When test 5 was modified by adding constraints test 7

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Unformatted text preview: are added to 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 for MODFLOW–2005. 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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