# Groundwater model

Groundwater models are computer models of groundwater flow systems, and are used by hydrogeologists. Groundwater models are used to simulate and predict aquifer conditions.

## Characteristics

An unambiguous definition of "groundwater model" is difficult to give. This section is therefore called "Characteristics" and it gives descriptions rather than stringent definitions

A groundwater model may be a scale model of the groundwater situation or of an aquifer. Usually, however, a groundwater model is meant to be a (computer) program for the calculation of groundwater flow and level. Some groundwater models include (chemical) quality aspects of the groundwater.

Groundwater models may be used to predict the effects of hydrological changes (like groundwater abstraction or irrigation developments) on the behavior of the aquifer and are often named groundwater simulation models. As the calculations are based on mathematical equations, often with numerical (approximate) solutions, these models are also called mathematical/numerical groundwater models.

For the calculations one needs (hydrological) inputs, parameters, initial and boundary conditions. This holds for many other models.

The input is usually the inflow into the aquifer or the recharge, which varies in time and from place to place. The parameters usually concern the physical properties used in the model that are more or less constant with time but variable in space.

Important parameters are the topography, thicknesses of soil layers and their horizontal/vertical hydraulic conductivity (permeability for water), porosity and storage coefficient, capillarity of the unsaturated zone (see hydrogeology).

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 called subsidence. The thickness, in this case, is variable in time and not a parameter proper.

Initial conditions and boundary conditions can be related to levels, pressures, and hydraulic head on the one hand (head conditions), or to recharge, discharge, inflow and outflow on the other hand (flow conditions).

The applicability of a groundwater model to a real situation depends on the accuracy of the input data and the parameters. Determination of these requires considerable study, like collection of hydrological data (rainfall, evapotranspiration), and determination of the parameters mentioned before including pumping 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 the if-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 a sensitivity 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 by calibration. 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 if salinity figures of the groundwater are available and the value of hydraulic 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 ones. 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.

Some groundwater models are two-dimensional, they apply to a vertical plane with a unit width wile it is assumed that the groundwater conditions repeat themselves in other parallel vertical planes.

Spacing equations of subsurface drains are an example of a two-dimensional groundwater model (see for example the page energy balance where drainage equations based on the energy balance of groundwater flow are discussed).

A three-dimensional model requires discretization of the flow domain as (mostly) the underlying mathematics cannot be solved analytically so that numerical methods are to be used.

To that end the flow region must be subdivided into smaller elements (or cells) within which the parameters are maintained constant.

The land surface may be divided into polygons (or triangles, squares, rectangles).

One may subdivide the underground also into cells of different forms or one may accept for the discretization horizontal layers (that may different from polygon to polygon).

In the latter case one may take the flow in the layer as horizontal and vertical: there is no flow in an intermediate direction. This is the reason why the layered models are called two-dimensional. In reality, the whole block of layered soil under a polygon has flow in two horizontal directions as well as in a vertical direction. Therefore, one might call such models "simplified 3-dimensional".

Models that are using cells throughout the soil body are truly 3-dimensional when in each cell there may flow in all directions. A well known model 3-dimensional model is Modflow (see modelling software).

## Groundwater modeling software & references

* GMS
* Visual MODFLOW

computer simulation, a computer model or a computational model is a computer program that attempts to simulate an abstract model of a particular system. Computer simulations have become a useful part of mathematical modelling of many natural systems in physics
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of lithologic formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water.
Hydrogeology (hydro- meaning water, and -geology meaning the study of the Earth) is the part of hydrology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust, (commonly in aquifers).
An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well.
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of lithologic formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water.
An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well.
Input is the term denoting either an entrance or changes which are inserted into a system and which activate/modify a process. It is an abstract concept, used in the modeling, system(s) design and system(s) exploitation. It is usually connected with other terms, e.g.
Hydraulic conductivity, symbolically represented as , is a property of vascular plants, soil or rock, that describes the ease with which water can move through pore spaces or fractures. It depends on the intrinsic permeability of the material and on the degree of saturation.
Hydrogeology (hydro- meaning water, and -geology meaning the study of the Earth) is the part of hydrology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust, (commonly in aquifers).
subsidence is the motion of a surface (usually, the Earth's surface) as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation.
In mathematics, in the field of differential equations, an initial value problem is an ordinary differential equation together with specified value, called the initial condition, of the unknown function at a given point in the domain of the solution.
boundary value problem is a differential equation together with a set of additional restraints, called the boundary conditions. A solution to a boundary value problem is a solution to the differential equations which also satisfies the boundary conditions.
Hydraulic head is a specific measurement of water pressure or total energy per unit weight above a datum. It is usually measured as a water surface elevation, expressed in units of length, but represents the energy at the entrance (or bottom) of a piezometer.
Recharge or deep drainage is a hydrologic process where water moves downward from surface water to groundwater. This process usually occurs in the vadose zone below plant roots, and is often expressed as a flux to the water table surface.
Discharge in the context to expel or to "let go" may refer to:
• A military discharge, issued when a member of the armed forces is released from service
• Termination of employment

Parameters, in the plural form, has recently become popular with non-technical users to mean limits, but this should not be confused with the word's technical meaning.

In mathematics, statistics, and the mathematical sciences, parameters (L: auxiliary measure
Rain is a type of precipitation, a product of the condensation of atmospheric water vapor that is deposited on the earth's surface. It forms when separate drops of water fall to the Earth's surface from clouds.
Evapotranspiration (ET) is a term used to describe the sum of evaporation and plant transpiration from the earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies.
An aquifer test (or a pumping test) is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" (drawdown) in observation wells.
Salinity is the saltiness or dissolved salt content of a body of water. Salinity in Australian English and North American English may refer to salt in soil (see soil salination).

## Definition

Water salinity
Fresh water Brackish water Saline water Brine
Hydraulic conductivity, symbolically represented as , is a property of vascular plants, soil or rock, that describes the ease with which water can move through pore spaces or fractures. It depends on the intrinsic permeability of the material and on the degree of saturation.
Energy balance has the following meanings in several fields:
• In physics, energy balance is a systematic presentation of energy flows and transformations in a system.

Three-dimensional space is the physical universe we live in. The three dimensions are commonly called length, width, and breadth, although any three mutually perpendicular directions can serve as the three dimensions. Pictures are commonly two dimensional, they lack depth.
discretization concerns the process of transferring continuous models and equations into discrete counterparts. This process is usually carried out as a first step toward making them suitable for numerical evaluation and implementation on digital computers.
POLYGONE is an Electronic Warfare Tactics Range located on the border between France and Germany. It is one of only two in Europe, the other being RAF Spadeadam.

The range, also referred to as the Multi-national Aircrew Electronic Warfare Tactics Facility (MAEWTF), is
The analytic element method (AEM) is a numerical method used for the solution of partial differential equations. It was initially developed by O.D.L. Strack at the University of Minnesota.