Why is ground effect greater over smooth hard surfaces? Helicopter Flying Handbook: Ground effect is the increased efficiency of the rotor system caused
  by interference of the airflow when near the ground. The air pressure
  or density is increased, which acts to decrease the downward velocity
  of air. Ground effect permits relative wind to be more horizontal,
  lift vector to be more vertical, and induced drag to be reduced. These
  conditions allow the rotor system to be more efficient. Maximum ground
  effect is achieved when hovering over smooth hard surfaces. When
  hovering over surfaces as tall grass, trees, brushes, rough terrain,
  and water, maximum ground effect is reduced. Rotor efficiency is
  increased by ground effect to a height of about one rotor diameter
  (measured from the ground to the rotor disk) for most helicopters.
  Since the induced flow vertices are decreased, the AOA is increased,
  which requires a reduced blade pitch angle and a reduction in induced
  drag. This reduces the power required to hover IGE


I understand ground effect and In Ground Effect versus Out of Ground Effect. Air pressure is increased below the helicopter, reducing induced flow, increasing AOA and decreasing drag.

But why is ground effect greater over smooth hard surfaces than over tall grass, trees, bushes, rough terrain, and water?

I've searched Aviation Stack Exchange, textbooks, and the front page of Google, but haven't found an explanation.

At first I would think friction from surfaces like tall grass would hold air below the helicopter, increasing ground effect. But we know this isn't true, since this would mean there's less ground effect over a runway.

Another guess would be the terrain disrupts the air from bouncing back toward the helicopter. Does the air reflect sideways instead, meaning the force of air bouncing back toward the helicopter is what increases lift? The Helicopter Flying Handbook contradicts this explanation with the following as the core source of ground effect:


  "The air pressure or density is increased"


Don't be afraid to include complicated aerodynamics if necessary. :)
From a conservation of energy perspective, the rotor is transferring energy to the air, which then moves down and is deflected by the ground. A soft or drag-inducing surface will absorb more of that energy, whereas a smooth hard surface will deflect it.

Think of it like running: If you run on a hard surface, your down force creates an equal force up. If you run on sand, some of the down force is lost by moving the sand, and you are not propelled up/forward as much. This is why it is harder to run on sand than a solid surface.