Temporal evolution of the injection efficiency derived from the radio spectral index (blue dots) and from the 3D model of SN1987A (Orlando et al. 2015). The black (red) line represents Q_t,num and corresponds to the case where the dense equatorial ring in the circumstellar nebula around SN1987A is accounted (neglected). The thick parts of the lines correspond to the time when the shock propagates in the uniform H ii region. The volume increase per unit time was calculated as Rhu₁ for the solid lines and as R²u₁ for the dashed line. The rapid increase in the black line shortly after Day 5000 corresponds to the shock interaction with the dense equatorial ring in the nebula around the SN. The rapid increase and following decrease of the red line right after the Day 6000 is due to the few ejecta clumps which protrude the forward shock and causes an artificial increase of the post-shock density and velocity around this time. Inset: evolution of different characteristics, in arbitrary units. This plot corresponds to the red line on the main figure. One can see that the main contribution to the evolution of Q_t,num comes from the surface increase, R·h.