In LiNbO_3 at light intensities above 10^6 W/m^2 much stronger refractive index changes can be induced, as expected from measurements at low intensities. Most experimental data have been published on iron-doped LiNbO_3 crystals. We propose a two-center charge-transport model for LiNbO_3:Fe that describes most results at low and high intensities quantitatively. It explains the intensity dependence of steady-state refractive index changes and enhanced holographic sensitivities at high light intensities as well as the presence of light-induced absorption changes.