Underground Cisterns: Reducing Evaporation in Water Storage

In many parts of the world, especially across arid and semi-arid regions, water is scarce – and what little rainfall does occur can quickly disappear through evaporation before it even gets a chance to be used. Open ponds and surface tanks lose enormous volumes of stored water to the sun and wind. Underground cisterns solve this problem directly. By keeping harvested water sealed beneath the ground, these covered storage structures dramatically cut evaporation losses, protect water quality, and provide a reliable supply long after the rains have stopped.

What is an underground cistern? 🔗

An underground cistern is a covered, watertight storage pit or tank built below the ground surface to collect and hold rainwater or runoff for later use. Unlike open surface storage, the cistern’s buried position and sealed cover keep water cool, protected from sunlight, and shielded from airborne contamination. According to Penn State Extension, most cisterns are located underground and supply water to households and farms through a pressurized plumbing system, and can even serve as an emergency water source during droughts. These systems are particularly well suited to hot, dry climates where water loss to evaporation is a constant challenge.

The concept is ancient but remains highly practical today. Wikipedia’s overview of cistern history notes that waterproof lime plaster cisterns are features of Neolithic village sites in the Levant, and that cisterns have been essential tools of dry-land farming communities for millennia. The core purpose – protecting stored water from loss – has not changed.

Why evaporation is such a critical problem 🔗

In arid and semi-arid regions, annual evaporation rates can far exceed annual rainfall. Open water storage exposed to heat and wind loses water rapidly – making even well-filled tanks or ponds unreliable over a dry season. Research on desert rainwater harvesting identifies evaporation as one of the most significant challenges in arid storage systems, alongside dust and debris contamination. The underground positioning of a cistern addresses both problems at once: the surrounding soil acts as natural insulation, maintaining lower and more stable temperatures, while the sealed cover prevents direct exposure to the atmosphere that drives evaporation.

Studies on water harvesting in desert environments confirm that underground cisterns keep stored water cool and clean, making them especially valuable where above-ground storage is impractical due to extreme heat or limited space. The result is that communities can store water collected during a brief rainy season and draw on it reliably for months afterward.

Construction materials for underground cisterns 🔗

The choice of material affects a cistern’s durability, cost, water quality, and suitability for the local environment. Three main materials are used today.

Concrete cisterns 🔗

Reinforced concrete is one of the most traditional and widely used materials for underground cisterns. Concrete cisterns offer load-bearing capabilities that allow them to be installed beneath driveways and parking lots, can be configured in any shape or layout, and maintain a relatively constant water temperature year-round. Concrete also has a slight benefit for slightly acidic rainwater since the walls can neutralize pH over time. The main drawbacks are that movement of soil over time can cause cracks, so lining the interior with a waterproof liner is recommended best practice. Additionally, precast concrete tanks may involve significant shipping costs and require heavy excavation equipment for installation.

High-density polyethylene (HDPE) and fiberglass cisterns 🔗

High-density polyethylene cisterns are a newer option that is rapidly growing in popularity. They are rotationally molded in seamless, one-piece construction, which means they will not rust, leak, or corrode. Storage volumes for polyethylene cisterns typically range from a few hundred gallons up to 20,000 gallons. Fiberglass cisterns are built to specification and used for volumes exceeding 20,000 gallons. Both polyethylene and fiberglass are superior to metal tanks because they resist the natural chemical properties of soil and tolerate low-pH, acidic conditions that metal would corrode in. Many are also certified to NSF standards for safe storage of potable water.

Lined pit cisterns and traditional materials 🔗

In lower-resource contexts, cisterns can be constructed as excavated pits lined with impermeable sheets of plastic, clay plaster, or lime mortar to prevent leakage and contamination. The One Million Cisterns Program in northeastern Brazil used cement-layered containers to harvest and store rainwater across nine states for small farm-holders. Each cistern, with a capacity of 16,000 liters, was covered with a lid to prevent evaporation, and water collected during 3-4 months of the rainy season sustained households through the rest of the dry year. By 2016, 1.2 million such cisterns had been implemented for human consumption alone – a powerful demonstration of the approach at scale.

Key design considerations 🔗

Building an effective underground cistern requires matching the structure’s size and layout to both the available water supply and the expected water demand. There is no single standard size – the right design depends on several interrelated factors.

Sizing based on water demand and catchment area 🔗

Research published in Environment, Development and Sustainability confirms that cistern capacity should be determined as a function of local rainfall data, roof or catchment area, and the anticipated non-potable or potable water demand. Oklahoma State University Extension advises that the capacity of the storage tank must be proportional to estimated monthly demand, the monthly rainfall in inches, and the size of the catchment area, accounting for a runoff efficiency factor. In simple terms, larger families or farms with greater daily consumption require proportionally larger cisterns, and a bigger catchment area – whether a rooftop, paved courtyard, or sloped field – allows more water to be collected per rainfall event.

A practical rule of thumb from water harvesting specialists is to account for a 10-15% storage loss due to system inefficiencies, and to always size slightly larger than the calculated minimum to handle dry spells longer than average. The Dalhousie University Centre for Water Resources Studies has even developed a free cistern sizing tool that uses historical daily rainfall records alongside cistern capacity, catchment area, and daily household demand to estimate how reliably a given system will meet needs.

Location and site selection 🔗

The cistern should be placed as close as possible to the catchment area to minimize pipe length, reduce contamination risk, and keep pumping costs low. It must be sited away from potential contamination sources such as septic systems, fuel storage, or industrial runoff. Good drainage around the cistern site is essential to prevent soil saturation that could compromise structural integrity, especially for concrete tanks. Local zoning permits and soil assessments should be completed before excavation begins.

Inlet, filtration, and cover design 🔗

A properly designed cistern includes more than just a tank. US EPA guidelines on rainwater harvesting emphasize that a key feature of any cistern system is a screen or filter at the inlet to prevent leaves, debris, and insects from entering the storage vessel. A first-flush diverter is typically used to discard the first flow of water from a catchment surface – which carries the most dust and contaminants – before cleaner water is directed into the cistern. The sealed cover is equally important: it keeps sunlight out (preventing algae growth), blocks mosquito breeding, and of course minimizes evaporation.

Pump and distribution system 🔗

Because the cistern is underground, water cannot flow out by gravity alone in most configurations. A submersible pump, well pump, jet pump, or booster pump is required to extract water from the tank and deliver it at usable pressure to irrigation lines, livestock troughs, or household taps. The pump selected must be sized to match the required flow rate and the distance between the cistern and end-use points.

Underground cisterns in practice: traditional and modern examples 🔗

Underground cisterns have been in continuous use across diverse cultures and geographies for thousands of years, and their relevance has only grown in the face of water stress driven by climate change.

In Rajasthan, India, underground cisterns known locally as tankas and kunds – covered, plastered underground tanks with a dedicated catchment – have collected rainwater for centuries. FAO documentation on water harvesting technologies describes kunds as covered underground tanks with plastered catchments found in the same arid region, providing a reliable water source for drinking, irrigation, and livestock through long dry seasons. These traditional systems embody the same principles as modern engineered cisterns: minimize evaporation, prevent contamination, and store the maximum possible volume from infrequent rainfall.

In the Northeastern Brazil semi-arid zone, the government-backed cistern program described above turned a fragmented, traditional practice into a national-scale water security intervention – demonstrating that underground cisterns are not just suitable for individual households but can be deployed systematically across entire regions facing chronic rainfall deficits.

Advantages of underground cisterns over surface storage 🔗

The benefits of burying a cistern rather than placing it above ground are substantial, particularly in warm climates. Underground positioning provides natural thermal insulation, keeping water temperature stable and reducing the biological activity that degrades water quality in warm storage. It eliminates the algae problem that plagues uncovered or translucent above-ground tanks exposed to sunlight. It also preserves surface space – the land above an underground cistern can still be used for parking, farming, or building.

From a long-term cost perspective, reduced evaporation, lower maintenance requirements, and improved water quality translate into meaningful savings over the life of the system. The higher upfront cost of excavation and installation is generally offset by the reliability and water volume preserved compared to open or above-ground alternatives in hot, dry conditions.

What do you think? In regions where open ponds and surface tanks have traditionally been the go-to water storage method, what would it take to shift communities toward adopting underground cisterns at scale? And given that cistern sizing depends so closely on local rainfall patterns and daily water demand, how should smallholder farmers in dryland areas approach the planning process before investing in construction?

References

1. https://extension.psu.edu/water/drinking-and-residential-water/supply-and-conservation
2. https://en.wikipedia.org/wiki/Cistern
3. https://homewatertreatmentsystems.com/rainwater-harvesting-in-desert-climates-a-case-study/
4. https://diversedaily.com/innovative-techniques-for-harvesting-water-in-desert-environments/
5. https://www.watercache.com/portfolio/underground-concrete-tanks
6. https://www.ntotank.com/blog/how-to-install-an-underground-water-tank
7. https://link.springer.com/article/10.1007/s10668-023-03480-x
8. https://extension.okstate.edu/fact-sheets/design-of-rainwater-harvesting-systems-in-oklahoma.html
9. https://www.aquabarrel.com/what-size-cistern
10. https://centreforwaterresourcesstudies.dal.ca/app/webroot/cistern/
11. https://www.epa.gov/system/files/documents/2022-04/green-infrastructure-toolbox_rainwater-harvesting.pdf
12. https://publikationen.bibliothek.kit.edu/24242000/3247