Ecohydrological modelling of Irish peatlands

Development of a semi-empirical hydrological model, reflecting climate, topographic conditions and surface/near surface hydrological processes, to 44 raised bogs with detailed vegetation mapping across Ireland identified that peat-accumulating vegetation types are typically restricted to areas with local surface slopes of between 0.2% and 0.6%, depending on rate of effective rainfall. In areas of localised focused-flow or flushes, peat-accumulating conditions can develop on slopes exceeding 1%.

Combining this information led to the development of a hydrological metric to predict restoration potential in areas outside of areas mapped as peat-accumulating vegetation types. The hydrological modelling approach has underpinned the development of a long-term restoration management plan for Irish raised bogs, supported by the Irish Government.<br><br>The modelling approach developed for raised bogs was subsequently adapted and applied to four blanket bog catchments across the island of Ireland with differences in climatic conditions.

Modifications to the modelling approach were required given differences in ecological and hydrological conditions between raised bogs and blanket bogs. Analysis indicated a log-linear relationship between adapted model outputs and summer (April-September) water tables (median and D90 levels (i.e., water levels equalled or exceeded (shallower) for 90% of the time)). Higher model outputs were associated with areas with shallower and more stable water tables.

Deviations between model predictions of surface wetness and observed water table levels were associated with hydrological pressures, such as artificial drainage or the occurrence of macropores/peat pipes, which lowered summer water tables. Model outputs also showed significant correlation with peat thickness; however, correlation with ecological variables such as Sphagnum spp. cover proved poor. The adapted modelling approach displays significant potential to assist peatland managers in optimising the use of resources in selecting high priority areas for restoration of blanket bogs in Ireland.

Furthermore, the success in applying a climatic correction factor suggests the approach may be applicable to blanket bogs outside Ireland, with comparable climatic conditions.<br><br>At one of the blanket bog study sites restoration comprising ditch blocking with peat dams resulted in a widespread and rapid change in hydrological conditions less than one year after restoration, contrasting with findings from similar studies of blanket bogs in the UK. 18 monitoring wells (test wells) were located in areas where drain blocking was carried out, while 3 monitoring wells (control wells) were located in an area where drain blocking had no influence.

Comparison between summer water tables before ditch blocking to water table levels after ditch blocking demonstrated median summer water table levels were shallower at 89% of monitored locations, while summer D90 levels were shallower at 78% of monitored locations. Application of machine-learning modelling demonstrated the ability to predict blanket bog water tables from meteorological inputs and supported the findings of a widespread change in water table dynamics post-ditch blocking.

Analysis revealed a shift towards more intact hydrological conditions. Drain orientation, surface slope and contributing catchment area proved to be a key influence on the impact of drainage and subsequent water table response to ditch blocking.<br><br>Overall, modelling approaches demonstrate the importance of topographic conditions in reflecting hydrological processes on both raised bogs and blanket bogs. This has important implications in understanding the restoration potential of these habitats and where resources should be best prioritised, not only to meet habitat restoration targets but also in achieving wider ecosystem services benefits such as a reduction in carbon emissions.<br>