The impact of semi-natural woodland and pasture on soil properties and
streamflow.
Abstract
The increased frequency of flood events has motivated interest in
natural flood management (NFM), in particular the potential for
woodlands to reduce flooding. 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 woodlands as a flood mitigation method due to
limited empirical data available, particularly for semi-natural
woodlands. We established a correlation catchment study in Haweswater,
Cumbria, UK. Nine small upland catchments, each less than 0.2 km2 in
area, were established on semi-natural broadleaf woodland sites where no
stock grazing occurs or pasture with varied grazing intensity. At each
site soil characteristics were investigated, namely soil moisture,
permeability and bulk density. In addition, a v-notch weir was installed
within in each catchment to calculate flow. The specific peak discharge
(SPD), peak runoff coefficient, volume runoff coefficient and time taken
to flow response was determined at each site for 28 storm events, of up
to 205 mm, identified over a 13-month period. We found that semi-natural
woodland reduced SPD by 33-52 % compared with pasture, reducing SPD by
36 % during larger storms (> 1 mm/hr peak discharge).
Woodland reduced the peak runoff coefficient by 31-52 % and the volume
runoff coefficient by 13-22 % compared to pasture. Additionally,
response to storm events took 1-4 hours longer in woodland. These
differences in flood response can be somewhat explained by the more
permeable woodland soils, 4.6 times greater than pasture soil. Our
analysis strengthens the argument that woodlands can reduce
rainfall-generated flooding as a land use management method of NFM. Data
collected here should be used to inform the parameters in flood
prediction models and contribute to the evidence base for NFM.