Biofuels may put unsustainable demands on water use

As the world moves towards renewable energy, it faces a challenge: water scarcity. The intensive water use in the coal, oil, gas and nuclear industries is well documented, but if we want to encourage a faster transition to renewables we must also contemplate the water use of the alternatives.
It is a great challenge to limit the drain on land and water resources now the transition has taken off. Bioenergy, hydropower, and wind, solar and geothermal energy all require substantial amounts of land and water resources. Given limitations to the availability of land and water, what energy scenarios are feasible in the long run?
With fossil fuels we have learned to worry about energy scarcity as a major concern for economic development and national security. In contrast, renewable energy seems inexhaustible: incoming solar radiation, for example, is far beyond what we need. The fact that renewable energy is available into infinity reinforces this idea of limitlessness. This, however, is a misunderstanding: we will replace energy scarcity by land and water scarcity.
Bioenergy production in particular requires vast amounts of land and water. Besides, with current energy-intensive agricultural practices, net energy output is far lower than gross energy production, sometimes even near zero. If only 10% of fossil fuels in the global transport sector were replaced by bioethanol from relatively efficient crops, global water demand would increase by 6-7%.
Hydropower, accounting for 16% of the world’s electricity supply, is regarded as a clean form of energy. However, we cannot simply increase hydroelectric capacity. Dams can heavily impact on ecosystems and societies, and any further damming of rivers should be subject to careful consideration.
Solar, wind and geothermal energy
Per unit of energy, the water footprint of bioenergy and hydroelectricity is two to three orders of magnitude larger than that of fossil fuels and nuclear. The water footprint of photovoltaic (PV) and wind energy is one to two orders of magnitude smaller.
Electricity from concentrated solar power has a similar water footprint to fossil fuels, while geothermal can be an order of magnitude smaller or even less. From a water consumption and scarcity perspective, it matters greatly whether we shift from fossil energy to bio and hydro or to solar, wind and geothermal energy.