Rainfall replaces Irrigation in Sid Meier's Alpha Centauri and it is done differently than in his other games.

There is 3 level of raininess : Arid (+0 Nutrient), Moist (+1 Nutrient) and Rainy (+2 Nutrients).

Rainy areas are found on the west of mountains. A river square may increase raininess.

Raininess can be increased through Terraforming, by using a terraformer to :

  • Drill an aquifer, which taps an underground aquifer, creating a river and may increase raininess. This also adds one energy point to each square.
  • Build a condenser increases the raininess of its square and all adjacent squares over 2 radiuses. It however greatly increases Eco-damage.
  • Raise the land, which creates mountains. Cautions must taken, because it considered a declaration of war to raise terrain near the borders of another faction, and it is impossible to use this terraforming option near a Pact Sibling, unless the Pact is declared null and void.