Soil characteristics and landcover relationships on soil hydraulic conductivity at a hillslope scale:a view towards local flood management

摘要

There are surprisingly few studies in humid temperate forests which provide reliable evidence that soil permeability is enhanced under forests. This work addresses this research gap through a detailed investigation of permeability on a hillslope in the Eddleston Catchment, Scottish Borders UK, to evaluate the impact of land cover, superficial geology and soil types on permeability using measurements of field saturated hydraulic conductivity (Kfs) supported by detailed topsoil profile descriptions and counting of roots with diameters >2 mm. Kfs was measured at depth 0.04–0.15 m using a constant head well permeameter across four paired landcover sites of adjacent tree and intensely grazed grassland. The measured tree types were: 500-year-old mixed woodland; 180-year-old mixed woodland; 45-year-old Pinus sylvestris plantation; and 180-year-old Salix caprea woodland. The respective paired grids of trees and grassland were compared on similar soil texture and topography. This study highlights the significant impact of broadleaf woodland at a hillslope scale on Kfs in comparison to grassland areas: median Kfs values under 180-year-old S. caprea woodland (8 mm/ h), 180-year- 43old mixed woodland (119 mm/ h) and 500-year-old Broadleaf woodland (174 mm/ h) were found to be respectively 8, 6 and 5 times higher than neighbouring grazed grassland areas on the same superficial geology. Further statistical analysis indicates that such Kfs enhancement is associated with the presence 46of coarse roots (>2 mm diameter) creating conduits for preferential flow and a deeper organic layer in the topsoil profile under woodlands. By contrast the P. sylvestris forest had only slightly higher (42 mm/ h), but not statistically different Kfs values, when compared to adjacent pasture (35 mm/ h). In the grassland areas, in the absence of course roots, the superficial geology was dominant in accounting for differences in Kfs, with the alluvium floodplain having a significantly lower median Kfs (1/ mm h) than surrounding hillslope sites, which had a range of median Kfs from 21 to 39 mm/ h.The data were used to infer areas of runoff generation by comparing Kfs values with modelled 15 min maximum intensity duration rainfall with a 1 in 10 year return period. Infiltration prevailed in the 180and 500 year old mixed and broadleaf woodland, whereas some grassland areas and the floodplain were 55inferred to generate overland flow. The significantly higher Kfs under deciduous mature forests suggest that the planting deciduous woodlands on hillslopes in clusters or as shelterbelts within grasslands would provide areas of increased capacity for rainfall infiltration and arrest runoff generation during flood-producing storm events.