While it is typical for many modelers to model only fixtures that require hot water, both EnergyPlus and OpenStudio do allow you to model cold water fixtures such as toilets and irrigation. Just set the target temperature lower than the water main temperatures. If you are modeling a xeriscape style landscape you may want to consider the water use the first few years when the plants are getting established as well as future years where the water use for irrigation should be very low.

I'll let someone more knowledgable on EnergyPlus address rainwater collection and water re-use. OpenStudio doesn't directly support this, so modeling these characteristics in an OpenStudio analysis would require using an EnergyPlus measure.