一些英文水土保持资料

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1、What is soil erosion?Soil is naturally removed by the action of water or wind: such background (or geological) soil erosion has been occurring for some 450 million years, since the first land plants formed the first soil. Even before this, natural processes moved loose rock, or regolith, off the Ear

2、ths surface, just as has happened on the planet Mars.In general, background erosion removes soil at roughly the same rate as soil is formed. But accelerated soil erosion loss of soil at a much faster rate than it is formed is a far more recent problem. It is always a result of mankinds unwise action

3、s, such as overgrazing or unsuitable cultivation practices. These leave the land unprotected and vulnerable. Then, during times of erosive rainfall or windstorms, soil may be detached, transported, and (possibly travelling a long distance) deposited.Accelerated soil erosion by water or wind may affe

4、ct both agricultural areas and the natural environment, and is one of the most widespread of todays environmental problems. It has impacts which are both on-site (at the place where the soil is detached) and off-site (wherever the eroded soil ends up).More recently still, the use of powerful agricul

5、tural implements has, in some parts of the world, led to damaging amounts of soil moving downslope merely under the action of gravity: this is so-called tillage erosion.Soil erosion is just one form of soil degradation. Other kinds of soil degradation include salinisation, nutrient loss, and compact

6、ion.Erosion processesSoil may be detached and moved by water, wind or tillage. These three however differ greatly in terms of: where and when they occur what happens to the area that is being eroded (on-site impacts) how far the eroded soil is moved, and if the soil is moved away from the place wher

7、e it was eroded, what happens as a result (off-site impacts).Water erosionSoil erosion by water is the result of rain detaching and transporting vulnerable soil, either directly by means of rainsplash or indirectly by rill and gully erosion.Water and soil splashed following a single raindrqprompiaWE

8、PP95 CD-ROM)RainsplashRain may move soil directly: this is known as rainsplash erosion (or just splash erosion). Spash is only effective if the rain falls with sufficient intensity. If it does, then as the raindrops hit bare soil, their kinetic energy is able to detach and move soil particles a shor

9、t distance.Because soil particles can only be moved a few centimetres at most by this process, its effects are solely on-site. Although considerable quantities of soil may be moved by rainsplash, it is all merely redistributed back over the surface of the soil (on steep slopes, however, there will b

10、e a modest net downslope movement of splashed soil). Thus a more descriptive term might be rainsplash redistribution.Because rainsplash requires high rainfall intensities, it is most effective under convective rainstorms in the worlds equatorial regions. Rainsplash is relatively ineffective where ra

11、in falls with a low intensity (e.g. because the rainfall is of frontal origin), such as in the north-west of the USA or in northern Europe.Rill and gully erosionRainfall may also move soil indirectly, by means of runoff in rills (small channels) or gullies (larger channels, too big to be removed by

12、tillage). In many parts of the world, rill and gully erosion is the dominant form of water erosion.That fraction of the rainfall which does not infiltrate (soak into) the soil will flow downhill under the action of gravity; it is then known as runoff or overland flow. Runoff may occur for two reason

13、s. Firstly, if rain arrives too quickly (i.e. with too high an intensity) for it to infiltrate: the runoff which results is then known as infiltration excess runoff, or Hortonian runoff. Secondly, runoff may occur if the soil has already absorbed all the water it can hold (i.e. because it is fully s

14、aturated, or if the soil is frozen). as saturation excess runoff.Diffuse overland flow. Note the raindrop impacts(Photo:)Runoff which results from this situation is knownAs runoff moves downhill, it is at first a thin diffuse film of water which has lost virtually all the kinetic energy which it pos

15、sessed as falling rain. Thus it moves only slowly, has a low flow power, and is generally incapable of detaching or transporting soil particles.The microtopography (i.e. small-scale pattern of irregularities) of the soils surface tends to cause this overland flow to concentrate in closed depressions

16、, which slowly fill: this is known as detention storage or ponding. Both the flowing water, and the water in detention storage, protect the soil from raindrop impact, so that rainsplash redistribution usually decreases over time within a storm, as the depth of surface water increases. There are, how

17、ever, complex interactions between rainsplash and overland flow.Detachment and transport processes associated with variations in raindrop and flow energies. et = critical raindrop energy to cause erosion. Line A ec prior to flow (increasing through crust development). Line B= ec when flow occurs (in

18、creasing drop energy used to penetrate flow).-critical stream power for transporting loose material. critical stream power for detaching soil from surface of soil matrix. RD - ST = raindrop detachment,splashtransport. RD RIFT = raindropdetachment,raindrop induced flow transport. RD FT raindrop detac

19、hment flow transport. FD FT = flow detachment, flow transportA diagram illustrating the complex interactions between rainsplash, anddetachment and transport by overland flow. Click for a larger version. (Source:Peter Kinnell, University of Canberra, Australia)If rain continues, the increasing depth

20、of water will eventually overtop the microtopogr aphic depressions. Overland flow that is released in this way is likely to flow downhill more quickly and in greater quantities (i.e. possessmore flow power as a result of its kinetic energy), and so may be able to begin transporting and even detachin

21、g soil particles. Where it does so, the soils surface will be lowered slightly. Lowered areas form preferential flow paths for subsequent flow, and these flow paths are in turn eroded further. Eventually, this positive feedback results in small, well-defined linear concentrations of overland flow (m

22、icrorills or traces).In many cases, individual microrills become ineffective over time due to sedimentation. A subset, however, grow further to become rills; and a smaller subset may go on to develop into gullies. This process of competition between microrills and rills leads to the self-organized f

23、ormation of networks of erosional channels (dendritic on natural soil surfaces; constrained by the direction of tillage on agricultural soils), which form efficient pathways for the removal of water from hillslopes. It is in such erosional channels that water erosion also operates most effectively t

24、o detach and remove soil by its kinetic energy. In most situations erosion by concentrated flow is the main agent of erosion by water.The flow-dominated erosional channels are separated by interrill areas where the dominant processes are rainsplash and diffuse overland flow; however, boundaries betw

25、een rill and interrill areas are both ill-defined and constantly shifting.Large rills (possibly big enough to be called gullies?) on an eroding hillslope. (Source of photo: unknown)In some circumstances subsurface flow may be important in determining where channel erosion will begin and develop (e.g

26、. at the base of slopes, and in areas of very deep soils such as tropical saprolites). Meltwater from thawing snow operates in a broadly similar way to rain-derived overland flow, detaching and transporting unfrozen soil in areas of concentrated flow. Snowmelt erosion is, though, less well studied a

27、nd less well understood.As erosional channels increase in size (i.e. grow to become large rills and gullies), processes such as gravitational collapse of channel walls and heads increase in importance. Runoff and sediment from rills and gullies may be moved into ditches, stream and rivers, and so tr

28、ansported well away from the point of origin. However, sediment may also be deposited within the rill or gully, or beyond the rill or gullys confines in a depositional fan, at locations where the gradient slackens. Here it may be stored for a variable period of time, possibly being reworked by tilla

29、ge activity, until a subsequent erosion event is of sufficient size to re-erode the stored sediment. It may then be redeposited further downstream, or make its way into a permanent watercourse and thence to lake or ocean.Wind erosionTillage erosionSoil erosion in the pastErosion of soil by water and

30、 wind has been occurring naturally since the first land plants formed the first soil, during the Silurian Period. Accelerated erosion is, from a geological perspective, of very recent origin; yet on a human timescale,accelerated erosion is old. There is considerable archaeological evidence from many

31、 parts of the world that accelerated erosion by water (in particular) is often associated with early agriculture.In a scientific context, water erosions association with unwise agricultural practiceswas first noted within during the early decades of the 20th century by pioneers of soil conservation

32、such as Hugh Hammond Bennett in the USA, and subsequently by workers in other parts of the globe.During the period of colonialism, the imposed adoption of European agricultural methods frequently led to accelerated erosion in developing countries. There, the problem often continues to the present da

33、y.A gully in Indiana, USA, in the 1(Si0frce of photo: unknown)In the last few decades of the 20th century, there was a worlwide move towards intensive agricultural technologies. These frequently leave the soil bare during times of heavy rainfall. As a result, previously problem-free areas of the wor

34、ld, such as north-west Europe, began to experience notable increases in water erosion.The extent of soil erosionDespite the global nature of the problem of erosion by water, even today we do not have good information regarding the global extent of erosion by water. Data on the severity of erosion is

35、 also often limited.The GLASOD study estimated that around 15 per cent of the Earths ice-free land surface is afflicted by all forms of land degradation. Of this, accelerated soil erosion by water is responsible for about 56 per cent and wind erosion is responsible for about 28 per cent.This means t

36、hat the area affected by water erosion is, very roughly, around 11 million square km., and the area affected by wind erosion is around 5.5 million square km.The area affected by tillage erosion is currently unknown.Because soil is formed slowly, it is essentially a finite resource. The severity of t

37、he global erosion problem is only now becoming widely appreciated.Soil degradationfery d&graded snilDegraded soilStable soilWithout veoetaiionThe GLASOD estimate of global land degradation: note that this includes all forms of soil degradation, not just erosion. (From UNEP-GRID)Erosion little and la

38、rgeOne of the things which makes soil erosion difficult to understand and so to predict and control is that it is affected by both common and rare events, and so must be studied over both short and long timespans. Erosion is also affected by factors on very small and very large spatial scales, and h

39、as its impacts over a similarly wide range of spatial scales.The timescales of erosionSoil erosion occurs both incrementally, as a result of many small rainfall or wind-blow events, and more dramatically, as a result of large but relatively rare storms. It is the large storms which produce the big h

40、ard-to-miss erosional featues such as deep gullies. But while erosion due to small common events may appear insignificant on the field, its cumulative impact (both on the eroding field, and elsewhere) may, over a long timescale, be severe.The spatial scales of erosionWater erosions complex hierarchy

41、 of processes mean that erosion by water operates (and is studied) over a wide range of spatial scales. Rainsplash redistribution and the initiation of microrills and rills occur at a scale of millimeters. Rill erosion on agricultural hillslopes operates at a scale of meters to tens of meters, while

42、 gully erosion can occur on a scale of hundreds of meters, or even kilometers. The offsite impacts of erosion can affect very large areas, sometimes hundreds or even thousands of square kilometers.At every spatial scale, however, erosion is highly patchy. Even in areas of severe erosion, rates of so

43、il loss can vary greatly from point to point on the landscape as the vagaries of topography and land use concentrate erosive flows on a wide range of spatial scales. Obvious erosion in one field can be found side-by-side with virtually untouched areas; and within an eroded field, the severity of ero

44、sion can vary markedly.On-site effects of soil erosionThe main on-site impact is the reduction in soil quality which results from the loss of the nutrient-rich upper layers of the soil, and the reduced water-holding capacity of many eroded soils. In affluent areas of the world, accelerated water ero

45、sions on-site effects upon agricultural soils can be mitigated by increased use of artificial fertilizers; however this is not an option for much of the earths population.Erosions removal of the upper horizons of the soil results in a reduction in soil quality i.e. a diminution of the soils suitabil

46、ity for agriculture or other vegetation. This is because the eroded upper horizons tend to be the most nutrient-rich. Also, because the finest constituents of eroded soil tends to be transported furthest, eroded soils become preferentially depleted of their finer fraction over time; this often reduc

47、es their water- holding capacity. In other words, “Erosion removes the cream of the soil”. Increased use of artificial fertilizers may to an extent, and for a time, compensate for erosion-induced loss of soil quality where economic circumstances are favorable. This is not usually feasible in develop

48、ing countries however. Loss of soil quality is a long-term problem; globally, soil erosions most serious impact may well be its threat to the long-term sustainability of agricultural productivity, which results from the the on-site damage which it causes.The on-site impact of erosion: severe rilling

49、 on a hillslope at Rottingdean on the UK South Downs in 1987. Photo: John BoardmanCrops are particularly reliant on the upper horizons of the soil, which are the most vulnerable to erosion by water and wind. In this sense, erosion removes the cream of the soil. Agricultural tillage also redistribute

50、s soil, resulting in thinner soils on topographically convex areas within a field.The damaging on-site effects of erosion, in terms of decreased agricultural yields, are well known in the developing countries ofAfrica and Asia. But even in the developed world there is cause for concern. Water erosio

51、n is locally severe in Australia, New Zealand, parts of the US, Southern Europe, and Eastern Europe (often as a result of the former large state-controlled farms).In erosion-prone areas of the more affluent countries, productivity may be maintained in the short to medium term by increased fertiliser

52、 input. The effects of erosion are thus rarely acknowledged by farmers in richer countries. This strategy is however infeasible with regard to erosion in developing countries.Off-site effects of soil erosionIn addition to its on-site effects, the soil that is detached by accelerated water or wind er

53、osion may be transported considerable distances. This gives rise to off-site problems.Water erosions main off-site effect is the movement of sediment and agricultural pollutants into watercourses. This can lead to the silting-up of dams, disruption of the ecosystems of lakes, and contamination of dr

54、inking water. In some cases, increased downstream flooding may also occur due to the reduced capacity of eroded soil to absorb water.Chinas Yangtze River at theThree Gorges, in Hubei province. Note the sediment-rich water. (Source of photo: unknown)A satellite view of the delta of the Yangtze River

55、as itMovement of sediment and associated agricultural pollutants into watercourses is the major off-site impact resulting from erosion. This leads to sedimentation in watercourses and dams, disruption of the ecosystems of lakes, and contamination of drinking water. Rates of erosion do not have to be

56、 high for significant quantities of agricultural pollutants to be transported off-site. This is a shorter-term impact than loss of soil quality; in the more affluent areas of the world it can be the main driver for present-day soil conservation policy initiatives. A more minor off-site effect can oc

57、cur in situations where eroded soil has a decreased capacity to absorb water: increased runoff may lead to downstream flooding and local damage to property.Another major off-site discharges into the East China Sea. The sediment plume is impact results from the clearly visible. (Image: NOAA)agricultu

58、ral chemicals thatoften move with eroded sediment. These chemicals move into, and pollute, downstream watercourses and water bodies.Where inputs of agricultural chemicals are high - as in the more affluent nations - costs of removing such pollutants from drinking water can be considerable.Therefore

59、the on-site impacts of soil erosion are a present-day problem for many of the developing nations. Such on-site impacts will be a problem only in the long term future for developed areas: as such they are outside the relatively short time horizon within which their policy makers work.In the short ter

60、m however, erosions off-site effects can be a notable problem for developed nations. Off-site impacts may therefore be the major driver for policy changes in such countries.Soil erosion: what we still dont knowGreater understanding of the occurrence, processes and impacts of soil erosion by water, w

61、ind and tillage is needed. Why? Both directly, in order to enhance mankinds ability to tackle the resulting environmental problems; and indirectly, in order to learn more about the processes of erosion and the conditions under which it occurs. Soil erosion in the future Erosion on other planetsSoil

62、erosion in the futureKnowledge gained about present-day soil erosion, and erosion in the past, can of course be a great help in suggesting where and how future erosion is likely to be a problem. However, it is likely that there will be some important differences.For example, future rates of water an

63、d wind erosionare likely to be affected both by climate change, as well as by land use change. Rates of water erosion, for example, are likely to respond to increases in rainfall in a non-linear manner, with disproportionately greater increases occuring in wet years. There are, though, still large l

64、arge gaps in our knowledge.v吧以*1850187519Q0192519501975 2DOOGlobal air lempeistijiei|2nri wnrmsl an rMn-rdjD-0tLft心The combined global land and marine surface temperature record from 1856 to 2003. The yea 2003 was the second warmest on recor. Also 2004 was the fourth warmest year globally. From the

65、Climatic Research UnitDave Favis-Mortlock, February 2005Where does soil erosion occur?Soil erosion in: Europe North America South America Africa Asia Australia/New Zealand ElsewherePreventing soil erosionIs more research needed on soil erosion?For further information: J. Poesen, J. Nachtergaele, G. Verstraeten and C. Valentin (2003): Gully erosion and environmental change: importance and research needs, CATENA, Volume 50, Issues 2-4, Page