Soil Intactness Monitoring Programme

An ongoing programme of soil intactness or erosion assessment has been implemented to help identify trends and issues relating to soil disturbance and land instability in the Tasman District. 

The first assessment was carried out on data collected off 2001 aerial photography. This photography covers the majority of privately owned land that may be used for land-based productive purposes. The data was collected from 6,005 points.

The survey indicates through the presence or absence of vegetation whether points are at risk of soil disturbance.  It also differentiates whether disturbance results from the shifting around of soil by land use activities, or erosion and accumulation by natural processes.  Finally, it assesses the extent of vegetative cover whether natural or planted ‑ and measures its effect with respect to soil conservation.

The 2001 Data 

The 2001 data indicates that:

  • 85% of the soil in the survey area is intact. 
  • 8% of the soil is disturbed by land use activities: tracking, roading, cultivation, harvesting and earthworks are present. 
  • 6.3% of the soil is unstable due to natural disturbance: landslide, slip, earthflow, gully, sheet, stream bank erosion, rockfall and windblown sand.

Fresh disturbance by land use related activities such as roading, cultivation, harvesting and earthworks is most intensive on dairy farms, where the majority of the disturbed area is due to the presence of farm tracks and lanes.  Although the majority of dairying is based on flat land where surface water run-off is less prevalent than, say, on hill country, localised sedimentation is known to occur and affect the in-stream values of small streams. Fresh disturbance in exotic forests also is significant.  Exotic forestry is the largest commercial-based land use type situated on hill country in the survey area hence the effect of the disturbance can potentially be significant, as silt in run-off from the disturbed sites is likely to enter water bodies.  The table below demonstrates the levels of disturbance.

Land Use

Percentage of Survey Area

Percentage Freshly Disturbed by Land Use

Intensive land use

1.3

5

Dairy pasture

4

18.5

Dry stock

15

6

Exotic forestry

17

11.4

Scrub

14.7

2.9

Native forest

40

0.2

Mountain scrub/tussock

3

0.4

 The survey also indicates that there is only a low level of natural erosion through the survey area and land use is not showing up as significantly accelerating this level. More detail can be found in the Report:

 Soil Intactness Monitoring Programme for the Tasman District 2001