What Of the total amount of available water on earth what prevent is useable?

Only about 2.5% of the total water on Earth is freshwater, and of that, roughly 70% is locked in glaciers and ice caps. This leaves approximately 1% of the world's water, or about 0.07% of the total, readily accessible for human use in rivers, lakes, and underground aquifers. Consequently, the vast majority of Earth's water is either saline or unavailable for direct consumption and use.