HYDROSPHERE

This includes water in liquid and frozen forms in groundwater, glaciers, oceans, lakes and streams. Saltwater accounts for 97.5% of this amount. Fresh water accounts for only 2.5%. Of this fresh water 68.7% is in the "form of ice and permanent snow cover in the Arctic, the Antarctic, and in the mountainous regions. Next, 29.9% exists as freshgroundwaters. Only 0.26% of the total amount of fresh waters on planet Earth is easily accessible. It is found in lakes, reservoirs and river systems, the principal elements of water ecosystems."The total mass of the Earth's hydrosphere is about 1.4 × 1018 tonnes, which is about 0.023% of the Earth's total mass. About 20 × 1012 tonnes of this is in the Earth's atmosphere (for practical purposes, 1 cubic metre of water weighs one tonne). Approximately 75% of the Earth's surface, an area of some 361 million square kilometers (139.5 million square miles), is covered by ocean. The average salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5%).