TM-6A30_hirez

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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, Calif. 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 pumping wells. • 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 wells. • 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 Implementation 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.

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