What it is
A tank, buried. Surrounded by earth. Sealed against light.
In hot climates, an open pond or sun-baked tank can lose 10 to 30 percent of stored water a year to evaporation. A buried cistern holds steady at 10 to 15 degrees Celsius. Evaporation drops to nearly zero.
Darkness kills algae. No light, no photosynthesis, no green slime on the walls. Pair it with a first-flush diverter and the water stays clean for months.
Why bury it
Surface space comes back. A driveway, a lawn, a vegetable bed can sit on top.
Heritage zones and dense urban lots reject above-ground tanks. They tolerate cisterns nobody can see. The footprint disappears.
Buried tanks also outlast their above-ground cousins. UV degrades plastic. Soil insulates concrete. The math favors going under.
Types
Precast concrete. The workhorse. Sections lowered by crane, 2,000 to 50,000 litres or more. Concrete is alkaline, which nudges pH up and slows bacterial growth. Fifty-year lifespan, heavy enough to resist floating in wet soils.
Ferrocement. Built on site with cement mortar over steel mesh and rebar. Geoff Lawton uses it in permaculture projects worldwide. A skilled crew builds a 10,000-litre tank in a few days. Thin walls, low cost, waterproof.
HDPE. Light, cheap, fast to install. Ribbed walls handle soil pressure. The catch: in saturated ground, an empty HDPE tank floats. Anchor it to a concrete pad. Twenty to thirty years before replacement.
Stone cisterns still hold water in the Mediterranean after centuries. Durability follows design, not materials.
Install it
Excavate wide. Leave 30 cm of clearance on each side for backfill. Use clean gravel or sand, never the spoil. Decomposing organics create voids.
Check the water table. This is the single biggest site question. If groundwater rises above the tank base in winter, an empty cistern floats out of the hole like a boat, cracking pipes on the way up. Anchor to a reinforced slab, or keep a minimum water level through the wet season. Ask neighbours what they hit when they dug.
Plan the load above. Driveway or parking? You need a traffic-rated lid and thicker walls. Garden or lawn? Pedestrian rating is fine. Bury the lid 300 to 600 mm deep. Shallower invites damage. Deeper makes inspection a chore.
Costs and lifespan
Buried storage runs 1.5 to 3 times the cost per litre of an above-ground tank. Excavation, engineering, and stronger walls drive the premium.
A 10,000-litre precast concrete cistern in stable soil: roughly $2,000 to $4,000 installed. Ferrocement at the same volume: half that in materials, with labour as the swing. HDPE sits between.
Then the math flips. Concrete and ferrocement last 50 to 100 years. Above-ground plastic tanks fade in UV and need replacing every 15 to 20. Cost per litre per year of service tilts toward burial.
Plumb it right
Inlets enter through the top or upper wall. This blocks soil moisture from backflowing in.
Use a calmed inlet. A vertical pipe directs water down to the floor without stirring sediment. The outlet draws from mid-depth, above the silt layer and below any floating debris.
For gravity-fed drip irrigation, site the cistern uphill of the garden. Otherwise, run a submersible pump. Connect to your catchment system with sized pipes.
Size the overflow to handle peak inflow. Send the surplus to a swale or rain garden. No rainfall event should leave the property.