6Socio-ecological transitions in the water system
Water covers more than 70 per cent of the Earth’s surface. However, only 2.5 per cent of the total volume is freshwater. The water system is in many regards special, as the natural and cultural functions of the resource are diverse and strongly influence each other. Human activity can affect water quantity and quality. Natural functions of water are life sustainment, habitat, and regulation like the energy balance, the hydrological cycle, and the matter balance (the self-purification of nature, solvent and transport medium, hydromorphology). The cultural functions of water are consumption and withdrawal for food and drinking, cleansing and agricultural as well as industrial production. Withdrawal of water reduces the quantity of the water resource and may also change water quality, as some original characteristics of the water can get lost. Likewise, water is used, but not withdrawn, for transportation, energy production, fisheries, aesthetics, and recreational or spiritual purposes. These types of use can also lead to pollution, and the habitat functions of the water can be impaired (cf. WBGU 1997, 45-48). Moreover, climatic changes can affect the hydrologic cycle (Richard G. Taylor et al. 2013), since the main water inflow happens by atmospheric deposition. Regional surface and groundwater resources are often interconnected.