Hydrology of a cultivated peatland in Northern Finland and implications for management

• Peat thickness affects the hydrological dynamics in controlled drainage peatlands. • Thicker peat (60–80 cm) yields more stable groundwater levels and slower drawdown. • Thin peat (<30 cm) hydrological dynamics resemble mineral soils. • Topsoil peat moisture responds to groundwater level rather than peat thickness • Non-uniform water management is recommended for fields with varying peat thickness. Peatlands are globally significant carbon storage and locally important for agriculture in Northern regions. However, cultivation on organic soils often results in nutrient leaching and greenhouse gas emissions. These impacts are strongly influenced by hydrology, yet the hydrological behavior of shallow peat soils (<1 m), which are common in many drained agricultural fields, remains understudied. In response, we studied hydrological processes in 6 fields with varying peat thickness (20–80 cm) over a mineral subsoil at the Ruukki NorPeat site in Northern Finland. The site features a controlled drainage system consisting of equally spaced perforated pipes installed at a depth of 120–130 cm within the mineral layer. Key hydrological variables, including groundwater table, soil moisture, drainage discharge, precipitation, evapotranspiration, and soil temperature, were continuously monitored from 2016 to 2021. Our results indicate that hydrological behavior strongly depends on seasonal climatic patterns, drainage management, and soil characteristics, particularly peat thickness. Plots with 60–80 cm peat thickness exhibited more stable groundwater tables and slower water table drawdown during dry periods, while thin peat plots (<30 cm) responded much more dramatically to hydrological inputs and losses, resembling behaviors of mineral soil. Despite prolonged drainage, the upper 30 cm peat layers remained a good buffer against short-term climatic variability and hydrological extremes. However, limited water storage in thinner peat makes these areas more susceptible to drought stress. Targeted measures such as earlier drainage closure or supplemental irrigation can be beneficial in these plots, though water table levels need to be balanced to maintain field trafficability.