环境科学专业英语---Unit4---

1、erosionerosion is an important natural process,resulting in the redistributes of the products of geologic weathering, and is part of both soil formation and soil loss. the world's landscapes have sculpted by erosion. when the results are spectacular enough, we enshrine them in national parks as

2、we did with the grand cany on. where erosion has worn down mountains and spread soil over the plains, or deposited rich alluvial silt on river beds, we gladly farm it.erosion is a disaster only when it occurs in the wrong place at the wrong time.in some places, erosion occurs so rapidly that anyone

3、can see it happen. deep gullies are created where water scours away the soil, leaving fence posts and trees sitting on tall pedestals as the land erodes away around them in most places, however, erosion is subtler. it is a creeping disaster that occurs in small increments. a thin layer of top soil i

4、s washed off fields year after year until eventually nothing is left but poor-quality subsoil that requires more and more fertilizer and water to produce any crops at all.the net effect, worldwide, of this general, wide spread top soil erosion is a reduction in crop production equivalent to removing

5、 about 1% of world cropland each yea匚 many farmers are able to compensate for this loss by applying more fertilizer and by bringing new land into cultivation. continuation of current erosion rates, however, could reduce agricultural production 25% in central america and africa and 20% in south ameri

6、ca by the year 2000. the total annual soil loss from croplands is thought to be 25 billion metric tons. about twice that much soil is lost from rangelands, forests, and urban construction sites each yea r.in addition to reduced land fertility, this erosion results in sediment-loading of rivers and l

7、akes, of reservoirs, smothering of wetlands and coral reefs, and clogging of water intakes and waterpower turbines. it makes rivers unnavigable, increases the destructiveness and frequency of floods, and causes gullies that turn good land into useless wasteland.wind and water are the main agents tha

8、t move soil around thin, uniform layers of soil are peeled off the land surface in a process called sheet erosion. when little rivulets of running water gather together and cut small channels in the soil, the process is called rill erosion. when rills enlarge to form bigger channels or ravines that

9、too large to be removed by normal tillage operations, we call the process gully erosion. stream bank erosion refers to the washing away of soil from the banks of established streams, creeks, or rivers, often as a result of re moving trees and brush along stream banks and by cattle damage to the bank

10、smost soil erosion on agricultural land is sheet and rill erosion. large amounts of soil can be transported by these mechanisms without being very noticeable a farm field can lose 20 metric tons of soil per hectare during winter and spring for runoff in rills so small that they are erased by the fir

11、st spring cultivation, that represent a loss of only a few millimeters of soil over the whole surface to the field, hardly apparent to any but most discerning eye. but it doesn't take much mathematical skill to see that if you lose soil twice as fast as it is being replaced, eventually it will r

12、un out.wind can equal or exceed water in erosive force, especially in a dry climate and on relatively flat land when plant cover and surface litter are removed from the land by agriculture or grazing, wind lifts loose soil particles and sweeps them away. windborne dust is sometimes transported from

13、one continent to another. scientists in hawaii can tell when spring plowing begins in china because dust from chinese farmland is carried by winds all the way across the pacific ocean. similarly, summer dust storms in the sahara desert of north africa create a hazy atmosphere over islands in the car

14、ibbean sea, 5,000 km away. it has been estimated that winds blowing over the mississippi river have one thousand times the soil-carrying capacity of the river itself.although data on soil condition and soil erosion in the world are not complete and difficult to access, it is evident that many places

15、 have very severe problems. china, for example, has a large area of loess (windblown silt) deposits on the north china plain that once was covered by forest and grassland. the forests were cut down and the grasslands were converted to cropland. this plateau is now scarred by gullies 30-40 m deep, an

16、d the soil loss is thought to be at least 480 metric tons per hectare per year. this would be equivalent to 3 cm of topsoil per year.one way to estimate soil loss is to measure the sediment load carried by rivers draining an area. the highest concentration of sediment in any river is in the huanghe

17、river (i.e. yellow river) that originates in the loess plateau of china. although its drainage basin is only one-fifth as big as that of the mississippi river, the huanghe river carries more than four times as much soil each year. this suggests that the average soil loss per hectare in china may be

18、twenty times that in the united states.in its middle reaches, the huanghe river carries about 700 kg of silt per cubic meter of water, or 50% by weight just under the level classified as liquid mud. as the river winds through northern china, much of this sediment settles out, raising the river botto

19、m above the level of the surrounding countryside only by building dikes to contain the river have the chinese been able to keep it in its course .in some places, the riverbed is now 10 m above the farmland through which it flows if the dikes give way during the summer rice season, millions of people

20、 would starve to death as a result of lost crops.next after the huanghe river in annual sediment load is the ganges river, which carries 1455 million metric tons of mud to the bay of bengal every yea匚 much of this sediment comes from the hill country of northern india, nepal, and bangladesh populati

21、on pressures and preemption of good bottomlands for cash crop production have forced farmers to try to grow crops on steep, unstable slopes fuel wood shortages also cause local people to cut down the forests that stabilize mountain soils. when the monsoon rains come, they wash whole hillsides away, destroying