The relationship of forest and improved grassland to soil water storage and its implication on Natural Flood Management in the Scottish Borders

An investigation was undertaken in the Eddleston Catchment, Scottish Borders, to evaluate the impact of land cover on soil permeability (measured as field saturated hydraulic conductivity, Kfs) and its implication on dominant stormflow pathways on hillslopes. Kfs was measured at a depth range of 4 to 15 cm and 15 to 25 cm using the Talsma Constant Head Well Permeameter across three paired sites of adjacent tree and grassland cover. The tree types were 500-year old mixed woodland, 180-year old mixed woodland, and 45-year old Scots pine plantation. Paired grids of different land cover were compared on similar soil texture and topography. Dominant stormflow pathways were inferred by overlaying rainfall-intensity-duration-frequency (IDF) data for different return periods on to Kfs boxplots. The resulting data from different land covers provide information for understanding sources and sinks for stormflow pathways under different land covers and topography in the context of reducing flood peak flows. It was found that the 500- and 180-year old mixed forests have an excellent capacity for rainfall infiltration where Kfs values were almost five to six times more than those for grassland areas and is a sink for 1-in-100 year rainfall IDF events. The study also discusses the importance of understanding the effect of forest cover of different ages, where the hydrological soil properties change through time.