Fen Management Handbook - Scottish Natural Heritage

the water level relative to the ground surface. Measurement of soil water levels will
therefore normally be an important element of a monitoring programme.
The soil water level in a dipwell will be at the level where the water pressure equals
atmospheric pressure in the surrounding soil (also termed the water table). This is
not the upper limit of saturated soil conditions, because a capillary fringe will extend
upwards from the water table in the soil. The height of the capillary fringe depends
on the pore size distribution in the soil and can be approximately calculated if
information regarding soil type is available.
The following points should be considered when deciding which of the wide variety
of techniques available for monitoring soil water levels is the most appropriate:
Access to the soil water surface for measurement. The simplest (and cheapest)
option is a hand-dug open hole or an open hand-augered hole, but the substrate
must be sufficiently cohesive for the sides of the hole to remain stable. If necessary,
holes should be cleared of any collapsed soil prior to measurement and time
allowed for the water level to equalise. Open holes allow direct addition of rainfall
to, and evaporation from, the soil water level, which means that the level measured
in the hole might not be representative of that in the surrounding soil. This method
is not therefore recommended where real accuracy is required, but such cheap,
‘quick and dirty’ measurements which yield virtually instantaneous results can be an
extremely valuable in getting a feel for a habitat and understanding whether more
detailed monitoring is required. Open holes are a hazard for small animals and
people, so should be filled once any observations on that individual occasion have
been made. A better option is installation of a ‘dipwell’, as described overleaf.
Establishment of a fixed vertical reference point (often termed a datum). For
measurements of the soil water level to be repeatable, a fixed vertical reference
point must be available. Where a dipwell is inserted into a firm substrate, such that it
is vertically stable, then its top lip can be used as the datum. Fen substrate is often
soft or even fluid (i.e. vertically unstable), but a wooden or metal stake hammered
into solid ground at depth will create a separate datum. The datum also enables the
measurement of fluctuations in the floating raft.
If the critical parameter is distance above or below the ground surface, a
measurement of the distance between the datum and the ground surface at each
dipwell is sufficient. If soil water levels across the site need to be related to each
other, for example if a water level transect or water level contours across the site is
required, the elevation of each dipwell relative to a common ordnance datum needs
to be established.
Method of measurement. Manual measurements can be made with a tape
measure if the water level will not fall below a level where it is visible, but preferably
with an electronic dipmeter. The measurement should be made from, or related to,
the datum; this is simple when the datum is the top of the dipwell, but can involve
the use of a spirit level and tape measure to check the relative elevations of a
dipwell and separate datum. Automated measurements are made using a combined
pressure-transducer and data-logger instrument. The pressure-transducer is
suspended in the water column, and the measured pressure is proportional to the
height of the column of water above it, minus the atmospheric pressure, which
must be measured in the locality. A typical pressure-transducer (0-10 m water
pressure range) will provide an accuracy of +/- 5 mm, a precision of +/- 1 mm,
and the period between downloads of monitoring data is constrained by the need
to safeguard data rather than the capacity of the instrument to store the data. It is
prudent to download transducers at least every three months to safeguard data.
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