EXERCISE B: The water of your islanders


In Exercise A, you tried to estimate the freshwater assets of your island. Based on what you have learned in this second lecture, the next question is how much you are using, as a society. It can be answered in two ways, depending on the viewpoint: either (a) the consumer´s perspective, or (2) the producer’s perspective.



a Human pressure

As mentioned in the Tilos example above, it is necessary to define the island’s “technical population”. This will let us understand the human pressure on the island’s water system.

First, for the residents, use census numbers. The residents don’t come and go with the seasons, their number is the same all year round. Multiply them by 365 to get the number of total person-days.

Second, for seasonal residents, there are no census figures. The municipality administration should be able to make a good guess. The post office often knows how many post boxes there are = households, and a household is typically 2.3 persons. Water or Energy Management should know how many seasonal clients (taps, meters) there are. It will let you make an educated guess.

Third, for tourists, there are statistics at the municipality office and the Chamber of Commerce should have these figures (maybe they do not have the same numbers as the municipality). Another source of data is the ferry/ferries: check the number of passengers in winter, it will let you figure out how many trips the residents do in a three-month period. In spring and autumn, the increase in traffic would mainly be the part-time residents. Last but not least, the summer traffic increase would be the tourists.

Check back on Tilos to see how the total sum of person-days was calculated and do it yourself:

b Water demand

You can get your national average water consumption statistics from Eurostat. For resident islanders, reduce the number to 75 %. For seasonal residents, let it be 100 %. For visitors, assume 100 l/p, d. With the figures from the previous table, you can now calculate the total, theoretical water need of your island:

Hotels, restaurants and bars use a lot of water, of course, but they are accounted for in the above tables. But there may be other industries on the island: fishing and fish processing, small-scale carpentry, shipyard, garage, service…? You can get data from the Water Agency, indirectly, or from the businesses, directly. Agriculture is a large consumer of water. Farm water may include water used in crop irrigation or livestock watering.

If possible, make an estimate of how water demand changes with the seasons, such as in this diagram for Ithaka.

Diagram 2: Human pressure on Tilos peaks in summer when rainfall is at its lowest yearly point.



The question can be viewed from the producers’ perspective, simply by checking data from the water production unit(s). Again, there can be two points of view: what is pumped/processed, and what is billed. The difference between water being produced for the customers and water being billed to the customers will tell you about leakages in the system, which is one of the subjects of our next lecture. Water produced and water billed does not include water from private drills and rainwater collection. Both can be quite extensive. In the case of Ithaka, described in the next lecture, it is estimated at one third of the water use.

At the end of the day, you must use your common sense to interpolate the different perspectives to reach a fairly correct figure.