This paper deals with the HYDROMAX real-time riverflow prediction system which has been developped by CESAME (Belgium) and is in permanent operation from 1995. HYDROMAX is based on a lumped grey-box rainfall-runoff model that requires onldy on-line rainfall and flow measurements in order to compute the riverflow predictions (Dalcin et al. 2003). The goal of the paper is to give a systematic comparison of the HYDROMAX performance when alternative estimations of the average rainfall over a river basin are used as model inputs : estimations from pointwise rain-gauge measurements, estimations from raw weather radar measurements and estimations obtained by combining both measurement systems using geostatistical methods.