WATER-LOK: Land Forms
Some of the richest plant areas are along stream borders in desert valleys. Desert land apparently lacks only water to be productive. Now, irrigation projects transport water for many miles to produce large crops in places like the interior desert valleys of California, Arizona, Colorado, and Texas in the United States. The deserts of the Middle East are in the beginning phases of large-scale agricultural production in such places as Egypt and Israel. A host of innovative irrigation techniques, such as drip irrigation, have originated in these areas.
Land Forms of Desert Regions
Desert areas differ greatly in the appearance and nature of their surface features, which range from mountains to plateaus to plains. These diverse arid landscapes occupy about 11,500,000 square miles (29,785,000 square kilometers) of the continental masses of the Earth--about 20 percent of the surface area--and occur on all continents except Europe. Although sand dunes are spectacular features of deserts, they are not as common or widespread as generally believed. In the deserts of the southwestern United States, dunes occupy less than 1 percent of the surface. In the most sandy of all deserts, the Arabian, dunes occupy only about 30 percent of the total area. If sand accumulations on plains are extensive and appear as a "sand sea," they are called ergs.
The more common type of desert consists of rugged mountains separated by basins called bolsons. The mountains receive most of the rains in downpours. As the water rushes down the steep slopes it cuts deep gullies and carries rock fragments, gravel, and sand to the bolson. These materials are freed from the water when it slows, and they are deposited as alluvial fans or cones. The rugged forms produced in this way, such as the terrain in Death Valley, Calif., are termed badlands. Sometimes the flood waters make a temporary shallow lake in the basin. The temporary lakes that form in basins with no outlet are called playa lakes. There are two general playa types: clay pan or clay flat playas in valleys where the water table, or underground water, is relatively deep, and salt pan or saline playas where the water table is within 10 feet (3 meters) of the surface. Those portions of playas that contain water throughout the year or that are kept moist are called salinas. In narrow basins, alluvial fans and badlands may extend to the edge of the playa. In broad valleys there is a surface of low relief and gentle downward slope that occurs between the playa and other alluvial fans of the mountain front. Such a surface is called a desert flat, or llano.
Other deserts consist of rocky plateaus, called hammadas, separated by sand-filled basins, or ergs. Here differences in altitude are usually slight. Many hammadas are broad, flattish, dome-shaped areas. Where streams or wind wear away the weaker rocks, strong rock formations stand out boldly as mesas or cuestas. Pinnacles, needles, and arches carved in colored rocks lend fantastic beauty to the deserts of the American Southwest. Gullies are cut deep into the hammadas by the wearing force of the torrents. Gullies are known as wadis in Arabia and arroyos in the Southwest.
The Food and Water Supply
All of man's food comes from the earth. Very little comes from the sea. Almost all of it comes from farms on the continents. But man can use only a small portion of the continents for farming. About 7 percent of the Earth's land is considered arable, or suitable for farming. The rest is taken up by the swamps and jungles near the equator, the millions of square miles of desert, the rugged mountain ranges, and--mostly in the Far North--the frozen tundra.
Man has been searching for ways to produce more food to supply the demands of the Earth's continually increasing population. Many persons have suggested that the oceans might supply more food. They point out that the oceans cover more than 70 percent of the Earth's surface and absorb about 70 percent of the sunlight. Since sunlight is a basic requirement for agriculture, it seems reasonable that the oceans could supply a great deal of food. But what seems reasonable is not always so.
Almost all the plants that live in the oceans and absorb sunlight as they grow are algae. Algae do not make a very tasty dish for man, but they are an important part of the food pyramid of the oceans. In this pyramid the algae are eaten by small sea creatures. These, in turn, are eaten by larger and larger ones.
Man now enters the pyramid when he catches fish, but the fish he catches are near the top of the pyramid. All the steps between are very inefficient. It takes about a thousand pounds of algae to produce a pound of codfish, less than a day's supply of food for a man. To feed the growing population of the world, man must find an efficient way to farm the sea. He cannot depend simply on catching fish.
Much of the Earth's land area is unusable for agriculture because of the lack of adequate water. Millions of acres of land have been converted into farmland by damming rivers to obtain water for irrigation. Some scientists have estimated that if all the rivers of the world were used efficiently, the amount of land suitable for farming might increase by about 10 percent.
Another way to increase the water supply would be to convert ocean water into fresh water. Man has known how to do this for more than 2,000 years. But the process has been slow, and even with modern equipment it is costly. The distillation plant for the United States naval base at Guantanamo, Cuba, produces more than 2 million gallons of water a day, but at a cost of $1.25 for every thousand gallons. In New York City, where fresh water is available, the cost is about 20 cents per thousand gallons.
Scientists have investigated the use of nuclear-powered distillation plants. One plant would produce 150 million gallons of water daily at a cost of 35 to 40 cents per thousand gallons. It also would provide nearly 2 million kilowatts of electricity.