Catchment-based approach for water table management with irrigation for cultivated peatlands

Controlled drainage and subsurface irrigation have been proposed to enable shallow-drained agriculture in organic soils and mitigate greenhouse gas (GHG) emissions from intensively cultivated peatlands. However, the effects of current drainage practices on peatland water table depth (WTD) and the potential of using runoff from upstream catchment areas to adjust WTD in northern conditions are still poorly understood. To address these issues, WTD monitoring was initiated on 13 cultivated peatlands with different drainage systems in the flat western coastal region of Finland. Monitoring locations with old subsurface drainage, new subsurface drainage, and open ditch drainage had average WTD of 0.51 m, 0.82 m, and 0.95 m, respectively, during the two monitoring years (11/2022 - 10/2024). For each field, we estimated the size of the upper catchment, median summer total runoff, and mean 7-day summer low flow rate. The water required to reach a 0.3 m target WTD was estimated from peat specific yield. Each 0.1 m decrease in mean WTD was estimated to require of 13.2 mm of additional water. Median summer total runoff from the upper catchment was sufficient to reach any target WTD, but the summer low-flow rate did not fulfil the daily water demand. Most runoff is available during early summer, thus creating a timing challenge with water availability even in region with excess annual precipitation. This highlights the importance of catchment-scale management for GHG mitigation. In this study, we propose a generally applicable framework to link peatland GHG mitigation with water resources and catchment-scale management.