Water is the most widespread substance to be found in the natural environment. Water exists in three states: liquid, solid, and invisible vapour. It forms the oceans, seas, lakes, rivers and the underground waters found in the top layers of the Earth's crust and soil cover. In a solid state, it exists as ice and snow cover in polar and alpine regions. A certain amount of water is contained in the air as water vapour, water droplets and ice crystals, as well as in the biosphere. Huge amounts of water are bound up in the composition of the different minerals of the Earth's crust and core. To assess the total water storage on the Earth reliably is a complicated problem because water is so very dynamic. It is in permanent motion, constantly changing from liquid to solid or gaseous phase, and back again. It is usual to estimate the quantity of water found in the so-called hydrosphere. This is all the free water existing in liquid, solid or gaseous state in the atmosphere, on the Earth's surface and in the crust down to a depth of 2000 metres. Current estimates are that the Earth's hydrosphere contains a huge amount of water - about 1386 million cubic kilometres. However, 97.5% of this amount are saline waters and only 2.5% is fresh water. The greater portion of this fresh water (68.7%) is in the form of ice and permanent snow cover in the Antarctic, the Arctic, and in the mountainous regions. Next, 29.9% exists as fresh groundwaters. Only 0.26% of the total amount of fresh waters on the Earth are concentrated in lakes, reservoirs and river systems where they are most easily accessible for our economic needs and absolutely vital for water ecosystems.
These are the values for natural, static, water storage in the hydrosphere. It is the amount of water contained simultaneously, on average, over a long period of time - in water bodies, aquifers, and the atmosphere. For shorter time intervals such as a single year, a couple of seasons, or a few months, the volume of water stored in the hydrosphere will vary as water exchanges take place between the oceans, land and the atmosphere. This exchange is usually called the turnover of water on the Earth, or the global hydrological cycle, as shown in the diagram below.
