Water resources are sources of water that are useful or potentially useful to humans. Activities that humans may use water for include agricultural, industrial, household, recreational, and environmental activities. Virtually all of these uses require fresh water. However, 97.5% of water on the Earth is salt water, leaving only 2.5% as fresh water. Over two thirds of this fresh water is frozen in glaciers and polar ice caps. The remaining unfrozen freshwater is mainly found as groundwater, with only a small fraction found above ground or in the air. Fresh water is a renewable resource, yet the world’s supply of clean, fresh water is steadily decreasing.