Geoscience Reference
In-Depth Information
may be a regular agricultural activity, sometimes
every two to five years in heavy agricultural land (i.e.
clay soils). Normal plough depth is around 30 cm,
so that the effects of mole draining last beyond a
single season.
The aim of tile and mole drainage is to hold less
water in a soil. This may have two effects on the
overall hydrology. It allows rapid drainage from the
field, therefore increasing the flashy response (i.e.
rapid rise and fall of hydrograph limbs) in a river.
At the same time the lack of soil moisture may lead
to greater infiltration levels and hence less overland
flow. Spaling (1995) noted that in southern Ontario,
Canada, land drainage alters timing and volume
of water flow at the field scale, but it is difficult to
detect this at the watershed scale. Hiscock
et al
.
(2001) analysed sixty years of flow records for three
catchments in Norfolk, UK to try and detect any
change in the rainfall-runoff relationship during
this time. The conclusion of their study was that
despite much land drainage during the period of
study the rainfall-runoff relationship 'remained
essentially unchanged' (Hiscock
et al
., 2001). This
lack of change in overall hydrology, despite the
known land drainage, may be due to the two hydro-
logical impacts cancelling each other out, or else
that the impact of land drainage is small, particularly
at the large catchment scale.
Land drainage can be a significant factor in
upland areas used for forestry. A common technique
in Europe is using the plough and furrow method
of drainage. A large plough creates drains in an area,
with the seedlings being planted on top of the soil
displaced by the plough (i.e. immediately adjacent
to the drain but raised above the water table). Like
all land drainage this will lower the water table and
allow rapid routing of stormflow. A study at a small
upland catchment in the north-east of England has
shown that land drainage effects are drastic, and
only after thirty years of afforestation has the impact
lessened (Robinson, 1998). This long recovery time
may be a reflection of the harsh environment the
trees are growing in; other areas have recovered
much faster.
Salination
Salination
is an agricultural production problem
that results from a build up of salt compounds
in the surface soil. Water flowing down a river is
almost never 'pure', it will contain dissolved solids
in the form of salt compounds. These salt com-
pounds are derived from natural sources such as
the weathering of surface minerals and sea spray
contained in rainfall. When water evaporates the
salts are left behind, something we are familiar with
from salt lakes such as in Utah, central Australia,
and the Dead Sea in the Middle East. The same
process leads to salinity in the oceans.
Salination of soils (often also referred to as
salinisation) occurs when there is an excess of salt-
rich water that can be evaporated from a soil. The
classic situation for this is where river-fed irrigation
water is used to boost agricultural production in
a hot, dry climate. The evapotranspiration of salt-
rich irrigation water leads to salt compounds
accumulating in the soil, which in turn may lead to
a loss of agricultural production as many plants fail
to thrive in a salt-rich environment. Although
salination is fundamentally an agronomic problem
it is driven by hydrological factors (e.g. water
quality and evaporation rates), hence the inclusion
in a hydrological textbook.
Salination of soils and water resources have
been reported from many places around the world.
O'Hara (1997) provides data on waterlogging and
subsequent salination in Turkmenistan, the direct
result of irrigation. Gupta and Abrol (2000) describe
the salinity changes that have occurred in the
Indo-Gangetic Plains on the Indian sub-continent
following increased rice and wheat production.
Flugel (1993) provides data on irrigation return flow
leading to salination of a river in the Western Cape
Province of South Africa. Prichard
et al
. (1983) report
salination of soils from irrigation in California, USA.
Irrigation water often has a high total dissolved
solids (TDS) load before being used for agricultural
production. Postel (1993) suggests that typical
values range from 200 to 500 mg/l, where water is
considered brackish at levels greater than 300 mg/l.
Postel also states that using this type of irrigation
