Generation and regulation of summer runoff in a boreal flat fen

Hydrometric measurements, electrical conductivity, water isotopic and hydrochemical components of stream water were used to study summer runoff generation in a flat fen. Different processes generated runoff at low- and high-flows. At storm-flows, fen runoff was generated from overland flow, originating from upland surface water. Temporary storage of water on the fen surface attenuated and delayed flow peaks. At low-flows, runoff at the fen outlet was generated from shallow subsurface flow in the Acrotelm. During low-flow periods, water originated mainly from peat storage water while during episodic events the wetland water storage was renewed by inflowing stream water. Assessment and modeling of hydrological effects of peatlands should be performed separately for low-flows and high-flows, based on the dominating runoff generating processes. Attenuation and retardation of storm-flows in fens by temporary surface storage will depend on the geometric properties of both storage sections and sections controlling outflow. A routing reservoir model adapted for flat fens can be used for simulation of attenuation and retardation in runoff events, and it is suggested that the model concept should be tested for a broader range of peatlands.