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EVAPOTRANSPIRATION PACKAGE

EVT6 2

EVAPOTRANSPIRATION PACKAGE

• The Evapotranspiration (ET) Package simulate the effects of plant transpiration and direct evapotranspiration in removing water from the saturated groundwater regime.

• In practice, the modeler treats evapotranspiration from a riparian zone with the ET Package, and the recharge from an application of irrigation water with the Recharge Package.

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EVAPOTRANSPIRATION PACKAGE

The approach used by the ET Package is based on the following assumptions,

1. When the water table is at or above a specified elevation, termed the ET surface, evapotranspiration loss from the water table occurs at a maximum rate (QETM) specified by the user.

2. When the depth of the water table below the ET surface exceed a specified interval, termed the Extinction depth (d), the evapotranspiration from the water table ceases.

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EVAPOTRANSPIRATION PACKAGE3. Between these limits,

evapotranspiration from the water table varies linearly with water table elevation.

Mathematically,RETij be the rate of loss per unit surface area (DELRj×DELCi) of the water table due to evapotranspiration [L/t].RETMij be the maximum possible value of RETij [L/t].hSij be the ET surface elevation.dij be the extinction depth [L].

And let,QETij = RETij× DELRj×DELCi

and QETMij = RETMij× DELRj×DELCi

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EVAPOTRANSPIRATION PACKAGE where

QETij is the evapotranspiration [L3/t] through the area, DELRj×DELCi.

QETMij is the maximum value of QETij and is in terms of volumetric discharge.Thus,

• ET is drawn from only one cell in the vertical column beneath the map area DELRj×DELCi.

0

( )

ij

ij

ij

ij

ETM

ET

ijk S ij

ETMij

Q

Q

h h dQ

d

( )ij

ij

ijk S ij

ETMij

h h dQ

d

0

ijETMQ

ijETQ ijijk Sh h

ijijk S ijh h d

( )ij ijS ij ijk Sh d h h

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EVAPOTRANSPIRATION PACKAGE• The maximum ET rate (QETM) of an

area is a function of,1. Species type (cottonwoods,

willows,…),2. Species size (small,

medium,…),3. Species size and type fraction (fraction of specie of type t

and size s, fsts),4. Surface density of total plant

life (Total number of all plants per unit area, DTot [1/L2])

5. Maximum Volumetric ET rate per species type and size (Qsetmts, [L3/t]),

6. Size of the area [L2],

Maximum Volumetric ET Rate (Qsetmts)

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EVAPOTRANSPIRATION PACKAGE

,ETM polygon ts Tot ts polygont s

Q fs D Qsetm AREA

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EVAPOTRANSPIRATION PACKAGE

• The area polygons are subdivided into grid cells.

• The i,j cell area that is designated to have ET is assigned the value RETMij,

where Anm is the area DELRn×DELCm

ΣAnm is the horizontal surface area of all the cells that cover the polygon.

ijETMQArea

,

,

1ijETM ETM polygon

nmAll n min polygon

R QA

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EVAPOTRANSPIRATION PACKAGE

• The extinction depth is a function of specie type and size.

• If a cell horizontal surface contains a mixture of specie type and size, the previous plot of volumetric ET as a function of head will over estimate some ET loss and under estimate others.

• Plant can be given too much water along with to little, therefore heads in the aquifer above the ET surface may produce a decline in ET (see two graphs on the right).

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EVAPOTRANSPIRATION PACKAGE

• There are two options for the user to designate the cell in a vertical column that receives the ET.

Option 1—ET is always drawn from the uppermost layer of the Model.

Option 2—the user specifies the cell, within the vertical column at i,j, from which the ET is to be taken.

Option 1

Option 2

Cross-section

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EVAPOTRANSPIRATION PACKAGE

NPEVT—is the number of evapotranspiration parameters.

NEVTOP—is the Evapotranspiration option code.NEVTOP=1, ET is only to the top grid layer. NEVTOP=2, vertical distribution of evapotranspiration specified in layer variable IEVT.

IEVTCB—is a flag and a unit number.IRCHCB > 0, the unit number to which cell-by-cell terms will be written when SAVE BUDGET or a non-zero value of ICBCFL is specified in output control.IEVTCB ≤ 0, cell-by-cell flow terms will not be written.

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EVAPOTRANSPIRATION PACKAGE

PARNAM—is the name of a parameter. This name can consist of 1 to 10 characters, and is not case specific.

PARTYP—is parameter type. There is only one parameter type for the ET Package—

the maximum ET flux, EVT.Parval—is the parameter value.NCLU—is the number of clusters

required to define a parameter. Each repetition of Item 4 is a cluster.There is usually only one cluster used to define a EVT parameter, but it is acceptable to have more.

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EVAPOTRANSPIRATION PACKAGE

Mltarr—is the name of the multiplier array to be used to define cell values that are determined by parameters.The name NONE means that there is no multiplier array, and the cell value will be set to Parval.

Zonarr—is the name of the zone array to be used to define cell values that are associated with a parameters.The name ALL means that there is no zone array, and all cells in the layer are associated with the parameter.

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EVAPOTRANSPIRATION PACKAGE

IZ—is up to ten zone numbers (specified by spaces) the define the cells that are associated with a parameter.These values are not used if Zonarr is specified as ALL.Values can be negative, but not zero.The end of line, a zero value, or a non-numeric entry terminates the list of values.

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EVAPOTRANSPIRATION PACKAGEINSURF—is the ET Surface (SURF)

read flag INSURF≥0, a layer variable containing the ET surface elevation, SURF, is readINSURF<0, the ET surface from the preceding stress period is reused.

INEVTR—is the maximum ET rate (EVTR) read flag whose function depends on whether or not parameters are being used.If no parameters are being used (NPEVT=0)INEVTR≥0, a layer variable containing the maximum ET rate, EVTR, is read.INEVTR<0, the maximum ET rate from the preceding stress period is reused.

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EVAPOTRANSPIRATION PACKAGE

If parameters are used (NPEVT>0)INEVTR>0, INEVTR is the number of parameters that will be used to define EVTR in the current stress period. Item 8 defines the names of the parameters.INEVTR<0, EVT parameters from the preceding stress period are used.INEVTR=0 is not allowed because when parameters are used, at least one parameter must be specified each stress period.

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EVAPOTRANSPIRATION PACKAGEINEXDP—is the extinction depth

(EXDP) read flag. INSURF≥0, a layer variable containing the ET surface elevation, SURF, is readINSURF<0, the ET surface from the preceding stress period is reused.

INIEVT—is the IEVT read flag, which is only read if NEVTOP=2INIEVT≥0, a layer variable of layer numbers (IEVT) is read.INIEVT<0, the variable IEVT from preceding stress period is used.

SURF—is the elevation of the ET surface [L]. Read only if INSURF≥0

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EVAPOTRANSPIRATION PACKAGE

EVTR—is the maximum ET flux [L/t]. Read only if INEVTR≥0 and if NPEVT=0.

Pname—is the name of the parameter the will be used to define the EVTR variable in the current stress period.Read INEVTR values if NPEVT>0 and INEVTR>0

IEVTPF—is an optional format code for printing the EVTR variable after it has been defined by parameters.The format codes are the same as those used in the U2DREL array reading utility module.

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EVAPOTRANSPIRATION PACKAGE

EXDP—is the extinction depth. This variable is read only if INEXPD≥0.

IEVT—is the layer number variable that defines the horizontal location where ET is removedRead only if NEVTOP=2 and if INIEVT≥0.

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EVAPOTRANSPIRATION PACKAGE

No Parameters

Option 1

Option 2

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EVAPOTRANSPIRATION PACKAGE

Parameters# EVT file for test case tc2#PARAMETER 1 Item 1: NPEVT 1 0 Item 2: NEVTOP IEVTCBETMAX EVT 4.0e-4 1 Item 3: PARNAM PARTYP Parval NCLUNONE RCHETM 2 Item 4: Mltarr Zonarr IZ 1 1 1 Item 5: INSURF INEVTR INEXDP, stress period 1CONSTANT 1000. Item 6: SURF arrayETMAX 20 Item 8: Pname print-codeCONSTANT 50. Item 9: EXDP array