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the system close to the multi-node well, local drawdowns
are increased and hydraulic gradients are somewhat steeper.
This condition (test 5) resulted in substantially greater computational effort, although the time per iteration remained
about the same. The next test (test 6) assessed the effects
of partial penetration calculations by modifying test 5 by
specifying the partial penetration fraction for the uppermost
node of the multi-node well to 0.2. This yielded a slight
reduction in the cumulative number of iterations and total
computational time. When test 5 was modified by adding constraints (test 7), the computational effort increased
by 15 percent for MODFLOW–2000 and 13 percent for
MODFLOW–2005. When test 5 was modified so that the
multi-node well was not vertical (test 8), the total number
of iterations and the total CPU time decreased slightly.
When test 5 was modified by activating the pump capacity option and constraining discharge accordingly (test
9), the total cumulative number of iterations increased by
about 7 percent and 8 percent for MODFLOW–2000 and
MODFLOW–2005, respectively. When test 5 was rerun but
output options were specified to eliminate writing optional
output files, the total computational time was reduced by
about 1 percent for MODFLOW–2000 and about 0.3 percent
The results of these tests give a sense of the computational burden associated with use of the MNW2 Package. Of
course, results will vary for different computers and for different problems. CPU time, in seconds Summary and Conclusions
The MNW2 Package allows MODFLOW to simulate
long wells (or boreholes) that extend beyond a single model
node, which allows more accurate and realistic representations
of field conditions for minimal computational costs. Because
long wells can be open or connected to different parts of a
ground-water flow system that have differing heads, flow can
occur within a borehole even if it is not pumped.
This update to the MNW1 Package simplifies the input data
structure and allows the calculation of partial penetration effects
in the borehole, the specification of the location of the pump
intake, discharge to change during a time step in response to
changes in pumping lift, improved conductance calculations for
nonvertical wells, the presence of a seepage face in the borehole,
and additional output options. The basic calculation procedure of
Halford and Hanson (2002), which for simplification assumes no
head loss within a borehole, remains unchanged. However, the
MNW2 Package routes flow within the borehole, which facilitates comparisons with borehole flowmeter surveys and enables
more accurate simulations of solute transport. The MNW2
Package is compatible with MODFLOW–2000 (Harbaugh and
others, 2000) and MODFLOW–2005 (Harbaugh, 2005).
In some cases, intraborehole flow and solute transport
through long boreholes can facilitate the movement of contaminants through a ground-water system and thereby need to
be recognized when calculating changes in concentration in
the system. The MNW2 Package is also compatible with the
MODFLOW–GWT solute-transport model to facilitate such
simulations. MNW2 routes solute, as well as flow, through the
borehole when MODFLOW–GWT is active. Acknowledgments
The authors appreciate the helpful review comments provided by USGS colleagues P.M. Barlow, C.E. Heywood, and 42 Revised Multi-Node Well (MNW2) Package for MODFLOW Ground-Water Flow Model C.D. Langevin. We also thank A.M. Provost of the USGS for
his helpful discussions in developing the method to estimate
conductance for nonvertical wells. References Cited
Anderson, M.P., and Woessner, W.W., 1992, Applied groundwater modeling—Simulation of flow and advective transport: San Diego, Calif., Academic Press, 381 p.
Barlow, P.M., and Moench, A.F., 1999, WTAQ—A computer
program for calculating drawdowns and estimating hydraulic properties for confined and water-table aquifers: U.S.
Geological Survey Water-Resources Investigations Report
99–4225, 74 p. Halford, K.J., and Hanson, R.T., 2002, User guide for the
drawdown-limited, Multi-Node Well (MNW) Package for
the U.S. Geological Survey’s modular three-dimensional
ground-water flow model, versions MODFLOW–96 and
MODFLOW–2000: U.S. Geological Survey Open-File
Report 02–293, 33 p.
Hanson, R.T., Li, Zhen, and Faunt, C.C., 2004, Documentation of the Santa Clara Valley regional ground-water/
surface-water flow model, Santa Clara County, California:
U.S. Geological Survey Scientific Investigations Report
2002–5231, 85 p.
Hanson, R.T., and Nishikawa, Tracy, 1996, Combined use of
flowmeter and time-drawdown data to estimate hydraulic
conductivities in layered aquifer systems: Ground Water, v.
34, no. 1, p. 84–94. Bennett, G.D., Kontis, A.L., and Larson, S.P., 1982, Representation of multiaquifer well effects in three-dimensional
groundwater flow simulation: Ground Water, v. 20, no. 3,
p. 334–341. Harbaugh, A.W., 2005, MODFLOW–2005, The U.S. Geol...
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