**5. Future Needs**

We describe an intricate relationship between water availability, food production, increase in the world population, contagious diseases.

In principle a human being needs only 2 L of water per day for drinking. Hunter-gatherers needed very little extra water but today the situation is drastically changed and human beings need enormous quantities of water: in the year 2000, according to the Environmental Outlook published by OECD [63], 2384 km<sup>3</sup> of water were consumed for irrigation; 504 km<sup>3</sup> were consumed for industrial purposes and 348 km<sup>3</sup> for domestic purposes. Note that water for irrigation is needed in specific places (where farms are present) and at specific times (when plant growth and ripening are occurring) [51], thus making this requirement more stringent. Moreover, as described in this Review, the use of domestic water is important to prevent the spreading of contagious diseases. It is worth noting that the human population increased because of agriculture production and developed where water was abundant; but today much water per person is needed to support people in areas where water is scarce. Globally, much more water will be needed in the future and the above quoted OECD document predicts that the total amount of water needed on the globe, 3236 km<sup>3</sup> in the year 2000, will increase to 5420 km<sup>3</sup> in the year 2050.

Water availability is not sufficient today [64]: in the poorest areas of the planet more than 2 billion people use on average 10 litres of water per day per person, thus generating migrations and wars. Moreover, it would be a common interest of poor and rich countries to provide enough water for sanitation to avoid the occurrence and spreading of infectious diseases [65].

The population of the planet increased in the last centuries mainly because of a decrease in mortality [49] and therefore it is unlikely that the present trend of population increase will change in the immediate future. Thus, the freshwater requirement will increase dramatically, not only to give water in an equitable way to everybody on the planet but also because demographic studies (see Table 1 in Ref. [26]) indicate that the total amount of people on the globe will increase to about 9800 million by the year 2050: therefore, we should be prepared to give enough water to an increased population [66,67]. Moreover, the density of people in certain areas will increase, thus increasing the danger of disease contagion; as a consequence, the use of water for sanitation will become more important and more attention should be given to the prevention of some diseases (for example through vaccination or better ecological management).

The water volume of rivers is about 2100 km<sup>3</sup> [3]; keeping in mind that part of this water is used to feed lakes and aquifers, we should begin to think in terms of total amount of rainfall water per year and compare the human needs to the figures assigned to the different components of the water repositories.

The provision of water will have a grea<sup>t</sup> social, financial and political relevance; all social sectors will have to be involved and many habits will have to change.

**Acknowledgments:** The author warmly thanks Anna Maria Aliperti for manuscript editing.

**Conflicts of Interest:** The author declares no conflict of interest.
