How does the smooth mountain rock surface type affect the amount of runoff?

The  smooth mountain rock surface has no physical barrier that slows runoff, nor does it have anything that helps water seep in rather than run off.

Explanation:

When the mountain has vegetation cover the runoff is reduced. This is because plants impose a physical barrier that prevents water from flowing at high speeds, and plant roots allow mane "channels" that allow water to seep into the soil. As a result, the flow rate is reduced and the water volume is much smaller. However, the smooth mountain rock surface has none of these things. Because it has no barrier and no way to seep, water flows at high speed and at a high volume.