  We present a phenomenological model for the initial conditions needed in a
one-fluid hydrodynamical description of ultrarelativistic nuclear collisions at
CERN--SPS. The basic ingredient is the parametrization of the baryon stopping,
i.e. the rapidity distribution, as a function of the thickness of the nuclei.
We apply the model to S + S and Pb + Pb collisions and find after
hydrodynamical evolution reasonable agreement with the data.
