A method of converting no-flow cells to variable-head cells for the U. S. Geological Survey modular finite-difference ground-water flow model

McDonald, Michael G.; Harbaugh, Arlen W.; Orr, Brennon R.; Ackerman, D.J.

doi:10.3133/ofr91536

Cited by 64 publications

(23 citation statements)

References 3 publications

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“…In the MODFLOW system, a cell is declared dry and made inactive if the calculated head in that cell drops below the cell's bottom elevation. A dry cell may optionally be allowed to ''rewet'' in a subsequent iteration or time step if the water level in a neighboring cell exceeds a user-defined threshold (McDonald et al 1991).…”

Section: Introductionmentioning

confidence: 99%

Robust Representation of Dry Cells in Single‐Layer MODFLOW Models

Painter¹,

Başağaoğlu

Liu³

2008

Groundwater

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Dewatered or "dry" grid cells in the USGS ground water modeling software MODFLOW may cause nonphysical artifacts, trigger convergence failures, or interfere with parameter estimation. These difficulties can be avoided in two dimensions by modifying the spatial differencing scheme and the iterative procedure used to resolve nonlinearities. Specifically, the spatial differencing scheme is modified to use the water level on the upstream side of a pair of adjacent cells to calculate the saturated thickness and hence intercell conductance for the pair. This makes it possible to explicitly constrain the water level in a cell to be at or above the cell bottom elevation without introducing nonphysical artifacts. Thus constrained, all initially active cells will remain active throughout the simulation. It was necessary to replace MODFLOW's Picard iteration method with the Newton-Raphson method to achieve convergence in demanding applications involving many dry cells. Tests using a MODFLOW variant based on the new method produced results nearly identical to conventional MODFLOW in situations where conventional MODFLOW converges. The new method is extremely robust and converged in scenarios where conventional MODFLOW failed to converge, such as when almost all cells dewatered. An example application to the Edwards Aquifer in south-central Texas further demonstrates the utility of the new method.

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Section: Introductionmentioning

confidence: 99%

Robust Representation of Dry Cells in Single‐Layer MODFLOW Models

Painter¹,

Başağaoğlu

Liu³

2008

Groundwater

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“…The major limitation of ground-water flow models is that the properties of the ground-water flow medium and recharge to and discharge from ground water are never precisely known and must be estimated. In this study, I have used MODFLOW (McDonald and Harbaugh 1988), a widely accepted numerical ground-water flow modellng program, with modi~cations documented in Prudic (1989), Hill (1990), andMcDonald et al (1991).…”

Section: Hydrologic Modeling and Discussionmentioning

confidence: 99%

Design of an urban, ground-water-dominated wetland

Winston

1996

Wetlands

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In the Barney Circle wetland in southeast Washington DC, a proposed site for wetland enhancement and creation, ground water supplied over 80% of the flow into the wetland from 25 March to 17 August 1994. In most months, the primary outflow from the wetland occurred as ground-water flow, but in April, almost all the outflow was surface water. Rain, runoff, and changes in storage account for less than a third of the observed surface-water discharge from the wetland to the Anacostia River in April and May. This indicates that ground water inflow was the major source of water. Although it is only a first order basin. the wetland had a small but definite flood-control function. Peak surface-water discharges out of the wetland during a storm event were reduced by 38 to 80% compared to surface water inflow into the wetland. The time during which storm outflow occurred was much longer than the time of storm inflow. Proposed construction of a highway will result in a 13% reduction in the recharge area of the Barney Circle watershed. A calibrated ground-water flow model of the watershed suggests that this will result in a 27@ decrease in baseflow discharge from the wetland. The hydroperiod of the wetland will only be slightly affected, however. By modifying the calibrated watershed model, it was possible to design a modified topography of the wetland that will favor desired plant communities. However, modifying the hydroperiod will require a much greater depth of excavation than would have been predicted based on the average hydroperiod either before or after highway construction. Excavation of the wetland wtll cause ground water in the surrounding uplands to drain more quickly than previously. This change in the hydrology had to be taken into account in the design. This study illustrates how failure to consider the effects of site alteration on local hydrology could result in project failure.

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“…This may be achieved via 'engineering out' the problems of drying by modifying the model (e.g. thickening of layers), altering the source code to remove the potential for drying (Hydrogeologic 1996;Doherty 2001;Painter et al 2008;Keating & Zyvoloski 2009) and/or by reducing the frequency or effect of rewetting by alteration of resaturation or solver-based parameters (McDonald et al 1991). A composite of these methods was useful in the development of the joined NEAC groundwater model.…”

Section: Overcoming Difficulties Of Large Joined Modelsmentioning

confidence: 99%

Crossing boundaries, the influence of groundwater model boundaries and a method to join and split MODFLOW models

Black¹,

Lewis²,

Grout

et al. 2012

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Neighbouring groundwater models often have large areas of overlap to avoid boundary issues in hydrogeological assessments and such overlap or artificial boundaries can lead to inconsistent representations of aquifers and processes. This paper presents the aggregation of six adjacent models spanning East Anglia, England, into one model without internal boundaries. This study principally discusses the effect of, and difficulties arising from model edge boundaries. In addition, a review of conceptual and numerical discontinuities at model boundaries is included and a more consistent and robust modelling approach over the whole area is demonstrated. The large, joined model is used to delineate groundwater divides, assess their transient migration, review edge boundary implications for water balances and investigate abstraction impacts without the influence of internal static boundaries. Computer codes developed in conjunction with this study facilitate joining adjacent models and, conversely, splitting of the joined model back into models at the scale of the original component models using simulated groundwater divides, or to smaller submodels incorporating edge boundary conditions calculated from the parent model.

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A method of converting no-flow cells to variable-head cells for the U. S. Geological Survey modular finite-difference ground-water flow model

Cited by 64 publications

References 3 publications

Robust Representation of Dry Cells in Single‐Layer MODFLOW Models

Robust Representation of Dry Cells in Single‐Layer MODFLOW Models

Design of an urban, ground-water-dominated wetland

Crossing boundaries, the influence of groundwater model boundaries and a method to join and split MODFLOW models

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