Top left: 24 h divergence field measured with the LCT applied to Dopplergrams. The supergranule boundaries detected by the watershed method are shown as white network. The circles represent the location of the 13 studied a persistent downflows. These persistent downflows are located in convergent flows and more particularly where several supergranules meet. Top right: 24 h mean Doppler map overlapped with the 13 studied persistent downflows (circles) and supergranule boundaries. Downflows are visible as dark structures in each circle. Far from the disc centre, in the lower right part of the figure, some large darker regions are visible. These are caused by the horizontal velocities which become a dominant contribution due to the projection effects. Bottom left: corks position (grey points) after 24 h diffusion by horizontal flows. Their locations are, in accordance with supergranule boundaries, found by the watershed method (superimposed white network). Bottom right: 24 h mean magnetic field and the location of the 13 studied persistent downflows (circles).