water butt 
noun
A large barrel used for catching and storing rainwater.Invest in a water butt to collect rainwater for use in the garden....
- I also emptied the bucket catching the drips from the new water butt onto the tomatoes, which were badly in need of watering.
- Thames Water is urging residents to make the most of April's heavy showers by installing a water butt in the garden in readiness for summer dry spells.