The Wetting and Drying frame uses a flag to select various options. These options can change depending on the model settings. Setting the Flag to zero (ISDRY=0) turns off wetting and drying; abs(ISDRY)>0 turns wetting and drying on, ISDRY>1 but <99 turns wetting and drying on with zero dimension volume balance and ISDRY=99 is the latest algorithm for multi-face wetting and drying. If ISDRY is a negative of the flag (e.g. -99) then EFDC uses cell skipping to improve computational speed. Most applications that need wetting and drying should use ISDRY = -99.
When using wetting/drying the Minimum Height in the Domain tab | Water Depth Setting (Legacy) frame should be less than the Dry Depth. If this is not the case the initial water depths everywhere will force all of the models cells to "wet", even if they should be dry.
The Dry Step is used if there is a wet cell that is isolated or perched and which has a very small depth. In this case, after a certain number of time steps set by this Dry Step value, EFDC will waste the water. EFDC continues to keep track of this cell and the volume of water removed from the system to calculate mass balance. In EFDCPlus, if the Dry Step is a positive value then it doesn't have much impact on the model. Set the value to negative to activate dry cell wasting. If the negative Dry Step is a small number then EFDC provide more reporting of cells going dry in the run screen. If a set a to larger negative number (eg -1600), then it will to report less often.