Variations in soil dispersivity across a gully head displaying shallow sub‐surface pipes, and the role of shallow pipes in rill initiation

Abstract A small bifurcating gully head displaying shallow pipe development was surveyed to explore how far three‐dimensional patterns of geochemistry and sediment size can be related to hydraulic gradients in the local marl bedrock (Almería, SE Spain). The crust, sub‐crust and parent materials were sampled every 20 cm across a 2 m by 3 m grid, and then analysed for dispersive and granulometric characteristics. Spatial patterns of sodium adsoption ratio (SAR) for each layer were plotted separately. In‐situ material at depths of 5–10 cm was only weakly dispersive, and the thin (0–2 cm depth) crust is also found to be mostly non‐dispersive, paralleling findings from other field sites in Almería. However, the ‘signature’ relating SAR to electrical conductivity for each layer shows that in places the immediate sub‐crust layer (2–5 cm) is highly dispersive. The pattern is not random; rather the SAR of this sub‐crust layer follows inferred hydraulic gradients, the dispersive ‘hot spots’ being located in the most incised part of the small gully, exacerbating the erodibility of that position. Patterns of sediment particle size and sorting do not correlate with inferred hydraulic gradients but surface material is slightly siltier than the sub‐crust. Clay fraction increased with depth, and SAR is shown to have a weak inverse relationship to particle size. This association between SAR and the increased clay fractions in the lower layers supports the inference that massive pipe enlargement in the Messinian‐Rich Unit is suppressed by sub‐surface swelling. Since a reduction in infiltration capacity (fc) with depth can be inferred from these results, infiltrating water must be deflected into the already vulnerable sub‐crust layer during rainfall events, explaining the development of shallow pipe forms at preferential depths. It is concluded that calcium replaces sodium in the crust during leaching, leaving a calcic crust, and a sub‐crust that is sodic and prone to subsequent pipe enlargement. Rill morphology in these materials also suggests that rills develop from these pipes when pipe roofs collapse (i.e. rill discontinuity; bridges; steep headwalls; and rills with excessively high depth‐to‐width ratios). Copyright © 2004 John Wiley & Sons, Ltd.