Methods - how we monitor
Where and how we collect the data
About 161 sites have been measured and re-measured for peat depth. Sites were selected where historical peat depth measurements had been undertaken.
There were 60, 45 and 56 measurement locations at Moanatuatua, Hauraki, and Rukuhia respectively.
Using the current methodology (peat probing) intervals between measurements need to be between 8 and 10 years to pick up a statistically significant change in peat thickness. 1
Peat thickness along transects were originally measured by land drainage engineers in the 1920s at Moanatuatua, Rukuhia, and Hauraki bogs. These transects were set out at 10 chain (200 m) intervals at Hauraki and Rukuhia, and at 20 chain intervals (400 m) at Moanatuatua.
In the early 2000s Landcare Research resurveyed peat depth along a number of these transects. The University of Waikato undertook a re-survey of these same points in 2012. Some of the original peat depth sites measured in the 1920s could not be re-surveyed during the early 2000s survey because all the peat had completely subsided and no longer remained, leaving a mineral soil at the surface.
Measurement technique 2
Peat depth measurements are recorded by pushing a narrow steel probe through the peat until the firm mineral substrate underlying the peat is struck, the depth of the probe is then recorded as peat depth.
For the re-survey in the early 2000s points from the peat depth transects in the drainage maps were transferred to topographical maps. ARC GIS was used to generate grid co-ordinates for each sampling site. These co-ordinates were located in the field using a hand held GPS and depth was re-measured using a 12 mm (diameter) steel probe. At Moanatuatua, peat depth measurements were collected at the identified co-ordinate and then 5 m either side, perpendicular to the farm or main drain. Therefore, a total of three depth measurements were completed at each site. This number was sometimes reduced if buried wood fragments were struck within the peat. At Hauraki and Rukuhia, one peat depth measurement was collected at each site.
For the survey in 2012, the same sites identified in the early 2000s survey were relocated and sampled with an 8 mm (diameter) stainless steel probe. Due to the shorter sampling time frame (about 10 years) five samples were collected at each site. The central point sample was based on the original co-ordinate and the other four samples were situated perpendicular to each other 3 m from the central point.
At Rukuhia, the number of samples included in the calculated rate of subsidence for the early 2000s to 2012 period were reduced due to hump and hollow land contouring having occurred at some sites between the measurement periods.
How this indicator is compiled
Coordinates for historical peat depth measurements from the 1920s were identified and the locations re-sampled in the early 2000s and 2012.
At each co-ordinate, peat depth was measured at up to 5 sub-locations.
Each sub-location depth point is entered into an Excel spreadsheet and the mean depth is calculated for each coordinate.
The depth information collected from the 2012 sampling was subtracted from the early 2000s sampling and the depth information from the early 2000s sampling was subtracted from the 1920s sampling for each sample point.
The result from the depth difference calculation is divided by the number of years between sampling to give an annual subsidence rate for each sample site.
Subsidence rates are averaged for each peat area (i.e. Moanatuatua, Rukuhia, and Hauraki) for the two time periods; 1920s to early 2000s and early 2000s to 2012.
The data presented in this indicator is from three peat areas in the Waikato region so does not represent the entire region. For example, there are no measurements from the Lower Waikato or Komakorau peat areas.
The indicator does not distinguish peat subsidence under different land uses.
Because the rate of peat subsidence is small on an annual basis the current methodology is susceptible to the following issues:3
- Due to undulating surface and sub-surface topography peat thickness can vary resulting in considerable variation in thickness over short distances.
- The transition between peat and the underlying mineral material can sometimes be comprised of thick muddy material or fluid silts and clays.4 This soft underlying lithology makes it difficult to distinguish the change from peat to mineral material and therefore the base of the peat.
- The depth of some peats, and narrow diameter of the sampling probe, mean that there is the potential for non-vertical probe orientation and rod curvature to occur below the peat surface during depth probing.
Further indicator developments
The intention is to add more monitoring sites which better represent the region and develop a common method for determining peat loss.
Preliminary investigations are underway to use a combination of peat probing and laser survey techniques for ongoing peat subsidence monitoring. This will hopefully improve the precision and accuracy of measurements and allow statistically significant changes in peat depth to be measured over shorter time intervals than currently possible.
The data presented in this indicator has been collected by research organisations (Landcare Research and the University of Waikato), published in various technical reports, and as a peer reviewed journal paper. See more, in Resources.