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.
Since Play the World it is possible to irrigate through the city square. In other words, if a city square has an adjacent irrigated tile, any other tile adjacent to that city square can be irrigated too. And unlike normal irrigation, cities can be built on hills.
|Flood Plain||3 food, 4 if irrigated|
|Grassland||2 food, 3 if irrigated|
|Plains||1 food, 2 if irrigated|
Workers are ordered to irrigate by pressing [I] or the "Irrigate" button.