The earth is called the blue planet – more than 70% of its surface is covered by water (PIDWIRNY 2006). Although water is the most widely occurring substance on earth, only ca. 2.5% is freshwater while the remainder is salt water. Some two thirds of this freshwater is locked up in glaciers and permanent snow cover (see graph below). In addition to the accessible freshwater in lakes, rivers and aquifers, man-made storage in reservoirs adds a further 8,000 cubic kilometres (km3). Water resources are renewable (except some ancient aquifers), with huge differences in availability in different parts of the world and wide variations in seasonal and annual precipitation in many places .