Increased use of fertilizers was another result of the Green Revolution. Particulate amounts of most fertilizers enter the hydrologic cycle through run off. As a result, bodies of water become enriched in dissolved nutrients, such as nitrates and phosphates. The growth of aquatic plants in rivers and lakes is over stimulated, and this results in the depletion of dissolved oxygen. This process of eutrophication can harm all aquatic life in these ecosystems.
Water Depletion
With an increasing reliance on irrigation, groundwater resources are mismanaged and overtapped. The rate of groundwater recharge is slow, usually between 0.1 and 0.3 percent per year. When the amount of water pumped out of the ground exceeds the recharge rate, it is referred to as aquifer overdraft. An aquifer is a water-bearing stratum of permeable rock, sand, or gravel.
In Tamil Nadu, India, groundwater levels dropped 25 to 30 meters during the 1970's due to excessive pumping for irrigation. In Tianjin, China, the groundwater level declines 4.4 meters per year. In the United States, aquifer overdraft averages 25 percent over the replacement rate. The Ogallala aquifer under Kansas, Nebraska, and Texas represents an extreme example of overdraft: Depletion is 130 to 160 percent above the replacement rate annually. At this rate, this aquifer, which supplies water to countless communities and farms, has been projected to become nonproductive by 2030.
Soil Salinization
In addition, continued irrigation of arid regions can lead to soil problems. Soil salinization is widespread in the small-grained soils of these regions, which have a high water absorption capacity and a low infiltration rate. Some irrigation practices add large amounts of salts into the soil, increasing its natural rate of salinization. This can also occur at the base of a hill slope. Soil salinization has been recognized as a major process of land degradation.
Although surface and groundwater resources cannot be enriched by technology, conservation and improved environmental management can make the use of precious freshwater more efficient. In agriculture, for example, drip irrigation can reduce water use by nearly 50 percent. In developing countries, though, equipment and installation costs often limit the availability of these more efficient technologies.
Urban Sprawl
As more farms become mechanized, the need for farmers and farm workers is being drastically reduced. From a peak in 1935 of about 6.8 million farmers farming 1.1 billion acres, the United States at the end of the twentieth century counted fewer than 2 million farmers farming 950 million acres.
Urban sprawl converts a tremendous amount of cropland into parking lots, malls, industrial parks, and suburban neighborhoods. If cities were located in marginal areas, then concern about the loss of farmland to commercial development would be nominal. However, the cities attracting the greatest numbers of people have too often replaced the best cropland. Taking the best cropland out of primary production imposes a severe economic penalty.
See also: Pollution and Silt
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