Geoscience Reference
In-Depth Information
size storms. The high interception losses were
experienced during small rainfall events and vice
versa. The interception loss from the Amazonian
rain forest is remarkably low, reflecting a high
rainfall intensity and high humidity levels. Overall
there is a high degree of variability in the amount
of interception loss that is likely to occur. While it
may be possible to say that in general a land use
change that has increased tree cover will lead to
a water loss, it is not easy to predict by how much
that will be.
Fahey and Jackson (1997) conclude that with the
loss of forest cover both low flows and peak flows
increase. The low flow response is altered primarily
through the increase in water infiltrating to ground-
water without interception by a forest canopy. The
peak flow response is a result of a generally wetter
soil and a low interception loss during a storm
when there is no forest canopy cover. The time to
peak flow may also be affected, with a more sluggish
response in a catchment with trees. In a modelling
study, Davie (1996) has suggested that any changes
in peak flow that result from afforestation are not
gradual but highly dependent on the timing of
canopy closure.
In Chapter 5 the issue of measurement scale was
discussed, and it is particularly pertinent for issues
of land use change. There has been considerable
debate in the hydrological research literature as to
how detectable the effects of deforestation are in
large river catchments. Jones and Grant (1996) and
Jones (2000) analysed data from a series of paired
catchment studies in Oregon, USA and concluded
that there was clear evidence of changes in inter-
ception rates and peak discharges. Thomas and
Megahan (1998) reanalysed the data used by Jones
and Grant (1996) and came to the conclusion that
although there was clear evidence of changes
in peak flows in the small-scale catchment pairs
(60-100 km 2 ) there was no change, or inconclusive
evidence for change, in the large catchments (up to
600 km 2 ). There has followed a series of letters
between the authors disputing various aspects of
the studies (see Water Resources Research volume 37:
175-183). This debate in the research literature
mirrors the overall concern in hydrology over the
scale issue. There are many processes that we mea-
sure at the small hillslope level that may not be
important when scaled up to larger catchments.
Land drainage
Land drainage is a common agricultural 'improve-
ment' technique in areas of high rainfall and poor
natural drainage. In an area such as the Fens of
Cambridgeshire, Norfolk and Lincolnshire in
England this has taken the form of drains or canals
and an elaborate pumping system, so that the
natural wetlands have been drained completely. The
result of this has been the utilisation of the area for
intensive agricultural production since the drain-
age took place in the seventeenth and eighteenth
centuries. Since that time the land has sunk, due
to the removal of water from the peat-based soils,
and the area is totally dependent on the pumping
network for flood protection. To maintain this net-
work vegetation control and clearance of silt within
channels is required, a cost that can be challenged
in terms of the overall benefit to the community
(Dunderdale and Morris, 1996).
At the smaller scale, land drainage may be under-
taken by farmers to improve the drainage of soils.
This is a common practice throughout temperate
regions and allows soils to remain relatively dry
during the winter and early spring. The most com-
mon method of achieving this is through a series of
tile drains laid across a field that drain directly into
a water course (often a ditch). Traditionally tile
drains were clay pipes that allowed water to drain
into them through the strong hydraulic gradient
created by their easy drainage towards the ditch.
Modern tile drains are plastic pipes with many small
holes to allow water into them. Tile drains are
normally laid at about 60 cm depth and should last
for at least fifty years or more. To complement the
tile drains ' mole drainage ' is carried out. This
involves dragging a large, torpedo-shaped metal
'mole' behind a tractor in lines orthogonal to the tile
drains. This creates hydrological pathways, at 40-50
cm depth, towards the tile drains. Mole draining
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