Guide
Fresh Water on a Private Island: Desalination, Rainwater and Wells
Water is the quietest determinant of whether an island can be lived on comfortably. There is no mains connection to fall back on, and the sea, though everywhere, is undrinkable until worked upon. A sound water strategy is almost always a blend of three sources — the sky, the ground and the sea — each with its own arithmetic. This is an orientation to that arithmetic.
Start with demand, not supply
Every water decision begins with a candid estimate of consumption. A frugal, conservation-minded household might live on 100 to 150 litres per person per day. A comfortable residence with gardens, guests and generous bathrooms runs to 250 to 400 litres per person per day, and a landscaped, resort-style operation with pools and irrigation can exceed that severalfold. Fix this figure before sizing anything, because both a tank and a desalination unit are dimensioned against daily demand and days of autonomy.
On an island you do not buy water, you buy the capacity to make and keep it. The tank is as important as the source that fills it.
A practical planning household of, say, four residents plus staff at a comfortable 300 litres per head implies well over a cubic metre a day, and a guest-receiving property several times that. Hold that number in mind through what follows.
Rainwater: the cheapest water on the island
Where rainfall is generous, catchment is the first and cheapest source. The arithmetic is elegant: one millimetre of rain on one square metre of roof yields one litre of water, before losses. The working formula is catchment area in square metres, multiplied by annual rainfall in millimetres, multiplied by a runoff coefficient that accounts for splash, evaporation and first-flush diversion. A metal roof runs a coefficient near 0.90; a tiled or shingled roof nearer 0.75 to 0.85.
The numbers become concrete quickly. A 300 m² roof under a tropical 2,000 mm of annual rainfall, at a coefficient of 0.85, yields roughly 510,000 litres a year — comfortably over a thousand litres a day averaged across the year. Many tropical islands are wetter still: parts of the Seychelles receive 2,300 to 3,500 mm annually, and our Seychelles guide discusses how that abundance is nonetheless seasonal.
| Roof area | Annual rainfall | Coefficient | Approx. annual yield |
|---|---|---|---|
| 150 m² | 1,200 mm | 0.85 | ~153,000 L |
| 300 m² | 2,000 mm | 0.85 | ~510,000 L |
| 500 m² | 2,500 mm | 0.85 | ~1,060,000 L |
The catch is that annual yield is not the constraint — seasonality is. Even generously-watered islands endure dry months, and the Seychelles, despite over 2,000 mm a year, is classed as water-stressed precisely because its soils recharge poorly and its streams run dry between April and September. The tank, therefore, is sized not to the average but to the drought.
Sizing the tank to bridge the dry season
Tank capacity should carry demand through the longest reliably dry spell. If a property needs 1,200 litres a day and faces a four-month dry season with negligible catchment, the bridging storage alone approaches 145,000 litres — before any margin. This is why island cisterns are large, often buried or built into the structure, and why storage, not collection, is usually the binding cost. First-flush diversion to discard the dirty opening minutes of a storm, mesh screening and periodic tank cleaning keep the stored water sound.
Wells and the freshwater lens
Some islands hold groundwater — a body of fresh water that, being lighter than seawater, floats as a lens on top of the salt beneath. On small coral and limestone islands this is described by the Ghyben-Herzberg relationship, which carries a sobering multiplier: for roughly every metre the water table sits above sea level, the fresh lens extends about forty metres below it. The lens is a thin, finite reservoir, and it is easily ruined.
The characteristic failure is upconing. Pump a well too hard and the salt interface beneath it is drawn upward in a cone toward the intake; the well first turns brackish, then saline, and the damage can be slow to reverse. The disciplines that protect a lens are therefore about restraint: modest, steady abstraction rather than hard pumping; shallow, wide wells that skim the top of the lens rather than deep ones that invite the cone; and monitoring of salinity so that the first rise in conductivity is a warning, not a crisis.
- Test the lens before relying on it; its thickness and quality vary enormously between and even within islands.
- Abstract gently and continuously rather than in hard bursts, which provoke upconing.
- Monitor electrical conductivity at the well; a rising trend is the early signal of saline intrusion.
- Treat groundwater as a supplement to catchment and desalination, rarely as the sole source on a small island.
Understanding the water beneath and around a site is part of reading it well; our field note Reading the Water and the Island Dossier we compile both carry what is known of a candidate island's hydrology.
Desalination: making the sea drinkable
Where rain is unreliable and the lens is thin or absent, reverse-osmosis desalination is the source of last resort that quietly becomes the source of first resort. Seawater is forced through a semi-permeable membrane at high pressure, leaving salt behind and clean permeate on the far side. The technology is mature, modular and, at the scales islands need, well understood.
Capacity
Units are rated by daily output. A compact system delivering around 460 litres per hour translates to roughly 11 m³ over a full day, which covers a substantial household. Small residential units producing a few thousand litres a day are common; a guest-receiving property may install 10 to 50 m³ per day of capacity. The design principle is to run a modestly-sized unit for a portion of the day into storage, rather than to buy a large unit that idles — the tank does the buffering.
Energy
Energy is the defining cost of desalination, and it is the number that ties the water system to the power system. Modern small seawater RO consumes on the order of 3 to 5.5 kWh per cubic metre of fresh water produced; efficient plants with energy recovery reach below 3, and the very best large plants approach 2.8. The thermodynamic floor is around 1 kWh/m³, which no real system meets. For an island producing, say, 5 m³ a day at 4 kWh/m³, desalination alone adds 20 kWh to the daily electrical load — a figure that must be carried straight into the power design covered in our guide to island power and energy.
| Metric | Typical small-island range | Note |
|---|---|---|
| Specific energy | 3 – 5.5 kWh/m³ | Below 3 with energy recovery |
| Capex (small plant) | ~USD 0.65 – 1.2M per 100 m³/day | Small plants cost more per unit |
| Opex (all-in) | ~USD 0.50 – 2.50 per m³ | Energy is 35–50% of opex |
| Membrane replacement | Every few years, not annual | A recurring, plannable cost |
Capital and running cost
Small plants carry a cost penalty per unit of capacity — very small units can cost several times per cubic metre what a large municipal plant achieves. All-in operating cost typically falls between USD 0.50 and 2.50 per cubic metre, with electricity the largest single component at 35 to 50 per cent. Membranes are replaced periodically rather than annually and should be budgeted as a foreseeable recurring expense, not a surprise. The economics improve markedly when the desalination unit runs on abundant midday solar rather than diesel, which is the strongest argument for coupling the two systems in design.
Treatment, storage and redundancy
No single source should stand alone. Rain fails in drought, the lens fails if overdrawn, and desalination fails if the power or a membrane fails. The resilient island water system layers all three, with generous storage as the shock absorber that lets any one source falter without interrupting supply.
Downstream of the source, treatment is common to all: filtration to remove particulates, and disinfection — ultraviolet or chlorination — to guarantee microbiological safety, particularly for stored rainwater and lens water. Desalinated permeate is often remineralised, since water this pure is flat to taste and mildly aggressive to pipework. Storage should be divided into more than one tank so that cleaning or repair never empties the reserve, and the whole system benefits from a simple hierarchy: rainwater first when it flows, the lens gently as a supplement, and desalination as the reliable backstop that guarantees the household is never dry.
- Layer at least two sources so no single failure interrupts supply.
- Size storage against the longest dry season, not the annual average.
- Split storage across tanks so maintenance never empties the reserve.
- Disinfect all stored water and remineralise desalinated permeate.
- Design the desalination load into the power system from the start, not after.
Because water and power are inseparable on an island, this guide is best read alongside those on power and energy, building and insurance and resilience. When a specific island is on the table, the water question is one we work through in the Acquisition Brief, and islands seeking a buyer can be introduced through Register an Island.
General orientation, not professional engineering, legal or tax advice. Enquiries: the enquiry form.