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Want to know how to understand, calculate and dispute the water needs of the islanders?         

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Roughly 2/3 of all water on Earth are used for irrigation in agriculture. Industries use large amounts of water for cooling, they need boiler make-up water, and industrial process water in pulp and paper, chemical, petrochemical, coal and cement industries.

Islands in general have undergone big changes. Residents are not so numerous, but visitors are. On many islands, agriculture has declined in favour of tourism, which is the new industry of the islands. Canneries have closed, shipyards are few, fishing has decreased significantly and manufacturing was never important. To understand modern water use on an island, we need to have a look at households, industry, agriculture and tourism.

Household use

We don’t consume water, we just use it in different ways. Water is essential for life but the amount of drinking water required by a human varies. It depends on physical activity, age, health issues, and environmental conditions. As a general rule, the human body needs about two litres of water per day to compensate for urine drainage and transpiration from the body. Most of the two litres are acquired by drinking, but some fluid is supplied to the body via the food we eat.

During a meeting in the European Parliament at the beginning of our project, Mairtin O´Méaloíd from the Irish team vividly illustrated how we drink a small bottle of water to quench our thirst and then use 10 litres of water to flush the toilet after urination.

Diagram 1: Average household water use distribution

The use of water in the household, daily hygiene, use of the toilet, dishing, dishwasher and laundry, cleaning, watering of flowers indoors etc. constitutes 75-150 litres per person per day (l/p, d). If there is a garden, water use can be even higher. Depending on the type of accommodation, water can also be used for other purposes (washing the car, a pool etc.). These other usages usually require between 75 and 150 l/p, d.

Water use in the household (per person) doesn’t only depend on what we need but also on technology, pricing and management of the production and distribution systems.

Nevertheless, water use for hygiene has increased sharply in many European countries over the past 50 years. This is partly due to the availability of hot water, and the creation of a subsequent culture in which you often take more than one shower per day and change clothes much more often than before.

Measuring water use in a house or an apartment to record different types of water usage is not easy, and it is not a priority research area for authorities or universities. In areas where water shortage occurs periodically or suddenly, there are reasons for the municipality to conduct investigations that can lead to a common water saving awareness among residents and visitors.

Water for the household – related to accommodation – may be municipally distributed water, groundwater coming from a well, drilling or fountain, or water from lakes and streams (which is rather unusual on islands). Historically, rainwater was used to a greater or lesser extent.


An island has industries that also use water: restaurants, hotels, laundry facilities, and the municipality’s waterworks that produce clean drinking water are only some examples of common activities. Small-scale industrial activities on islands, such as fishing industry and shipyards, are another kind of water users. The same goes for schools and offices, shops, hospitals or healthcare services, small craft businesses and sports facilities.


Agriculture with cultivation and animal husbandry often requires large amounts of water. Not ‘manufactured’ water, but still freshwater from the same reserves as we use. A cow needs from 10 to 100 litres per day depending on age, body size (weight), stage of production, and the environment. Lactating cows drink nearly twice as much water compared to dry cows. The same applies to goats who need 4-5 litres of water per day and up to 10 litres per day when lactating, and who refuse to drink dirty or contaminated water. Sheep: a weaner needs 2-4 l/p, d, whereas an adult sheep needs 2-6 litres if grazing on grassland, 4-12 litres on saltbush.

It takes 995 days to grow cattle. Globally, the “food animal” population equates about 2.85 animals per human, which might also apply to islands.


Inis Oirr, which we visited in the first chapter. As the island, a barren rocky outcrop, originally had very little soil, the islanders created soil – using sand and seaweed. They sheltered the crops by building stonewalls, dividing the land among them. The result of their labour is evident in an amazing myriad of stonewalls with a total length of 360km.

Counting humans, the island has a resident population of 260, some 400 summer residents in 100 houses, 130 students during 3 x 3-week courses, 100,000 day-trippers and 50,000 weekenders (who typically stay 3 days). To make a living, residents work in tourism, as seamen, in social services and in farming. The tourism season is long, from March to November. There is a post office, a summer college, a church, a community centre, a hotel, a hostel, eight B&B’s, two self-catering facilities, one healthcare centre, one café and two bars/restaurants.

The island needs about 40 million litres (40,000 m3) of freshwater per year. The pressure on the water scheme is very uneven, peaking in summer.


Visitors make up important segments of water use on islands. One segment consists of summer residents, staying a large part of the year in their own properties on the island. They use the same amount of water per person and per day as the permanent residents on the island do. Another segment are those who stay for a number of days in rented houses or apartments and have more lavish water habits. The third segment is made up of day-trippers, who are only on the island for a part of the day. They use water from restaurants, campsites, bathing places and public toilets. The fourth segment are visitors who come by their own boats to the islands.  These visitors have different water habits: a day-tripper uses 20 l/p, d, while a 5-star hotel guest uses 400 l/p, d.

Although visitors and part-time residents use water only for a short period of the year, their numbers drive a large-scale need for water in the summertime which might exceed the municipality’s capacity of producing water. This leads to water shortages affecting residents, visitors and the tourist business who can lose customers, short-term or long-term, if the guests decide to shorten their stay on the island.


Going back to Tilos, where people have been living for 10,000 years, maybe along with the island’s famous dwarf elephants. Minoans, Mycenaeans, Dorians, Sicilians, Egyptians, Romans, crusaders, Ottomans, Italians and Germans had ruled over Tilos until it joined Greece in 1948, together with all the Dodecanese islands. After that, population rapidly declined as many Tilians emigrated to USA or Australia.  There has been a recovery since year 2000 due to the improvement of the island’s sea connection to Rhodes and an improvement in tourism, which is now the main source of income for the majority of the islanders.

Today, there are 600 permanent residents on Tilos, whereas there are approximately 500 seasonal residents – people who, although they have a residence, stay on the island only a few months a year. In addition, the island has a capacity of approximately 1,300 beds (hotels, rooms, etc.), while there are two free camping sites on the beaches of Eristos and Plaka, peaking during the summer season with 500 campers.

Let us recalculate these people into person-days:

The total number of person-days per year on Tilos is 492,300 - almost half a million. Dividing 492,300 by 365, we get an equivalent of a population counting 1,349 permanent residents. This is what we call the “technical population”, a number which describes the human pressure on Tilos’ healthcare, mobile-phone, rescue, fire prevention, waste, sewage, energy, transport and water supply systems more accurately than the census figure.

Water consumption in Tilos is 54,000 m3 as measured by water meters installed at the final consumers of the water supply network and recorded every three months of the year. There are some small supplies that are not recorded, because they are not priced, as in churches, schools and municipal buildings. There are also 5-6 private boreholes for irrigation purposes with small (~5 m3/day) supplies that are not recorded.

This fits well with a cross-check calculation of person-days with the average 125 l/p, d consumption in Greece:

Of these 54 million litres of water, only 4 million (7%) is for drinking, over 21 million litres is for showers and over 10 million litres is used to flush toilets.

Wasted water

Wasted water – not wastewater – is water used without intention and benefit. This is caused by mismanagement and leaks.

Water use is partly a function of how water is priced. If the energy price for heating water increases, it can cause a reduction in water use. On the other hand, the cost of water for most European households is far too low to motivate important water-savings. There are only a few examples of our islands where the water price is used to calibrate water use.

Water use also depends on the amount of leakage in the water production and distribution systems – an unwanted and large part of municipal water use. There are examples when leaks represent more than 60% of the total water production in the municipality. The normal value would be about 20%, including the water processed in freshwater production. If one excludes the waterworks purification water, leakage on average in one year should not exceed 15% of the total production.

Leakage typically derives from old water mains and pipes, where smaller corrosion holes in the water pipe or leaky joints are not visible above the ground surface and are also difficult to track, although there is sound measuring equipment for detecting the location of smaller water leaks. Larger water leaks create pressure drops in the water distribution so that the water cannot reach the taps of the consumers. This is first visible in the apartments situated higher up. Large leaks usually create a hole in the ground and water flows onto the ground surface. Unfortunately, most municipalities are not actively trying to track leaks.


Sein is an island outside the coast of Brittany, France. On average, the island rises but five feet above the water and is tiny: a 0.6 km2 settlement flanked by two flippers of rock and heath.

Sein became an island some 9,000 years ago. It is a string of islets connected by dykes and pebbles, well known for the dangers of its waters, the Chausée de Sein, a vast zone of reefs stretching far out at sea, requiring numerous lighthouses, beacons, and buoys.

The average yearly rainfall on Sein is 787 mm. Estimated groundwater recharge is 300-350 mm per year. Due to the low level of the island's surface, the pumping of water from boreholes lowers the groundwater level with an extremely high risk of saltwater intrusion, and groundwater outlets on the island can only be accepted to a very small extent.

Vegetation is scarce. There are no trees or bushes but only fields, now mostly left to fall into fallow and delineated by ancient dry-stone walls acting as windbreakers. On the pebble beaches, shorebirds make their nests. There are herds, sheep and rabbits. On the other hand, the sea is rich with conger, lobsters, berniques, mackerels and dolphins.

The island’s smallness and lack of relief exposes it to floods and marine erosion. Some experts say the islands get smaller with each heavy storm. Natural erosion is amplified by tourism erosion. The proliferation of rabbits is another major issue, as the burrows they dig allow the sea to rush under the ground.

Sein has 216 all-year residents. In summer (about 10 weeks), the population rises to 1,360 plus some 500 day trippers. The minute parcels of land have not been cultivated since the 1960s (which is also when the last cow left the island).

Sein is a municipality which manages its water resources locally and produces its freshwater in a local seawater desalination plant and then distributes it through a pipe network 8 km long with 328 subscribers. All households are covered by the network. Water on Sein is very expensive: 6.78 euro per m3.

The yearly production of freshwater is 31,000 m3. The plant uses 6-10 kWh to produce one litre. But this demand is not analogous to the need: the true need of Sein is 17,000 m3/year. The roads on the island are made with concrete slabs with the water mains buried underneath. Big construction works have been going on for 3-4 years on the jetties of Sein, and the heavy machinery has crushed the pipes and is causing major leaks.