I found a couple of computer images that seem to support the idea that pressure does increase under the airfoil in ground effect quite dramatically.
Figure 3. shows a computer simulation of a conventional wing profile both in ground effect and free flight.
By comparing the total Cl (Co-efficient of lift) of both, it can be seen that the same wing in Ground Effect has an increase in lift of approximately 75%.
You can also see that the pressure below the wing has increased dramatically, this is called the dynamic air cushion.
Here's another simulation showing the air pressure gradients as a wing travels over uneven terrain. As the terrain rises the ground effect is increased and pressure builds as indicated by the color chart.
-Chiplight