Changes to the dispersive characteristics of soils along an evolutionary slope sequence in the Vera badlands, southeast Spain: implications for site stabilisation

Threshold behaviour in hydrological systems as (human) geo-ecosystems: manifestations, controls, implications

“…Lateral preferential flow in soil pipe systems, which crucially determines hillslope scale subsurface runoff generation, is intermittent and is likely to be a threshold phenomenon as well (Adams and Parkin, 2002;Buttle and McDonald, 2002;Negishi et al, 2007;Weiler and McDonnell, 2007). Threshold behaviour is also discussed in the context of the long-term development of soil structures and landforms (Newson and Newson, 2000;Phillips, 2004Phillips, , 2006, fluvial morphology (Willgoose et al, 1991a(Willgoose et al, , 1991bGrant, 1997;Piégay et al, 2000;Fryirs et al, 2007;Harnischmacher, 2007), rill and gully erosion (Bull and Kirkby, 1997;Kirkby et al, 2003;Poesen et al, 2003;Shao et al, 2005;Parkner et al, 2006Parkner et al, , 2007, badlands formation (CalvoCases and Harvey, 1996;Howard, 1997;Faulkner et al, 2000Faulkner et al, , 2003Boardman et al, 2003;Faulkner, 2008), and formation and growth of channel networks (Willgoose et al, 1991a(Willgoose et al, , 1991bRinaldo et al, 1996;Rodriguez-Iturbe et al, 1998;Rodriguez-Iturbe and Rinaldo, 2001). Threshold behaviour and feedbacks between vegetation patterns and redistribution of abiotic resources determine the resilience of geoecosystems in semi-arid regions (Grant, 1997;Piégay et al, 2000;Wilcox et al, 2003a;Wilcox and Newman, 2005;Fryirs et al, 2007;…”

“…Will an expected rainfall event trigger lateral preferential flow/subsurface storm flow so that upslope parts of the landscape or groundwater will contribute to flood formation (McDonnell, 1990;Kirnbauer et al, 2005;Wenninger et al, 2004;Graeff et al, 2009), or not? Will an expected rainfall event cause erosion rills to connect to the main channel which in turn triggers gully formation (Faulkner, 2008), or not? Will an expected rainfall event trigger a landslide (van Asch et al, 1996(van Asch et al, , 1999Boogard et al, 2002;, or not?…”

“…Change of hydrological functioning -that may manifest through changes in any one of these indices -can either be in response to external climate change, or due to persistent, substantial often human-induced changes in geo-ecosystem properties that determine either local process thresholds or the topology/structures in the system and thus response thresholds. Illustrative examples are a die-back of earthworms and related substantial changes in infiltration properties and local bioturbation (Edwards et al, 1990b;Lavelle et al, 1997Lavelle et al, , 2004Shipitalo and Butt, 1999;Milcu, 2005) the onset of gully erosion and badlands formation (Howard, 1997;Boardman et al, 2003;Boardman and Foster, 2008;Faulkner, 2008;Maerker et al, 2008), leaky vegetation patterns due to overgrazing in arid runoffrunon systems (Saco et al, 2007), or as a classic example, the results of past regulation of the upper Rhine river (Dister et al, 1990;Hofius, 1991). Again, the underlying control for this form of threshold behaviour is more complex when compared to the response level, since it involves long-term feedbacks between biotic and abiotic geo-ecosystem components or morphological processes, as well as substantial disturbances that go beyond the resilience of the system.…”

“…This is well known from many places around the world such as Spain (Bull and Kirkby, 1997;Kirkby et al, 2003), Chile (Maerker et al, 2008), France (Maquaire et al, 2003;Rey et al, 2005) New Zealand (Hicks et al, 2000;Parkner et al, 2007), and South Africa (Boardman et al, 2003;Achten et al, 2008;Boardman and Foster, 2008). Initiation of erosion rills/pipes and their possible evolution into gullies is a crucial factor (Bull and Kirkby, 1997) that is strongly controlled by connectivity of surface flow paths in combination with the occurrence of extreme rainfall events (Fryirs et al, 2007;Faulkner, 2008). Formation of erosion gullies is commonly associated with agricultural landscapes where vegetation clearance increases runoff, leading to fluvial incision (Bull and Kirkby, 1997).…”

“…Identification of cause-effect relations in the context of threshold behaviour is made difficult as a result , especially when these are accompanied by structural or morphological changes as well (Newson, 1980;Newson and Newson, 2000;Phillips, 2004Phillips, , 2006Faulkner, 2008). Hence, threshold behaviour in hydrological and geoecosystems becomes much more difficult to detect, understand and predict in comparison to elementary threshold phenomena, which can be predicted on the basis of well observable dimensionless variables such as the Reynolds number.…”