With the terrible flooding that has been
experienced around the UK this last winter, I have just taken part in a panel debate on Radio 4’s Farming Today, aired on 19th April 2014, Easter Saturday. My role
was to provide a view about the ‘science’ of flooding and farming, what we know
and what we do not know. I hope the
following is a useful précis of the position…..
To
what extent does agriculture contribute to flooding events?
We know quite a lot about what happens in the
soil and even at the field or paddock scale, but as the scale increases so does
the uncertainty about what happens at the
catchment and basin scale – so the larger the catchment, the more uncertain is
the link between cause and effect. Local
flooding can be reasonably attributed to local agricultural sources, such as
the treading by animals or compaction arising from trafficking, all of which
affect the soil hydraulic conductivity, but larger than this there are other
things that come into play – topography, local geology, urban inputs etc..
To help the debate it is important to
consider the science of flooding. A
flood occurs because the rate input if water to the river channel exceeds the
capacity of the channel to hold the water, hence the catastrophic effects we
can often see when the rivers burst their banks. So, anything that can slow down the water
flow of the inputs is a good thing for flooding, anything that speeds it up is
not…...
About 60-70% of UK land is under
agriculture so clearly this is an opportunity for agriculture to play a
positive role in flood management (note also, conversely there are other
responsibilities too in non agricultural land 30-40% of the land area). Consider the analogy of a sponge. A sponge is good at holding water and that is
desirable and, by and large, is reasonable to say that soil can help act in
this manner – having pores spaces and a hydraulic conductivity that can help
slow down and store water. Conversely a concrete
car park is the opposite extreme as it has virtually no hydraulic conductivity
below the surface, and the speed of water flow away from a concrete car park to
he river channel is fast. This is very
undesirable, but on some occasions the same can happen to agricultural soil, as
soil becomes compacted, overland (fast flowing) runoff is increased and to
return to the original analogy, the sponge becomes destroyed.
At the soil
and field scale there has been lots of good research that supports this,
for example:
1.
There is a significant positive
relationship between saturated hydraulic conductivity and macroporosity. Conversely, trading by livestock has been
shown to reduce macroporosity.
Presence of animals, as opposed to forestry and trees, generally reduces
the hydraulic conductivity.
2.
Trafficking by farm vehicles
has a similar effect on soil storage.
3.
Some research has also shown an
affect of compaction in urban parks, due to foot, bicycle and urban traffic.
4.
Recent work has shown that
maize, plus other late sown crops, over wintering of livestock can all degrade
soil structure in SW England.
However, and this is so important in the
debate, at the catchment and basin scale
there is considerable uncertainty about linking the field scale affects to the
flood peak, as many contributing factors come into play. A great study published by Miles Marshall et
al in Hydrological Processes in 2009 focussed on the Pontbren Catchment in
Wales, a small tributary of the River Severn.
Here, they tried to overcome the issues of scale by studying plots as
well as the catchment up to 10 km2.
They say (I quote):
“there is little evidence to suggest that these
effects propagate downstream. This does not imply that these effects do not
exist, but rather that there has been little work to quantify this (O’Connell
et al ., 2007) and that the effects are difficult to distinguish from
catchment-scale data (Beven et al ., 2008).”
Miles Marshall et al in Hydrological
Processes, in 2009
So, in summary, there
is strong evidence at the field farm and plot scale, less so when we get to the
large scale – just because of the complexity of real catchments.
What solutions can farming offer?
There is no silver
bullet! However, I think that because
of the sheer land are where farming is so dominant (60-70%) then there are lots
of positive things that farmers can do. As
well as being food producers, farmers are inevitably custodians of the
landscape and my view is that nobody knows how to manage their land better than farmers do! So I start from the proviso that farmers know
best and they do not want to lose soil and water from their land. Back to the science
briefly, it is all about slowing down the runoff, increasing the soils
hydraulic conductivity – because water that is stored in the soils is better
than fast overland flow.
Things that can be done:
1.
Soil mechanical management, to
include use of tine to disrupt tramlines, topsoil lifting, sub-soiling, use of
tine (especially on tramlines) – all of these are mechanical things that a
farmer can do to improve structure and enhance pore space at depth.
2.
Landscape management, to
include hedgerows, berms, riparian areas, ponds. Ponds offer great potential, historically
there were many across the country, I am pleased to say numbers are now
increasing, we have about 500000 ponds in the UK today, and more can be
helpful. In the upland, blocking upland
drainage ditches can also be helpful, there is on-going work on Exmoor that is
demonstrating this from Richard Brazier.
3.
Animal management. In areas where the localised threat is
obvious, over wintering of livestock outdoors is to be avoided, and measures to
avoid the concentrating of livestock, for example around feed and drink
troughs, should be encouraged, by regular movement of the troughs.
4.
Tramline management. A single pass of a shallow tine has been
shown to have a positive effect from work by Martyn Silgram, spreading of straw
and vegetation of tramline is also helpful.
In extreme cases, , a gantry system may be helpful to avoid creating
tramlines.
5.
Naturalizing - in some cases the
use of trees can help, silvopasture systems are being talked about as having
benefits, and strategic use of trees, tree shelter belts around the
landscape.
6.
‘Hi Tech Plants’ – this is slightly
‘blue’ skies, but I
was recently involved in a project where soil scientist and plan scientists
collaborated to select grass plants with improved rooting properties that could
be used to better retain water – there may be some hi – tech biological
solutions out there.
7.
Partnerships, Tools, Strategic
Assistance can be a great way forward.
In the South West (cited in Palmer and Smith, December 2013) a great
initiative apparently hosted by The EA purchased and shared a subsoiler, and on
50% of the sites where it was used the soil structure was improved. Palmer and Smith also suggest a field
assessment tool for assessing soil structural stability – this has to be
useful. A great example is the National
Defra Demonstration Test Catchments as a way of bringing farmers, catchments
managers, residents, the EA, Rivers trusts and academics together to find a
common long-term solution. This has to
be the way ahead.
How
can we prioritise between the protecting of farmland and the protection of
homes?
This is difficult but we must try to do both. What is very obvious, local hydrological
connection between the farm and the flood, then we have to determine risk, and
roll out strategies (outlined in the above) to cope with this. If the risk is high and the hydrological
linkage is compelling, then more of the measures above need to be used. It is also about raising awareness and
communities to deal with the flooding together.
I was recently involved in a collaborative project called the NERC Virtual Observatory
where we tried to bring together farmers, agencies, and residents, to
understand the issues and potential solutions, together.
Is there choice between
flood defence and food security?
Absolutely not, I do not see that it is at al helpful to polarise
the debate here. Clearly there are
high-risk vulnerable flood areas that we need to take a close look at, but the
most sensible solution is remembers that soil and farms are by and large well suited
to retaining water, so if we can find an integrated solution with food
production and reduced flooding, this has to be the way ahead. We need to value and respect farmers as
custodians of the landscape and help them, in partnerships with agencies and
academics, use tools to assess risk and to find solutions together.
P. M. Haygarth, Lancaster University, 16th April 2014
