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Land

• Land cover
• Land use
• Soil health
• Soil intactness

Soil intactness of erosion-prone land

Erosion-prone land is mainly hill country with a slope of more than 21 degrees. The most erosion-prone hill-country lands are in pastoral land cover and have soils that are known as ‘yellow-brown earths’ situated on weakly consolidated mudstones and sandstones.

Land cover can be used to assess the soil intactness of erosion-prone hill country (that is, the ability of soils to stay in place on erosion-prone hill country).

The risk of soil erosion on hill-country areas in pasture is based on information from the Land Cover Database and the Land-Use Capability survey classes from the New Zealand Land Resource Inventory (NZLRI).

Erosion risk is classified by degree: severe, very severe, or extreme. This classification is based on:

• Soils and underlying geology – areas with yellow-brown earths situated on weakly consolidated mudstones and sandstones pose greater erosion risk.
• Slope or altitude – areas with slopes more than 21 degrees or situated 1,000 metres above sea level pose greater erosion risk.
• Land cover – whether erosion-prone areas are in pasture or other more appropriate vegetation to hold the soils in place.

Current situation

In 2002, just over 1.14 million hectares of erosion-prone land was in pastoral land cover. Many areas are underlaid by soft, erodible materials. About 10 per cent of New Zealand is classed as severely erodible. The removal of native forest between the 1880s and 1920s for pastoral land use increased erosion rates. In 2002 about 4.25 per cent of New Zealand's land area was hill-country erosion-prone land in pasture.

Many parts of the country are prone to mass movement soil erosion (ie, extreme landslip, tunnel gully, gully, and earth-flow forms of soil erosion). The loss of soil through erosion, and transport by rivers to the sea, was estimated in 1996 to be 400 million tonnes a year.

Hill-country pasture areas at risk of erosion – North Island (LCDB 2, 2002)

Note: Mapping limited to developed pasture with no woody vegetation.
Data source: Landcare Research.

The magnitude of soil erosion on the east coast of the North Island is far greater than elsewhere in New Zealand. The impacts of erosion (flooding and sedimentation) in the Gisborne region are considered greater than in other areas of New Zealand, particularly for the southern part of the region, the Waipoa catchment.

Other regions that have erosion-prone hill country include Waikato, Bay of Plenty, Hawke’s Bay, Taranaki, Manawatū–Wanganui, Wellington, Tasman, and Marlborough.

Hill-country pasture areas at risk of erosion – South Island (LCDB 2, 2002)

Note: Mapping limited to developed pasture with no woody vegetation.
Data source: Landcare Research.

Recent trend

The percentage change from pasture for areas at risk of erosion between 1997 and 2002 is small. The results show a reduction in pasture on hill-country erosion-prone land of 36,400 hectares nationally.

Just over half of this total was in the Gisborne, Hawke’s Bay, and Manawatū–Wanganui regions (17,481 hectares in total). In the South Island, the Marlborough and Tasman regions experienced a combined pastoral land-cover change of 4,119 hectares.

Area (in hectares) of pasture on hill-country erosion-prone land between 1997 (LCDB 1) and 2002 (LCDB 2)

Note: Pasture classes from the erosion risk data used for this analysis limited to the LCDB Database ‘Primarily Pastoral’ classes for reporting.
Data source: Landcare Research.

LCDB analysis shows that of the 36,400-hectare reduction in pasture on erosion-prone hill country, 36,300 hectares were converted to exotic forestry or retired and left to revert to scrub.

The changes in the land area under pastoral land cover highlight efforts to replace pasture with more stabilising vegetation on erosion-prone hill country. For example, planting bare land in trees has a noticeable impact on erosion rates. In four to five years, the tree roots will have intertwined, the tree foliage will intercept the strike of heavy rainfall, and the soil will begin to be protected. This process is complete when the tree canopy closes.

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Last updated: December 2007