Geology Reference
In-Depth Information
2
2
h
R s t
-
-
h
-
h
G
Kh
Kh
=
(16)
x
y
2
2
-
-
x
-
y
where h is the water level function of x , y .
Boundary Conditions
Boundary conditions specify how an aquifer interacts with the environment
outside the model domain. To ensure a unique solution to a problem at least
one unique boundary condition is specified. There are four basic types of
boundary conditions:
No Flow Boundaries
There are physical or hydrological barriers that prevent water from flowing
into or out of the model domain. No flow boundaries are specified either
when defining the boundary of the model grid or by setting grid blocks as
inactive (i.e. hydraulic conductivity = 0). This is a very special type of the
prescribed flux boundary and is also referred to as no-flux, zero flux,
impermeable, reflective or barrier boundary. In the analysis of model results,
no flow boundaries are identical to streamlines. The natural groundwater
divides or streamlines act as no flow boundaries.
Constant Head Boundary
A source of water has an invariant water level at the model boundary. This
condition is used to model an aquifer in good communication with a lake,
large river, or another external aquifer. This is also known as first type
boundary condition, which is mathematically referred as Dirichlet boundary.
The constant head boundary or prescribed boundaries can occur when surface
water bodies such as rivers, lakes, canals, seacoast, impoundments and drains
interact freely with the aquifer.
Mathematically, the constant head boundary condition is stated as
h ( x )= h 0 ( x ), x @H 1 Dirichlet
where h 0 is the specified head along the boundary segment
H 1 of the
modelled domain I
Constant Flux Boundary
Entering or leaving the aquifer is constant/prescribed flux. This boundary
condition is used to simulate rainfall or distributed discharge such as
evaporation. It is also useful for specifying known recharge to the aquifer
due to induced recharge or reticulation. This boundary condition is also
referred to as second type boundaries, Neumann's condition or recharge
boundaries.
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