Connecting fields to the river

By John Boardman, Environmental Change Institute

River pollution is a major problem with few rivers in the UK measuring up to the ‘Good’ standard under Water Framework Directive criteria.  Concerns are about ecological damage, voiced by the angling community, and the costs of purification borne by water companies and therefore consumers.

Pollution comes in many forms – when my grandson (8) swims in the Thames he asks me ‘Will I find a Shopping Trolley?’ In rural England, N and P, pesticides and metaldehyde (slug pellets) are pollutants. Damage to fisheries is often the result of fine sediments coating the beds of former gravel-based streams. Over 70% of fine sediment in watercourses in England and Wales is from agricultural sources.

The problems are acute and challenging in areas of intensive arable farming, an example being the Rother valley in West Sussex (Figure 1). Fertile, easily worked but erodible soils are used to grow winter cereals, maize, potatoes, vegetables, salad crops and asparagus.  Most fields in the low-lying belt near to the river are under crops, some with irrigation adding to the risk of runoff.

Figure 1. The Rother valley and fields with a history of erosion since 1987

Erosion in the form of gullies, rills or wash is a frequent occurrence and we have a database of almost 200 fields with a history of erosion since 1987 (Figure 2). This is based on one-off surveys, remote sensing, including Google Earth, and systematic monitoring of all the fields in the last five years.

Figure 2. Erosion near Petworth on a winter cereal field, February 2014

The threat to the river is not so much a result of high erosion rates (these occur occasionally), as the degree of connectivity between arable fields and the river.  Of the 200 fields about 68% are potentially connected to the river: runoff and sediment reaching the river by various routes (Figure 3). At times of exceptional erosion and during storms it is possible to map routeways and points at which sediment enters the river. This is not a task for the faint-hearted or for modellers. Maps and models are poor predictors of pathways of flow. They tell us little about field boundaries: are they permeable or impermeable?  We also need field evidence for the condition of ditches (cleaned or overgrown?) and the presence of culverts between fields and under roads. This is not to deny the usefulness of technology. Google Earth, when one is fortunate with the date of the image, is brilliant!

Figure 3. Routes from the fields to the river

Understanding patterns of connectivity gives us a chance to design mitigation measures to protect the river. In the Rother valley this is an ongoing challenge because with a predominance of high value crops it is not easy to persuade farmers to adopt less risky land use options. Also, detaining coarse sediment (sand) is not difficult but fine sediment tends to travel with the runoff and reach the river via leaky mitigation measures.

This research has been a co-operative effort. I thank Professor Ian Foster (Northampton), South Downs National Park, the Arun and Rother Rivers Trust, Catchment Sensitive Farming, Southern Water and the Environment Agency for help and data.

Further reading

Boardman, J. 2016. The value of Google Earth for erosion mapping. Catena 143, 123-127

Boardman, J. Vandaele, K., Evans, R., Foster, I.D.L. 2019. Off-site impacts of soil erosion and runoff: why connectivity is more important than erosion rates. Soil Use and Management 35(2), 245-256DOI: 10.1111/sum.12496