This same capability is incorporated into mnw2 with

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Unformatted text preview: el might be constrained by the land surface or the maximum injection head. Halford and Hanson (2002) recognized that a drawdown (or water level) constraint on pumping or injection rates is especially useful for predictive scenarios and ground-water management analyses where the future stresses and hydraulic interference among wells are not known, and they built this capability into MNW1. This same capability is incorporated into MNW2 with minor modifications; the capability is activated by setting input variable Qlimit > 0 in the MNW2 input file. As stated by Halford and Hanson (2002), the maximum discharge rate for an individual well may be limited by the drawdown (change in head or water level) within that well, which is a function of the hydraulic conductivity of the surrounding aquifer, frictional energy loss owing to formation damage from drilling, and energy losses due to flow through the well screen. Nearby wells also can contribute to the drawdown in a pumped well and thereby additionally limit the discharge from a well. For example, well BM1 (fig. 23) is screened deeper and discharges more water than do the neighboring wells PA1 and PA2. Because of the water-table decline caused by discharge from well BM1, the maximum discharge rate for well PA1 might be reduced, and well PA2 would be rendered inoperative. MNW2 computes a drawdown-dependent decrease in net discharge (or in net recharge rate for an injection well) 22 Revised Multi-Node Well (MNW2) Package for MODFLOW Ground-Water Flow Model Figure 21. Computed flow from the aquifer into the uppermost node of the multinode well for the modified Reilly problem, both with and without a correction for the development of a seepage face. Figure 22. Computed distribution of flows from the aquifer into all nodes of the multi-node well during the 11th time-step (at about 114 days) for the modified Reilly problem, with and without a correction for the development of a seepage face. Negative values of Qn indicate that flow represents a discharge from the aquifer. Node numbers increase with depth and nodes are located 5 feet apart. if constraints are imposed and if a limiting head in the well is reached or exceeded. Furthermore, if the option to apply constraints is activated, then the user can specify a minimum pumping rate (Qfrcmn) that represents the lower limit of the fixed range of pump capacity for each well (the upper limit is the desired flow, Qdes). Discharge is reduced to zero if the computed net discharge falls below the specified minimum pumping rate. In MNW1 this condition was checked with lagged heads; that is, the potential net discharge for a time step was computed at the beginning of that time step using heads at aquifer nodes computed at the end of the previous time step. Thus, it was possible that the computed net discharge could have been less than the specified minimum for one time step before the pump shuts off in the model. In MNW2, the condition is checked using the most recent estimates for heads within a time step. Recharging (injection) wells are limited in the same manner but the signs are reversed (and Qfrcmn represents a minimum injection rate). Model Features and Processes 23 Figure 23. Hypothetical cross-section illustrating limitations on well discharge rates owing to aquifer characteristics, well construction, and influence of other wells (from Halford and Hanson, 2002). In transient flow systems, it is possible for heads at a particular location in an aquifer to cycle between rising and falling stages. This means that a well for which constraints caused pumping to cease as water levels fell can have pumping and discharge restart if water levels subsequently rise sufficiently. In the model, pumpage from a constrained well is restored if the potential pumping rate exceeds a user-specified threshold (Qfrcmx). The absolute value of the threshold Qfrcmx must be different from and greater than the absolute value of the minimum pumping rate Qfrcmn to help avoid oscillating numerical solutions, which could produce instability and lack of convergence in solving the ground-water flow equation. Unlike MNW1, MNW2 also uses the most recent calculated head values to check this condition (instead of head values lagged from the previous time step). Qfrcmn and Qfrcmx can be specified by the user either as explicit volumetric rates or as percentages of the specified net discharge for the well (Qdes). If the option to impose constraints is activated (that is, Qlimit > 0), then the net discharge from a withdrawal well becomes limited when the water level in the well, hWELL, reaches or falls below a user-defined lower limit, hlim (Hlim in input dataset 2f or 4b). (Note that MNW1 allowed a user to set a constraint either in terms of a critical water level or in terms of a drawdown and a reference elevation. For simplicity, MNW2 only enables the former approach.) If hWELL remains above or equal to hlim, then the flow rate will be estimated as...
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This document was uploaded on 01/20/2014.

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