Ecohydrology of wetlands and remote sensing monitoring
Updated: Oct 13, 2021
Prof Laurence Gill, of Trinity College Dublin, is principal investigator on the EPA-funded research project, EcoMetrics, developing ecohydrological metrics to support the conservation and restoration of our wetlands. He writes:
Wetlands provide important regulating ecosystem services such as water purification, carbon capture and storage, flood protection etc., in addition to providing rich habitats for biodiversity including many protected species. However, many of the world's wetland ecosystems have been removed, whilst others are under threat due to proximal land degradation, water quality pollutant impacts and/or water supply pressures (caused by drainage etc.), making them among Europe’s most threatened ecosystems.
Special area of conservation...Ballymore fen, County Westmeath
Understanding the eco-hydrogeological connectivity and environmental supporting conditions is critical to their management. The Environmental Engineering research group in Trinity College Dublin have been carrying out an EPA-funded research project, EcoMetrics, over the past four years to evaluate and develop methods for the assessment and definition of appropriate ecohydrological metrics in order to help policymakers conserve and/or restore such wetlands, as well as to achieve Water Framework Directive objectives as applied to groundwater-dependent terrestrial ecosystems (GWDTEs) in Ireland.
The project has focussed on three main wetland types: raised bogs, turloughs and calcareous fens, using a combination of existing data and additional field studies where required, particularly with regards to the fens. In addition, there has been an overarching work package on the use of remote sensing (RS) techniques to identify GWDTEs as well as to monitor changes in their ecohydrological health over time.
Four fens were instrumented and monitored over a two-year period, investigating both their hydrology and hydrochemistry in relation to their different vegetation habitats. The sites were Ballymore fen, Co Westmeath, Scragh Bog (fen), Co Westmeath, Tory Hill fen, Co Limerick and Pollardstown fen, Co Kildare. The fens were selected to represent a range of different conditions with at least one fen wetland considered to be under water quantity pressure (i.e., under pressure by drainage), one site considered to be under water quality pressure (i.e., under pressure from nutrient pollution), and one site that is considered to be still in a relatively pristine (near-intact) state.
Hard at work...EcoMetrics Team member's hydrometric monitoring of fens
Regular water quality monitoring was carried out across different transects on the fens comparing samples taken at the free water surface, at depths in the underlying sediments and from groundwater in the aquifers feeding the fens. The results of the monitoring provided a conceptual model of groundwater feeding the fens at discrete points, helping to maintain high water levels even across drought periods in the summer, as well as supplying relatively high concentrations of nutrients which appear to be picked up by the fen vegetation, thereby leaving lower nutrient concentrations in the surface water of the fen which leaves via surface runoff, eventually combining into a stream. The vegetation dies back annually, accumulating to form peat as well as partially degrading, thereby performing a key role within the in-fen nutrient and organic cycles within the underlying substrate.
In terms of water level, the field investigations across all fen habitats have a defined threshold water level envelope required for healthy fen vegetation: for example, the PF1 (rich fen and flush) habitat seems to require a higher envelope of water levels always above the ground surface from approximately 100 mm to 400 mm depth of flooding all year round.
In terms of water quality, the setting of thresholds is more challenging and is an area that needs further research, particularly with respect to the link between the wider supporting catchment groundwater water quality and quantity metrics and those metrics defined in the wetlands for different vegetation types.
Whilst envelopes of nutrients in the surface water of the fen associated with healthy fen vegetation can be defined, these levels should be regarded as the water quality after the fen vegetation has effectively treated the higher incoming nutrient levels in the groundwater. At the four fens studied the generally higher levels of nutrients in the groundwater feeds did not appear to be causing any ecological stress to the fen ecosystems and so groundwater threshold values cannot yet be defined with any confidence.
View from above...Mosaicking of drone images from Clara bog.
The RS study used existing habitat and vegetation mapping and began with a pixel-based approach using freely available Sentinel-2 data to map vegetation communities across raised bogs and fens using ensemble classifiers such as bagged tree. The pixel-based approach only takes into account the spectral properties of the area. Therefore, the study was further extended to segment-based learning approach which also considers the textural information on top of the spectral information.
Wetland mapping...Clara bog ecotope distribution (a) field survey reference map (2018) (b) drone augmented S2 classified map (2019).
This segmentation process worked well for the wetland maps and is termed a mapping vegetation communities (MVC) algorithm. A total of up to 18 classes were mapped temporally inside 13 wetlands using the MVC algorithm with an average accuracy of 84% for the years 2017 and 2018. The algorithm works very well for larger vegetation communities, but due to the restriction of spatial resolution (10 m) of Sentinel-2 data, some small communities were not adequately identified. Therefore, to gain high spatial resolution, an unmanned aerial vehicle (UAV) (i.e., a drone) was employed and a comparison was made between Deep Learning and Machine Learning methods.
The study then extends the ability of remote sensing-based monitoring of wetlands by combining the high spatial resolution of drones with the global coverage of the satellite data to create seasonal maps of vegetation communities within the wetlands. This nested methodology incorporates geo-referenced land-cover maps, scaled at the drone resolution level, and up-sampled to Sentinel-2 imagery level through interpolation.
More information on the project, including details on the raised bog and turlough aspects of the project, can be found in the EPA final and synthesis reports soon to be published.
Open access publications from the research project so far:
Nested drone-satellite monitoring
Monitoring vegetation communities using drone images
Monitoring vegetation communities using Sentinel-2 satellite images