Trees and Flood Risk in Cumbria — What the Evidence Says and What Landowners Can Do

Flooding is not an abstract risk in Cumbria. The events of December 2015, when Storm Desmond brought unprecedented rainfall across the county and caused catastrophic flooding in Carlisle, Keswick, Appleby, Kendal and dozens of other communities, made the relationship between upland land management and downstream flood risk a live and urgent question across the region. Trees and woodland are a significant part of that conversation.

As a team that works in the landscape of Cumbria every week, including in the upland catchments above many of the affected communities, we have watched the debate about trees and flooding develop with considerable interest. The evidence base has grown substantially over the past decade and the picture is now considerably clearer than it was. This article summarises what we know, what the evidence shows and what it means for landowners and land managers in Cumbria.

How Trees Affect Water Movement Through the Landscape

The relationship between trees and water is more complex than either enthusiasts or sceptics tend to acknowledge. Trees affect water movement through the landscape in several distinct ways, and it is worth understanding each of them separately.

Interception is the capture of rainfall by the canopy before it reaches the ground. A significant proportion of total rainfall in a wooded catchment never reaches the soil at all, being evaporated directly from leaf surfaces. Studies in the UK have estimated interception losses of between fifteen and forty percent of total rainfall in woodland, varying by species, season and rainfall intensity. Interception is highest for conifers, which retain their needles year-round, and lowest for deciduous broadleaf woodland in winter when the canopy is bare.

Infiltration is the rate at which water moves from the soil surface into the soil profile. Woodland soils, with their root channels, earthworm activity and organic matter, typically have infiltration rates many times higher than the equivalent area under improved grassland or bare ground. This means that water arriving at the soil surface under woodland moves into the soil rapidly rather than running off the surface, reducing peak flows into watercourses.

Evapotranspiration is the combined loss of water through evaporation from the soil surface and transpiration through plant leaves. Woodland has higher evapotranspiration rates than grassland, meaning that woodland catchments lose more water to the atmosphere over the year, reducing total water input to watercourses.

Root water uptake affects soil moisture content over time. Trees extract water from the soil profile through their roots, reducing soil moisture and maintaining greater capacity to absorb rainfall during the next precipitation event. This effect is most significant during the growing season.

What the Evidence Says About Woodland and Flooding

The evidence from academic research and practical demonstration projects now points fairly consistently in one direction: woodland in the right location, managed well, does reduce flood peaks in upland catchments. The key phrase is in the right location.

The most significant flood risk reductions from woodland come from planting in the upper catchment areas of rivers, where the relationship between land surface management and runoff generation is most direct. In the headwaters of rivers like the Kent, the Lune, the Eden and their tributaries, woodland and scrub cover slows water movement and increases infiltration in a way that measurably reduces peak flows in the watercourses downstream.

The Pontbren project in mid-Wales, one of the most extensively studied examples of farm-scale woodland and hedge planting for flood risk reduction, demonstrated reductions in peak flow at the field scale of up to fifty percent from relatively modest areas of woodland planting on critical runoff pathways. Scaling these results to whole catchments is complex and contested, but the direction of effect is clear.

In Cumbria, the Cumbria Woodlands partnership and Natural Flood Management programmes led by the Environment Agency, the Lake District National Park and various catchment partnerships have been working to increase tree cover in key locations in the catchments most affected by the 2015 flooding. The Holnicote Estate in Somerset, though not in Cumbria, provides one of the most detailed long-term evidence sets for natural flood management at landscape scale and points consistently toward the value of woodland and scrub as part of an integrated flood management approach.

What Landowners in Cumbria Can Do

For farmers, estate managers and other landowners in the Cumbrian uplands and river catchments, the practical implications of this evidence are becoming clearer. Several things are worth considering.

Woodland creation on critical runoff pathways, particularly on steeper ground in upper catchments where surface runoff is generated most rapidly, can make a meaningful contribution to flood risk reduction downstream. Grant funding through the England Woodland Creation Offer, and additional payments available for woodland that contributes to water quality and flood risk reduction, makes this financially more viable than it has been previously. See our article on creating woodland in Cumbria for more on the practicalities.

Riparian woodland, trees along watercourses and stream banks, provides multiple benefits including bank stability, water temperature regulation, in-stream habitat and some contribution to slowing water flow at peak events. Alder and willow are the primary species for riparian planting in Cumbria, and their establishment on bare riverbanks is relatively straightforward where deer pressure is managed. Our tree planting services include riparian planting across our coverage area.

Retaining and managing existing woodland in the catchment, rather than converting it to other land uses, is as important as creating new woodland. The infiltration capacity and interception function of existing woodland is a valuable natural asset that, once lost, takes decades to rebuild.

Managing woodland in the floodplain itself is more complex. Floodplain woodland, sometimes called wet woodland or carr, has significant wildlife value and does contribute to flood attenuation through flow resistance during flood events, but large woody debris in the wrong location can also contribute to blockages at bridges and culverts. Management needs to be carefully considered in relation to the specific watercourse.

A Note on What Trees Cannot Do

We want to be honest about the limits of what woodland can achieve. Trees are not a complete solution to flood risk in Cumbria, and anyone who presents them as such is oversimplifying. The rainfall events that caused the 2015 flooding were of an intensity that would have produced significant flooding even from fully wooded catchments. At the scale of a major storm event, the contribution of woodland to flood risk reduction is real but modest relative to the total water volumes involved.

Trees are one component of an integrated approach to managing the landscape for water, alongside better drainage design, floodplain reconnection, traditional flood defences in settlements and changes to grazing management. They are an important component, and one whose value is increasingly recognised and funded, but they work alongside other measures rather than replacing them.

Advice for Cumbrian Landowners

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