Depending on the severity of the fire, the soils may become so hot that the become "hydrophobic", or water repellent. 

Not all wildfires cause soils to become hydrophobic, but it is common with moderate to high intensity fires on sandy, course soils.  When soils become hydrophobic, soil erosion increases, as does water runoff and flooding.  Over time, soils slowly regain their water absorbancy properties, but it can take several years for the water repellent tendencies to diminish. 

According to an article by eXtension.org,

"To check for water repellent soil, scrape away the ash layer to expose the soil. Place one drop of water on the soil surface. If water droplets bead on the surface for 10 to 40 seconds, it is considered moderately water repellent. If it beads for more than 40 seconds, it is considered strongly water repellent."

If you find that your soils are hydrophobic/water repellent, there are actions you can take to reduce soil erosion and increase water absorbancy.

• Try to break up the water repellent soil layer with a rake or plow.  Often, the water repellent layer is not very thick and can by broken up.
• Use fallen trees, silt fences, mulch, straw bales, silt fences, straw wattles, and culverts to reduce erosion and direct water flow.
• Reseed with a desired reclamation seed mix to help stabilize the soil. 
• Stay on top of your noxious weed management.  Weeds will likely be the first to establish.

References

Soil Erosion Control After Wildfire. Colorado State University Extension. http://extension.colostate.edu/soil-erosion-control-after-wildfire-6-308/
Wildfire and Water Repellent Soils.  eXtension.org. Wildfire and Water Repellent Soils