Soil Erosion

It is important that we have reliable information on the amount and type of soil erosion in our region. This is because:

• Soil loss accelerates sedimentation and nutrient run-off and degrades water quality in adjacent or downstream water bodies;
• Downstream erosion cause rivers to become filled in with silts and gravel increasing the risk of flooding;
• The gradual loss of topsoil affects the general heath of the soil and reduces fertility and the productive capacity of the soil resource;
• Some types of erosion can affect land stability for housing and other uses.

To gain a better understanding of the amount and type of soil erosion across the Marlborough Region, in 2009 a survey was undertaken to assess Soil State using region-wide aerial photography. Soil State characterises whether soil at a given site is on:

• stable surfaces i.e. vegetated
• erosion-prone unstable surfaces i.e. inactive vegetated surfaces
• eroded, unstable surfaces i.e. recently disturbed and re-vegetating
• eroding, unstable surfaces i.e. freshly disturbed and bare

What have we found out?

• 52.5% of the region’s sample points are on stable surfaces
• 11.5% of the region’s sample points are on erosion-prone but inactive surfaces
• 36.0% of the region’s sample points are on actively eroded and eroding surfaces
• We also found out that 11.56% of the region has bare soil
• However most of the bare soil in the region (10.08%) was due to natural processes of which the most important by far was bare rock and scree (7%), with lesser amounts due to streambank deposition (1%) and sheetwash (1%) (Figure 1).
• Significantly less bare soil in the region was due to land use activities (1.47%). By far the most important landuse activity responsible for bare soil was tracks (0.87%), with lesser amounts due to cultivation and grazing pressure (Figure 2).

Soil State in the Marlborough Region, 2002 – 2005 (499 Kb)

Figure 1

Figure 2