Water Storage

Part of Irrigation

Every reliable irrigation system depends on storage — a reservoir, cistern, or tank that absorbs water during periods of surplus and releases it during periods of demand. Without storage, you are dependent on the precise coincidence of available water and crop need, which rarely aligns. Sizing storage correctly, constructing it to prevent seepage and evaporation, and managing water quality within the storage are the practical skills that turn an intermittent water source into a year-round irrigation resource.

Storage Types Overview

Storage Type	Volume Range	Best Use	Key Advantage
Household tank	500–10,000 L	Domestic, garden	Quick to build, sealed
Farm cistern	10,000–200,000 L	Small irrigation	Below-ground, evaporation-free
Small reservoir/pond	200,000–1,000,000 L	Field irrigation	Cheap per litre, flexible
Large earthen dam	>1,000,000 L	Community irrigation	Maximum volume, gravity supply

Sizing Calculations

The fundamental sizing question: how many days of irrigation can you provide from stored water if no inflow arrives?

Calculation steps:

1. Determine daily crop water demand (mm/day × field area in m²)
2. Determine source flow rate and reliability
3. Set a design buffer period (how many dry days must storage cover?)

Example calculation:

• Field area: 1 hectare (10,000 m²)
• Crop demand: 5 mm/day
• Daily volume required: 5 mm × 10,000 m² = 50,000 litres/day
• Design buffer: 14 days without inflow
• Storage required: 50,000 × 14 = 700,000 litres minimum

Add 30% for evaporation and seepage losses: 700,000 × 1.3 = 910,000 litres (910 m³)

A reservoir 30 m × 20 m × 1.5 m average depth holds 900 m³ — suitable for this example.

Quick Sizing Table

Field Area	Crop Demand	7-Day Buffer	14-Day Buffer	30-Day Buffer
0.1 ha	5 mm/day	35,000 L	70,000 L	150,000 L
0.5 ha	5 mm/day	175,000 L	350,000 L	750,000 L
1.0 ha	5 mm/day	350,000 L	700,000 L	1,500,000 L
2.0 ha	5 mm/day	700,000 L	1,400,000 L	3,000,000 L

For livestock water (drinking only), allow 50 litres/day per large animal (cattle), 5 litres/day per small animal (sheep, goat), 15 litres/day per person.

Method 1: Ferrocement and Masonry Tanks

For household to small-farm scale (500–50,000 litres), constructed tanks offer the best combination of durability, sealing, and construction from local materials.

Ferrocement Tank Construction

Ferrocement uses a thin shell of cement mortar reinforced with wire mesh. Tanks 5,000–50,000 litres are practical for small farms.

Materials per 10,000-litre tank (cylindrical, 3 m diameter × 1.4 m height):

• Cement: 10 bags (50 kg each)
• Sand: 25 bags (clean, fine)
• Chicken wire mesh (1-inch hex): 50 m²
• Mild steel wire (2–3 mm): 20 kg
• Plaster reinforcing mesh: 50 m²

Construction sequence:

1. Cast a reinforced concrete ring footing 20 cm wide × 20 cm deep around the tank perimeter
2. Construct a cylindrical wire framework (chicken wire stiffened with vertical and horizontal steel bars) to the tank shape
3. Apply cement mortar (1 part cement : 2 parts fine sand, minimum water) by pressing it through the wire mesh from both sides simultaneously — this ensures full penetration
4. Apply in layers: first coat 10 mm, second coat 10 mm after 24 hours
5. Plaster smooth inside surface with a fine finishing coat (1:1 cement:sand slurry)
6. Cure by keeping wet under shade for 21 days minimum — cover with wet sacking
7. Test with water before backfilling or using

Curing is Non-Negotiable

Cement that dries too fast is weak and porous. A ferrocement tank that is not cured for at least 14 days will crack and leak. During hot, dry, or windy conditions, rewet the tank surface every 2–4 hours for the first 7 days.

Brick or Stone Masonry Tanks

Brick or stone tanks are more durable than ferrocement but require more labour and skilled masonry. Suitable for permanent installations with access to good masonry materials.

• Walls: minimum 20 cm thick fired brick laid in 1:3 cement:sand mortar
• Internal render: 20 mm thick, 1:2 cement:sand (two coats)
• Base: 15 cm reinforced concrete, watertight mix (1:1.5:3 cement:sand:gravel)
• Waterproof additive in render: potassium silicate or ferrous sulphate at manufacturer’s rate

Method 2: Earthen Ponds and Reservoirs

For volumes above 50,000 litres, earthen ponds offer the most economical storage per litre on a liveable budget. See the Pond Sealing article for sealing strategies.

Site Selection for Storage Ponds

• Locate in a natural depression or at the low end of a catchment to maximise rainfall capture
• Avoid sites with permeable sandy or gravelly subsoil unless you plan to line the pond
• A perennial spring feeding the pond provides year-round inflow
• Position the pond upslope from irrigated fields so gravity can distribute water

Embankment Construction

For reservoir embankments (earthen dams), basic principles:

• Embankment slope: not steeper than 2:1 (horizontal:vertical) on either face
• Embankment crest width: minimum 2 m for small dams, 4 m for larger ones
• Embankment must extend at least 0.5 m above the maximum expected water level (freeboard)
• Core: compact the centre of the embankment with the finest, most impermeable soil available
• Spillway: every earthen dam must have an overflow spillway rated for the maximum expected inflow; a dam without a spillway is a dam that will eventually overtop and fail

Never Skimp on the Spillway

The most common earthen dam failure is overtopping — water rising above the crest, eroding the embankment, and catastrophic failure. Size the spillway to pass the maximum expected storm inflow. A simple grass-lined spillway 2–3 m wide through a saddle in the embankment is often adequate for small farm dams.

Method 3: Underground Cisterns

Underground cisterns store water below ground, eliminating evaporation losses and protecting water quality from sunlight, wind-blown contamination, and algae growth. Traditional in arid regions of the Mediterranean, Middle East, and India.

Advantages

Feature	Above-ground tank	Underground cistern
Evaporation	Significant	Negligible
Algae growth	Common problem	Minimal (no light)
Construction material	Ferrocement or masonry	Masonry or rock
Excavation required	No	Yes
Water quality	Needs lid	Superior
Construction complexity	Moderate	High
Cost per litre	Moderate	High

Construction (Rock-Cut or Brick-Lined)

1. Excavate to the cistern volume required; in stable rock, walls may be left as-is
2. Line walls with 20 cm brick or stone masonry in waterproof mortar
3. Apply two coats of 1:2 cement:sand render on all internal surfaces, 15 mm thick each coat
4. Waterproof the final coat with a slurry of 1 part cement, 1 part fine sand, and waterproofing additive
5. Cast a reinforced concrete roof slab with access hatch and vent pipe
6. The vent pipe allows air movement without contamination — cap with a fine mesh screen

Cisterns traditionally collect roof runoff via a first-flush diverter (the first 10–20 litres of rain after a dry period, which contains the most contamination, are automatically diverted away before clean water enters the cistern).

First-flush diverter: A vertical standpipe connected to the downpipe. Volume = 2 mm of rain × collection roof area in litres. For a 50 m² roof, the first-flush chamber holds 50 mm × 50 m² × 0.001 = 100 litres. Once this chamber fills, subsequent clean rain overflows into the cistern.

Evaporation Prevention

Evaporation from open water surfaces in hot, dry climates can remove 5–10 mm/day — equivalent to the irrigation demand it was meant to supply.

Method	Evaporation Reduction	Notes
Shade cover	50–80%	Trees on west/south sides, shade cloth over small tanks
Floating cover (polystyrene panels)	70–90%	Partial coverage acceptable
Reflective floating material	60–85%	Foil or white polythene
Underground storage	Near 100%	Eliminates the problem
Windbreaks	20–40%	Reduces surface turbulence

For small tanks (under 10,000 L), a simple fitted lid is most effective and prevents contamination simultaneously.

Water Quality Management

Stored water can deteriorate if not managed. Key threats:

Algae: Grow in sunlit water. Make water taste bad and can be toxic (blue-green algae). Prevent by covering storage to exclude light. If algae appear, reduce sun exposure and increase flushing rate.

Sediment: Suspended fine particles cloud water and clog drip emitters. Install a sediment trap (a settling chamber where inflow enters and heavier particles settle before water moves on) between the inflow and the main storage.

Mosquitoes: Breed in any still water with access. Cover all tanks and cisterns with tight-fitting lids or mesh. For open ponds, introduce mosquito fish (Gambusia) or encourage bats.

Animal contamination: Fence livestock away from open water storage. Install a separate animal trough fed from storage rather than allowing direct access.

Tapping Near the Bottom

Draw irrigation water from 20–30 cm above the reservoir floor, not from the very bottom. This leaves a dead storage zone where sediment settles, keeping drawn water cleaner and extending the service life of filtration systems downstream.

Inlet and Outlet Infrastructure

• Inlet: Include a screen or coarse filter (gravel) at the intake to exclude debris and fish
• Outlet pipe: Position at 20–30 cm above the floor; fit a gate valve for control
• Overflow: Set overflow pipe at maximum design level; route to a ditch or controlled outlet
• Drain: A bottom drain valve allows complete emptying for inspection and cleaning — essential for cisterns and tanks, optional for large ponds

Water Storage Summary

Size storage based on daily crop demand multiplied by the design dry period you need to cover, adding 30% for evaporation and seepage. Ferrocement or masonry tanks serve household to small-farm scale (up to 50,000 L) and are fully sealable. Earthen ponds are cost-effective for large volumes (50,000–1,000,000+ L) but require sealing and a properly designed spillway. Underground cisterns eliminate evaporation and protect quality in arid settings. Evaporation in hot climates can remove 5–10 mm/day from open surfaces — cover or shade all storage wherever feasible. Maintain a first-flush diverter on any roof-catchment cistern and a sediment trap on any inflow to preserve water quality.