ir·ri·gate / ˈirigāt/ • v. [tr.] supply water to (land or crops) to help growth, typically by means of channels. ∎  (of a river or stream) supply (land) with water. ∎  Med. apply a continuous flow of water or liquid medication to (an organ or wound).DERIVATIVES: ir·ri·ga·ble / -gəbəl/ adj.ir·ri·ga·tion / ˌiriˈgāshən/ n.ir·ri·ga·tor / -ˌgātər/ n.ORIGIN: early 17th cent.: from Latin irrigat- ‘moistened,’ from the verb irrigare, from in- ‘into’ + rigare ‘moisten, wet.’

irrigate XVII. f. pp. of L. irrigāre, f. IR-1 + rig¯re wet, water; see -ATE3.
So irrigation XVII. — L.