  In hydrodynamics, the momentum distribution of particles at the end of the
evolution is completely determined by initial conditions. We study
quantitatively to what extent anisotropic flow is determined by predictors such
as the initial eccentricity in a set of realistic simulations, and we also show
the importance of nonlinear terms in order to correctly predict the
quadrangular flow. This knowledge will be important for making a more direct
link between experimental observables and hydrodynamic initial conditions, the
latter being poorly constrained at present.
