The amount of water on earth has been constant over centuries. It is in permanent circulation and regeneration and is therefore a renewable resource. However, most of the water on earth is salty (97%) and thus not suitable for the majority of uses. Only roughly 2.5% is freshwater in rivers, lakes, groundwater, fixed in soil or frozen in icecaps and a mere 0.5% of the total is easily accessible for human use (INFORESOURCES FOCUS 2006). This liquid water travels the earth constantly in the water cycle. Humans largely influence the water cycle today, be it quantitatively, by using large parts of the water available, or be it qualitatively, by changing the quality of water (e.g. pollution). On the earth, water never occurs in pure form – it always contains dissolved substances – be it minerals, nutrients, and also pollutants. The water cycle is thus inherently linked to the nutrient cycle.