Is a water cistern part of you water system? When should a cistern be part of your water system? It all depends on your water source, usually a well, and how much water you need at any given time. Having a large supply of good water can come in handy for watering a garden, or for emergency fire protection. Cisterns to store rainwater are also growing in popularity. Here are a few tips from our experience with our 3000 gallon above ground poly cistern.