Arid environments - Environmental Sedimentology - page 181

connection of the hillslope elements to the first-order channel, and this occurs extremely rarely
(Puigdefabregas et al. 1999), and was not witnessed during the project.
The IBERLIM managed site covers a 3.68 ha gully catchment in central Spain, located on the
Raña Formation (Pliocene alluvial sediments). Prior to the 1940s the area was used as agricultural
land, which then became abandoned and taken over by Cistus mattoral vegetation (Ternan et al.
1996a). In an attempt to combat erosion problems in the area an afforestation programme was
undertaken and in the 1970s the catchment was bench terraced and planted with Pinus . The
IBERLIM team began monitoring in 1992, on a variety of scales (Williams et al. 1995) from (i)
gully (3.5 ha), (ii) erosion plot (10 -21.5 m 2 ) to (iii) rainfall simulation plot (1 m 2 ). The research
determined that the bulk soil had an overall low aggregate stability (Ternan et al. 1996a), which
was exacerbated locally by high clay content (especially if expandable clays were present). Not
all sediment and water erosion occurred at the surface, with a good deal of erosion occurring
through subsurface pipes, which developed in layers within the sediment where dispersion,
shrinkage and swelling were prolific (Case Fig. 5.3). This again coincided with clay horizons
lacking in coarse material (typically less than 40% sand). The sediment dispersion was found to
link closely with the chemistry of the porewater, soil chemistry and clay minerals. Horizons with
sodium absorption ratios (SAR) of more than 0.4 and exchangeable sodium percentage (ESP)
of more than 1.5 were found to be susceptible (Ternan et al. 1998). These horizons had been
exposed with the excavation of the bench terraces within the woodland (Ternan et al. 1996b).
These areas were thus sources of runoff and associated surface erosion early in a rainstorm
event. Continuous runoff pathways were more prolific under wet weather conditions. It was
demonstrated that to manage erosion in such areas, and also to assist in the minimization of
flooding, both the overall wetness threshold at which runoff is generated throughout the catch-
ment should be considered together with the spatial mosaic of runoff-generating source areas
and sinks (Fitzjohn et al. 2002). If sinks could be used to disrupt continuous hydrological flow
pathways both slope sediment and water transfer could be minimized.
The MEDALUS and IBERLIM studies serve to emphasize the mosaic nature of runoff generation
on hillslopes in arid environments. The resulting data demonstrate how human intervention
can exacerbate problems by increasing linkages of source areas on slopes and indicates that a
knowledge of the linkages could be used to mitigate runoff problems by using sink areas to break
up flow pathways.
Relevant reading
Fitzjohn, C., Ternan, J.L.,Williams, A.G., et al. (2002) Dealing with soil variability: some insights from land
degradation research in central Spain. Land Degradation and Development 13 , 141-50.
Fitzjohn, C., Ternan, J.L. & Williams, A.G. (1998) Soil moisture variability in a semi-arid gully catchment:
implications for runoff and erosion control. Catena 32 , 55-70.
Puigdefabregas, J., Sole, A., Gutierrez, L., et al. (1999) Scales and processes of water and sediment redistribution
in drylands: results from the Rambla Honda field site in Southeast Spain. Earth Science Reviews 48 , 39-70.
Puigdefabregas, J., del Barrio, G., Boer, M.M., et al. (1998) Differential responses of hillslope and channel
elements to rainfall events in a semi-arid area. Geomorphology 23 , 337-51.
Ternan, J.L., Elmes, A., Fitzjohn, C., et al. (1998) Piping susceptibility and the role of hydro-geomorphic controls
in pipe development in alluvial sediments, Central Spain. Zeitschrift Für Geomorphologie 42 , 75-87.
Ternan, J.L., Williams, A.G., Elmes, A., et al. (1996a) Aggregate stability of soils in central Spain and the role of
land management. Earth Surface Processes and Landforms 21 , 181-93.
Ternan, J.L., Williams, A.G., Elmes, A., et al. (1996b) The effectiveness of bench-terracing and afforestation for
erosion control on Raña sediments in central Spain. Land Degradation & Development 7 , 337-51.
Williams, A.G., Ternan, J.L., Elmes, A., et al. (1995) A field study of the influence of land management and soil
properties on runoff and soil loss in central Spain. Environmental Monitoring and Assessment 37 , 333- 45.