Progression of Drought in the Mor–Peat Profile of Transformed Drained Peatlands—a Modelling Study

Drained peatland forests cover a substantial fraction of boreal landscapes and are increasingly exposed to drought under a warming climate. After drainage, a mor humus layer has gradually developed on top of the peat—these sites are called transformed drained peatlands. This transformation fundamentally alters soil hydraulic properties and may increase drought sensitivity by weakening hydraulic connectivity between soil layers. However, the hydrological role of the mor layer and its impact on rootzone moisture availability remains poorly represented in current peatland ecohydrological models.

This study investigates how the mor layer regulates soil moisture and water table dynamics during dry periods. We compare the soil moisture and water table dynamics of a mor–peat profile using three different hydrological modelling approaches: i) a hydrostatic equilibrium model for the whole profile, ii) a mor-layer bucket model coupled to peat in hydrostatic equilibrium, and iii) a full 1D numerical solution of the Richards equation.

Preliminary results indicate that the mor layer can substantially modify near-surface soil moisture during drying events by limiting upward capillary flow from deeper peat layers. This hydraulic decoupling leads to faster topsoil drying and altered soil moisture profiles compared to simulations with an assumption of hydrostatic equilibrium. On the other hand, the drying mor layer rapidly starts to restrict evapotranspiration and protects water storage in the underlying peat. These effects are most pronounced during prolonged summer droughts, when evapotranspiration demand is high and capillary upflux becomes critical for sustaining root-zone moisture.

The proposed improvements to the description of hydrological interactions within the mor–peat profile advance process-based simulation of drought responses in transformed peatland forests. The results contribute to a better mechanistic understanding of soil moisture dynamics under meteorological extremes and provide a foundation for assessing drought risk and adaptive water management strategies in peatland forestry.