For nearly all human purposes, we need freshwater as it occurs on land. Salt water as it occurs in the ocean is not useful for drinking, washing, cooking, field irrigation or for most applications in industry. Salt water can be desalinized, but this is a costly and energy-intensive process, feasible for a limited number of applications only. Besides, salt water is available at the coast, while much of the water needs are inland, so that transport uphill becomes an issue as well. In short, humans mainly depend on freshwater as it occurs on land. Although water forms a cycle, so that freshwater on land is continuously replenished, its

availability is not unlimited. Per year, people need a certain volume of water for domestic, agricultural and industrial purposes, which cannot exceed the annual replenishment rate. The major question is therefore: how much freshwater is available over a certain period and what is man’s actual appropriation of this flow in this period? Water footprint accounting provides the data for answering the second half of the question. The water footprint basically expresses human appropriation of freshwater in volume terms. Comparing man’s water footprint with the actual freshwater availability is part of a water footprint sustainability assessment, which is the subject of Chapter 4.