The Evapotranspiration package causes water to leave the system at rate that depends on the head in the cell with the evapotranspiration boundary. If the head is above a defined value, the rate of water leaving the system, is set to some constant rate. If the head n the cell drops low enough, no water will be removed from the groundwater system through the boundary. Between those extremes, the rate of water loss varies linearly from zero to the maximum rate. The pane for this package is on the MODFLOW Packages and Programs dialog box under Head Dependant Flux.
On the EVT: Evapotranspiration package pane, the user can specify where the evapotranspiration will be applied. Possible choices include (1) the Top layer, (2) a Specified layer, and (3) the Top active cell. If Specified layer is selected, the user can choose whether to use the layer of the object used to assign the rates for all the stress periods or to have the layer be specified separately for each stress period (Time varying EVT layers). If Top active cell is specified, the location of evapotranspiration can move up or down as cells at the surface convert between dry and wet. The user can also specify parameters for the maximum evapotranspiration rate. The final maximum evapotranspiration rates will be the parameter value times a multiplier. Objects must be used to define the maximum evapotranspiration rates or maximum evapotranspiration rate multipliers.
Parameters are not required to define the maximum evapotranspiration rates but if parameters are defined, parameters must be used to define all the maximum evapotranspiration rates.
If more than one parameter of the same type applies to the same cell, the input value for MODFLOW will be the sum of the input values determined for that cell for all the parameters that apply to the cell. For example, suppose that there are two HK parameters defined: HK1 and HK2. They have Values of 10 and 20 respectively. If they both apply to the cell in column 1, row 1, layer 1 and neither has multiplier data sets, the hydraulic conductivity for that cell would be 10 + 20 = 30. Now suppose that the multiplier for HK1 was 5 at that cell. The hydraulic conductivity for that cell would be 10*5 + 20 = 70.
Parameters can also be edited in the Manage Parameters dialog box.
The Evapotranspiration Segments package is similar to the Evapotranspiration package except that it allows a more complicated relationship between head and evapotranspiration rate.
