Soil conditions play a crucial role in determining water runoff and retention in forested sites, according to new research. Soil characteristics, such as compaction, play a greater role than forest type in determining water dynamics.
Forests help retain water and mitigate flooding by slowing peak water flows during storm events. However, forest management practices, such as clear cutting and heavy equipment use, can reduce their effectiveness. Climate change is predicted to increase the number and severity of extreme weather events, raising the question of how to prevent future flooding at the catchment level.
Research funded through the EU WaReLa(1) and ForeStClim(2) projects, investigated the influence of soil conditions on rainwater runoff. The study, carried out in the low mountain ranges of southwest Germany, conducted sprinkling experiments on 11 test sites within two forested catchments to simulate rainfall. Sprinkling occurred on three consecutive days and simulated 10mm of rainfall over a 50m2 area, reflecting natural levels that cause flooding in this area. The scientists measured the bulk density and water content of soil samples together with hydraulic conductivity (a measure of how water moves through the soil) and infiltration capacity.
The sites covered a range of different conditions, including established forest stands with deciduous and coniferous species, arable land and afforestation sites following agricultural land-use. Soil types also varied, but in all cases subsoil layers were compacted due to Ice Age periglacial dynamics, limiting deep percolation of water into the sites.
In the first catchment, runoff was highest in a relatively young (one year old) afforestation site (20% of total simulated rainfall). Runoff was generally less in forested areas but higher run off in deciduous than coniferous forests (17% compared to 5%) was attributed to differences in the depth of the compacted soil layer, rather than tree type. Runoff was considerably lower in the second catchment (less than 4%) due to water being able to flow vertically through the compacted soil layer.
The compacted layer of soil was found near the surface within an afforestation site created 30 years ago, leading to similarly high runoff levels to an arable plot. This shows that the effects of cultivation are long lasting and that forest managers could use machinery to break up the compacted soil layer before afforestation to improve root penetration and water infiltration.
Approaches to flood protection should reflect the fact that runoff characteristics are site-specific and involve a range of stakeholders, including forestry, agriculture, water management and spatial planning representatives. An ecohydrological approach could be adopted to reverse ecosystem degradation while enhancing forests' ability to accommodate human impacts.
The researchers recommend a number of forest management practices, including sustainable soil management, using low-impact harvesting methods and avoiding clear cuts, drainage systems and extensions to forest roads. Further insight into the influence of soil properties on water retention is important for forest planning and large scale flood prevention.
- WaReLa, Water Retention by Land Use, was funded by the European Commission through the INTERREG IIIB NWE programme. See: http://3b.nweurope.eu/page/
projet.php?p=31&id=524 - ForeStClim was funded by the European Commission through the INTERREG IVB NWE programme. See: http://forestclim.eu/
fuente: Science for Environment Policy
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