Make it so flying super low actually increases lift moderately so it's more efficient during takeoff and super-low flying.
R e a l i s m . :D
The air being forced under the aircraft would normally disperse downwards, but when an obstacle (water or pavement) is directly under it in close proximity, it increases the pressure of the air under the aircraft which increases lift.
Also, this lack of underside dispersion for the air prevents advanced wingtip vortices from forming properly, so it significantly decreases drag.
This would affect each plane differently so it'd be kind of daunting to program. I can imagine a simple algorithm being used though since it doesn't need to be extremely accurate.
Generally, just use the wingspan across as the affect area and make it so the affect falls to zero rapidly beyond that.


