A double-duct conduit for irrigation and drainage comprises an inner small-diameter duct secured eccentrically inside an outer large-diameter duct with the walls of the ducts closely adjacent to one another along their tops. Ports through the wall of the inner duct are spaced above the bottom thereof, to provide a sump in its bottom and are spaced below its top to provide an air chamber above the sump. The outer duct also has ports through its wall, at its opposite sides, spaced below the level of the bottom of the inner duct but a little above the bottom of the outer duct, providing a shallow sump in the bottom of the outer duct and an air chamber thereabove. During irrigation, water falls from the inner duct to the sump in the outer duct, then flows out through the outer duct ports. For drainage, water rises in the outer duct to the level of the ports in the inner duct, while slime and sediment remain in the sump in the outer duct.