The swirling turbulent flow in a hydroturbine draft tube is computed with a nonintrusive uncertainty quantification (UQ) method coupled to Reynolds-Averaged Navier- Stokes (RANS) modelling in order to take into account in the numerical prediction the physical uncertainties existing on the inlet flow conditions. The proposed approach yields not only mean velocity fields to be compared with measured profiles, as is customary in Computational Fluid Dynamics (CFD) practice, but also variance of these quantities from which error bars can be deduced on the computed profiles, thus making more significant the comparison between experiment and computation.