1. EVT61 EVAPOTRANSPIRATION PACKAGE

2. EVT62 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.

3. EVT63 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 ( Q ETM ) 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.

4. EVT64 EVAPOTRANSPIRATION PACKAGE 3.Between these limits, evapotranspiration from the water table varies linearly with water table elevation. Mathematically, R ETij be the rate of loss per unit surface area ( DELR j ×DELC i ) of the water table due to evapotranspiration [L/t]. R ETMij be the maximum possible value of R ETij [L/t]. h Sij be the ET surface elevation. d ij be the extinction depth [L]. And let, Q ETij = R ETij × DELR j ×DELC i and Q ETMij = R ETMij × DELR j ×DELC i

5. EVT65 EVAPOTRANSPIRATION PACKAGE where Q ETij is the evapotranspiration [L 3 /t] through the area, DELR j ×DELC i. Q ETMij is the maximum value of Q ETij and is in terms of volumetric discharge. Thus, ET is drawn from only one cell in the vertical column beneath the map area DELR j ×DELC i.

6. EVT66 EVAPOTRANSPIRATION PACKAGE The maximum ET rate (Q ETM ) 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, fs ts ), 4.Surface density of total plant life (Total number of all plants per unit area, D Tot [1/L2]) 5. Maximum Volumetric ET rate per species type and size ( Qsetm ts, [L 3 /t]), 6.Size of the area [L 2 ], Maximum Volumetric ET Rate (Qsetm ts )

7. EVT67 EVAPOTRANSPIRATION PACKAGE

8. EVT68 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 R ETMij, where A nm is the area DELR n ×DELC m ΣA nm is the horizontal surface area of all the cells that cover the polygon.

9. EVT69 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).

10. EVT610 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

11. EVT611 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.

12. EVT612 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.

13. EVT613 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.

14. EVT614 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.

15. EVT615 EVAPOTRANSPIRATION PACKAGE INSURF —is the ET Surface (SURF) read flag INSURF≥0, a layer variable containing the ET surface elevation, SURF, is read INSURF<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.

16. EVT616 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.

17. EVT617 EVAPOTRANSPIRATION PACKAGE INEXDP —is the extinction depth (EXDP) read flag. INSURF≥0, a layer variable containing the ET surface elevation, SURF, is read INSURF<0, the ET surface from the preceding stress period is reused. INIEVT —is the IEVT read flag, which is only read if NEVTOP=2 INIEVT≥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

18. EVT618 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.

19. EVT619 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 removed Read only if NEVTOP=2 and if INIEVT≥0.

20. EVT620 EVAPOTRANSPIRATION PACKAGE No Parameters Option 1 Option 2

21. EVT621 EVAPOTRANSPIRATION PACKAGE Parameters