  We develop a drift-diffusion equation that describes electron spin
polarization density in two-dimensional electron systems. In our approach,
superpositions of spin-up and spin-down states are taken into account, what
distinguishes our model from the traditional two-component drift-diffusion
approximation. The Dresselhaus and Rashba spin-orbit coupling mechanisms are
incorporated into consideration, as well as an applied electric field. The
derived equation is applied to the modelling of relaxation of homogeneous spin
polarization. Our results are consistent with previous studies.
