More than meets the eye

Posted on May 26, 2009

Although it’s a commonplace that surface runoff is an important source of sediment in freshwater streams, the relative importance of sediment that enters via subsurface (drainage) pathways is much harder to gauge.  Researchers in theU.K.simultaneously measured surface and subsurface sediment flows in a small agricultural catchment and found that subsurface sources dominated the sediment supply.  Although rainfall, and hence soil saturation and surface runoff, were low during the monitoring period, the study highlighted the need for comparative data from other catchments to help clarify how subsurface pathways deliver sediment and pollutants to streams.  For example, do subsurface inputs derive mainly from sediment originally eroded at the surface, or from sediment mobilized below the surface?  Subsurface passage is likely to be most important in soils such as artificially-drained cracking clays, where macropore networks are well-defined.

Reference:  Deasy, C.,  Brazier, R.E., Heathwaite, A.L. & Hodgkinson, R. 2009.  Pathways of runoff and sediment transfer in small agricultural catchments.  Hydrological Processes 23, 1349–1358.