Time-Variant Specified-Head Package (CHD1)

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Last modified on Monday, January 15, 1996 - 5:20:50 AM

Skip forward to Input instructions for the Time-Variant Specified-Head Package.

The original instructions for the Time-Variant Specified-Head Package are on page 60 of Leake and Prudic (1991).

Introduction: As it's name implies, the Time-Variant Specified-Head Package is used to specify heads that vary with time. It is possible to specify heads that vary among stress periods using the General-Head Boundary Package but only at the risk of large, meaningless errors in the water budget. Specified-head cells in which the head remains constant throughout the simulation can be designated as a constant head cells using the Basic Package

The Time-Variant Specified-Head Package works by changing a cell's designation to a constant head cell and then varying the cells head within each stress period. Once a cell has been designated as a constant head cell, it remains one for the rest of a simulation. Thus if you specified a cell as a time-variant specified-head cell in one stress period and ignore it in a later stress period, it will be treated as a constant head cell in that later stress period with a head equal to the head it had at the end of the last stress period. If this is unacceptable, you could try using the General-Head Boundary Package instead.

Analysis of the source code of module CHD1FM suggests that the time-variant specified-head used in calculations is the head at the end of a time step. To me, it would seem more appropriate to use an average of the heads at the beginning and end of a time step.

For those who wish to review the source code, there is an error in comment 7 of CHD1RP; PERLEN is a local variable within subroutine BAS1ST not BAS1AD.

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Input Instructions

• Line 1
  • Spaces 1-10, MXCHD, Integer, Maximum number of specified-head cells
• Lines 2+, Use one copy of Line 2 followed by ITMP copies of Lines 3+ for each stress period
  • Spaces 1-10, ITMP, Integer,
    • If < 0, specified-head data from the last stress period will be reused
    • If >= 0, ITMP is the number of specified-head cells during the current stress period.
• Lines 3+, In each stress period use ITMP copies of Lines 3+
  • Spaces 1-10, Layer, Integer, Layer of specified-head cell
  • Spaces 11-20, Row, Integer, Row of specified-head cell
  • Spaces 21-30, Column, Integer, Column of specified-head cell
  • Spaces 31-40, Start Head, Real Number, Head of specified-head cell at the beginning of the stress period
  • Spaces 41-50, End Head, Real Number, Head of specified-head cell at the end of the stress period

Line 1, Spaces 1-10, MXCHD, Integer

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Lines 2+, Spaces 1-10, ITMP, Integer

• If ITMP < 0, specified-head data from the last stress period will be reused. Normally you would not use this option because you would want the beginning head in the current stress period to equal the ending head in the last stress period.
• If ITMP >= 0, ITMP is the number of specified-head cells during the current stress period.

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Lines 3+, Spaces 1-10, Layer, Integer

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Lines 3+, Spaces 11-20, Row, Integer

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Lines 3+, Spaces 21-30, Column, Integer

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Lines 3+, Spaces 31-40, Start Head, Real Number

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Lines 3+, Spaces 41-50, End Head, Real Number

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