Irrigation water tank

A water drain tank or channel module (10) includes a structure having a top platen (20) and a bottom platen (22), a plurality of generally vertical support members (26) for supporting at least the top platen (20), the support members (26) being retained in sockets (28) on at least one of the bottom platen (22) and the top platen (26), and optional side walls (19) each including a water-permeable lattice member (24) that is adapted to support an impermeable membrane (32) or water-permeable geotextile material (30) that is capable of controlling the flow of drain water at least out of the module (10) and restricting sediment from entering the module (10). An assembly (11) of such modules (10, 10', 10", 10a, 10b, 10c, 10d) to form a drain tank, channel or storage reservoir is also disclosed.