Evapotranspiration Package (EVT)

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Last modified on Wednesday, January 31, 1996 - 5:41:48 PM

Skip forward to Input instructions for the Evapotranspiration Package.

The original instructions for the Evapotranspiration Package are on pages 10-8 to 10-9 of McDonald and Harbaugh (1988).

Introduction: The Evapotranspiration Package is used to model the effects of evapotranspiration in MODFLOW. If the head in a cell exceeds some maximum (The "ET surface"), evapotranspiration will proceed at a maximum rate. If the head is below the "extinction depth" no evapotranspiration will occur. The evapotranspiration rate between the extinction depth and the ET surface is a linear function of head. The user must specify from which cell in each vertical column evapotranspiration will be extracted. If a cell from which evapotranspiration is supposed to occur goes dry, no evapotranspiration will take place in that vertical column. Unlike the Recharge Package, there is no way to cause evapotranspiration to take place from some a lower cell when an upper one goes dry except by explicitly changing the cell in the vertical column from which evapotranspiration should occur. Setting the extinction depth lower than the base of a cell may cause the model to fail to converge to a solution when used with the BCF2 or BCF3 packages due to repeated drying out and rewetting of some cells. With the BCF1 package, MODFLOW is likely to converge to an incorrect solution under the same conditions. If you wish to have multiple layers within the rooting zone and only require a uniform evapotranspiration rate, you can do it by using negative values of recharge in the Recharge Package.

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Input Instructions

• Line 1
  • Spaces 1-10, NEVTOP, Integer, Evapotranspiration option code
    • if NEVTOP = 1, Evapotranspiration is only calculated for cells in the top grid layer
    • if NEVTOP = 2, The layer in each vertical column from which evapotranspiration occurs is specified in IEVT (Lines 6+).
  • Spaces 11-20, IEVTCB, Integer, (You must also set ICBCFL in the Output Control option to print or save cell-by-cell flow terms.)
    • if IEVTCB > 0, IEVTCB is the unit number for cell-by-cell flow terms;
    • if IEVTCB <= 0, cell-by-cell flow terms will not be printed or saved;
• Line 2, For each stress period, enter Lines 2+ to Lines 6+ before entering data for the next stress period.
  • Spaces 1-10, INSURF, Integer
    • If INSURF >=0, an array representing the evapotranspiration surface is read SURF(NCOL,NROW) is read as Lines 3+, For each stress period, enter Lines 2+ to Lines 6+ before entering data for the next stress period.
    • If INSURF <0, the evapotranspiration surface for the preceding stress period is reused.
  • Spaces 11-20, INEVTR, Integer
    • If INEVTR >=0, an array representing the maximum evapotranspiration rate is read EVTR(NCOL,NROW) is read as Lines 4+.
    • If INEVTR <0, the maximum evapotranspiration rates for the preceding stress period are reused.
  • Spaces 21-30, INEXDP, Integer
    • If INEXDP >=0, an array of extinction depths EXDP(NCOL,NROW) is read as Lines 5+
    • If INEXDP <0, extinction depths for the preceding stress period are reused.
  • Spaces 31-40, INIEVT, Integer, used only if NEVTOP = 2.
    • If INIEVT >=0, an array of layer indicators IEVT(NCOL,NROW) is read as Lines 6+. Used only if NEVTOP=2.
    • If INIEVT <0, layer indicators for the preceding stress period are reused.
• Lines 3+, SURF array entered using U2DREL. Repeated for each stress period in which INSURF >= 0.
• Lines 4+, EVTR array entered using U2DREL. Repeated for each stress period in which INEVTR >= 0.
• Lines 5+, EXDP array entered using U2DREL. Repeated for each stress period in which INEXDP >= 0.
• Lines 6+, IEVT array entered using U2DINT. Used only if NEVTOP=2 and INIEVT >= 0. Repeated for each stress period in which INIEVT >= 0. Indicates the layers from which evapotranspiration should be extracted.

Line 1, Spaces 1-10, NEVTOP, Integer, Evapotranspiration option code

• if NEVTOP = 1, Evapotranspiration is only calculated for cells in the top grid layer (layer 1). Lines 6+ are skipped.
• if NEVTOP = 2, The layer in each vertical column from which evapotranspiration occurs is specified in IEVT (Lines 6+).

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Line 1, Spaces 11-20, IEVTCB, Integer

• if IEVTCB > 0, IEVTCB is the unit number for cell-by-cell flow terms;
• if IEVTCB <= 0, cell-by-cell flow terms will not be printed or saved;

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Line 2, Spaces 1-10, INSURF, Integer

• If INSURF >=0, an array representing the evapotranspiration surface is read SURF(NCOL,NROW) is read as Lines 3+.
• If INSURF <0, the evapotranspiration surface for the preceding stress period is reused.

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Line 2, Spaces 11-20, INEVTR, Integer

• If INEVTR >=0, an array representing the maximum evapotranspiration rate is read EVTR(NCOL,NROW) is read as Lines 4+.
• If INEVTR <0, the maximum evapotranspiration rates for the preceding stress period are reused.

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Line 2, Spaces 21-30, INEXDP, Integer

• If INEXDP >=0, an array of extinction depths EXDP(NCOL,NROW) is read as Lines 5+
• If INEXDP <0, extinction depths for the preceding stress period are reused.

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Line 2, Spaces 31-40, INIEVT, Integer, used only if NEVTOP = 2.

• If INIEVT >=0, an array of layer indicators IEVT(NCOL,NROW) is read as Lines 6+.
• If INIEVT <0, layer indicators for the preceding stress period are reused.

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Lines 3+, SURF(NCOL,NROW) array entered using U2DREL.

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Lines 4+, EVTR(NCOL,NROW) array entered using U2DREL.

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Lines 5+, EXDP(NCOL,NROW)) array entered using U2DREL.

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Lines 6+, IEVT(NCOL,NROW) array entered using U2DINT.

1 2 2
2 3 4
4 5 5

indicates that
evapotranspiration will be extracted from layer 1 at (1,1)
evapotranspiration will be extracted from layer 2 at (2,1)
evapotranspiration will be extracted from layer 2 at (3,1)
evapotranspiration will be extracted from layer 2 at (1,2)
evapotranspiration will be extracted from layer 3 at (2,2)
evapotranspiration will be extracted from layer 4 at (3,2)
evapotranspiration will be extracted from layer 4 at (1,3)
evapotranspiration will be extracted from layer 5 at (2,3)
evapotranspiration will be extracted from layer 5 at (3,3)
For each stress period, enter Lines 2+ to Lines 6+ before entering data for the next stress period.

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