Soil Erosion in New Zealand

Once the whole of the two islands was covered with temperate forests, but with colonization by humans and the removal of these forests by slashing and burning, their replacement by clover and rye grass pasture gives the impression of a clean green land, and that everything is healthy.The picture above shows slips that have 'healed'. This whole area was once covered in thick temperate forest.The sheep have grazed the grass short and transformed the the hills into little terraces. The short grass means shallower root growth and less to hold the soil together.At each heavy rain they fill up rapidly bringing down soil and stone because the water is no longer held in the catchments to be released slowly afterwards.he sheep are dependent on the clover and rye grass, and the grass and rye grass grow mainly on the hill slopes of the hill country, from whence all the run off comes. 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.

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. 48% of that risk area is being farmed.

Decades of research and experience have provided, the New Zealand Department of Conservation, 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.