If this option is used then the model will compute

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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 2d-2. 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 length-weighted average of the cell-to-well conductances will be used to define the well-node 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 single-node 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 Multi-Node Well (MNW2) Package for MODFLOW Ground-Water 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, ft-1] 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 no-flow 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 5-ft 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 no-flow 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 2×10-6 ft-1 2×10-4 (dimensionless) 0.99 ft 96,000 ft3/day The...
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