Groundwater models arecomputer models ofgroundwater flow systems, and are used byhydrologists andhydrogeologists. Groundwater models are used to simulate and predictaquifer conditions.
An unambiguous definition of "groundwater model" is difficult to give, but there are many common characteristics.
Agroundwater model may be ascale model or an electric model of a groundwater situation oraquifer. Groundwater models are used to represent the natural groundwater flow in the environment. Some groundwater models include (chemical) quality aspects of the groundwater. Such groundwater models try to predict the fate and movement of the chemical in natural, urban or hypothetical scenario.
Groundwater models may be used to predict the effects of hydrological changes (like groundwater pumping or irrigation developments) on the behavior of the aquifer and are often named groundwater simulation models. Groundwater models are used in various water management plans for urban areas.
As the computations inmathematical groundwater models are based ongroundwater flow equations, which aredifferential equations that can often be solved only byapproximate methods using anumerical analysis, these models are also calledmathematical, numerical, or computational groundwater models.[1]
The mathematical or the numerical models are usually based on the real physics the groundwater flow follows. These mathematical equations are solved using numerical codes such asMODFLOW, ParFlow,HydroGeoSphere,OpenGeoSys etc.Various types ofnumerical solutions like thefinite difference method and thefinite element method are discussed in the article on "Hydrogeology".
For the calculations one needs inputs like:
The model may have chemical components likewater salinity,soil salinity and other quality indicators of water and soil, for which inputs may also be needed.
The primary coupling between groundwater and hydrological inputs is theunsaturated zone orvadose zone. The soil acts to partition hydrological inputs such as rainfall or snowmelt intosurface runoff,soil moisture,evapotranspiration andgroundwater recharge. Flows through theunsaturated zone that couple surface water tosoil moisture andgroundwater can be upward or downward, depending upon the gradient ofhydraulic head in the soil, can be modeled using the numerical solution ofRichards' equation[2] partial differential equation, or the ordinary differential equation Finite Water-Content method[3] as validated for modelinggroundwater andvadose zone interactions.[4]
The operational inputs concern human interferences with thewater management likeirrigation,drainage, pumping fromwells,watertable control, and the operation ofretention orinfiltration basins, which are often of an hydrological nature.
These inputs may also vary in time and space.
Many groundwater models are made for the purpose of assessing the effectshydraulic engineering measures.
Boundary conditions can be related to levels of thewater table,artesian pressures, andhydraulic head along the boundaries of the model on the one hand (thehead conditions), or to groundwater inflows and outflows along the boundaries of the model on the other hand (theflow conditions). This may also include quality aspects of the water like salinity.
Theinitial conditions refer to initial values of elements that may increase or decrease in the course of the timeinside the model domain and they cover largely the same phenomena as the boundary conditions do.
The initial and boundary conditions may vary from place to place. The boundary conditions may be kept either constant or be made variable in time.
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The parameters usually concern thegeometry of anddistances in the domain to be modelled and those physical properties of the aquifer that are more or less constant with time but that may be variable in space.
Important parameters are thetopography, thicknesses of soil / rock layers and their horizontal/verticalhydraulic conductivity (permeability for water),aquifer transmissivity andresistance,aquifer porosity andstorage coefficient, as well as thecapillarity of the unsaturated zone. For more details see the article onhydrogeology.
Some parameters may be influenced by changes in the groundwater situation, like the thickness of a soil layer that may reduce when the water table drops and/the hydraulic pressure is reduced. This phenomenon is calledsubsidence. The thickness, in this case, is variable in time and not a parameter proper.
The applicability of a groundwater model to a real situation depends on theaccuracy of the input data and theparameters. Determination of these requires considerable study, like collection of hydrological data (rainfall,evapotranspiration,irrigation,drainage) and determination of the parameters mentioned before includingpumping tests. As many parameters are quite variable in space, expert judgment is needed to arrive at representative values.
The models can also be used for theif-then analysis: if the value of a parameter is A, then what is the result, and if the value of the parameter is B instead, what is the influence? This analysis may be sufficient to obtain a rough impression of the groundwater behavior, but it can also serve to do asensitivity analysis to answer the question: which factors have a great influence and which have less influence. With such information one may direct the efforts of investigation more to the influential factors.
When sufficient data have been assembled, it is possible to determine some of missing information bycalibration. This implies that one assumes a range of values for the unknown or doubtful value of a certain parameter and one runs the model repeatedly while comparing results with known corresponding data. For example, ifsalinity figures of the groundwater are available and the value ofhydraulic conductivity is uncertain, one assumes a range of conductivities and the selects that value of conductivity as "true" that yields salinity results close to the observed values, meaning that the groundwater flow as governed by the hydraulic conductivity is in agreement with the salinity conditions. This procedure is similar to the measurement of the flow in a river or canal by letting very saline water of a known salt concentration drip into the channel and measuring the resulting salt concentration downstream.
Groundwater models can be one-dimensional, two-dimensional, three-dimensional and semi-three-dimensional. Two and three-dimensional models can take into account theanisotropy of the aquifer with respect to thehydraulic conductivity, i.e. this property may vary in different directions.
In semi 3-dimensional models thehorizontal flow is described by 2-dimensional flow equations (i. e. in horizontal x and y direction).Vertical flows (in z-direction) are described (a) with a 1-dimensional flow equation, or (b) derived from awater balance of horizontal flows converting the excess of horizontally incoming over the horizontally outgoing groundwater into vertical flow under the assumption that water isincompressible.
There are two classes of semi 3-dimensional models:
An example of a non-discretized radial model is the description of groundwater flow moving radially towards adeep well in a network of wells from which water is abstracted.[7] The radial flow passes through a vertical, cylindrical, cross-section representing the hydraulicequipotential of which thesurface diminishes in the direction of the axis of intersection of the radial planes where the well is located.
Prismatically discretized models likeSahysMod[8] have a grid over the land surface only. The 2-dimensional grid network consists of triangles, squares, rectangles orpolygons. Hence, the flow domain is subdivided into vertical blocks orprisms. The prisms can be discretized intohorizontal layers with different characteristics that may also vary between the prisms. The groundwater flow between neighboring prisms is calculated using 2-dimensional horizontal groundwater flow equations. Vertical flows are found by applying one-dimensional flow equations in a vertical sense, or they can be derived from the water balance: excess of horizontal inflow over horizontal outflow (or vice versa) is translated into vertical flow, as demonstrated in the articleHydrology (agriculture).
In semi 3-dimensional models, intermediate flow between horizontal and vertical is not modelled like in truly 3-dimensional models. Yet, like the truly 3-dimensional models, such models do permit the introduction of horizontal and verticalsubsurface drainage systems.
Semiconfined aquifers with a slowly permeable layer overlying the aquifer (theaquitard) can be included in the model by simulating vertical flow through it under influence of an overpressure in the aquifer proper relative to the level of the watertable inside or above the aquitard.