The development of a water repellent layer in soils is a common side effect of wildfires.  The water repellent layer is casued by the volitilization and subsequent condensation of organic compounds on the surface of soil particles.   After the first post-fire precipitation event the layer is destroyed and may or may not re-establish itself as the soil dries out.  A series of experiments incorporating ash/char created at tempertures ranging from 180C to 480C indicates that the natural incorporation of ash and char into the soil profile that occurs over the weeks and months following a fire are like to affect the hydraulic properties of the soil in the soil to a far greater degree than short lived water repellent layer.