If present trend continue, by 2025, 2/3 of the world's population will live in water stressed areas.- U.N.E.P Global 2000 Report
The situation is even worse with ground water supplies. The Ogallala Aquifer in the central of US which waters 1/5 of the irrigated land in the U.S., is being mined at a rate of 12 billion cubic meters per year, a quantity equivalent to 1/6 of the annual flow of the Colorado River. As the water table falls, the cost of deepening wells and pumping becomes prohibitive, and land is taken out of production. Since the peak in 1978, the amount of land irrigated by the aquifer has fallen by 20% with a predicted drop of another 20% by 2020. India and China face similar shortages. The water table in the northern plains of China where 40% of the country's grain is produced, is dropping at between 1-1.5 meters per year. Many of the world's aquifers will recharge and can be utilized at a sustainable level, but countries in North Africa and the Middle East, Saudi Arabia and Libya most notably, are dependent on fossil aquifers, water left from ancient times when climates were wetter. These aquifers have no water source from which to recharge. Both countries have tried experiments with growing grain on the desert, but once the government subsidies ended, the economics of pumping 3000 tons of water to get one ton of grain quickly shut down the farming activities.
With water deficits estimated to be 200 billion cubic meters per year globally, it is clear that either a significant amount of land will come out of production, or will be switched from grain crops to less water thirsty crops. At a normal ratio of 1000 tons of water to one ton of grain this implies that up to 10% of the world's grain harvest is being produced with deficit water. So not only do we need to find a source for water to increase the production of food for the growing population, but we need to replace the water that is already being consumed at an unsustainable rate.
As the the process of urbanization continues world wide, the demand for water for cities and manufacturing will continue to grow. Most countries cannot escape the economic incentives to use water for manufacturing, and divert it from farming. In this country San Diago recently purchased 247 million cubic meters of water from the Imperial Irrigation District for between $.20- $.27 per cubic meter, roughly 25 times what it could get selling the water to farmers. Similar swaps are occurring in other countries which may make economic sense in the short run, but in the long term will have an effect on food supply.
Another aspect of industrial demand that takes water away from agricultural use is pollution. Some rivers in China are so polluted that is is illegal to use the water even for irrigation. In other areas the build of of chemicals in the soil from polluted irrigation water could have long term health consequences.
In the developed countries there is an increased awareness of the damage done by dams to the habitat for fish and wildlife. Some investigators list up to 1/3 of fresh water fish species as in danger of extinction. Many dams are now being managed to allow more water to stay in the rivers to protect freshwater species. Of the 68 species of fish listed as threatened or endangered in the western states, 50 of them are threatened in part by agriculture and dams. Where such measures are being taken, the amount of water available to farmers is being reduced by 10-25%. In other areas, the Aural Sea in Russia, for instance, there is an awareness of the need to restore water to the rivers, but the demand for food production prevents any meaningful level of restoration. Another movement, to remove dams from rivers in order to restore fish runs, will also have a cost in agricultural production. In cases where that has been done, the benefits of preserving the Salmon and other anadromous fish have outweighed the cost to farmers, but again, there is a reduction in food production.
Over the years of irrigation there is a tendency for the salts in the water to build up in the soil. In addition, the water table tends to rise closer to the surface so there is inadequate drainage to carry the salts past the root zone of the plants. The result is a gradual decrease in output until only highly salt tolerant plants will grow. Salinization has been listed as a major if not the primary cause of the collapse of many civilizations in the past, in particular the Sumarians, Babylonians, and Assyrians. According to Sandra Postel: "Salinization could be a time bomb waiting to explode upon the agricultural scene, or it could remain a quiet but steadily increasing suppressor of crop yields. Either way, salt remains one of the greatest threats to irrigated agriculture and food security in a world that will be striving to feed 8-9 billion people within 50 years."
The diagram to the left shows some of the demands on the water supply and the interrelationships of these demands.
Click on diagram to enlarge and activate links.