Maybe we should consider that the downwash produced by the lifting wing doesn't change in ground proximity, but is reflected by the ground as an upwash. The wing benefits from the upwash in a manner that is somewhat similar to an airplane (or glider) flying in an updraft, or the migrating birds flying in a V-formation. Replace the ground surface by the virtual mirror wing (what Serebrisky&Biachuev call the "method of images") and the upwash it 'induces'.
P.S.
It is perhaps of interest to note that S&B's test setup uses a 'mirror' wing opposite to the test wing to simulate a ground plane.
Last edited by HazelNuts39; 3rd Dec 2012 at 14:36.
Reason: P.S.