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Unformatted text preview: each model node to the total flow rate (or net discharge) of the
well (such as described in Bennett and others, 1982; Neville
and Tonkin, 2004).
Neville and Tonkin (2004) reviewed several alternative
numerical methods to represent multiaquifer wells with the
widely used U.S. Geological Survey (USGS) ground-water
simulation model MODFLOW (McDonald and Harbaugh,
1988; Harbaugh and McDonald, 1996; Harbaugh and others,
2000; Harbaugh, 2005). They demonstrated that the MultiAquifer Well (MAW1) Package (McDonald, 1984) closely
matched exact analytical solutions for pumping and nonpumping conditions, and noted that the MNW Package (Halford and
Hanson, 2002; hereafter referred to as MNW1) expanded on
the capabilities of the earlier MAW1 Package to simulate flow between a long well and the adjacent ground-water system.
The value and importance of the MNW1 Package has been
illustrated by its application and use in the simulation of many
aquifer systems. For example, Hanson and others (2004)
applied the package in their model of the Santa Clara Valley,
The MNW1 Package was designed for maximum compatibility with the original WEL Package of MODFLOW
(McDonald and Harbaugh, 1988). However, the input data
structure to enable this compatibility was relatively complex
and required the user to develop a single long list of all nodes
contained in all multi-node wells. Use of this type of nodeoriented structure also therefore required the redundant specification of well properties for each node, sometimes entailing
unnecessary duplication of effort and information. The MNW1
Package was also designed to be as general as possible and
included a number of features, processes, and parameters that
would only be used in a limited number of very special cases.
Although this generality has value, it also increases the complexity and difficulty of the input structure and data preparation for most users.
The purpose of this study was to develop a new MNW
Package, MNW2, that would be simpler and easier to use
(primarily through a restructured input data format), to add
new features and processes that would be applicable in common situations, to eliminate rarely used or highly specialized features, and to improve the efficiency of the code. This
report describes the underlying theory of the processes that
are implemented in the MNW2 Package, provides detailed Conceptual Model and Numerical Implementation
instructions for preparing the input file necessary for a simulation that includes the MNW2 Package, and gives examples
of the application of the package for hypothetical conditions.
Because of the large number of changes that have been made
in the transition from the MNW1 to the MNW2 Packages and
because the input file for the MNW2 Package is not backwards
compatible with that for the MNW1 Package, the MNW2
Package supersedes the MNW1 Package. This report provides
documentation for the MNW2 Package.
The main features added to the MNW2 Package include:
• Restructured input formats, including the separate
specification of properties associated with individual
wells, an option to specify vertical locations of open
intervals (or well screens) by their top and bottom
elevations, and an option to define skin effects in terms
of the hydraulic conductivity of the well skin.
• The option for the model to calculate additional head
changes due to partial penetration effects in vertical
• The capability to calculate flow into a borehole through
a seepage face that develops when the water level in
the well drops into a cell below the layer containing
the water table.
• Improved capability to simulate slanted and horizontal
• The option to adjust pumpage for changes in lift over
time using pump performance (head-capacity) curves.
• The option to specify the pump intake at any depth.
• Additional output options, including the development
of a Multi-Node Well Information (MNWI) Package,
which includes options for writing data to separate
output files. For example, a well may be designated as
an “observation well,” for which changes in selected
variables with time are recorded in a separate output
file. Conceptual Model and Numerical
The MNW1 Package (Halford and Hanson, 2002) allows
MODFLOW to simulate wells that extend beyond a single
model node, and the model user has to specify a group of
nodes that are associated with a single well. This allows the
simulated well to penetrate more than one model layer, more
than one aquifer, or to represent a slanted or horizontal well.
The net flux in or out of the well can be negative (representing a withdrawal well), positive (representing an injection
well), or zero (representing a nonpumping well or a longscreened observation well). The net flux represents the addition or removal of water from the ground-water system and 3 corresponds with the flow at the wellhead. MNW2 follows the
same basic conceptual model and numerical equation-solving
implementation as documented for MNW1.
Regardless of the net flux, when a well is linked to multiple nodes of the finite-differenc...
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This document was uploaded on 01/20/2014.
- Winter '14