Compare and Contrast the erosional processes occurring in semi-arid and temperate regions

and briefly describe possible ways of ameliorating soil erosion on an arable field in North West England

1. History of Soil Erosion

Soil erosion, defined as where soil removal is greater than formation, is an essential part of the geological process, but with the evolution of man and increased need for land, demands on soils have increased, which in turn leads to greater soil erosion, both through farming and poor land management.

There is much geological and historical evidence of soil erosion. Sedimentary sequences such as flood plain alluvium and physical features like soil creep are all results of soil erosion. Other signs of past erosion are seen through such techniques as pot fragments, pollen analysis and 14C dating, these providing date ranges from pre-history man to 60ka before present. Evidence of man induced erosion can be seen where hedges around enclosures, a process commenced in the 12th century, have trapped eroded sediment from farmed fields. Furthermore, podzols, which form under various tree and heather types, are easily bleached and the underlying iron and humic enriched horizons eroded away, the remaining soil being evidence of clearance and the subsequent erosionp>

Removal of vegetation, in particular forested areas, is a key propagator of soil erosion. Some 5,000 years ago, woodland was predominant in Britain and this ensured relative stability in river channels and lesser water flows. Neolithic man started minor deforestation in the south of England but this made little impact. Organised government with the Roman occupation some 2,000 years ago brought the first real deforestation, both for farming, land development and raw materials and this was subsequently continued by the Anglo Saxons in central England and the Vikings in northern England. Stops and starts in deforestation coincided with population dips such as during the Plague during the 14th century and national demand, as was the case with the need of the 16th century Elizabethan navy for wood for shipbuilding.

2. Contrasting Climates

Semi-arid land by definition is partly dry and generally steppe or savannah, that is dry, grassy and often treeless low gradient land, where precipitation is between 250mm and 500mm per year, focused over a single 4 - 6 month growing season and a dry summer. This, coupled with temperatures of between 22 and 32 degrees C, puts much pressure on people living in these regions, especially those trying to farm the land. Locations of semi-arid land lie broadly between 18 and 35 degrees N and 18 and 35 degrees S, including the Midwest USA, sub Saharan Africa, central Asia, and parts of South America and Australia.

Temperate literally means without extremes. Precipitation ranges between 500mm and 1500mm spread throughout the year with an annual temperature range of 0 - 22deg C. Bearing in mind the extremes of rainfall and temperature experienced globally, this climate type falls to the centre of the scale. Because of the lack of extremes, development of agriculture has not been inhibited, producing the most productive farm land. Temperate zones fall roughly between 40 N - 50 N and 40 S - 50 S, covering most of Europe, parts of north America, Australasia and South America. As with the latitudes given above for semi-arid zones, these are very much a general guide. Although not true in all cases, the definitions of temperate and semi-arid often align themselves with 'developed world' and 'developing world'. Figure 1 shows the world climatic regions.

Figure 1 - World Climatic Regions

(after Bradshaw and Weaver)

CLICK TO SEE ENLARGEMENT

Plate 1 - Dense plume of loamy sand being blown across the A1 trunk road near Blyth, Nottinghamshire

    (Bayliss-Smith & Owens)

3. Overview of Soil Erosion

The natural agents of soil erosion are water, wind, ice or other geologic agents. Water has generally greater effect in both semi-arid and temperate climates although wind can be significant in certain areas of the latter. Plate 1 shows a dense plume of loamy sand being blown across the A1 trunk road near Blyth, Nottinghamshire. Few measurements have been made of wind erosion although Lal (1993) gave values of up to 20t/ha/yr for wind compared with up to 50t/ha/yr for water in the semi-arid West African Savannah. Outlier values for wind erosion of as high as 200t/ha/yr are also given but these are far from the norm. Boardman (1990) monitored erosion by water in the South Downs, southern UK and gives median erosion figures of between 0.6 and 5.0 m3/ha/yr. Assuming a bulk soil density of 1.5 t/m3, this gives mass of lost soil due to water as between 0.9 and 7.5t/ha/yr.

4. Erosion In Semi-Arid and Temperate Regions

Although natural processes can increase likelihood, the main cause of soil erosion is ultimately of human origin. Figure 2 outlines erosion sources. In reality, erosion is usually a combination of different causes, such as with the case in the American Mid-West mentioned below. Reduction of vegetation cover is perhaps the single most means by which soil erosion increases. Reasons for removal are many fold.

Figure 2 - Outline of erosion sources

(after Pickering and Owen)

4.1 Drought

A natural mechanism, drought, is the phenomenon of much lower than mean rainfall for a temporary period. It is much more frequent in semi-arid than in temperate environments . Once vegetation dies off, the surface resistance of the soil decreases and, with breakdown of root structure that would normally bind the soil, removal of the soil by wind or rain becomes much easier. Prolonged drought combined with poor semi-arid land farmed with a monoculture and freak wind conditions caused the 'Dust Bowl' conditions of the 1930's in the American Mid-West, as shown on Plate 2 . Human activities leading to vegetation reduction are livestock grazing, arable farming, deforestation and urbanisation.

Plate 2 - 'Dust Bowl' conditions of the 1930's in the American Mid-West

(Press & Siever)

4.2 Livestock

Grazing not only reduces vegetation but also compacts the ground. This can reduce rainwater infiltration, increasing runoff and causing sheet or gully erosion, especially along paths frequently used by livestock. Although this does occur in temperate climates, it is far more prominent in semi arid land where the system is not only under pressure, but also poorly managed.

4.3 Arable Farming

The nature of crop farming initially involves removal of natural vegetation which had adapted over time to the location, with something perhaps much less well suited. Problems experienced more in the temperate climates, as a result of more developed agricultural practices include deep ploughing of land and provision of fine seedbeds to improve germination, both enabling easier erosion of soil particles.

Also 'tramlines' left between crops for later pesticide application and vehicle wheelings encourage gully erosion. Autumn crops are one of the principle causes of erosion in the UK as the ground is left clear of developed vegetation during the higher rainfall winter months. Plate 3 shows severe erosion in East Sussex in the form of rills and gullies, following recent winter crop drilling. Second crops are not a luxury afforded semi-arid climates where there is only one growing season. In either climate, irrigation of crops may cause problems if the water drops are too large, the flow is too high or irrigation water not contained properly.

Plate 3 - Severe erosion in the form of rills and gullies, following recent winter crop drilling

(Boardman)

4.4 Deforestation and Urbanisation

Deforestation for timber and generally any vegetation clearance for urban expansion initiates erosion. As shown above, semi-arid climates have a lower annual rainfall, at best half that of temperate climate, and a higher erosion rate, over 10 times. These conditions mean that desertification, where less than 35% of land is vegetated on a long term basis (Binns, 1990), is much more possible in semi-arid than temperate climates. Urbanisation may not be an instigator of desertification as satellite and ground surveys have not shown village expansion to necessarily increase arid lands (Press and Siever, 1997). Plate 4 shows semi-arid land that has been affected by long term drought and human pressure.

Plate 4 - Long term drought and human pressure adversely affect semi-arid and arid land

(Press and Siever)

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