Cracking the code

Peat soils are particularly effective for carbon storage. Teagasc research is assessing its drainage status, which is key to harnessing its potential for emissions reduction.

Drone technologies offer a complementary solution to capturing geospatial data. Photo credit: Teagasc

Per Ireland’s National Inventory Reporting, there are 339,000ha of peat soils in grassland. Of this total, approximately 140,000ha are considered drained. When drained, peat soils are at least 30cm thick and have a minimum organic matter content of 20%. Greenhouse gas emissions from this area are determined by several factors, including drainage status.

Drainage status is defined as either ‘shallow drained’ or ‘deep drained’, where the position of the water table in the soil is less than or greater than 30cm, respectively.

Both drainage statuses receive a different emissions factor in National Inventory Reporting, explains Owen Fenton, a Principal Research Officer at Teagasc Johnstown Castle. “Unfortunately, at a national level, we don’t have a lot of monitoring points to record the current and actual drainage status of peat soils in grassland.”

Owen is one of several researchers involved in the D-TECT project, which is currently tasked with cracking the code for drainage status in agricultural peat soils and advancing Ireland’s national drainage status knowledge.

Setting the table

Establishing the average drainage level of a soil requires consistent measurement. A temporary monitoring well – or dipwell – is inserted into the soil, down to the deepest position of the water table – the standing water level in the ground. Typically, the water table is at its deepest during summer months. The dipwell consists of plastic slotted piping, into which is inserted a pressure sensor. This sensor measures the position of the water table every 15 minutes, for up to three years. The dipwell is installed flush with the field’s surface level and sealed off, allowing farm management to continue as normal.

Every few months, the sensor data is downloaded by D-TECT project’s technical team, allowing them to map the water table’s daily position, explains Owen. “We’re able to establish an average once we’ve gathered over a year’s worth of daily data. Once the water table’s average resting position is known, we can accordingly allocate its drainage status of ‘shallow’ or ‘deep’.”

However, in practical terms, researchers can only install a limited number of dipwells to cover all the different peat soil types – fen, raised bog, blanket bog – currently used for grassland. Therefore, they need to develop models for predicting the temporal and spatial resting position of the water table using their limited monitoring network, Owen continues.

“The next step is seeing how our predictive model performs in peat soil areas where we have no monitoring points. For example, survey data on seasonal vegetation and man-made drainage can be collected and compared against real water table position data. Further, seasonal drone flights can capture highly detailed information on various useful proxies for water table prediction, such as soil moisture or elevation of the ground surface.”

High-resolution drone images used to identify soil drainage features. Photo credit: Teagasc

Settling the score 

Seasonal field surveys are used to analyse vegetation and land cover as indicators of the water table position at the grassland site on peat soils. Different scorecards are employed for peatland (raised bog, blanket bog), grassland, and scrub/woodland fields. These scorecards provide data related to the ecological health of the fields, such as the presence of wet species like mosses and heather in the peat soils and grassland. They also include information about nutrient-rich vegetation such as clover, docks, and ryegrass.

The drainage section of the scorecard offers details about boundary and in-field drains affecting the sites, Owen explains.

“The depth, width, and flow condition of these drains are recorded to evaluate their influence on the fields. This can help us infer the water table position at the time of the survey.”

Besides the field scorecard, vegetation quadrant scoring is conducted near the installed dipwells. Scoring in these quadrants provides a detailed analysis of vegetation, aiding in the sub-classification of peatlands and grasslands with wet species. Initial summer survey results suggest that wet grassland and bog areas are dominated by moss, a key indicator of peat soil wetness. Conversely, dry grasslands are covered with nutrient-rich species such as ryegrass, clover, and docks.

“The vegetation survey data will be correlated with the water table position dipwell data to determine drainage status in each field across our sites,” adds Owen. “Additionally, vegetation surveys are conducted alongside drone surveys to cross-validate the vegetation data.”

339,000ha

there are 339,000ha of peat soils in grassland.
Of this total, approximately 140,000ha are considered drained

Multi-faceted methods

Drone technologies offer a complementary solution to the challenge of monitoring water table levels across peat soils. These geospatial technologies enable the capture of data over extensive areas at any user-specified time, allowing for the assessment of water table levels throughout different seasons.

The information from these data can provide key insights, including vegetation type, topography (e.g. surface elevation and slope), location of drainage features, and surface temperature. These parameters can be correlated with the water table levels in that area.

Through the integration of drone data with the existing point-based water table observations, models of water table levels over time and space can be created. This process also quantifies the relative importance of each site characteristic, highlighting factors that most strongly correlate with measured water table levels. These geospatial datasets may deliver much-needed information for drained and undrained areas, Owen explains.

“This will enhance our understanding of the subtle transition between these categories and, at a larger scale, the complex nature of the soil drainage status. In addition, this geospatial modelling could support the scaling up of this approach to a national level, using satellite imagery. This would result in a multi-scale framework for evaluating drainage status.”

Combining monitoring data with vegetation, drainage and drone surveys is helping to provide a more complete image of water table positions in agricultural grassland peat soils, concludes Owen.

“We now have a methodology that brings together many technologies and approaches – with other project partners expected to deploy further technologies. Together, this will enable us to crack the drainage status code little by little in the coming years.”

Funding

The D-TECT project (Geospatial drainage status detection mapping of organic rich soils for NIR and policy support needs.,Grant No: 2023RP959) is funded by the Department of Agriculture, Food and the Marine under the Research Stimulus Programme with co-funding from the Environmental Protection Agency.

Acknowledgements

The authors acknowledge the research performed by colleagues John Connolly, Eoin McCarthy, Martin Donoghue, Asaf Shnel, Lilian O Sullivan, Réamonn Fealy, Eve Daly and Stuart Green.

Contributors

Owen Fenton, Principal Research Officer, Teagasc Johnstown Castle.

owen.fenton[at]teagasc.ie

Patrick Tuohy, Senior Research Officer, Teagasc Moorepark.

Charmaine Cruz, Postdoctoral, Research Fellow, Trinity College Dublin.

Muhammad Inam Bari, PhD Walsh Scholar, Munster Technological University.