Estimated highly erodible land in the North Island

  • Image, Estimated highly erodible land in the North Island.

    Some areas of New Zealand’s North Island are classified as highly erodible land. They have steep slopes and are at high risk of mass soil movement because they lack woody vegetation cover with deep roots to hold the soil in place. This can lead to soil erosion. It is important to identify areas of land at risk of severe erosion to inform land-use decisions and help prioritise soil conservation work.

    We classified Estimated highly erodible land in the North Island as a national indicator.

    Key findings

    Trend not assessed

    Seven percent (840,000 ha) of the North Island (11,443,000 ha) was classified as highly erodible land at risk of severe erosion in 2012.

    • Of the 840,000 ha of highly erodible land at risk of severe erosion in the North Island, 61 percent (515,000 ha) had a high risk of landslides.
    • Gisborne had the highest proportion of its area classified as highly erodible land at risk of severe erosion (16.5 percent, 138,000 ha) of all North Island regions.
    • Manawatu-Wanganui had the largest area of highly erodible land at risk of severe erosion (254,000 ha, 11.5 percent) of all North Island regions.
    • Hawke’s Bay and Northland regions each had more than 100,000 ha of highly erodible land at risk of severe erosion.
    Figure 1

    Map, Estimated highly erodible land, 2012

    Note: Landslide erosion is the shallow (approximately 1m) and sudden failure of soil slopes during storm rainfall. Gully erosion is massive soil erosion that begins at gully heads and expands up hillsides over decadal time scales. Earthflow erosion is the slow downward movement (approximately 1m/year) of wet soil slopes towards waterways.

    Figure 2

    Figure 3

    Note: Landslide erosion is the shallow (approximately 1m) and sudden failure of soil slopes during storm rainfall. Gully erosion is massive soil erosion that begins at gully heads and expands up hillsides over decadal time scales. Earthflow erosion is the slow downward movement (approximately 1m/year) of wet soil slopes towards waterways.

    Definition and methodology

    New Zealand experiences high rates of soil erosion. In the North Island, this is mostly due to the historical clearance of forest for agriculture (see also Estimated long-term soil erosion). In contrast, erosion in the South Island is mostly due to natural processes, primarily high rainfall and steep mountain slopes.

    Highly erodible land comprises land at risk of landsliding, gullying, or earthflow erosion if it does not have protective woody vegetation (Dymond et al, 2006). Landsliding occurs on steep slopes where the soils do not have the support of tree roots.

    Gullying and earthflow erosion can occur on all slopes, irrespective of steepness, but the land is only considered at risk if it does not have woody vegetation.

    Landslide erosion is the shallow (approximately 1m) and sudden failure of soil slopes during storm rainfall. Gully erosion is massive soil erosion that begins at gully heads and expands up hillsides, over decadal time scales. Earthflow erosion is the slow downward movement (approximately 1m/year) of wet soil slopes towards waterways.

    Regional boundaries are based on 2012 regional boundary data.

    Data quality

    We classified Estimated highly erodible land in the North Island as a national indicator.

    Relevance

    relevance-direct This national indicator is a direct measure of the ‘Land and soil condition’ topic.

    Accuracy

    accuracy-high The accuracy of the data source is of high quality.

    See Data quality information for more detail.

    References

    Dymond, J, Ausseil, A-G, Shepherd, J, & Buettner, L (2006). Validation of region-wide model of landslide susceptibility in the Manawatu-Wanganui region of New Zealand. Geomorphology, 74, 70–79.

    Archived versions

    See Estimated highly erodible land in the North Island (archived April 2018)

    Published 19 April 2018

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