Soil Characteristics and Landcover Relationships on Soil Hydraulic Conductivity at a Hillslope Scale: A View towards Local Flood Management. Archer, N. A. L., Bonell, M., Coles, N., MacDonald, A. M., Auton, C. A., & Stevenson, R. Journal of Hydrology, 497:208–222, August, 2013.
doi  abstract   bibtex   
We evaluate woodland/grassland cover and soil types to reduce local flooding. We measured field saturated hydraulic conductivity under grassland and woodland. Established broadleaf woodland had significantly higher infiltration rates than grassland. 1 in 10~year storm events would cause infiltration-excess overland flow on grassland. We suggest deciduous shelterbelts upslope could locally reduce overland flow. 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 $>$2~mm. Kfs was measured at depth 0.04-0.15~m using a constant head well permeameter across four paired landcover sites of adjacent tree and intensely grazed grassland. The measured tree types were: 500-year-old mixed woodland; 180-year-old mixed woodland; 45-year-old Pinus sylvestris plantation; and 180-year-old Salix caprea woodland. The respective paired grids of trees and grassland were compared on similar soil texture and topography. This study highlights the significant impact of broadleaf woodland at a hillslope scale on Kfs in comparison to grassland areas: median Kfs values under 180-year-old S. caprea woodland (8~mm~h-1), 180-year-old mixed woodland (119~mm~h-1) and 500-year-old broadleaf woodland (174~mm~h-1) were found to be respectively 8, 6 and 5 times higher than neighbouring grazed grassland areas on the same superficial geology. Further statistical analysis indicates that such Kfs enhancement is associated with the presence of coarse roots ($>$2~mm diameter) creating conduits for preferential flow and a deeper organic layer in the topsoil profile under woodlands. By contrast the P. sylvestris forest had only slightly higher (42~mm~h-1), but not statistically different Kfs values, when compared to adjacent pasture (35~mm~h-1). In the grassland areas, in the absence of course roots, the superficial geology was dominant in accounting for differences in Kfs, with the alluvium floodplain having a significantly lower median Kfs (1~mm~h-1) than surrounding hillslope sites, which had a range of median Kfs from 21 to 39~mm~h-1. The data were used to infer areas of runoff generation by comparing Kfs values with modelled 15~min maximum intensity duration rainfall with a 1 in 10~year return period. Infiltration prevailed in the 180- and 500-year-old mixed and broadleaf woodland, whereas some grassland areas and the floodplain were inferred to generate overland flow. The significantly higher Kfs under broadleaf mature forests suggests that planting broadleaf woodlands on hillslopes in clusters or as shelterbelts within grasslands would provide areas of increased capacity for rainfall infiltration and arrest runoff generation during flood-producing storm events. [Excerpt: Conclusions] Overall this study highlights the significant impact of broadleaf woodland on a hillslope that increases Kfs in comparison to grassland areas. In particular, Kfs under 180 and 500 year old broadleaf forest was found to be respectively 6 and 5 times higher than neighbouring grazed grassland areas on the same superficial geology. This was attributed to the significantly deeper organic layer in the topsoil profile providing greater available water storage and the presence of coarse roots ($>$2 mm diameter) creating conduits for preferential flow. On the other hand, the Kfs under P. sylvestris had only slightly higher Kfs values than the adjacent grassland area, and were statistically not significantly different. This result is surprising considering that root numbers and diameter under P. sylvestris is similar to the 180 year old broadleaf forest. Causal factors may have been due to enhanced illuviation of available organic colloids from pine needles, which could cause soil repellency, ultimately reducing Kfs. The floodplain broadleaf woodland had significantly lower Kfs values than the other woodland area, illustrating the problem of poor soil drainage relating to the gley soils in the floodplain. The coarse roots that did exist within the floodplain woodland were shallow and were spatially less extensive (in clumps), but they did provide some high outlying Kfs values and an associated large mean variance in this parameter. The low median Kfs values at 0.04-0.15 m soil depth under all four measured grassland ranged from median 1 to 39 mm h-1. The highest grassland Kfs values were in head deposits (high gravel content) and the lowest in the alluvium floodplain deposits (higher silt and clay content). The diverse particle size distribution of clay, silt, sand and gravel within the hillslope indicated the significant effect of glaciation and fluvial transport in depositing and sorting which in turn, affected the topsoil Kfs under grassland. Moreover no coarse roots ($>$2 mm diameter) were observed in the soil profiles in the grass areas. Thus at these sites the superficial geology was a more dominant influence on Kfs than biological factors, which were more dominant in broadleaf woodland. Low Kfs values in the floodplain were also observed to be influenced by soil compaction caused by high stocking rates. Of all the soil description parameters (total root numbers, number of roots $>$5 mm, number of roots between 2 and 5 mm, humus depth, stone size, depth of gravel layer), an increase in Kfs was determined by regression analysis to be most associated with total root numbers $>$2 mm diameter. By contrast no statistical relationship between gravel size and Kfs was evident. [\n] Mapping in situ Kfs measurements inferred possible areas of sources and sinks for overland flow during high intensity duration rainfall (the example used was I15max 1 in 10 year rainfall event). A source of overland flow (infiltration-excess and saturation-excess) occurs in most grassland areas and particularly in the floodplain silty soils. Broadleaf woodland areas are likely to act as sinks to overland flow during high intensity rainfall, but at a depth $>$0.15 m sub-surface storm flow is likely to occur. [\n] In terms of NFM, this study suggests that older broadleaf forests on pastoral hillslopes could mitigate local flooding because of the significantly higher infiltration rates and sub-soil Kfs under these forested areas in contrast to the heavily, grazed grasslands. However, as indicated earlier, such deciduous forests occupy only $\sim$7\,% of woodland in the Scottish Borders ( Anon, 1999) and a paradigm shift in forestation practice in terms of species is thus required.
@article{archerSoilCharacteristicsLandcover2013,
  title = {Soil Characteristics and Landcover Relationships on Soil Hydraulic Conductivity at a Hillslope Scale: A View towards Local Flood Management},
  author = {Archer, N. A. L. and Bonell, M. and Coles, N. and MacDonald, A. M. and Auton, C. A. and Stevenson, R.},
  year = {2013},
  month = aug,
  volume = {497},
  pages = {208--222},
  issn = {0022-1694},
  doi = {10.1016/j.jhydrol.2013.05.043},
  abstract = {We evaluate woodland/grassland cover and soil types to reduce local flooding. We measured field saturated hydraulic conductivity under grassland and woodland. Established broadleaf woodland had significantly higher infiltration rates than grassland. 1 in 10~year storm events would cause infiltration-excess overland flow on grassland. We suggest deciduous shelterbelts upslope could locally reduce overland flow. There are surprisingly few studies in humid temperate forests which provide reliable evidence that soil permeability is enhanced under forests. This work addresses this research gap through a detailed investigation of permeability on a hillslope in the Eddleston Catchment, Scottish Borders UK, to evaluate the impact of land cover, superficial geology and soil types on permeability using measurements of field saturated hydraulic conductivity (Kfs) supported by detailed topsoil profile descriptions and counting of roots with diameters {$>$}2~mm. Kfs was measured at depth 0.04-0.15~m using a constant head well permeameter across four paired landcover sites of adjacent tree and intensely grazed grassland. The measured tree types were: 500-year-old mixed woodland; 180-year-old mixed woodland; 45-year-old Pinus sylvestris plantation; and 180-year-old Salix caprea woodland. The respective paired grids of trees and grassland were compared on similar soil texture and topography. This study highlights the significant impact of broadleaf woodland at a hillslope scale on Kfs in comparison to grassland areas: median Kfs values under 180-year-old S. caprea woodland (8~mm~h-1), 180-year-old mixed woodland (119~mm~h-1) and 500-year-old broadleaf woodland (174~mm~h-1) were found to be respectively 8, 6 and 5 times higher than neighbouring grazed grassland areas on the same superficial geology. Further statistical analysis indicates that such Kfs enhancement is associated with the presence of coarse roots ({$>$}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-1), but not statistically different Kfs values, when compared to adjacent pasture (35~mm~h-1). 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-1) than surrounding hillslope sites, which had a range of median Kfs from 21 to 39~mm~h-1. 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 180- and 500-year-old mixed and broadleaf woodland, whereas some grassland areas and the floodplain were inferred to generate overland flow. The significantly higher Kfs under broadleaf mature forests suggests that planting broadleaf 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.

[Excerpt: Conclusions]

Overall this study highlights the significant impact of broadleaf woodland on a hillslope that increases Kfs in comparison to grassland areas. In particular, Kfs under 180 and 500 year old broadleaf forest was found to be respectively 6 and 5 times higher than neighbouring grazed grassland areas on the same superficial geology. This was attributed to the significantly deeper organic layer in the topsoil profile providing greater available water storage and the presence of coarse roots ({$>$}2 mm diameter) creating conduits for preferential flow. On the other hand, the Kfs under P. sylvestris had only slightly higher Kfs values than the adjacent grassland area, and were statistically not significantly different. This result is surprising considering that root numbers and diameter under P. sylvestris is similar to the 180 year old broadleaf forest. Causal factors may have been due to enhanced illuviation of available organic colloids from pine needles, which could cause soil repellency, ultimately reducing Kfs. The floodplain broadleaf woodland had significantly lower Kfs values than the other woodland area, illustrating the problem of poor soil drainage relating to the gley soils in the floodplain. The coarse roots that did exist within the floodplain woodland were shallow and were spatially less extensive (in clumps), but they did provide some high outlying Kfs values and an associated large mean variance in this parameter. The low median Kfs values at 0.04-0.15 m soil depth under all four measured grassland ranged from median 1 to 39 mm h-1. The highest grassland Kfs values were in head deposits (high gravel content) and the lowest in the alluvium floodplain deposits (higher silt and clay content). The diverse particle size distribution of clay, silt, sand and gravel within the hillslope indicated the significant effect of glaciation and fluvial transport in depositing and sorting which in turn, affected the topsoil Kfs under grassland. Moreover no coarse roots ({$>$}2 mm diameter) were observed in the soil profiles in the grass areas. Thus at these sites the superficial geology was a more dominant influence on Kfs than biological factors, which were more dominant in broadleaf woodland. Low Kfs values in the floodplain were also observed to be influenced by soil compaction caused by high stocking rates. Of all the soil description parameters (total root numbers, number of roots {$>$}5 mm, number of roots between 2 and 5 mm, humus depth, stone size, depth of gravel layer), an increase in Kfs was determined by regression analysis to be most associated with total root numbers {$>$}2 mm diameter. By contrast no statistical relationship between gravel size and Kfs was evident.

[\textbackslash n] Mapping in situ Kfs measurements inferred possible areas of sources and sinks for overland flow during high intensity duration rainfall (the example used was I15max 1 in 10 year rainfall event). A source of overland flow (infiltration-excess and saturation-excess) occurs in most grassland areas and particularly in the floodplain silty soils. Broadleaf woodland areas are likely to act as sinks to overland flow during high intensity rainfall, but at a depth {$>$}0.15 m sub-surface storm flow is likely to occur.

[\textbackslash n] In terms of NFM, this study suggests that older broadleaf forests on pastoral hillslopes could mitigate local flooding because of the significantly higher infiltration rates and sub-soil Kfs under these forested areas in contrast to the heavily, grazed grasslands. However, as indicated earlier, such deciduous forests occupy only {$\sim$}7\,\% of woodland in the Scottish Borders ( Anon, 1999) and a paradigm shift in forestation practice in terms of species is thus required.},
  journal = {Journal of Hydrology},
  keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-13908859,~to-add-doi-URL,ancient-forests,broadleaved,flood-control,floods,forest-resources,grasslands,land-cover,mature-forests,mitigation,pinus-sylvestris,plantation,runoff,salix-caprea,soil-resources,storm,vegetation,water-resources},
  lccn = {INRMM-MiD:c-13908859}
}
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