The recharge package defines the quantity of water (0.001L/T) from precipitation that percolates to the groundwater by one IDF (or a constant value). The RCH package is comparable to the RCH package of MODFLOW. Remember that the units are mm/d for iMODFLOW.
The RCH package may be replaced by the CAP module which makes a more sophisticated simulation of the recharge process through the unsaturated zone.