World Water Resources and Achieving Water Security

Water is the elixir of life and a principal indicator of sustainable development. It is important to understand the magnitude of water resources, the hydrological cycle, and the impact of land use and management. Large area of Earth's surface is covered by water, but renewable fresh water resources are finite (similar to 2.5% of the total), unequally distributed geographically, and prone to eutrophication and pollution. Conceptually, water resources comprise of four categories: (i) blue water is the fresh surface and ground water (e.g., water in lakes, rivers, and aquifers), (ii) green water is the precipitation on land that is stored in the soil for plant use, (iii) gray water is contaminated by human use, and (iv) virtual water is embedded in agricultural and industrial produce. Water scarcity, when demand exceeds supply, occurs wherever the per capita availability of renewable freshwater is <1700 m(3)/yr. In contrast, water stress occurs when the per capita availability of renewable freshwater is <1000 m(3)/yr. Therefore, water security exists when all people at all times have physical and economic access to sufficient, safe, and clean water that meets their basic needs for an active and healthy life. Water footprint, the amount of water required to produce goods and services for human consumption, is increasing with increase in world population and its growing affluence. Th us, sustainable management of water involves technologies to increase the green water storage in soil, purify the gray water, reduce export of virtual water, increase water use efficiency, and desalinize brackish water.