Coast
Fresh water sets the real capacity of a small island
Islands sit on a thin lens of rainwater floating above salt, and how much of it can be drawn without spoiling the rest quietly limits how many people the place can hold in August.
By Anjali Raghunathan3 min read
Rain that falls on an island mostly floats
Fresh water is slightly less dense than salt water, so rain soaking into a permeable island does not simply mix with the sea beneath it. It forms a lens, thickest under the middle of the island and thinning towards the shore, sitting on top of the salt water that saturates the rock below.
The lens is the island’s reservoir, and it is deeper than it looks in the sense that a modest thickness of fresh water above sea level implies a much greater thickness below. That relationship is why even low, flat islands can hold usable quantities of fresh water.
It is also fragile in a specific way. Pumping too hard from a well draws the salt water upward in a cone beneath it, and once salt reaches the well the water is spoiled for a long time, because the lens takes years of rainfall to recover. Islands therefore tend to use many shallow wells rather than a few deep ones.
Geology decides whether an island has any groundwater at all
Not every island has a lens. Limestone and sand islands are permeable and store water well; hard volcanic or granite islands may shed most of their rainfall to the sea through streams and hold very little, which is why two neighbouring islands with the same rainfall can have completely different water situations.
Where the rock does not hold water, the traditional answer is to catch it. Roof catchment into cisterns is the oldest island technology in many parts of the world, and the size of the tank rather than the annual rainfall is what determines whether a household gets through the dry months.
Some islands have neither option and have always imported water, historically by boat and more recently by pipeline where a neighbour is close enough. That dependence tends to be invisible to visitors and entirely obvious to residents, and it usually shows up in the price.
The demand peak and the supply low arrive together
On Mediterranean-type islands and many others, the dry season and the visitor season are the same months. Rainfall stops replenishing the lens at exactly the point when the population multiplies, which is the central water problem of tourist islands and the reason restrictions appear in late summer rather than in winter.
Visitor demand is also higher per head than resident demand, because of showers, laundry, pools and gardens that would not exist for the resident population alone. That is not a moral observation; it is a design load that the water system has to be sized for and paid for.
The result is that many islands run water services designed around a peak they experience for a few weeks. That is expensive infrastructure to keep idle for ten months, which is one reason island water charges frequently look high relative to the mainland.
Desalination changed the constraint and moved it to energy
Reverse osmosis has made fresh water available on islands that could never have supported their current population otherwise. It is a genuine transformation and it is why some very dry islands sustain substantial towns and agriculture.
What it does is convert a water problem into an energy problem, since the process is power-hungry and island electricity is often the most expensive in a country. Where the plant runs on imported fuel, the cost of water tracks the cost of that fuel, and where it runs on wind or solar, the economics look very different.
It also creates a discharge to manage. The concentrated brine left over has to go somewhere, and how it is dispersed matters to the seabed near the outfall, which is a real if manageable engineering consideration rather than an argument against the technology.
What a visitor sees, and what it means
Signs asking guests to be careful with water are usually not decorative. On a small island in late summer they generally reflect an actual constraint on the system, and the household or business displaying them may be paying for tankered water when the supply runs short.
The visible clues are worth reading. Rooftop cisterns and catchment aprons indicate an island that stores its own rain; a desalination plant near the harbour indicates one that manufactures its supply; a hotel with lawns and a large pool on a dry island indicates a considerable draw on whichever of those it uses.
None of this requires a visitor to behave heroically. Shorter showers, reusing towels and not running taps are small in isolation, but on an island where the constraint is a lens that recharges slowly, the aggregate of a summer’s worth of visitors is precisely the quantity in question.
Common questions
Is tap water on small islands safe to drink?
It depends entirely on the island and its treatment arrangements rather than on being an island as such. Some island supplies are treated to the same standard as any mainland system; others are cistern water intended for washing. Local advice is the reliable source, and it is usually given readily.
Why is bottled water so expensive on some islands?
Because it is freight rather than water. Every bottle arrived by boat, and the crossing cost is spread across everything in the shop. Where a refill point or a treated public supply exists, using it is usually both cheaper and considerably less wasteful.
Do water restrictions affect visitors directly?
Sometimes. Islands under pressure have imposed limits on filling pools, watering gardens and washing vehicles, and in extreme dry periods supply can be intermittent. These measures are more commonly applied to businesses than to individuals, but they can affect what a place is able to offer.
Editor, The Next Postcard
Anjali has been reporting on cities, slow routes, rail & road since long before it was fashionable and reads the small print so you do not have to.
