Highly erodible land: Data to 2022
What is measured
Highly erodible land (HEL) refers to land that is very susceptible to erosion. This indicator measures landslide, earthflow, and gully erosion risk across Aotearoa New Zealand using the HEL model (see About the data for more details) and is based on two different datasets:
- HEL using the ‘Woody layer’
- HEL using the Land Cover Database (LCDB).
The ‘Woody layer’ and LCDB are land cover datasets that describe the vegetation cover across New Zealand. This indicator uses the ‘Woody layer’ for the first time because it provides higher resolution vegetation cover and is updated annually.
We report on how much land is classified as highly erodible in 2022 from the HEL model using the ‘Woody layer’.
We present maps of hot and cold spots, showing statistically significant clusters of locations with high or low risk of five major types of erosion in 2022 from the HEL model using the ‘Woody layer’.
We also present how much land is classified as highly erodible in 1996, 2001, 2008, 2012, and 2018 from the HEL model using each of the years LCDB is available (the latest year being 2018).
Why it is important
Soil erosion can have negative consequences on land productivity, water quality (via increased deposited sediment and turbidity), the natural form of the land, and infrastructure (Davies-Colley et al., 2003; Kemp et al., 2011; Owens et al., 2005; Page et al., 2000; Vale et al., 2021; Vale et al., 2023; Westrich and Förstner, 2007; Wood & Armitage, 1997).
New Zealand experiences high levels of soil erosion due to steep terrain, high rainfall, and tectonic activity (Basher, 2013; Hicks et al., 2011; Soons & Selby, 1992). While soil erosion is a natural process, it can be accelerated due to climatic conditions, or when human activities modify soil or vegetation, including from farming, construction, and mining (Wantzen & Mol, 2013). Generally, soil erosion in the South Island is more likely due to high rainfall and vulnerable, steep, mountainous terrain, while in the North Island it is due to the historical clearance of forest on steep slopes for pastoral agriculture (Koons, 1990; Page et al., 2000).
It is important to identify areas of land at risk of severe erosion to inform land-use decisions and help prioritise regional soil conservation work.
Key findings
Text alternative for Hot and cold spots for landslide, earthflow, and gully erosion risk, 2022
These five maps of New Zealand show hexagonal grid cells coloured according to whether an area is a hot spot or a cold spot for gully risk, high landslide risk (delivery to stream), high landslide risk (non-delivery to stream), moderate earthflow risk, and severe earthflow risk in 2022. Red represents a very likely hot spot with 90 percent confidence. Light red represents a likely hot spot with 66 percent confidence. Grey represents no significant hot spot or cold spot. Light blue represents a likely cold spot with 66 percent confidence. Dark blue represents a very likely cold spot with 90 percent confidence.
Based on the HEL model using the ‘Woody layer’:
- Of Aotearoa New Zealand’s 267,338 km2 of land, 5 percent (12,693 km2) was classified as highly erodible land at risk of mass-movement erosion (all classes aggregated together) in 2022.
- 60 percent of the area at risk was in the North Island, despite the North Island comprising only 43 percent of the total land area of New Zealand.
- Risk of erosion by landslide was the most common class of erosion risk, representing 75 percent (9,575 km²) of areas at risk of erosion, or 4 percent of New Zealand’s total land area.
- Manawatū-Whanganui had the largest area of highly erodible land at risk of erosion (2,208 km², 17 percent of total highly erodible land at risk of erosion in New Zealand) and had the greatest area in the country with high landslide risk (non-delivery to stream) (547 km²).
- Of all regions, Gisborne had the highest proportion of its area classified as highly erodible land at risk of erosion (15 percent, 1,280 km²). Gisborne also had the greatest area in New Zealand with severe earthflow risk (228 km²) and gully risk (161 km²).
- Hot spots were identified in many areas across New Zealand representing areas with significantly higher proportions of highly erodible land at risk of erosion compared to New Zealand on average.
- Of all regions, Gisborne had the highest proportion of area identified as very likely hot spots (47 percent) for high landslide risk (delivery to stream).
Based on the HEL model using LCDB:
Text alternative for Estimated land area at risk of landslide, earthflow, and gully erosion by region, 1996–2018
These five bar graphs show the area of land (km2) estimated to be at risk of gully, high landslide, and earthflow erosion by region in New Zealand for the year of measurement (1996, 2001, 2008, 2012, and 2018). Each bar represents a different year, and each year is a different colour. The data is available from Estimated land area at risk of erosion by region, 1996–2018 (CSV, 22 KB).
Where this data comes from
Manaaki Whenua – Landcare Research
View data table
Highly erodible land: Data to 2022
Related indicators
Coastal and estuarine water quality
Estimated long-term soil erosion: Data to 2022
Related content
Hill Country Erosion Programme
Technical report
About the data
Highly erodible land model
The highly erodible land (HEL) model identifies land at risk from the main forms of mass-movement soil erosion in New Zealand (landslide, earthflow, and gully erosion) if it does not have protective woody vegetation (Dymond et al., 2006).
The HEL model identifies five classes of land at risk of erosion:
- high landslide risk – delivery to stream
- high landslide risk – non-delivery to stream
- moderate earthflow risk
- severe earthflow risk
- gully risk.
These classes are not ranked in severity, except for earthflow risk, which has severe and moderate classes of risk.
Landslide erosion is the sudden failure of soil slopes during storm rainfall. Earthflow erosion is the slow downward movement of wet soil slopes towards waterways. Gully erosion is massive soil erosion that begins at gully heads and expands up hillsides over decadal time scales. Estimated soil erosion is modelled from three factors:
- slope
- land cover
- erosion terrain.
The model uses a digital elevation model (DEM) to identify slopes, incorporates different erosion terrains and erosion thresholds based on these terrains, and uses land cover mapping from either the LCDB or the ‘Woody layer’, which is an automated annual update of basic land cover produced by applying spectral rules to satellite imagery (Dymond & Shepherd, 2004). Land cover information describes the extent of vegetation, built environments, water bodies, and bare natural surfaces across New Zealand. The HEL model that uses the LCDB provides data for five years (1996, 2001, 2008, 2012, and 2018). The HEL model that uses the ‘Woody layer’ provides data for seven consecutive years (2016–2022), but we only report on 2022.
Some areas identified by the model as being at risk of soil erosion lie above the natural elevation limit where woody vegetation would normally grow (the bush line). However, the model does not make this distinction, so care is needed when interpreting the data, as not all areas at risk of soil erosion are suitable for planting trees.
The model used to identify areas at risk of soil erosion does not consider whether an area has space-planted trees (an erosion mitigation activity) because this information is not available nationwide. Furthermore, slope failure from earthquakes is not considered in the model.
The HEL model identifies where highly erodible land is located, but not the amount of sediment moved. While patterns of highly erodible land generally drive patterns of soil erosion, there are some areas in the North Island (particularly Gisborne) where erosion rates are excessively high in comparison with everywhere else due to very soft rock. As a consequence, erodible land in Gisborne will be producing an order of magnitude more sediment into rivers in the long term than land at risk of landsliding in other areas, such as Manawatū-Whanganui (Dymond & Shepherd, 2023).
Highly erodible land 2024 – DataInfo+ provides more information about the highly erodible land model.
For estimating long-term rates of soil erosion (tonnes/km²/year) and the resulting sediment loads in rivers (tonnes/year), the national indicator estimated long-term soil erosion should be used.
Hot spot analysis
Hot spot analysis identifies locations of statistically significant hot spots and cold spots in data by aggregating points of occurrence into polygons. This is one way to account for spatial autocorrelation. The analysis groups features when similar high (hot) or low (cold) values are found in a cluster. We used hot spot analysis to analyse the data from the HEL model that uses the ‘Woody layer’. The data covers all of New Zealand at a resolution of 10m by 10m pixels and for seven consecutive years (2016–2022).
Highly erodible land 2024 – DataInfo+ provides more information about hot spot analysis.
Data quality
The accuracy of the data source is estimated to be of high quality at a regional level.
Highly erodible land is a direct measure of the ‘Land and soil condition’ topic.
Stats NZ and the Ministry for the Environment must report on topics related to the five environmental domains: air, atmosphere and climate, fresh water, land, and marine. These topics identify key issues within each domain.
Topics for environmental reporting describes the topics for each domain.
Data quality information has more information about the criteria we use to assess data quality.
References
Basher, L. (2013). Erosion processes and their control in New Zealand. In J. R. Dymond (Ed.), Ecosystem services in New Zealand - conditions and trends (pp. 363–374). Manaaki Whenua Press. https://doi.org/10.7931/DL1MS3
Davies-Colley, R. J., Vant, W. N., & Smith, D. G. (2003). Colour and clarity of natural waters. Blackburn Press.
Dymond, J., & Shepherd, J. (2023). Update to Highly Erodible Land and Estimated Long-term Soil Erosion data sets for Environmental Reporting (Contract Report: LC4365). Manaaki Whenua – Landcare Research. https://environment.govt.nz/publications/update-to-highly-erodible-land-and-estimated-long-term-soil-erosion-data-sets-for-environmental-reporting
Dymond, J. R., Ausseil, A.-G., Shepherd, J. D., & Buettner, L. (2006). Validation of a region-wide model of landslide susceptibility in the Manawatu–Wanganui region of New Zealand. Geomorphology, 74(1–4), 70–79. https://doi.org/10.1016/J.GEOMORPH.2005.08.005
Dymond, J. R., & Shepherd, J. D. (2004). The spatial distribution of indigenous forest and its composition in the Wellington region, New Zealand, from ETM+ satellite imagery. Remote sensing of Environment, 90(1), 116–125. https://doi.org/10.1016/j.rse.2003.11.013
Hicks, D. M., Shankar, U., McKerchar, A. I., Basher, L., Lynn, I. H., Page, M., & Jessen, M. (2011). Suspended Sediment Yields from New Zealand Rivers. Journal of Hydrology. New Zealand, 50(1), 81–142. https://www.hydrologynz.org.nz/journal/volume-50-2011
Kemp, P. S., Sear, D., Collins, A. L., Naden, P. S., & Jones, J. I. (2011). The impacts of fine sediment on riverine fish. Hydrological Processes, 25(11), 1800–1821. https://doi.org/10.1002/hyp.7940
Koons, P. O. (1990). Two-sided orogen: Collision and erosion from the sandbox to the Southern Alps, New Zealand. Geology, 18(8), 679–682. https://doi.org/10.1130/0091-7613(1990)018<0679:TSOCAE>2.3.CO;2
Owens, P. N., Batalla, R. J., Collins, A., Gómez, B., Hicks, D. M., Horowitz, A. J., Kondolf, G. M., Marden, M., Page, M., Peacock, D. H., Petticrew, E. L., Salomons, W., & Trustrum, N. A. (2005). Fine-grained sediment in river systems: environmental significance and management issues. River Research and Applications, 21(7), 693–717. https://doi.org/10.1002/rra.878
Page, M., Trustrum, N. A., & Gómez, B. (2000). Implications of a century of anthropogenic erosion for future land use in the Gisborne-East Coast region of New Zealand. New Zealand Geographer, 56(2), 13–24. https://doi.org/10.1111/j.1745-7939.2000.tb01571.x
Soons, J., & Selby, M. (Eds.). (1992). Landforms of New Zealand (2nd ed.). Longman Paul.
Vale, S. S., Smith, H. G., Davies-Colley, R. J., Dymond, J. R., Hughes, A. O., Haddadchi, A., & Phillips, C. J. (2023). The influence of erosion sources on sediment-related water quality attributes. Science of The Total Environment, 860, 160452. https://doi.org/10.1016/j.scitotenv.2022.160452
Vale S. S., Smith H. G., Matthews A., & Boyte S. (2021). Determining sediment source contributions to overbank deposits within stopbanks in the Oroua River, New Zealand, using sediment fingerprinting. Journal of Hydrology (NZ), 59(2), 147–172.
Wantzen, K. M., & Mol, J. H. (2013). Soil erosion from agriculture and mining: a threat to tropical stream ecosystems. Agriculture, 3(4), 660–683. https://doi.org/10.3390/agriculture3040660
Westrich, B., & Förstner, U. (Eds.). (2007). Sediment dynamics and pollutant mobility in rivers: an interdisciplinary approach. Springer Science & Business Media.
Wood, P. J., & Armitage, P. D. (1997). Biological effects of fine sediment in the lotic environment. Environmental Management, 21(2), 203–217. https://doi.org/10.1007/s002679900019
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