How does dehydration effect hematocrit?

Dehydration decreases hematocrit.

Normally plasma and RBC exist in a certain ratio 55:45% or Hct of 45%.

In dehydration , there is loss of water due to various physiological or pathological circumstances resulting in increased RBC% , increased PCV or increased Hct (all mean the same). Thus there may be increased viscosity as well.