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Flood related soil erosion - impact of farming practices

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Flooding in December 2015 impacted many farms with various degrees of severity. Whilst the immediate problems associated with the recent flooding is to deal with the damage to property and people’s lives, it is worth considering how land management can affect surface runoff volumes and whether, in some circumstances, better soil management can help reduce flooding risk.

Flood related soil erosion  - impact of farming practices

Recent extreme rainfall events in parts of the UK had devastating impacts on rural areas in parts of North Wales, Northern England and Scotland, outlined in a recent ADAS article “Two thousand livestock drowned, infrastructure damaged and agricultural land destroyed in recent flooding”. Severe flooding led to damage to transport routes, collapse of infrastructure (including bridges, walls and buildings), accelerated soil erosion and loss of livestock.

The way soil and land are managed can influence water movement and some farming practices may have contributed to increasing surface runoff and the speed with which flow increases in ditches, streams and rivers.

David Harris, an experienced ADAS consultant in this area explains that “soil compaction on arable land has been a concern for some time and changes in soil management practices may help reduce its impact on production and the environment.  Recently, it has become apparent that many grassland soils are in moderate or poor condition,  and this may be due to increases in the size of machinery in recent decades, the extent of tracking, treading by livestock and the greater area that larger farmers need to manage in often narrow ‘windows’ of good weather.”

Recent work involving ADAS has looked at tyre choice and management, tramline management and removing compaction in grassland. 

We have been able to increase water infiltration in grassland where topsoil structure has been degraded using techniques such as mechanical loosening, and on arable land to reduce surface runoff by over 90% by methods such as tramline disruption” says David.

Although there is much talk of the damage caused by surface runoff it is not always easy to see what the problem is or to relate it to your own farm.  When high intensity or persistent rain falls on bare soil and/or compacted ground, the nutrients and sediment in surface runoff and the organic loading from recently applied organic materials can have significant impacts on the ecology of rivers and lakes.

“For the farmer, the losses of soil and nutrients may seem minor” David explains, “however, recent work from the ‘Demonstration Test Catchment’ (DTC) project (Defra) has indicated that soil and nutrient losses can be worth around £100 per hectare for a typical dairy farm.  Furthermore, surface runoff can also result in the loss of soil organic matter, making soils more difficult to manage and less resistant and resilient to further damage from compaction and erosion”.

Soil losses from a field are not necessarily apparent when the erosion is in the form of a sheet of water moving across the soil surface (i.e. ‘sheet wash’).  This is particularly the case in grassland.  Nevertheless, losses from sheet wash over a number of hectares can still be significant. Rill erosion in the form of small channels is more common in arable soils and for a farmer may be no more than a minor embarrassment that is easily rectified through cultivation at the end of the season, but a number of small rills can add up to large volumes of transported soil.

 “Recent work in the DTC project has shown that soil losses from grassland can be as high as 1.3t/ha and from arable crops up to 30t/ha.  Whilst this may not look too disastrous spread over a whole field, it would look far worse if focused as gully erosion in a confined area” explains David.

In grassland, surface compaction can reduce infiltration of rain water and the proliferation of roots through the topsoil and into the subsoil. In winter such soil compaction can result in prolonged waterlogging, while in a dry summer reduced rooting depth and proliferation can result in the earlier onset of drought.  Both situations can impact the yield from more productive grass varieties and can over time can also favour the survival and persistence of less productive grassland plant species.

If you have been affected by the recent flooding, or have concerns about flood risk on your farm, talk to an ADAS adviser to understand more on the practices to build resilience and reduce the likelihood of your land being affected in the future. David Harris can be contacted on 01332 776150 or david.harris@adas.co.uk

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