Cisterns: Storage and Design

Cisterns are containers for storing harvested rainwater. They range in scale from small above-ground poly tanks of a few hundred liters to large underground reinforced concrete cisterns holding tens of thousands of liters. The choice of cistern type depends on the available space, the climate, the amount of storage needed, and the construction budget.

In the Pangea approach, cisterns are often built into the building itself: integrated into the interior as structural and thermal mass elements, with a water feature (small waterfall or fountain) that both aerates the water and creates ambient sound. An interior cistern also benefits from the building’s conditioned temperature, preventing the freezing that is a risk for exterior cisterns in cold climates and the algae growth that occurs in sunlit exterior tanks. Cisterns integrated into the building’s structure also provide significant thermal mass that contributes to passive temperature regulation.

For exterior cisterns, underground installation is preferred in most climates: soil temperature is more stable, the cistern is protected from freezing and from sunlight (which promotes algae growth), and the cistern can be gravity-fed to interior distribution points if properly positioned. Alternatively, an above-grade cistern positioned above the building’s water service connection can provide gravity-fed pressure without a pump, eliminating the energy cost of pressurization.

Cistern Sizing Rule of Thumb

Determine your longest expected dry period (days between significant rain events).

Multiply by your daily water consumption per person and number of occupants.

Add a 20–30% safety margin.

Example: 60 dry days × 50 L/person/day × 3 people = 9,000 L minimum; size to 11,000–12,000 L.

In arid climates, size for the full dry season, not just the typical inter-storm interval.