Cistern Building

Part of Irrigation

Cisterns store water during wet periods for use during dry periods. They bridge the gap between when water is available and when crops need it — the fundamental challenge of irrigation in seasonal climates.

In most climates, water supply and crop demand are mismatched. Rain falls heavily in spring but crops need the most water in midsummer. Streams run high during snowmelt but drop to a trickle during the growing season. A cistern solves this mismatch by capturing surplus water and holding it until you need it.

Underground cisterns have been used for at least 8,000 years across the Middle East, Mediterranean, and Asia. They remain one of the most practical water storage solutions for small-scale agriculture because they are built from local materials, keep water cool and clean, resist evaporation losses, and require no mechanical components.

Planning Your Cistern

Sizing

The cistern must hold enough water to bridge the gap between supply and demand. Start with your irrigation deficit — the difference between what rain provides and what crops need.

Example calculation for a 500 m² garden in a climate with dry summers:

• Crop water need: 6 mm/day × 500 m² = 3,000 L/day
• Dry period: 60 days (July-August)
• Rain during dry period: approximately 0 mm
• Total storage needed: 3,000 × 60 = 180,000 L = 180 cubic meters

That is a very large cistern. In practice, you would combine cistern storage with other sources (wells, streams) and grow drought-tolerant crops that need less water. A more practical target:

Garden Size	Supplemental Storage	Cistern Volume	Approximate Dimensions
100 m²	30 days backup	18,000 L (18 m³)	3 × 3 × 2 m deep
250 m²	30 days backup	45,000 L (45 m³)	4.5 × 4.5 × 2.2 m deep
500 m²	15 days backup	45,000 L (45 m³)	4.5 × 4.5 × 2.2 m deep
1,000 m²	15 days backup	90,000 L (90 m³)	6 × 6 × 2.5 m deep

Start Small

Build a small cistern first (5,000-10,000 liters) to learn the techniques before committing to a large structure. A 2 × 2 × 1.5 meter cistern holds about 6,000 liters, takes one person a week to build, and will reveal any waterproofing problems before you scale up.

Site Selection

Choose a location that is:

• Higher than the fields it will serve, so water flows to crops by gravity
• Close to the water source (roof catchment, spring, or channel) that will fill it
• In stable soil — avoid areas with a high water table (external water pressure can collapse an empty cistern), expansive clay that swells and shrinks, or fill material
• Away from latrines, animal pens, and waste areas — at least 15 meters separation, preferably uphill from contamination sources
• Accessible for maintenance — you will need to enter the cistern periodically for cleaning and repair

Excavation

Shape

Cylindrical cisterns are structurally strongest (no corners to crack) but harder to excavate. Rectangular cisterns are easier to dig and line but require thicker walls to resist soil pressure. A practical compromise is a rounded-rectangle: rectangular with rounded corners, about 1.5:1 length-to-width ratio.

Wall Slope

Never dig vertical walls in an unlined cistern — they will collapse. Slope the walls inward from the bottom at about 10-15 degrees from vertical (a 2-meter deep cistern should be about 50-70 cm wider at the bottom than at the top on each side). This inward slope also helps resist the soil pressure that pushes inward on the walls when the cistern is empty.

Digging Procedure

1. Mark the outline on the ground, adding 30-40 cm on each side beyond the finished interior dimensions to allow for wall thickness.

2. Remove topsoil separately and save it — you will use it later for backfilling and landscaping.

3. Dig in layers: Remove soil in 30-40 cm horizontal layers across the full floor area. Do not dig one corner deep while leaving others shallow — uneven excavation causes wall collapse.

4. Check dimensions every 50 cm of depth using a plumb line and measuring stick. It is far easier to correct a misshapen cistern before you reach full depth.

5. Flatten the floor at final depth. The floor should be level or slightly sloped (1-2%) toward one corner where a small sump collects sediment for easy cleaning.

6. Let the excavation stabilize for 2-3 days before lining. Watch for any wall slumping, water seepage, or cracking that indicates problem soils.

Excavated volume for common sizes:

Interior Dimensions	Depth	Excavated Volume	Approximate Person-Days
2 × 2 m	1.5 m	~8 m³	4-6 days
3 × 3 m	2.0 m	~24 m³	12-16 days
4 × 4 m	2.5 m	~52 m³	25-35 days
5 × 5 m	2.5 m	~80 m³	40-55 days

Waterproofing

The waterproofing layer is the most critical component. A cistern that leaks is useless — worse than useless, because it wastes all the effort of construction while failing to store water.

Clay Lining (Puddled Clay)

The most accessible method. Requires a source of clay subsoil (at least 30% clay content — soil that can be formed into a ball that holds its shape when squeezed).

Application process:

1. Prepare the clay: Mix dry clay with water to a thick, sticky paste. Remove all stones, roots, and organic matter. The consistency should be like thick bread dough — wet enough to work but not runny.

2. Apply the floor first: Spread a 15-20 cm layer of puddled clay on the cistern floor. Compact it thoroughly by walking on it, stamping, or pounding with a flat timber. Work out all air pockets.

3. Apply wall layers: Start from the floor and work upward. Press clay firmly against the wall in layers of 5-8 cm thickness. Each layer must bond to the one below — score the surface of the previous layer before applying the next.

4. Total wall thickness: Build up to 15-20 cm of compacted clay on all walls.

5. Cure slowly: Cover the clay with damp cloth or grass and allow it to dry very slowly over 2-3 weeks. Rapid drying causes cracking that destroys the waterproof seal. Mist the surface daily if the weather is hot and dry.

Clay Cracking

The greatest enemy of a clay-lined cistern is drying cracks. Once the cistern is built, keep it full whenever possible. An empty clay-lined cistern exposed to summer heat will crack within days. If the cistern must be emptied for maintenance, cover it and refill within a week. Repair cracks by wetting the area, scoring both sides of the crack, and pressing fresh puddled clay into the gap.

Lime Plaster

Lime plaster creates a harder, more durable waterproof layer than clay, but requires limestone and the knowledge to burn and slake it.

Preparation:

1. Burn limestone in a kiln at high temperature until it converts to quickite (calcium oxide). The stone will lose about 40% of its weight and become white and crumbly.

2. Slake the quicklite by slowly adding water. This is an exothermic reaction — the mixture will heat and steam violently. Add water gradually in a safe, open area. The result is slaked lime (calcium hydroxide), a white paste.

3. Mix slaked lime with clean, sharp sand at a ratio of 1 part lime to 2-3 parts sand. Add water to achieve a workable plaster consistency.

Application:

1. Dampen the cistern walls thoroughly — lime plaster will not bond to dry surfaces.
2. Apply a scratch coat: a thin (5 mm) layer pressed firmly into the wall surface. Score the surface with horizontal lines.
3. After the scratch coat firms (12-24 hours), apply the finish coat: a 10-15 mm layer smoothed to a uniform surface.
4. Keep the plaster damp for 7-10 days while it cures. Lime plaster gains strength through carbonation — a slow reaction with atmospheric CO2 — and must not dry out during this process.

Waterproofing Method	Thickness	Durability	Seepage Rate	Materials Needed
Puddled clay	15-20 cm	5-10 years	Low-moderate	Clay soil
Lime plaster	15-25 mm	15-30 years	Very low	Limestone, sand, fuel
Stone + lime mortar	15-20 cm wall	30-100 years	Minimal	Stone, limestone, sand
Clay + lime combined	15 cm clay + 15 mm lime	15-25 years	Very low	Clay, limestone, sand

Combined Method

For the most reliable waterproofing with readily available materials, apply a clay lining first (as a backup seal), then finish with a lime plaster coat. The clay provides bulk waterproofing; the lime provides a hard, crack-resistant surface. If the lime develops hairline cracks, the clay behind it continues to hold water.

Inlet and Outlet Design

Inlet

Water enters the cistern through an inlet channel or pipe at the top of one wall. The inlet should include:

• A first-flush diverter (for roof catchment): The first few minutes of rain wash dirt, bird droppings, and leaves off the roof. Divert this dirty water away from the cistern. A simple diverter is a standpipe that fills with the first flush; once full, subsequent water overflows into the cistern inlet.

• A filter: Pack a wooden frame with layers of gravel (5 cm), coarse sand (5 cm), and fine sand (5 cm). Water passing through this filter arrives in the cistern relatively clean. Clean or replace the sand annually.

• A splash guard: Where inlet water falls to the cistern floor, place a flat stone to prevent the stream from eroding the floor lining.

Outlet

Install the outlet 10-15 cm above the cistern floor (not at the very bottom) so sediment that settles stays in the cistern rather than flowing to your fields. The outlet passes through the wall into an irrigation channel or pipe.

Seal around the outlet with extra lime plaster or clay — wall penetrations are the most likely leak points.

Overflow

An overflow pipe or channel near the top of the cistern prevents overfilling, which would erode the soil around the cistern rim. Direct overflow water to a drainage area away from the cistern foundation.

Covering the Cistern

An open cistern loses water to evaporation (potentially 5-10 mm per day in hot climates) and collects debris, insects, and algae. Covering the cistern is essential.

Timber and earth cover: Lay strong beams (minimum 15 cm diameter, spaced 40-50 cm apart) across the cistern opening. Cover with smaller poles or planks, then a layer of brush or straw, then 30-40 cm of earth. Leave an access hatch (60 × 60 cm minimum) for cleaning and inspection.

Stone slab cover: For permanent cisterns, flat stone slabs spanning the opening provide the most durable cover. Support long spans with intermediate walls or columns inside the cistern.

Leave a ventilation gap or pipe — a completely sealed cistern can develop anaerobic conditions that make the water smell and taste bad.

Cleaning and Maintenance

Task	Frequency	Method
Check water level	Weekly	Visual through access hatch
Clean inlet filter	Monthly	Remove, rinse, replace sand
Remove floating debris	Monthly	Skim with a net through access hatch
Inspect for leaks	Seasonally	Check surrounding soil for damp patches
Full cleaning	Annually	Drain, remove sediment from floor, inspect lining
Replaster/re-clay	Every 3-10 years	Drain, dry, repair all cracks, recoat

Confined Space Hazard

Never enter a cistern alone. Cisterns can contain low-oxygen air or toxic gases (hydrogen sulfide from decaying organic matter). Before entering, ventilate the cistern by removing the cover for at least 24 hours. Lower a candle on a string — if it goes out, the air is unsafe. Always have a second person at the surface with a rope tied to the person inside.

Summary

Underground cisterns store water from wet periods for dry-season irrigation. Size your cistern based on irrigation deficit — typically 15-30 days of backup supply. Excavate with sloped walls, waterproof with puddled clay (15-20 cm, lasts 5-10 years) or lime plaster (15-25 mm, lasts 15-30 years), or combine both for maximum reliability. Include an inlet filter, an elevated outlet, and an overflow pipe. Cover the cistern with timber and earth or stone slabs to prevent evaporation and contamination. Clean annually, inspect seasonally, and never enter a cistern alone. Start with a small cistern (5,000-10,000 liters) to learn the techniques before building larger.