GMS:EVT Package

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The Evapotranspiration package is used to simulate the effect of plant transpiration and direct evaporation by removing water from cells during a simulation. Evapotranspiration (ET) is typically defined by specifying values for each stress period for each vertical column in the grid. The values consist of an elevation, an ET extinction depth, and a maximum ET rate. The elevation is an absolute elevation and the ET extinction depth (measured positive downward) is relative to the specified elevation. If the water table rises above the specified elevation, the evapotranspiration occurs at the maximum ET rate. If the water table falls below the ET extinction depth, evapotranspiration ceases. If the water table elevation lies between these two extremes, the evapotranspiration rate varies linearly with depth. The ETS package, a later extension of the EVT package, allows the rate to vary non-linearly.

From OFR00-466[1]


To learn more about evapotranspiration in MODFLOW go here: http://water.usgs.gov/nrp/gwsoftware/modflow2000/ofr00-466.pdf

EVT Package Dialog

The MODFLOW EVT Package dialog.

The dialog for editing the evapotranspiration package input data is identical to the Recharge Package dialog except that rather than editing the flux and layer indicator arrays, the evapotranspiration arrays are edited: elevation, ET extinction depth, maximum ET rate, and layer indicator.

Evapotranspiration Option

Two ET options are supported by MODFLOW: ET only at the top layer, and ET at specified vertical cells.

Multiplier

The multiplier is a constant which can be written to the package file with each stress period array. Each value in the array is scaled by the multiplier as the array is imported to MODFLOW. The format button brings up a dialog listing the standard MODFLOW formats. This format is used for displaying the values in the spreadsheet and it controls how the values are written to the package file.

Stress Periods

The values displayed in the spreadsheet are for an individual stress period. The values associated with other stress periods can be edited by entering the ID of the desired stress period in the Stress period edit box in the center of the dialog. If the Use previous option is selected for a given stress period, the values from the previous stress period are used and the spreadsheet is dimmed.

Edit All Use previous/Multipliers

This button brings up a spreadsheet allowing the user to edit the Use Previous values and the multipliers for the active array for each stress period.

2D Dataset → Array

The 2D Dataset → Array button brings up the Data Browser listing all of the current datasets associated with the current 2D grid. In order for this button to be active, the 2D grid must have the same number of rows and columns as the 3D grid. The selected dataset is copied to the recharge array. Datasets are typically generated with the 2D Scatter Point module. The 2D Scatter Point module can be used to interpolate from a scattered set of rainfall measurements to the cell locations. If the dataset is transient, the values in the dataset are linearly interpolated, if necessary, to each stress period as the dataset is copied to the array.

Array → 2D Dataset

The Array → 2D Dataset button copies the array to the 2D dataset list associated with the existing 2D grid.

Constant → Array

The Constant → Array button brings up a dialog which prompts for a single value. This constant is then assigned to each item in the array for the given stress period.

Spreadsheet

The spreadsheet allows editing for a 2D array of values, the type depending on the selection in the view/edit combo box. The array types include elevation, et extinction depth, and max ET rate.

Notes

  1. Banta, Edward R. (2000), MODFLOW-2000, The U.S. Geological Survey Modular Ground-Water Model-Documentation of Packages for Simulating Evapotranspiration with a Segmented Function (ETS1) and Drains with Return Flow (DRT1). Open-File Report 00-466., Denver, Colorado, http://water.usgs.gov/nrp/gwsoftware/modflow2000/ofr00-466.pdf