At least one billion people or nearly 20 percent of the world's population, lack safe drinking water. The W.H.O. considers 53,000 gallons of good water per year to be the minimum for a healthful life. Some forty countries (including island nations, Middle East countries) in the world fall below this level.
1. Natural forces
Deficits are caused by natural forces such as poor rain fall and hot winds, rivers changing courses.
2. Human causes
Include increased population, rapid urbanization, over grazing by cattle, improper cultivation methods, poor sewage systems, inadequate finances for providing infra structures.
3. Depleting ground water
Ground water is the source of nearly 40 % water for agricultural and domestic use in most of the countries. Nearly 95% of rural population depends on groundwater for drinking and other domestic purposes. Over
use of the supplies causes several kinds of problems, including drying of wells, natural springs and disappearance of surface water features such as wetlands, rivers and lakes.
In many parts of the world, groundwater is being withdrawn from aquifers faster than natural recharge can replace it. On a local level this causes a level of depression in the water table. A heavily pumped well can lower the ground water table so that nearby shallower wells go dry. On a broader scale, heavy pumping can deplete a whole aquifer. Many aquifers have slow recharge rates, so it will take thousands of years to refill them once they were emptied.
4. Salt water intrusion
Many parts of the world are losing freshwater sources due to saltwater intrusion. Over use of under ground freshwater reservoirs often allows salt water to intrude into aquifers and affect the water table.
5. Loss of free flowing rivers
Loss of free flowing rivers that are either drowned by reservoir impoundments or turned into linear, sterile irrigation channels is yet another cause for freshwater crisis.
6. Evaporations, leakage and siltation
It happens in freshwater lakes, ponds and dams.