In the presence of a strong magnetic field (B) applied perpendicular to the interface of a two-dimensional electron system (2DES), the density of states for a 2D electron gas differs sharply from that at zero-magnetic field, which results in the total number of electrons in the structure differing from that at B = 0 and consequently in the variation of the ionization of the dopants with magnetic field. In this paper, we observe this effect through calculating self-consistently the electronic structure in a 2DES in strong magnetic fields. By considering Si modulation-doped A1 x Ga~_ x As-GaAs-AlyGal-y As single quantum wells, we study the dependence of 2D electron density and depletion length on the applied magnetic field. Our results show that: (1) the sawtooth-shaped total electron density (nr) and depletion length (Za) can be viewed by plotting nr and z d as a function of B; (2) in the quantum Hall plateaus n r and z a increased linearly with magnetic field; (3) in the steepness regimes of the quantum Hall effect the step changes of nr and z a can be observed and the shape of the step changes depends on the shape of the density of states, and (4) the above mentioned (1)-(3) depends very little on the asymmetric modulation doping and on the applied gate voltage.