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