Described is an axial fan in which an impeller (1) comprises a hub (2), motor-powered so that it rotates about its own central axis (3) and a plurality of blades (5) extending from the hub (2) in directions transversal to the hub (2) axis of rotation (3); the hub having a cup shape and formed by a ring-shaped side wall (7) and a bottom wall (6) which has a plurality of long ribs (20) arranged in a radial direction and extending in height towards the hub (2) cavity, each of the ribs (20) abutting on the internal face of the ring-shaped side wall (7) and delimiting, together with said ring-shaped side wall, a corner area of the bottom wall (6) positioned in front of the rib (20) with reference to the hub (2) direction of rotation (V); each corner area having a through-hole (9) for discharging any debris, in particular water, sand, soil or sludge, from the hub (2) cavity to the outside of the hub (2) The through-holes (9) are evenly distributed along a circle centered on the hub (2) axis of rotation (3) and have a substantially triangular prismatic shape with a vertex wall (15) facing forwards according to the hub (2) direction of rotation (V).