The impact of semi-natural broadleaf woodland and pasture on soil properties and flood discharge

Woodlands can reduce the risk of rainfall-generated flooding through increased interception, soil infiltration and available storage. Despite growing evidence, there is still low confidence in using woodlands as a flood mitigation method due to limited empirical data, particularly for broadleaf woodlands. We measured soil properties and streamflow for nine small (<0.2 km2) upland catchments and compared mature semi-natural broadleaf woodland where no stock grazing occurs to pasture with varied grazing intensity. We compared streamflow across 28 storm events including a 1 in 10-year event, two 1 in 4-year events and five 1 in 1.5-year events, identified over a 13-month period. We found that semi-natural broadleaf woodlands reduce specific peak discharge by 23%–60% and peak runoff coefficients by 30%–60% compared with pasture. Response to storm events took 14–50% longer in woodland compared to pasture. These differences in flood response are partly explained by more permeable woodland soils, 11–20 times greater than pasture soil. The more muted response of wooded catchments to storm events is consistent across the storms investigated, including Storm Ciara, a 1 in 10-year event. Our analysis strengthens the argument that semi-natural woodlands can reduce rainfall-generated flooding contributing to the evidence base for natural flood management.