Asia is the world’s largest and most populous continent with a population in 2011 of 3,879,000,000 (3.8 billion). It covers 8.6% of the Earth’s total surface area (or 29.9% of its land area) and with approximately 4 billion people, it hosts 60% of the world’s current human population, But only about 36 percent of the renewable fresh water. So, even on a continental scale, it is clear that water is a serious constraint in Asia. China, India and Pakistan are three of the four top irrigators in the world, suffering serious water problems.
Pakistan has in most areas of agriculture a monsoon climate, and there might be abundant rainfall during the wet season and then a very long dry season where crop production depends very heavily on irrigation water. Groundwater is a very important source of irrigation for farmers. Ground water is being over-pumped extensively in order to meet current demands for food production but if our demands exceed that renewable supply, then we must be in the situation that we might be over-pumping groundwater to satisfy the demand, or taking too much water from the river basin systems. The result will be the formation of salinity and barren land that in the long run will cause food scarcity. Over-pumping of groundwater for agriculture, industry or domestic use comes at a sharp ecological price. It disrupts the natural hydrologic cycle,causes Rivers and wetlands to dry up, the ground to collapse and fish and wildlife and trees to die.
Water and agricultural sectors are likely to be the most sensitive to climate change. Fresh water availability is expected to be highly vulnerable to the anticipated climate change while the frequency and severity of floods would eventually increase in river deltas. The arid and semi-arid regions could experience severe water stress.
According to an estimate today 1.2 billion people lack safe drinking water and 2.4 billion lack access to basic sanitation. United Nations officials say that if we continue with business as usual, two-thirds of the world's population will be living in moderate to severe water stress by 2025. More than two billion people worldwide live in regions facing water scarcity and in Pakistan this is a particularly acute crisis. Millions of Pakistanis currently lack access to clean drinking water, and the situation is only getting worse. Pakistan has approximately 35 million acres (140,000 km2) of arable land irrigated by canals and tube wells, mostly using water from the Indus River.
Pakistan luckily had the largest irrigation system, but water losses from the system were the highest in the World, due to which its agricultural sector has been badly affected. At the same time the population of the country is increasing rapidly. Rapid industrialization is also playing an important role in the reduction of agricultural sector.
Chashma, Mangla, and Tarbela are mainly use for irrigation purposes but the gross capacity of these dams has decreased because of sedimentation, a continual process. Per-capita surface-water availability for irrigation was 5,260 cubic meters per year in 1951. This has been reduced to a mere 1,100 cubic meters per year in 2006. The water shortage will cause a wheat deficit of 12 million tons per year by 2012–13.