The Recharge package allows the user to specify a recharge rate over an area. The total flux entering each cell will be the rate times the horizontal area of the cell. (The UZF package can also simulate recharge but in a somewhat different way.) The pane for this package is on the MODFLOW Packages and Programs dialog box under Specified Flux.
On the RCH: Recharge package pane, the user can specify where the recharge 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 recharge layers). If Top active cell is specified, the location of recharge can move up or down as cells at the surface convert between dry and wet. The user can also specify parameters for the recharge rate. The final recharge rates will be the parameter value times a multiplier. Objects must be used to define the recharge rates or recharge rate multipliers.
ModelMuse has an option to determine how objects are used to define the recharge rate. If Objects overwrite values of previous objects is selected and more than one object defines the recharge rate for a cell, only the recharge rate from the last object that assigns a value will be used. The rates from previous objects will be ignored. If Sum values of all objects is used, the recharge rates from all the objects that assign a rate to a cell will be added together. The latter option is helpful for combining recharge due to different processes. For example, it might be convenient to define recharge due to rainfall with a polygon and recharge from a river with a polyline and then have the two added together.
In MODFLOW 6 models, "multipliers" can be used with recharge boundaries. Multipliers will be used if the "Use Multiplier (MODFLOW 6)" checkbox is checked.. Ordinarily, multipliers are not needed. If multipliers are used, the recharge rate is multiplied by the value of the multiplier. One use for multipliers might be when the recharge rates are defined using a time-series and it is desired to multiply the recharge rate values in the time series by a different amount at different locations.
Parameters are not required to define the recharge rates but if parameters are defined, parameters must be used to define all the recharge 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.
Negative recharge rates are allowed. A negative recharge rate might be used to simulate a constant evapotranspiration rate.
