As in mnw1 the mnw2 package assumes that the

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: e grid, then the flow between the model domain and the well can vary greatly in magnitude (and perhaps in direction) among the various nodes linked to the single well. As in MNW1, the MNW2 Package assumes that the hydraulic head within the well will equilibrate to a single representative value. Because the heads in the aquifer at various model nodes encompassing a multi-node well will vary depending on local and regional aquifer properties and boundary conditions, a well can have nonuniform borehole flow (or intraborehole flow), and the maximum borehole flow rate can exceed the net withdrawal or injection rate specified for the well. The MNW2 Package partitions the flux among the various nodes connected to a multi-node pumping well on the basis of relative heads and hydraulic conductances (the product of hydraulic conductivity and cross-sectional area of flow divided by the length of the flow path). The conceptual model for flow through a long borehole that is connected to multiple nodes of the model grid represents a substantial simplification of the actual hydrodynamics of such a system. A rigorous representation of the flow dynamics within the borehole, such as analyzed by Cooley and Cunningham (1979), is neither developed nor applied. In fact, for simplicity and computational efficiency, it is assumed that there are no head gradients within the borehole and that a single value of hydraulic head (and water level) is effective over the entire length of the borehole (Bennett and others, 1982; Fanchi and others, 1987), although Rutledge (1991) discusses some hypothetical examples in which he calculates head differences within a borehole of several feet [for example, a range of about 7 feet (ft) relative to a mean drawdown of about 21 ft]. Under the simplifying assumptions of the MNW1 and MNW2 Packages, at any level of the open or screened borehole, the flow between the well and the adjacent porous media would be controlled by the head difference and the hydraulic conductance between the well and the porous media for any particular location (grid cell) where the well and porous media are connected. When a well is open to two or more different intervals in which the aquifer heads are different from each other, as illustrated in figure 1, the well provides a pathway for flow between the aquifers, and flow will occur in the borehole, even in a nonpumped well, in response to head gradients in the aquifer (and not to head gradients within the borehole, which are not computed). Simulation of Pumping and Nonpumping Wells In MODFLOW’s standard WEL Package, the discharge of a well (Q) must be specified explicitly by the user (where Q has dimensions of L3/T and is assumed to be negative in sign for discharge). Also, a WEL Package well can only be 4 Revised Multi-Node Well (MNW2) Package for MODFLOW Ground-Water Flow Model connected to a single node of the grid. If a well is known to discharge from a length of aquifer equivalent to more than one model node or layer, then the user must determine an approach for allocating the total discharge among the multiple model nodes or layers such that the total flow (or net discharge) from all model layers equals QNET: (1) where n is the total number of model nodes open to the well, and m is the index of sequential node numbers of the multinode well. In the MNW2 Package, however, the user specifies the net discharge rate for the multi-node well, and the code then determines the layer-by-layer (or node-by-node) flow rates between the well and the aquifer using methods described in the next section of this report. The total flow or discharge from a well at the land surface is referred to as a “net” discharge because, in complex hydrogeological settings, a single well can have inflow in some parts of its open interval and outflow at other parts, with the difference being equal to the net discharge. QNET can be negative (a pumping well), positive (an injection well), or zero (an unpumped well or observation well). The total flow rate of a well (or net discharge) that is specified in MNW2 is referred to in the input file as variable Qdes, or the maximum desired flow rate for the well. This designation is used because the actual flow rate at the wellhead can be constrained by user-specified maximum and minimum water levels at the well and by possible pump-capacity constraints, as described in later sections of this report. If constraints are not specified, then the actual net discharge for the well will equal the specified value of Qdes. In constructing a well and installing a pump, the actual location of the pump intake may be located at a depth or position determined for the conditions of that specific well. Therefore, the model allows the user to specify where within the length of the borehole the pump intake is located. This is set using the input variable PUMPLOC in input dataset 2b (appendix 1) and related variables in optional dataset 2e. If the user does not specify a pump intake location...
View Full Document

This document was uploaded on 01/20/2014.

Ask a homework question - tutors are online