Well Package (WEL1)

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Last modified on Monday, January 15, 1996 - 6:55:58 AM

Skip forward to Input instructions for the Well Package.

The original instructions for the Well Package are on page 8-3 of McDonald and Harbaugh (1988).

Introduction: The Well Package is used to simulate the withdrawal or recharge of water from or to aquifers by wells. The well package can also be used to simulate any other source of withdrawal or recharge which occurs at a known rate including specified flow boundaries. Unlike some other packages such as the Recharge or Evapotranspiration packages in which the withdrawal or recharge rates have units of length/time, the Well Package recharge rates are in units of length^3/time.

When preparing input files for the well package, remember that discharge values must be negative. Positive values represent recharge.

It's important to remember when interpreting MODFLOW output files, that the heads given for cells containing pumping wells are not the heads at those wells. The modeled heads more closely resemble the average head in the cell. Typically, the head observed at a pumping well is an extreme head; either a minimum head for pumping wells or a maximum head for recharge wells. Anderson and Woessner (1992) give the following formulas (Eq. 5.1 and 5.7) relating head in the well to modeled head in its cell for confined and unconfined aquifers.
For confined aquifers, hw = hm - (Q/2{pi}T)*ln(re/rw).
For unconfined aquifers, hw = SQRT[hm^2-(Q/{pi}K)*ln(re/rw)].
Where

• hw = head in the well,
• hm = head in the model cell,
• Q = pumping rate,
• T = aquifer transmissivity,
• re = the effective well block radius,
• rw = the well radius, and
• K = hydraulic conductivity.

Wells which are open to more than one layer can not be simulated directly by MODFLOW. Instead they must be treated as separate wells each of which is open to only one layer. The total discharge from the well must be divided among the layers. One way to do this is using the equation q = Q(t/T) where

• q = the discharge from layer i
• Q = the total discharge from the well
• t = the transmissivity of layer i
• T = the sum of the transmissivities of all the layers.

Input Instructions

• Line 1
  • Spaces 1-10, MXWELL, Integer, the maximum number of wells used at any time in the simulation
  • Spaces 11-20, IWELCB, Integer, (You must also set ICBCFL in the Output Control option to print or save cell-by-cell flow terms.)
    • if IWELCB > 0, IWELCB is the unit number for cell-by-cell flow terms;
    • if IWELCB = 0, cell-by-cell flow terms will not be printed or saved;
    • if IWELCB < 0, Well recharge will be printed on the main output file.
• Lines 2+, One copy of Lines 2+ followed by 0 or ITMP copies of Lines 3+ are read for each stress period.
  • Spaces 1-10, ITMP, Integer
    • If < 0, well data from the last stress period will be reused
    • If >= 0, ITMP is the number of active wells during the current stress period.
• Lines 3+
  • Spaces 1-10, Layer, Integer, The layer number of the cell containing the the well
  • Spaces 11-20, Row, Integer, The row number of the cell containing the the well
  • Spaces 21-30, Column, Integer, The column number of the cell containing the the well
  • Spaces 31-40, Q, Volumetric recharge rate, Positive values indicate recharge, negative values indicate discharge.

Line 1, Spaces 1-10, Variable = MXWELL, Integer

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Line 1, Spaces 11-20, Variable = IWELCB, Integer

• if IWELCB > 0, unit number for cell-by-cell flow terms;
• if IWELCB= 0, cell-by-cell flow terms will not be printed or saved;
• if IWELCB< 0, well recharge will be printed in the main output file.

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

• If ITMP < 0, well data from the last stress period will be used. There will be 0 copies of Lines 3+ immediately following this copy of Line 2.
• If ITMP >= 0, ITMP is the number of active wells during the current stress period. There will be ITMP copies of Lines 3+ immediately following this copy of Line 2.

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

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

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

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Lines 3+, Spaces 31-40, Variable = Q

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