The hydrology of cultivated peatlands in Norway

Peatlands cover only ∼3% of the earth’s land surface, but store ∼30% of the world’s soil carbon. Owing to the limited availability of mineral soils, particularly in the northern latitudes, some peatlands have been drained for various purposes, including agriculture, forestry, horticultural peat extraction, and livestock grazing. Although drainage lowers the water table and improves plant growth, it causes significant environmental consequences, including increased greenhouse gas emissions, nutrient leaching, and land subsidence all which result from peat decomposition.

Drainage and hydrology of agricultural peatlands in Norway are not well understood. To address this knowledge gap, this dissertation investigated the water table dynamics, soil moisture, soil hydraulic properties, and grass yield in Norwegian agricultural peatlands drained by peat inversion, profiling (the surface is sloped), and pipe drainage (sub surface pipes are placed). This is also the first comprehensive assessment of hydrology in inverted peatlands, a unique drainage method in Norway. In this method, the underlying mineral soil is excavated and placed on top of peat and formed as a cover layer to reduce peat decomposition. Furthermore, this work presents the first investigation of drainage performance under Arctic conditions, providing new insights into drained peatland hydrology and management in cold climate environments.

Water table dynamics were monitored using in-situ sensors at four agricultural peatland sites across a climatic gradient from wet and warm coastal regions to dry and cold continental regions. Soil samples from both peat and mineral layers were studied for hydraulic properties. The inverted peatlands had the lowest water table than graded fields and pipe-drained fields. At the site receiving more than 2000 mm of annual precipitation, the inverted peatland produced higher grass yields than the adjacent pipe-drained field.

The results indicate that the mineral cover layer in inverted peatland resulted in deeper and more prolonged soil frost than graded or pipe-drained fields. Moreover, drainage systems in continental climates, characterized by long winters, should be designed to efficiently accommodate spring snowmelt, whereas in coastal climates should prioritize rapid drainage and reduced surface saturation due to autumn and winter rainfall events. Furthermore, climate change is expected to bring warmer winters, more frequent mid-winter snowmelt events, and increased precipitation in the northern hemisphere. Therefore, drainage design and management should incorporate anticipated future climatic conditions to ensure long-term effectiveness.

While several advanced hydrological and hydrogeological models are available to simulate water table dynamics in agricultural peatlands, a key finding of this study is that a freely available, and easy-to-implement sequential coupling of the HBV-light (the hydrologic model) and MODFLOW (the groundwater model) models can be effectively used to simulate water table dynamics in peatlands with complex soil structures. Moreover, this novel approach requires relatively less computational effort and simulation times than advanced models (which uses an integrated approach considering two-way feedback between surface and groundwater processes), making it a practical alternative for peatland hydrological studies.