Peat subsidence

Why we monitor peat subsidence

Waikato Regional Council monitors peat subsidence to get an overview of the rate at which drained peat soils are subsiding in the Waikato region. We can see if subsidence is changing over time due to land management practices or natural processes. Monitoring a range of peat areas also helps understand the spatial variability of peat loss.

Peat is almost entirely made up of organic matter. Drainage of peat results in on-going land subsidence and carbon loss.

Subsidence is the gradual lowering of the land surface due to the drained peat shrinking, consolidating and becoming denser. As time passes the slower process of oxidation (microbial breakdown of the organic material) produces carbon dioxide (CO₂), a greenhouse gas, and therefore carbon originally stored as peat is lost to the atmosphere.

Other processes that can contribute to peat subsidence include fire, compaction due to stock treading and machinery, wind and water erosion.

Peat subsidence generally decreases over time, but will continue as long as the soils remain drained, until the drained peat disappears.

Drained peat soils, although altered from their original state, are important economically. For example, about 60 per cent¹ of peat soils are used for dairy farming. This represents approximately 10 per cent² of the region’s dairy farming land use.

The negative effects of subsidence include the loss of ecosystem services, a need to upgrade flood protection and drainage assets, damage to infrastructure, and impacts on farm productivity. There is limited understanding on the eventual environmental and economic impacts of complete peat loss.

In some areas, the effects of peat shrinkage will be compounded by sea level rise, especially where the underlying sediments are near or below sea level. The level of the Waikato River relative to some areas of peat also causes issues as subsidence progresses.

The subsidence of drained peat soils may impact on the sustainability of adjoining wetland ecosystems.

Oxidation of peat results in the loss of the greenhouse gas, carbon dioxide (CO₂) to the atmosphere. Drained peat is also a source of the greenhouse gas, nitrous oxide (N₂O).

Peat subsidence may impact on soil processes and have an effect on nutrient cycling and loss, therefore impacting on freshwater quality.

As the regional coverage of this indicator improves (and we build a better understanding of how peat subsidence changes with time, land use, and peat type) we can use the information to help understand the likely impacts of peat loss in the future and plan for these impacts or develop strategies to reduce them.

What's happening?

The Waikato region has about 94,000 ha of peat soils. The mean depth of peat is about 4 m, but this can range from less than 1 m to as much as 12 m.

Originally, these soils represented large areas of unique wetland ecosystems. However, from the late 19th century to the mid-20th century, about 80 per cent of the original area of these wetlands were drained for agriculture, horticulture, and peat mining (for horticultural products).

During the most recent measurement period (early 2000s to 2012) the annual rates of subsidence at Rukuhia, Moanatuatua, and Hauraki were approximately 1.7, 2.1 and 1.7 cm per year.

The current average subsidence rate was about 1.9 cm per year. These three areas represent about 30 per cent of our region's peat area.

At Moanatuatua and Rukuhia, annual subsidence rates have decreased between the earlier (1920s to early 2000s) and later time periods.

The higher rates of subsidence during the earlier time period (1920s to early 2000s) at Moanatuatua and Rukuhia compared to Hauraki is probably due to drainage intensification being more recent at Rukuhia (1940s) and Moanatuatua (1950s)³ than at Hauraki (early 1900s)⁴.

If subsidence continues at the current rate, about 1 m of peat will be lost every 50 years.

Water table depth has the single greatest impact on subsidence rate. Therefore, subsidence is most pronounced close to drains and deeper drains have been associated with greater subsidence.

Preliminary data suggests that peat subsidence does not proceed at a constant rate and is linked to time since initial drainage.

Drain deepening to keep ahead of peat loss continuously draws down the water table, resulting in further subsidence and a requirement for even deeper drainage. This cycle results in continued subsidence and, if allowed, this process will continue until all the peat is lost.

Net carbon loss of 2.9 t C ha^-1 yr^-1 has been reported from one study⁵ on a Waikato pastoral agricultural peat soil.

Under the current situation, peat subsidence and carbon loss will continue until all the drained peat has gone. This would result in the loss of a valuable and unique resource.

In the long term we would like to see the regional rate of peat subsidence significantly slow or stop. This will require changes to the way we manage drained peat soils.

How we monitor

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.
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.
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.
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.

Data and trends

View the data file

The Excel spreadsheet below contains the source data for this indicator's graph and information.

The data were collected in the 1920s, early 2000s, and 2012.

Download the data file
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When this indicator is updated

The intention is to eventually update the indicator every 5 years. However, current sampling methods mean this is not possible, andat the moment updates are every 8 to 10 years. New methods are being investigated to reduce the time period between measurements.

Technical reports

Subsidence Rates of Peat Since 1923 in the Hauraki Plains Area

Subsidence Rates of Peat Since 1924 in the Rukuhia Swamp

Subsidence Rates of Peat Since 1925 in the Moanatuatua Swamp Area

Peat subsidence near drains in the Waikato Region

Best practice guides

For Peat's Sake (parts 1 and 2) - found on Managing peat webpage

Guidelines for landowners in peat lake catchments

Thesis

Pronger J, 2013. Historic and contemporary subsidence rates of drained agricultural peat, Waikato, New Zealand. Unpublished BSc (hons) dissertation, University of Waikato, Hamilton, New Zealand.

Published papers

Campbell DI, Wall AM, Nieveen JP, Schipper LA, 2015. Variations in CO₂ exchange of dairy farms with year-round rotational grazing on drained peatlands. Agriculture Ecosystems & Environment 202: 68 – 78.

Pronger J. Schipper LA, Hill RB, Campbell DI, Mcleod M, 2014. Subsidence rates of drained agricultural peatlands in New Zealand and the relationship with time since drainage. Journal of Environmental Quality 43: 1442 – 1449.

Schipper LA, McLeod M, 2002. Subsidence rates and carbon loss in peat soils following conversion to pasture in the Waikato Region, New Zealand. Soil Use and Management 18: 91 – 93.