Throughout the world, serious water deficits are emerging. Irrigation demand for crops is one of the prime causes of heavy water demand worldwide. In the US 80% of the water used is for irrigation and worldwide it is 70%.
All living organisms require significant amounts of freshwater to sustain themselves. At present the 'total' amount of water made available by the world hydrologic cycle is sufficient to provide the current world population with adequate freshwater. Yet, world water supplies are concentrated in some areas, while others experience severe shortages.
Rainfall replenishes the water found in streams, rivers, lakes, and oceans and it is a vital part of the hydrologic cycle. When surface water is not managed effectively, water shortages and pollution result, both of which threaten humans and the aquatic biota that depend on freshwater.
Disputes concerning freshwater distribution are increasing between states and regions within states. The Colorado River, for example, is used so heavily by Colorado, California, Arizona, and other adjoining states, that by the time it reaches Mexico, it is usually no more than a trickle running into the Gulf of California.
In addition to rivers and lakes, rainfall is also stored in enormous underground aquifers. Their slow recharge rate from rainfall is usually only between 0.1% and 0.3% per year. At this slow recharge rate, groundwater resources must be carefully managed to prevent overuse and depletion, but frequently this is not the case. For example, in Tamil Nadu, India, groundwater levels declined 25-30 m during the 1970s, because the pumping of irrigation water was excessive. Similarly in Beijing, China, the groundwater level is falling at a rate of about 1 m yr~ ; while in Tianjin, China, it drops 4.4 m yr~ .
In the US, the great Ogallala aquifer located under Nebraska, Texas, and several other states is being pumped three times faster than its recharge rate. In general in the US, groundwater overdraft is high, averaging 25% greater than replacement rates. An extreme case is now occurring in Arizona, where several aquifers are being pumped ten times faster than the recharge rate of the aquifers.
Rapid population growth and associated increased total water consumption combine to rapidly deplete water resources. The present and future availability of adequate supplies of freshwater for human and agricultural needs is already critical in many world regions. This is especially critical in the Middle East and parts of North Africa where low rainfall is endemic.
All vegetation requires enormous quantities of water during the crop growing season. For example, an average US corn crop that produces about 9000 kg ha- of grain uses more than 6 million 1 ha-1 of water during its growing season. To supply this much water to the crop, approximately 1000 mm of rainfall per hectare must reach the plants. If irrigation is necessary, about 10 million l (more than 1 million gallons per acre) of irrigation water is required during the growing season.
Irrigation supports crop production in arid regions, provided there is an adequate source of freshwater, plus fossil energy to pump and apply the water. Currently, approximately 70% of the water removed from all sources worldwide is used solely for irrigated crop production. Of this amount, about two-thirds are consumed by growing plant life and is nonrecoverable. For example, an irrigated corn crop requires about 10 million lha-1 of water and uses about three times more energy to produce the same yield as rainfed corn.
The limitation of surface and groundwater resources for irrigation, and the high economic costs for irrigation, such as the large energy inputs, will tend to limit future agricultural irrigation. This will be especially true in developing nations, where economics cannot support major expenditures for water and energy.
A major threat to maintaining ample freshwater resources for all human needs is pollution. Although considerable water pollution has been documented in the US, this problem is of greatest concern in countries where water regulations are not rigorously enforced or do not exist. This is common in many developing countries that discharge approximately 95% of their untreated urban sewage directly into surface waters. For instance, of India's 3119 towns and cities, only 209 have partial sewage treatment facilities and a mere eight possess full wastewater treatment facilities. Downstream, the polluted water is used for drinking, bathing, and washing.
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