Farms grow crops, and <b>crops</b> can be used in various places but mainly to feed your people. But crops don’t grow for free, they consume <b>water</b> and <b>fertility</b>.
Soil water level is replenished by rain and this is mostly sufficient for crops with low water demands (such as potatoes). However, rain is not a reliable water source and crops can yield less or even dry out and die when not having enough water.
Having an external source of water is vital for reliable farming. This requires upgrading farms with irrigation systems (unlocked via research). <b>Irrigated Farms</b> only consume water when the soil gets dry so externally supplied water does not get wasted.