[en] We developed a reduced description of kinetic effects that is included in a fluid model of stimulated Brillouin backscattering (SBS) in low Z plasmas (e.g. He, Be). Following hybrid-PIC simulations, the modified ion distribution function is parametrized by the width (delta) of the plateau created by trapping around the phase velocity of the SBS-driven acoustic wave. An evolution equation is derived for (delta) , which affects SBS through a frequency shift and a reduced Landau damping. This model recovers the linear Landau damping value for small waves and the time-asymptotic nonlinear frequency shift calculated by Morales and O'Neil. Finally we compare our reduced model with Bzohar simulations of a Be plasma representative of experiments that have shown evidence of ion trapping