  The building block of the surface detector of the Pierre Auger Observatory is
a water Cherenkov tank. The response to shower particles is simulated using a
dedicated program based on GEANT4. To check the simulation chain, we compare
the simulated signals produced by cosmic muons at various zenith angles with
experimental data from a special Cherenkov detector equipped with a muon
hodoscope. The signals from muon-decay electrons and the evolution of the
charge with water level are also studied.
