effects of AGRICULTURE ON RIVERS

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