This preview shows page 1. Sign up to view the full content.
Unformatted text preview: user needs to specify
a single spatial location for the well in terms of its row and
column location in the grid and the elevations of the tops and
bottoms of the open intervals (or well screens). If this option is
used, then the model will compute the grid layers in which the
open intervals occur, the lengths of the open intervals, and the
relative vertical position of each open interval within a model
layer. The top and bottom elevations (Ztop and Zbotm) are
specified in dataset 2d2. The elevations must be referenced to
the same datum as the model grid.
If open intervals are defined by elevations, then the list
of intervals must be ordered so that the first interval listed is
the shallowest, the last interval listed is the deepest, and all
intervals are listed in sequential order from the top to the bottom of the well. If an interval defined by elevations partially
or fully intersects a model layer, then a node will be defined in
that cell. If more than one open interval intersects a particular
layer, then a lengthweighted average of the celltowell conductances will be used to define the wellnode characteristics;
the cumulative length of well screens will be assumed to be
centered vertically within the thickness of the cell. Additional
details related to such complex situations are discussed below
in the section on “Well Screen Variability.” If the open interval
is located in an unconfined (or convertible) layer, then its position in space remains fixed although its position relative to the
changing water table will be adjusted over time, as discussed
later in the section on “Partial Penetration.” Finally, if the
well is a singlenode well, as defined by setting LOSSTYPE =
NONE, and the specified open interval straddles more than one
model layer, then the well will be associated with the one cell
where the vertical center of the open interval is located. 8 Revised MultiNode Well (MNW2) Package for MODFLOW GroundWater Flow Model Model Features and Processes Table 1. Properties of the confined aquifer system analyzed in
the Lohman test problem. This section describes the basis for and evaluation of the
major features and processes of MNW2. Much of the testing
and evaluation was done using representative test problems.
Two basic test problems are described first. [Abbreviations used: feet, ft; feet per day, ft/day; square feet, ft2; cubic feet
per day, ft3/day; per foot, ft1] Description of Test Problems
This section describes two sample problems with which
some of the basic functions, and input variables, of the MNW2
Package are tested, evaluated, and demonstrated. Where possible, calculations made by the MNW2 Package are compared to known analytical solutions or previously published
simulations. Fully Penetrating Pumped Well in Ideal Confined
Aquifer (Lohman Problem)
The first test case is based on the properties of a hypothetical aquifer system described by Lohman (1972, p. 19) as
an example of a system with transient radial flow without vertical movement and amenable to solution by the Theis (1935)
equation. That is, the test problem represents an ideal nonleaky
confined aquifer of infinite areal extent and homogeneous
and isotropic properties and analytical solutions are available
for both fully penetrating and partially penetrating wells. The
basic properties of this system are listed in table 1.
A MODFLOW–2000 model was constructed to represent
and simulate this relatively simple, hypothetical, confined
aquifer system. The size of the domain was set at 300,000 ft
long by 300,000 ft wide so that the boundaries would not
influence drawdowns near the well during the expected simulation times. The well was located at the center of the model
domain. The outer boundaries of the grid represent noflow
conditions, and zero recharge is assumed. The areal grid
includes 438 rows and 420 columns of cells and has variable
spacing, with the finest part of the mesh having a horizontal
spacing of 5 ft and located in the central part of the grid, near
the pumping well. At a distance 200 ft or more from the well,
the 5ft grid spacing increases gradually by a factor of about
1.2 to a maximum spacing of 5,000 ft at the outer edges of
the domain. Vertically, the domain was subdivided into one or
more model layers, depending on the scenario being evaluated.
The first test was conducted to assure that the grid was
sufficiently large so that the peripheral noflow boundaries of
the model domain did not appreciably affect the calculated
heads and drawdowns at or near the pumping well at the
center of the grid, and that the numerical solution adequately
matched an appropriate analytical solution. The analytical
solution for the base case of a fully penetrating pumping well
is derived from the classic Theis (1935) solution and generated
using the WTAQ Program (Barlow and Moench, 1999). Parameter
Horizontal hydraulic conductivity
Vertical hydraulic conductivity
Saturated thickness
Transmissivity
Specific storage
Storage coefficient
Well radius
Well discharge Symbol
Kh
Kz
b
T
Ss
S
rw
Q Value
140 ft/day
140 ft/day
100 ft
14,000 ft2/day
2×106 ft1
2×104 (dimensionless)
0.99 ft
96,000 ft3/day The...
View Full
Document
 Winter '14

Click to edit the document details