Three- fourth surface of earth (about $71\; \%$ of total) is occupied by ocean, which contains $97.5 \;\%$ of total water: This is marine water with about $3.5 \;\%$ salt contents. Rest water; i.e, $2.5\; \%$ is fresh water, which occurs on land. Most amount of this water (about $1.97\; \%$, i.e, more than $70\; \%$ of world's total freshwater) occurs as frozen polar ice caps and glaciers and $0.5 \;\%$ freshwater occurs as source water: