Editorial 133

Copyright © 2006 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 31, 133–134 (2006)

Earth Surface Processes and LandformsEarth Surf. Process. Landforms 31, 133–134 (2006)Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/esp.1318

Editorial

Gully erosion in mountain areas: processes,measurement, modelling and regionalizationJohn Wainwright,1* Nicolle Mathys2 and Michel Esteves3

1 Sheffield Centre for International Drylands Research, University of Sheffield, Winter Street, Sheffield, S10 2TN, UK2 CEMAGREF Grenoble, Unité de Recherche ‘Erosion Torrentielle Neige et Avalanches’ (ETNA), BP 76, 38 402 Saint Martin d’Hères, France3 Laboratoire d’Étude des Transferts en Hydrologie et Environnement, BP53, 38 041 Grenoble Cedex 9, France

The papers in this special issue arise from the conference ‘Gully erosion in mountain areas: processes, measurement,modelling and regionalization’, held in Digne, France, in October 2003. The meeting had a truly international flavour,with presentations of work from scientists from 22 countries over five continents. A second set of papers (Mathysand Poesen, 2005; Raclot et al., 2005; Rey, 2005; Cohen and Rey, 2005; Gauché, 2005; Collinet and Zante, 2005)has been published in Géomorphologie: Relief, Processus, Environnement, the journal of the Groupe Français deGéomorphologie (http://dossier.univ-st-etienne.fr/gfg/www/).

Gully erosion is a particularly important process both from a perspective of landform evolution and in relation toapplied aspects of land degradation, water quality and reservoir capacity. Poesen et al. (2003) note that gully erosiontypically makes up 10 to 94 per cent of the total sediment production within catchments, and that topography is animportant control in gully initiation, but that topographic control of sediment production by gullies is poorly known.However, research presented at the meeting suggests it is important in mountain areas. For example, Ghimire et al.(this issue) record a proportion of 59 per cent sediment yield from gully erosion in the Siwarlik Hills, Nepal, over atwo-year period. But many studies emphasize the temporal variability of such rates (e.g. Poesen et al., 2003; Trimble,1999), requiring longer data series. In part, difficulty of access has meant that these environments have been difficultto measure. The development of techniques such as stereophotogrammetry of time series of images has allowedlonger-term records of gully erosion of upland areas to be estimated (e.g. Veyrat-Charvillon and Memier, this issue),but there are still issues of reliably estimating catchment-scale sediment production in this way.

Gully erosion is particularly important above a threshold catchment size of 1 to 10 ha (Poesen et al., 2003). Thus, afurther reason for the lack of progress in research on gullies has been the increasingly small-scale, process-basednature of studies carried out in geomorphology over the last few decades (see also Wainwright et al., 2000). Anincreasing focus on the spatial scaling of processes is fortunately now leading to a reverse of this trend. However, itpresents a whole host of new research problems in terms of linking detailed process understanding with landscapeevolution. Parsons et al. (2004) have demonstrated that there is a significant mismatch between erosion rates estimatedusing plot-based techniques and rates of landscape evolution. They point out that the discrepancy arises due to a lackof understanding of transport processes which means the source area for much monitored sediment is typically veryclose to the measurement point, rather than being spread out over the whole catchment. Veyrat-Charvillon and Memier(this issue) demonstrate that understanding source areas is critical for interpreting sediment-transport data, providingfurther support for this idea, and further supporting the need for a solid understanding of catchment connectivity andhillslope–channel linkages (Helming et al., 2005; Wainwright et al., 2002). A further conceptual issue is the need toconsider gullies as complete systems, rather than simply focusing on a single type of process. Gullies form as thecomplex interplay of both surface and subsurface (e.g. Farifteh and Soeters, 1999; Zhu et al., 2002) hydraulic erosion,debris flows (e.g. Berti et al., 1999), various forms of slope and channel-bank instability (e.g. Harvey, 2001; Collisonand Simon, 2001), and thermal cycling (Sidorchuk, 1999), the latter being a process that is likely to be particularlyimportant in upland areas. The approach of Parsons et al. (2004) provides an underlying framework for integratingand scaling these process domains to landscape scale, but there is a need for a significant amount of field andlaboratory investigation that will be required to underpin this approach.

*Correspondence to: J. Wainwright, Sheffield Centre for International Drylands Research, University of Sheffield, Winter Street, Sheffield, S102TN, UK. E-mail: J.Wainwright@sheffield.ac.uk

134 Editorial

Copyright © 2006 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 31, 133–134 (2006)

Temporal scaling of gully processes is also an area that requires significant attention. Again, the aspect of obtainingreliable data is critical in this respect. The unlocking of photographic archives has already been mentioned in thisrespect, but other approaches seem likely to be fruitful. Nyssen et al. (this issue) have developed a semi-quantitativeassessment method derived from social science techniques and demonstrated that it can be applied effectively. Temporaldynamics of gullies also tend to be complex. Scaled laboratory experiments tend to emphasize a rapid developmentof gully systems – Kosov et al. (1978, cited in Sidorchuk, 1999) report that during the first 5 per cent of the lifetime ofa gully, it will typically have developed >90 per cent of its length, 85 per cent of its depth, 60 per cent of its area and35 per cent of its volume – that is typically unilinear. Longer time-scale field studies, on the other hand, report morecomplex histories, with phases of activity and relative dormancy, depending on a range of controlling factors such asland use and climate variability (e.g. Parkner et al. and Nyssen et al., this issue). Copard et al. (this issue) use novelmeasurement techniques to demonstrate the impact of gullies on longer-term recycling of sediments and the potentialimpacts of gully erosion on the global carbon cycle. One reason that process descriptions of gully initiation may failis that gullies tend to reactivate in previously eroded areas (e.g. Palacios et al., 2003), even over time scales of tens ofthousands of years (e.g. Rienks et al., 2000). These observations suggest that understanding the role of historicalevents (see also Wainwright and Thornes, 2003) and contingency is of fundamental importance. To evaluate modern-day processes and their impacts, we need to redevelop an integrated discipline of geomorphology in which field,laboratory and modelling techniques are applied within strong, holistic conceptual frameworks.

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