | Absolute value of the decimal logarithm of the hydrogen-ion concentration (activity) . Used as an indicator of acidity (pH < 7) or alkalinity (pH > 7). |
Pollutant | A substance which impairs the suitability of water for a considered purpose. |
Photosynthesis | chemical process carried on by green plants through which light energy is used to produce glucose from carbon dioxide and water, and oxygen is released as a by-product. UNSD |
Photochemical Air Pollution | Pollution caused by the reaction of unsaturated and saturated hydrocarbons, aromatics and aldehydes (emitted owing to the incomplete combustion of fuels) with light. It causes eye irritation.UNSD |
Physically based models | Models using fluid mechanics laws and principles to describe the systems structure and behaviour. |
Plume | A zone containing predominantly dissolved and sorbed contaminants in equilibrium with the dissolved contaminants. |
Pluviogram | The pluviograph diagram that records the rain characteristics (quantity, rain time, rain intensity). |
Pollution | The alteration of the physical, chemical, or biological properties of water, or a discharge of any substance into water, which adversely affects any beneficial water use. |
Pore space | Open space in rock or granular material (Syn: pore, interstice, void space). |
Porosity | represents the ratio of the volume of voids (Vv) to the total volume of the soil (V), expressed as a decimal fraction or as a percentage. |
Potential evapotranspiration | see module 1a, 4.2.3 |
Porous medium | is the solid particles-voids entity representing the medium through which groundwater movement occurs, being pervious or impervious depending on the communicating degree of the porous space. |
Precipitation | Liquid or solid products of the condensation of water vapour falling from clouds or deposited from the air on the ground. For example, rain, sleet, snow, hail. |
Principle of mass conservation | Equality between the net inflow (inflow minus outflow) and the variation of storage in a given period of time |
Production function | see module 1a, 8.2 |
Purification | Treatment of water (or sewage) to change harmful or undesirable physical properties and remove harmful and undesirable chemical substances and living organisms. |
Q | |
R | |
Rain gauge | see module 1a, 4.2.3 |
Real evapotranspiration | see module 1a, 9.1.1 |
Representative elementary volume | represents the minimum volume randomly selected from a porous medium, which keeps yet porosity features of the entire volume of the site. |
Residual porosity | represents that porosity characterizing the noncommunicating voids unable to allow flow of underground water. |
Residual soil | is that soil resulted if the weathering products remains at their original location. |
Reservoir routing | Computation of the downstream discharge from a reservoir using the continuity equation and an empirical relation: the stage-discharge curve of the spillways. |
Retardation | The movement of a solute through a geologic formation at a velocity less than that of the groundwater. |
Ribonucleic Acid (RNA) | Form of nucleic acid. Ribonucleic acid is used by most organisms to read the genetic information found in DNA and to produce specific organic molecules used in the development and functioning of cells. FPG |
Rising limb | see module 1a, 8.2 |
Risk assessment | An evaluation of the potential for exposure to contaminants and the associated hazard. |
Routing |
|
Runoff |
see module 1a, 6.2 |
Runoff coefficient |
|
Runoff hydrograph | The hydrograph constituted by the surface runoff and subsurface runoff. |
S | |
Saturated zone | see module 1a, 7.2.1 |
Saturation ration | see degree of saturation |
Sediment | Material transported by water from the place of origin to the place of deposition. In watercourses, sediment is the alluvial material carried in suspension or as bed load. |
Sediment concentration | Ratio of weight of dry solids to the weight of a water/sediment sample. |
Sediment yield | Total sediment outflow from a watershed or past a given location in a specified period of time. It includes bed load as well as suspended load. Usually expressed in weight per unit of time. syn. total load |
Sewage | Water supply from a community after it has been fouled by various uses. It may be a combination of the liquid or water-carried domestic, municipal and industrial wastes, together with such groundwater, surface water and storm water as may be present. |
Simulation |
|
Simulation model | A model which is formulated by using arithmetic and algebraic relations along with non-mathematical logical processess. It usually consists of five major components: state variables, exogenous variables, functional relations, inputs and outputs. |
Soil | In pedology, that Earth material which has been so modified and acted upon by physical, chemical and biological agents that it will support rooted plants. |
Sorption | see module 1a, 5.2.2 |
Specific flux | The rate of flow of a groundwater stream tube divided by its normal cross-sectional area (Syn: specific discharge). |
Specific storage | The volume of water that a unit volume of the aquifer under a unit decline in the average head releases from storage due to expansion of water and compression of the aquifer. |
Specific yield | The quantity of water which a unit volume of aquifer, after being saturated, will yield by gravity (Syn: effective porosity; storage coefficient). |
Staff gauge | see module 1a, 9.4.1 |
State of the system | Amount of matter, energy or information existent in the system at a certain time. |
State variables | Variables used to characterize the quantify of matter, energy or information of the system. |
State vector | Vector having as components state variables. |
Stationary point | A point where the first derivative is null; it can be a minimum, a maximum or an inflection point. |
Stochastic model | Model using stochastic variables. |
Stochastic variable | A variable whose value as the result of an experiment has a statistical distribution. |
Storage | Volume of water stored. |
Storage coefficient | The volume of water that a vertical column of the aquifer
of unit cross-sectional area releases from or takes into storage as the
average head within this column changes a unit distance. |
Stream hydrological regime | see module 1a, 6.1.1 |
Structure imitating model | A model designed to use meaningful principles of fluid mechanics and hydraulics to facilitate the transformation of the rainfall signal (input) into streamflow (output). It is opposite to the black box model. (syn. white box model) |
Sublimation | The process of the water vapours passage directly into the solid state (ice crystals). It takes place at low temperatures well below 0 °C, associated with the emission of a large quantity of latent heat. |
Subsurface flow | see module 1a, 6.3 |
Subsurface water | All water occcuring below the eart's surface in the liquid, solid or gaseous state. It consists of various waters : suspended water or vadose water (soil water, pellicular and gravitational water, cappilary water) and groundwater. see module 1a, 7.2 |
Sub-system | A part of a system, having interactions with other sub-systems and with the system of superior rank. |
Suction | A negative-pressure head of water which exists within the vadose zone (a tension in positive sense). |
Suction head | In an unsaturated porous medium, the part of the total energy possessed by the fluid due to the soil suction forces. The suction head will vary with the moisture content of the medium. |
Sulphur Dioxide (SO2) | heavy, pungent, colourless gas formed primarily by the combustion of fossil fuels. It is harmful to human beings and vegetation, and contributes to the acidity in precipitation.UNSD |
Surface retention capacity | see module 1a, 7.1.1 |
Surface-runoff | see module 1a, 7.1.1 |
|
|
Finding the best possible model to describe the system’s structure and behaviour. |
T | |
Total dissolved solids (tds) | Total weight of dissolved mineral constituents in water per unit volume (or weight) of water in the sample. |
Torrential rain | Frontal, intense rain with long duration. |
Trajectory of the system | The evolution in time of the state vector. |
Transfer function | An equation representing a transformation of the input into the output in a system: where: (syn. transformation equation) see module 1a, 8.2 |
Transpiration | The process by which water from vegetation is transferred into the atmosphere in the form of vapour. |
Transportated | is that soil resulted if the weathering products are transported from their original location. |
Trial and error | A method of calibration, the searching process being oriented through individual simulations followed by the evaluation of the results. |
U | |
Ultraviolet Rays | radiation in the wavelength range between visible light and X-rays, divided into wave length bands A, B, C. Much of the ultraviolet radiations in bands B and C are prevented from reaching the Earth's surface by the ozone layer present in the atmosphere. UNSD |
Unconfined aquifer | An aquifer in which the water table serves as the upper surface of the zone of saturation (Syn: phreatic aquifer; water-table aquifer). |
Unit hydrograph |
The hydrograph of direct runoff at a given point on a given stream resulting from 1 unit of excess rainfall occuring uniformly over the contributing drainage area at a uniform rate duting a unit period of time. see module 1a, 7.2.1 |
Unsaturated zone | see module 1a, 7.2.1 |
Urea (CO(NH2)2) | Urea is an organic N compound that is manufactured in large quantities. It is used as a fertilizer, including controlled-release fertilizers, and in many industrial processes. The U.S. production of urea was 9,330,000 tons in 1999 (Anon. 2000). Urea is the soluble form of N that is excreted by mammals. Therefore, large amounts of urea are added to the surface soils of feedlots and dairies. However, urea is rapidly hydrolyzed to NH3 and CO2, so it is not expected to make up a significant part of organic N in water bodies, soils, or sediments. (Ilinois State Water Survey.) |
V | |
Volumetric water content or volumetric humidity (θ) | is the ratio of pore water volume to the total volume of soil, expressed as percentage: |
W | |
Wash-out (precipitation) | Removal of substances such as dust, aerosols and gases from the atmosphere by solid or liquid precipitation. IGH. |
Wastewater | Water containing waste, i.e. liquid or solid matter discharged as useless from a manufacturing process. |
Water conservation | Measures introduced to reduce the amount of water used for any purpose, and/or to protect it from pollution. |
Water content | or mass humidity (w) is the ratio of the mass of water to the mass of solids in the soil, usually expressed as percentage: |
Water budget | see module 1a, 1.4 |
Water quality | Physical, chemical, biological and organoleptic properties of water. |
Watershed | see module 1a, 2.1.1 |
Water table | The upper surface of the unconfined groundwater body which is under atmospheric pressure and separates the upper unsaturated from the lower saturated soil zone. |
Weathering | disintegration of rocks into small soil particles through the physical and chemical action of atmospheric agents, for example, rain, water, frost, wind, temperature changes, plant and animals. UNSD |
Weighting function | The response of a linear system to a unit impulse. Syn: unit response function; kernel function |
White box model | A concept involving the use of fluid mechanics models for obtaining the best mathematical description of the system. |
X | |
Y | |
Sources