  Several nonlinear stochastic differential equations have been proposed in
connection with self-organized critical phenomena. Due to the threshold
condition involved in its dynamic evolution an infinite number of
nonlinearities arises in a hydrodynamic description. We study two models with
different noise correlations which make all the nonlinear contribution to be
equally relevant below the upper critical dimension. The asymptotic values of
the critical exponents are estimated from a systematic expansion in the number
of coupling constants by means of the dynamic renormalization group.
