Fen Management Handbook - Scottish Natural Heritage

3. Understanding Fen Hydrology
Quantity and quality of water are critical to fens, playing a major
role in determining the type of fen, vegetation and wildlife, what
products can be harvested, and further social and economic
benefits provided by fens. Understanding the basics of fen
hydrology is therefore essential to successful fen management.
This section provides an overview of fen hydrology and
hydrological assessment. The case studies at the end of the
section illustrate how the hydrological regime has been assessed
for three different fens. Guidance on managing fen hydrology,
including troubleshooting and finding sustainable solutions, is
covered in Section 5: Fen Management and Restoration.
3.1 The basic principles of fen hydrology
Fens are terrestrial wetlands fed by surface water and/or groundwater, as well as
direct input from rainfall. They are characterised by high soil water levels for all or
part of the year, and are often based on peat. The landscape setting combined
with the presence of ground- or surface water largely defines the hydrological
functioning of a fen. The availability of water from the various sources will obviously
vary throughout the year, and will differ between years. Both water inputs to the
fen (the source and method of supply which feeds the fen) and water distribution
through the site need to be considered.
3.2 The importance of water
Fens require:
– a reliable source throughout the year of surface water and/or groundwater in
quantity and quality appropriate to the specific type of fen;
– a location with a perennial supply of water (such as a groundwater-fed spring)
or which retains water (such as a dip in the landscape with impermeable soil),
either or both of which can result in high levels of surface ‘wetness’ for all or part
of the year.
The amount of water required to maintain a particular type of fen, and how this
varies throughout the year, is described as the hydrological regime, which
might include the level of groundwater or the duration and depth of a flood. The
hydrological regime is important for the ecological functioning of fens because:
a. High water tables generates physical conditions that exclude some species.
For example, prolonged flooding can reduce the anchoring capacity of the
soil and make it more difficult for trees to establish.
b. Higher water tables deprive soil of oxygen (i.e. generate anoxia) which
limits species which have not adapted to these conditions. Anoxia can
also increase concentrations of phytotoxic chemicals such as sulphide and
ammonia.
c. Water is the main carrier of dissolved chemicals to a fen and thus strongly
influences the acidity and fertility of the site (see Section 4: Understanding
Fen Nutrients), which in turn affects the type and growth of vegetation and
fauna which it supports (see Section 2: Fen Flora and Fauna).
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