This change is justified by thinking that water-free grounded ice locations should have a pressure (for the absent water ...) determined by the steady state value of the pressure.  Thus there is a difference between having any sliding and not having any sliding.  This change makes a huge difference in runs initialized with W=0 at the start.  Experimentation is needed to determine if it affects W>0 steady states much at the boundary where W->0, in particular.