Information and Advisory Note Number 20
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Rivers and their catchments: physical impacts of agriculture
1 Introduction
1 1 This Information and Advisory Note
highlights the physical impact of agricultural
activities on rivers, and outlines ways in which
these impacts can be minimised. It addresses
those agricultural activities that affect the water
and sediment regime of rivers It does not
examine pollution of watercourses, which is
dealt with in the Code of Good Practice for the
Prevention of Environmental Pollution from
Agricultural Activity (PEPFAA code, SOAEFD,
1997), or the impact of farming waste, which is
dealt with in Nature Conservation and Pollution
from Farm Wastes (NCC, 1991). This note
should be read in association with Rivers and
their Catchments: an Overview (Information
and Advisory Note No. 18)
1.2 Agricultural activities often comprise a significant proportion of land use
in a catchment, and thus constitute an issue that must be addressed as part of
an Integrated Catchment Management (ICM) approach (see Information and Advisory
Note No 18) ICM comprises a catchment management plan to which all major
interests in the catchment voluntarily subscribe. There is currently no
legislative requirement for ICM in Scotland
2 Physical impacts of agriculture on rivers
and impact mitigation
2 1 Land drainage
2.1.1 Agricultural drainage impacts on the water and sediments of the river
system. Land drainage has been used since the mid 18th century in Scotland to
increase the suitability of land for cultivation. As a consequence, it has been
suggested that in times of flooding or heavy rain, water levels have risen and
fallen
much faster (i.e. floods have become more flashy) as a result of drainage
2 1 2 Drainage and ditching lower the water table to enable agriculturally
desirable plants to grow more productively. Water falling on the land is
transported away more effectively. Thus, in drained catchments, river flow matches
the rainfall profile more closely than in a natural catchment (although there
will be a time-lag relating to catchment characteristics and the preceding soil
saturation level). Drainage reduces the natural buffering capacity of the
catchment against floods, so the magnitude and frequency of floods downstream
may be increased. Conversely, as the natural 'sponge' effect of the land is
reduced, drained land may be drier for longer. Lowering of the water table can be
detrimental to wetland communities
2 1 3 Drains (infilled ditches with plastic or clay pipes in them, either with or
without gravel backfill) are used in fields to encourage subsurface flow,
whereas open ditches are more common in upland areas where overland flow
predominates. Infilled drains allow ploughing
2 1 4 Generally, field drains are not responsible for large sediment inputs, but
do cause an accelerated throughput of water. They constitute a long lasting
alteration of the natural flow regime and have become part of the 'natural'
process of surface water run-off
2 1.5 Ditch maintenance involves cleaning out deposited sediment and vegetation
growth. To limit impacts on habitat and damage to receiving streams, it is
recommended that ditch maintenance is done infrequently, at times that avoids
sensitive periods in the life cycles of aquatic species, and that only one side
of the ditch is cleared per year. This
allows some continuity of habitat. Minimising impacts to vegetation during
maintenance works also limits erosion Shallow bed gradients and vegetated side
slopes constitute good ditch construction techniques
2.1 6 The construction of flood embankments to protect agricultural land from
inundation is common. Rivers can breach these banks in periods of high flood,
often repeatedly at the same point. To reduce the likelihood of repeated
breaching, care should be taken during construction and repair to avoid altering
the natural morphology. Flood damage costs to the farmer include loss of crops
and topsoil, embankment reconstruction costs, and unnaturally high rates of
channel change. Embankments may cause the height of flooding to be increased
downstream since they prevent water occupying the floodplain
2 2 Abstractions for irrigation
2 2 1 Unregulated water abstraction for crop spraying and irrigation reduces the
flow left in the stream. This is particularly significant during periods of low
flow when a given extraction rate takes a greater percentage of the total
discharge. Aquatic flora and fauna will thus lose habitat, and pollution will be
less diluted in periods of abstraction and low flow.
2 2.2 Farmers should consult SEPA prior to abstracting water for irrigation,
since an abstraction licence may be needed. Farmers should also be encouraged to
abstract as little water as possible during low flow conditions, thus allowing
an adequate 'hands off discharge. This is particularly true for salmon spawning
rivers where an adequate intragravel flow is necessary to supply eggs and fry
with oxygenated water and to remove metabolites. Farmers can be encouraged to
build reservoirs which fill naturally during winter and provide a water source
for irrigation in summer
2 3 River engineering works
2 3 1 Piecemeal bank protection is often undertaken at sites where farmland is
being eroded. Methods include dumping boulders, concrete and car bodies against
the eroding bank. The use of such 'hard' material tends to alter instream flow
dynamics, deflecting the main current and causing bank erosion immediately
downstream. Traditionally, gabion baskets and rip-rap have been used to provide
long-term bank protection. They are now
widely questioned on environmental grounds as they are visually intrusive,
reduce bankside habitat availability and alter natural instream and bankside
processes. Other 'hard' bank protection methods include, the use of current
deflectors (though this may cause erosion on the opposite bank), the use of a
stone berm at the toe of the bank, and bank reprofiling to reduce bank slope
Soft engineering involves planting and the use of geotextile filter layers, and
aims to dissipate rather than deflect the river's energy. It is less obtrusive
than 'hard' engineering. Good practices for river engineering are detailed in Hoey
et al (1995).
2 3 2 Dredging for flood alleviation or drainage improvement may occasionally be
undertaken by farmers. Information and Advisory Note No 23 covers dredging
operations and mitigation measures in detail
2 4 Ploughing and crop cultivation
2 4 1 Soil erosion occurs when there is a combination of heavy rain or wind, and
exposed soil. Erosion by water most commonly occurs where there are clear routes
for the run-off to follow, such as plough furrows and tyre wheelings running up
and down the slope. The impact of erosion upon the river system is greater during
winter, when an increased sediment load due to exposed soil or high winds, and
faster run-off due to a saturated or frozen substrate, results in the arrival of
'coffee coloured' sediment laden run-off in streams
2 4 2 During storms, less than 1 % of the total number of arable fields in
Scotland are affected by rills and gullies. Some rainfall events do have a
significant erosive effect locally. These storms can be either intense or
prolonged, and preceding catchment conditions help determine the erosive outcome
of the storm. Rill and gully erosion occurs when run-off becomes concentrated by
irregularities in the soil surface.
2.4.3 Fields can be particularly vulnerable to soil erosion where crops such as
potatoes, swedes and turnips have been grown in rows. Erosion can be serious
where conditions have been wet at harvest and the ground between the rows is
rutted. Run-off may then become concentrated between the rows. Soil may also be
vulnerable to erosion after row crops are planted when there is a high
percentage of bare ground. Crop rotation may lead to different levels of soil
protection in different years, and ploughing-in of the crop residue after
harvest will improve the organic matter content of the soil. Cultivation of steep
slopes, and the compaction of land by heavy machinery also exacerbate erosion
problems
2 4.4 The amount of erosion and thus sediment entering a river, can be greatly
reduced by sensitive cultivation practices MAFF and SOAEFD have issued Codes of
Practices to encourage appropriate practices (MAFF 1993, SOAEFD 1997) Erosion
control is dependent upon good management, including sufficient crop cover,
appropriate crop selection, and good tillage practices
2.4.5 Land management techniques can be used to control the movement of water and
wind over the soil surface and hence reduce erosion. These methods are normally
used in conjunction with agronomic (plant cover) measures. Management techniques
include: ploughing, planting and cultivation along the contour, which can reduce
soil loss from a slope by up to 50%, contour bunds (earth banks) across the
slope acting as barriers to run-off; windbreaks (usually stands of trees but
also hedges, stone walls, brush fences etc.), and geotextiles on slopes to
provide temporary stability until vegetation becomes established.
2.4.6 Fields next to rivers should not be ploughed before or during winter when
erosion is exacerbated by periods of heavy, prolonged rain and flooding. If a
crop is sown in late summer or early autumn, e g. winter oil-seed
rape or winter barley, there is a good chance that there will be an adequate
crop cover to prevent serious winter erosion. Undersowing cereal crops with a
grass mixture will mean that there is a good cover of vegetation over the winter
after the crop is harvested, which will reduce the potential for erosion.
2 4 7 Direct inputs of sediment can be avoided by leaving buffer strips adjacent
to watercourses. The appropriate width for the buffer strip depends upon the
nature of the soils, the vegetation and the river. Treatment using fertilisers,
insecticides, pesticides and herbicides must avoid buffer strips. Water margins
can be an important wildlife habitat, and their value is increased if they are
protected by a buffer strip. If there is no buffer strip, there is a danger that
the margins will become just a nutrient and sediment sink, and that the
diversity of plants will be reduced as the area becomes dominated by plants that
can take advantage of these nutrients
2.5 Livestock
2.5 1 Trampling by cattle and sheep can compact the soil causing increased
run-off. Livestock also trample and feed on vegetation, the root systems of which
bind soil together. The destruction of vegetation reduces both the interception
of rainfall by plants, and the resistance to run-off created by the plants
themselves. This means that more of the rain falling on the land runs off into
the watercourse immediately after it has fallen, increasing soil erosion and
sediment transport. Heavy grazing of the riparian zone will affect the plant
community, reduce shading for fish and perhaps reduce beneficial inputs to the
stream
2.5.2 Livestock can break banks down by trampling. They can increase bed
roughness by repeatedly crossing the stream at certain points (this locally
increases flow height), and they disturb the bed, temporarily increasing
suspended sediment concentrations Increased bed roughness and loose soil from
trampling, increases the susceptibility of banks to erosion, particularly during
flood flows. In addition, the shear weight of livestock on a heavy saturated bank
could cause slumping
2 5 3 Livestock can be excluded from river banks by fencing off erosion prone
areas. This also allows a natural riparian zone to develop, improving marginal
shading of the stream and riparian habitat diversity. There is however,
some concern that larger riparian shrubs increase flood damage by increasing
frictional resistance and roughness, producing eddying Some managed access or
riparian zone grazing may form an effective compromise and allow livestock
continued access to water
3 Legislation and incentives
3 1 Some legislation and incentives relevant to agriculture are briefly
discussed below
3.2 Whilst local authorities have discretionary powers to undertake works to
prevent flooding of non-agricultural land, flood prevention on agricultural land
is generally the responsibility of the landowner. Any person proposing to
undertake drainage works must consult the Scottish Environment Protection Agency
(SEPA) to determine the necessary precautions to prevent water pollution. SEPA
has the statutory responsibility of forestalling or remedying the pollution of
water but has no statutory role in flood prevention schemes
3 3 The Countryside Premium Scheme (CPS) was launched in March 1997. It is a
conservation scheme for farmers and crofters that extends the range of
conservation incentives currently available and also contributes to the
rationalisation of the agri-environment programme. Drainage is not included in
this scheme. CPS can be used to create habitats and features that can help
mitigate or reduce soil erosion
3 4 Farm conservation plans (FCPs) can give comprehensive management advice to
help farmers identify practices which would reduce soil erosion and nutrient
run-off. FCPs are more comprehensive than the conservation audit requested for a
CPS application, and in cases where a significant conservation benefit might be
achieved, grants from SNH may be available to assist with the production costs.
3 5 Grants for drainage of in-bye land are available under the Agricultural
Business Improvement Scheme (ABIS) launched in 1994, which is part of the
Highlands and Islands Agricultural Programme (HIAP). The HIAP scheme is jointly
funded by the UK government and the EU under Objective 1 funding
3 6 Under the Crofting Counties Agricultural Grants Scheme (CCAGS), grants are
available to crofters and other eligible occupiers living in
the former crofting counties of Argyll, Ross and Cromarty, Inverness,
Sutherland, Caithness, Orkney, and Shetland, towards the cost of certain works
on their holdings, including ditching, under drainage, hill drainage, arterial
drainage and river works. Written authority from SOAEFD is required before
commencing work on arterial drainage and river works
3 7 Until 1999, the Secretary of State retains the responsibility for
maintaining schemes initiated by the Land Drainage (Scotland) Act of 1930,1935
and 1941. This legislation has now been repealed.
3 8 It is currently under debate whether planning permission is required for
river engineering works Under the Town and Country Planning (General Permitted
Development) (Scotland) Order 1992 (GDO), agricultural businesses can undertake
works without planning permission, although local authorities may need to be
notified before certain works begin
3 9 The Conservation (Natural Habitats etc)
Regulations (1994) are relevant to proposed
works in or affecting a European site (Natura
2000 sites), a Special Area of Conservation
(SAC) or Special Protection Area (SPA).
Article 6(2) of the EU Habitats. Directive states
that Member States are required to take steps
to avoid significant deterioration and
disturbance of natural habitats and species at
these sites
4 Further reading
Hoey, T.B , Smart, D W J , Pender, G & Metcalfe, N 1995 Alternative Methods of
River Management for Scottish Rivers Scottish Natural Heritage Review No 33
MAFF 1993 Code of Good Agricultural Practice for the Protection of Soil London,
HMSO
NCC 1991 Nature Conservation and Pollution from Farm Wastes Peterborough, Nature
Conservancy Council
SOAEFD 1997 Prevention of Environmental Pollution from Agricultural Activity Code
of Practice London, HMSO
5 Author
Author: Jonathan Clark, RPS Cairns Ltd
Editor: Katherine Leys, Earth Science Team
Scottish Natural Heritage
2 Anderson Place
EDINBURGH
EH6 5NP
Tel 0131-447-4784
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