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Soil erosion

Soil erosion is a natural process that has been accelerated in New Zealand by deforestation and unwise land use practices (for example, overgrazing). Accelerated erosion is the most serious and the least reversible of soil degradation problems.

The main forms of erosion in New Zealand are:

  • Mass movement erosion – occurs when heavy rain or earthquakes cause whole slopes to slump, slip or landslide. Most hill slopes steeper than 15 degrees are susceptible to mass movement, and those steeper than 28 degrees generally have severe potential for erosion. Storms are the primary triggers. Mass movement also damages fences, tracks, drains and occasionally buildings. This is the most common form of erosion in the hill country.
  • Fluvial erosion – occurs when running water gouges shallow channels or deeper gullies into the soil. On sloping land the gullies can cut deep into the subsoil or undermine surrounding soils. The sediment is washed into streams. Gullies do not cause much loss of crop or pasture but can disrupt stock and vehicle movement around farms. This is a common form of erosion on loessal soils such as those found in Marlborough and Manawatu.
  • Surface erosion – occurs when wind, rain or frost detach soil particles from the surface, allowing them to be washed or blown off the paddock. Surface erosion can occur on any land that is exposed to wind and rain but occurs largely outside the hill country.
  • Streambank erosion – is a special case of fluvial erosion that occurs when banks that have been cleared of tree cover become unstable.

Hill country erosion

Hill country is defined as all Class V, VI, VII and VIII land from the NZ Land Resource Inventory (NZLRI) with D slopes and above (more than 16 degrees), below an altitude of around 1000 metres above sea level.

Hill country erosion decreases the productivity of farms. Although vegetation returns within a few years on an eroded site, growth is generally less productive than before because the underlying soil is thinner and holds fewer nutrients.

After hill country slip erosion, pasture production takes around 20 years to recover to within 70-80% of its pre-erosion levels. In severely eroded areas only a few stress tolerant weeds may survive. After repeated erosion, sites may become barren.

In promoting the concept of land use to land capability in the hill country, Government’s policies focus on areas that are at risk of mass movement and fluvial erosion.

Streambank erosion, also a significant form of erosion in the hill country, is being addressed through the riparian management work being undertaken by the Ministry.

Surface erosion, while a significant problem in some regions, occurs largely outside the hill country.

Downstream, debris causes rivers to aggrade, increasing flood risk. Erosion can also contribute to many water quality problems such as loss of aquatic habitat and increased sediment loads.

Regions with hill country land prone to erosion include: Waikato, Bay of Plenty, Gisborne, Hawkes Bay, Taranaki, Manawatu-Wanganui, Wellington, Tasman and Marlborough. Together they have approximately 7,075,000 hectares at risk of erosion (1993). 48% of that risk area is being farmed (State of the Environment Report 8:53)

Erosion control techniques

Decades of research and experience have provided us with techniques to reduce the impacts of erosion on pastoral land. These include:

  • maintaining adequate vegetative cover (e.g. avoiding over-grazing and maintaining a dense pasture sward through regular applications of fertiliser and grass seed)
  • spaced or close tree planting
  • retiring land from pasture
  • fencing off and planting river banks
  • building debris dams to slow water flows in gullies.

The short-term costs of soil conservation to farmers include soil conservation work and possible loss of production when stock is excluded from newly established plantings. The major benefits include:

  • avoided stock losses and extra income from timber
  • recreational
  • aesthetic benefits to people in the region either from improved landscape from conservation plantings or improvement and maintenance of water quality
  • less disruption to road and rail links.

The Ministry for the Environment and the Ministry for Agriculture and Forestry have jointly funded the Soil Conservation Technical Handbook published by the New Zealand Association of Resource Management. This handbook is a valuable resource for anyone dealing with soil erosion or landslips.

Last updated: 17 September 2007