The rotor disc becomes more efficient within close proximity of the ground, so it takes less power to provide the same amount lift.
The benefits of ground effect are received when the rotor disc is about ½ its diameter from the ground, usually about 3-4ft skid height. The benefits of ground effect are the greatest over hard surfaces.
When over grass or similar surfaces, more power will be required then when over a smooth hard surface. When over a hard surface, such as a taxiway, there is some pressure received which lowers the velocity of the induced flow which reduces the angle of attack. With a lower angle of attack, more power is available for lift. In addition, the tip vortices are reduced, which lowers the induced drag, providing more power available for lift. When in ground effect over grass or similar surfaces, the horizontal movement of the air along the ground is reduced, increasing the induced flow, and requiring more power.
Another factor with grass or non-hard surfaces, pilots tend to slightly increase their altitude. Any increase in altitude, significantly lessens the benefit of ground effect.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-10
Principles of Helicopter Flight, 2nd Edition, pg. 62
FM 3-04.203-2007 Fundamentals of Flight pg. 1-34