In Madeira all the water is on the side where almost nobody lives

There are islands where water is a problem and others where it is an excess. Madeira is both at once, and in very little space: 801 square kilometres in which one side gets almost three and a half times more rain than the other.

What turns that figure into a story is where people settled. The capital, Funchal, with 105,795 inhabitants, is on the south coast, the dry one. The water is on the north. And the consequence of that contradiction is the most extraordinary piece of public works in the Atlantic: more than two thousand kilometres of hand-dug channels.

01 A six-kilometre mountain of which you see one thousand eight hundred metres

It is worth starting with what you cannot see. The island is the top of a shield volcano rising some six kilometres from the floor of the Atlantic. Of those six kilometres, the sea covers most: what emerges reaches 1,862 metres at Pico Ruivo, followed by Pico das Torres at 1,847 and Pico do Areeiro at 1,818.

Volcanic activity began in the Miocene, some five million years ago, and continued into the Pleistocene, some 700,000 years ago. The most recent eruptions took place around 6,500 years ago in the west-central area. In other words: geologically recent, but already extinct.

Position counts too. The island lies 805 kilometres south-west of mainland Portugal and 520 west of Morocco, in the middle of the Atlantic, with the Canary and Gulf Stream currents passing by. None of that is decorative: it explains why Funchal's annual mean temperature is around 20 degrees and why it barely moves.

02 The ridge splits the island into two climates

What is decisive is the orientation. The ridge crosses the island from east to west, side to side, and stands between north and south like a wall almost two kilometres high. The moist winds from the north and north-west strike that wall, rise, cool and unload.

The figures for the split are conclusive. The north receives up to 2,800 millimetres of rain a year. At the high-altitude station of Pico do Areeiro, at 1,610 metres, measured annual precipitation is 2,927.8 millimetres. Funchal, on the south coast, gets 817.7 millimetres over 80.9 days.

Between those two stations there are some twenty kilometres in a straight line. One receives three and a half times more water than the other. The south, in fact, has a climate described as semi-arid in its lowest stretches, and a gentler relief as well.

03 The forest that manufactures the water

On the northern slope, between about 400 and 1,200 metres, grows the laurisilva: a laurel forest that is a remnant of a vegetation which millions of years ago covered much of southern Europe and northern Africa. The island's palaeobotanical record shows that this forest type has existed here for at least 1.8 million years.

It covers some 15,000 hectares, 20 % of the island, and is considered the largest surviving area of laurel forest in the world, with around 90 % primary forest. UNESCO inscribed it as a World Heritage Site in 1999.

Its function in this story is not scenic. Much of the precipitation on the northern slope does not arrive as rain but as drizzle and fog, and the forest's leaves intercept it and channel it to the ground. A forest like this does not only receive water: it captures it from the air. It is the first link in the system.

04 Two thousand one hundred and seventy kilometres of channel

The solution to the imbalance is called a levada, from the verb to carry. It is a narrow built channel that follows the contour line with a minimal gradient and transports water from where there is a surplus to where there is a shortage. In Madeira there are more than 2,170 kilometres of them, accompanied by some 40 kilometres of tunnels.

The first large channels were cut in the late fifteenth century and the first half of the sixteenth, during the sugar boom, when irrigating the cane made necessary a quantity of water the south did not have. The system has been extended ever since.

But there is a chronological limit worth remembering: until the end of the eighteenth century, irrigation in the south was done only with levadas that did not cross the central range. Bringing water from the north was technically risky and hugely expensive. Getting through the wall came later, and that is what explains the tunnels.

05 Why people settled on the dry side

The obvious question remains: if the water is in the north, why does the island live in the south? The answer has three parts and none of them is water.

The first is light. Funchal accumulates 2,316 hours of sunshine a year, with average highs running from 19.8 degrees in January to 26.8 in August. The north, under cloud, has considerably less. The second is relief: the southern slope is gentler and takes terraces; the northern one drops to the sea as cliff.

And the third is the sea. The southern coast offers sheltered anchorages and the northern one takes the North Atlantic swell head-on. On an island that lived off the sugar and wine trade, the harbour weighed more than the spring.

Hence the final logic: the side with sun, workable land and a harbour was chosen, and then the water was brought over. A quarter of a million inhabitants live inside that decision today.

06 How to read the island

The first practical consequence is that in Madeira the north and the south are different destinations and it pays to treat them as such. Crossing the ridge is not changing valley: it is changing climate, vegetation and light in forty minutes by car.

The second is that levadas are not footpaths: they are infrastructure. The path that runs alongside them exists because somebody had to walk the channel to keep it clear and distribute the water. Walking a levada is walking an island's irrigation network.

And the third is the one that reorders the trip. Once you understand that the northern forest captures water from the fog, that this water enters a channel and that the channel carries it twenty kilometres to a banana terrace in the south, the island stops being a catalogue of viewpoints. It becomes a single machine, very large and very old, that is still running.