The RES: Reservoir package pane is on the MODFLOW Features tab of the Object Properties dialog box.
The data that can be specified for the Reservoir package are the Starting time, Ending time, Starting head, and sometimes Ending head.
Starting time and Ending time are explained in the help for the MODFLOW Features tab.
Starting head (Ststage) is the value of reservoir head (stage) at the start of the stress period. Because the package assigns stage values on the basis of linear interpolation to the end of each time step, the assigned reservoir head will never equal Starting head unless Starting head and Ending head are equal.
Ending head (Endstage) is the value of reservoir head (stage) at the end of the stress period. Ending head is the value that will be assigned as head for the corresponding reservoir for the last time step in the stress period.
Reservoir stage is measured relative to the same datum used for the groundwater heads. Reservoir cells will be active only when the stage exceeds the reservoir elevation.
