Rainwater Harvesting and Cistern Storage

What is it?

Rainwater harvesting is the deliberate collection of precipitation, most commonly from a roof, and its storage for later use. Water sheets off the roofing material into a gutter, runs down a downspout, passes through a first-flush diverter that discards the initial, dirtiest pulse of runoff, and then fills a cistern — an above-ground tank or a buried, usually concrete or ferrocement, tank — from which it is drawn as needed. It is one of the oldest and most reliable ways for a household to decouple its water supply from a well, a pipe network, or a river.

What is it good for?

• Irrigation and livestock water, where no treatment is required and yield is the only real constraint.
• Non-potable household use — laundry, flushing, washing — with only coarse filtration.
• Potable supply, if the catchment is a clean, non-toxic roofing material and the stored water goes through additional filtration and disinfection (see water-purification) before drinking.
• Drought buffering: a sized cistern carries a household through weeks of dry weather using nothing more than gravity and stored rain.
• Reducing site runoff and erosion, a secondary benefit valued in urban and peri-urban settings.

The physics behind it

The available yield is a simple product:

Yield (liters) = catchment area (m²) × rainfall depth (mm) × runoff coefficient

One millimeter of rain falling on one square meter of roof is one liter of water, so a 100 m² roof in a 20 mm rain event sheds roughly 2,000 liters before losses. The runoff coefficient accounts for evaporation, splash, and absorption by the roofing material — around 0.8–0.9 for metal or tile roofs, lower for porous or heavily textured surfaces. This is the same catchment-area logic used to size a fog net, except the driving variable is rainfall depth rather than fog liquid-water content and wind exposure.

The first-flush diverter works on a simple volumetric principle: the first portion of any rain event, typically 0.5–1 liter per square meter of catchment, carries the dust, bird droppings, pollen, and debris that accumulated on the roof between rains. A vertical side-leg pipe fills first and traps this dirty water; once it is full, subsequent runoff — now cleaner — spills over into the main downspout toward the cistern. A slow bleed valve, float, or ball at the base of the diverter drains it between storms so it is ready to catch the next first flush.

Inside the cistern, sediment settles and the water column stratifies; drawing from a foot valve suspended above the tank bottom, rather than a bottom outlet, avoids pulling up settled silt.

History

Roof and courtyard rainwater catchment into cisterns is documented across most pre-industrial civilizations independently — Roman impluvium courtyards draining into underground cisternae, rock-cut cisterns across the Mediterranean and Middle East, monsoon-region tank systems in South Asia, and rooftop-to-barrel collection throughout rural Europe and colonial-era settlements without piped supply. It declined wherever centralized municipal water networks became reliable and cheap in the 20th century, then re-emerged from the 1970s onward — first in drought-prone and off-grid regions, more recently as a mainstream practice for irrigation, resilience, and reducing demand on municipal systems.

Simple version

A single downspout feeding a food-grade barrel or an IBC tote through a simple leaf screen at the gutter inlet and a basic mesh cap over the tank opening, with an overflow pipe set below the tank lid.

Advanced version

A properly sized first-flush diverter ahead of the tank, a sealed, insect-proof tank inlet, an inline sediment or carbon filter on the draw line, a floating foot valve, and a calculated overflow that routes excess water safely away from the building's foundation.

Industrial version

Large buried or above-ground concrete or ferrocement cisterns (tens to hundreds of cubic meters) feeding pressurized distribution through a pump and pressure tank, often paired with automated first-flush and filtration trains, UV or chlorine disinfection for potable use, and a mains-water backup switch for dry spells.

Building your own

1. Measure the catchment: roof footprint (plan area, not slope area) in square meters, and your region's typical rainfall depth per event and per year.
2. Size the gutter and downspout: match pipe diameter to your peak rainfall intensity so the gutter doesn't overflow in a heavy storm.
3. Build the first-flush diverter: a vertical capped pipe teed off the downspout, sized to roughly 0.5–1 L per m² of catchment, with a slow-bleed valve or small drilled weep hole at its base.
4. Set the tank: an IBC tote or barrel on a level, load-bearing platform (water is heavy — about 1 kg per liter), or a buried/partially buried ferrocement or concrete cistern for larger volumes.
5. Screen every opening: gutter inlet, tank lid, and overflow outlet all need mesh fine enough to exclude mosquitoes and debris but not so fine it clogs.
6. Add an overflow: a pipe near the top of the tank, sized at least as large as the inlet, directed well away from any foundation.
7. Install a foot valve or elevated draw point: keep the outlet above the sediment layer that settles at the bottom.

Common mistakes

How to measure

Track collected volume against a simple rain gauge and the catchment-area formula (area × rainfall × runoff coefficient) to check the system's real efficiency against the theoretical yield. Periodically test stored water for turbidity, and for potable systems, for microbial contamination, to confirm the first-flush and filtration stages are doing their job.

Videos

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Downloadable PDF

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