A simple analytical treatment based on the hydrodynamical model of plasmas is developed to study both steady-state and transient stimulated Raman and Brillouin scattering processes in centrosymmetric or weakly noncentrosymmetric semiconductors. Gain constants, threshold-pump intensities, and optimum pulse durations for the onset of Raman and Brillouin instabilities are estimated. The qualitative behaviour of transient gain factors is found to be in agreement with the experimental and other theoretical observations. The analysis explains satisfactorily the competition between stimulated Raman and Brillouin processes in the short-and long-pulse duration regimes.