Irrigation increases food production. Only squares adjacent to fresh water (river or lake) or to another irrigated square may be irrigated. After the discovery of Electricity, you can irrigate a tile without proximity to fresh water. Terrain Produces Floodplains 3 food, 4 if irrigated. Grasslands 2 food, 3 if irrigated. Plains 1 food, 2 if irrigated. Workers are ordered to irrigate by pressing I or the "Irrigate" button.