Groundwater model
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Groundwater models are computer models of groundwater flow systems, and are used by hydrogeologists. Groundwater models are used to simulate and predict aquifer conditions.
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[edit] Characteristics
An unambiguous definition of "groundwater model" is difficult to give, but there are many common characteristics.
A groundwater model may be a scale model or an electric model of a groundwater situation or 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, (hydraulic) parameters, initial and boundary conditions.
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 groundwater 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.
[edit] Dimensions
Groundwater models can be one dimensional, two dimensional, semi three dimensional and truly three dimensional.
One dimensional models can be used for the vertical flow in a system of parallel horizontal layers.
2-dimensional models apply to a vertical plane while 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 Groundwater energy balance where drainage equations based on the energy balance of groundwater flow are discussed).
3-dimensional models require discretization of the entire 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), in horizontal and vertical sense, within which the parameters are maintained constant (Fig. 2). The flow of groundwater between neighboring cells can be calculated horizontally and vertically using groundwater flow equations.
Semi 3-dimensional groundwater models have a grid over the land surface only. The 2-dimensional grid network consists of polygons, rectangles, squares, or triangles (Fig. 3). Hence, the flow domain is subdivided into vertical prisms. The prisms can be discretized into horizontal layers with different characteristics that may also vary between the prisms. The groundwater flow between neighboring prisms is calculated using horizontal groundwater flow equations. Vertical flows are found by applying one-dimensional flow equations in a vertical sense or from the water balance: excess of horizontal inflow over horizontal outflow (or vice versa) is translated into vertical flow. Like the truly 3-dimensional models, such models also permit the introduction of horizontal and vertical subsurface drainage systems (Fig. 1).
