Hydrological thresholds of soil surface properties identified using conditional inference tree analysis Matthew Tighe, 1 * Carlos Muñoz-Robles, 1,2 Nick Reid, 3 Brian Wilson 3,4 and Sue V. Briggs 5 1 Agronomy and Soil Science, School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia 2 Instituto de Ecología, A. C. Carretera antigua a Coatepec 351, El Haya, Xalapa 91070, Veracruz, Mexico 3 Ecosystem Management, School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia 4 NSW Office of Environment and Heritage, Armidale, NSW 2351, Australia 5 Institute for Applied Ecology, University of Canberra, ACT 2601, Australia Received 27 January 2011; Revised 20 November 2011; Accepted 28 November 2011 *Correspondence to: M. Tighe, Agronomy and Soil Science, School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia. E-mail: mtighe2@une.edu.au ABSTRACT: There has been limited success in determining critical thresholds of ground cover or soil characteristics that relate to significant changes in runoff or sediment production at the microscale (<1m 2 ), particularly in semi-arid systems where management of ground cover is critical. Despite this lack of quantified thresholds, there is an increasing research focus on the two-phase mosaic of vegetation patches and inter-patches in semi-arid systems. In order to quantify ground cover and soil related thresholds for runoff and sediment production, we used a data mining technique known as conditional inference tree analysis to determine statistically significant values of a range of measured variables that predicted average runoff, peak runoff, sediment concentration and sediment production at the microscale. On Chromic Luvisols across a range of vegetation states in semi-arid south-eastern Australia, large changes in runoff and sediment production were related to a hierarchy of different variables and thresholds, but the percentage of bare soil played a primary role in predicting runoff and sediment production in most instances. The identified thresholds match well with previous thresholds found in semi-arid and temperate regions (including the approximate values of 30%, 50% and 70% total ground cover). The analysis presented here identified the critical role of soil surface roughness, particularly where total ground cover is sparse. The analysis also provided evidence that a two-phase mosaic of patches and inter-patches identified via rapid visual assessment could be further delineated into distinct groups of hydrological response, or a multi-phase rather than a two-phase system. The approach used here may aid in assessing scale-dependent responses and address data non-linearity in studies of semi-arid hydrology. Copyright © 2012 John Wiley & Sons, Ltd. KEYWORDS: erosion; thresholds; ground cover; surface roughness; woody encroachment Introduction The conceptual links between vegetation patterns and hydro- logical processes in semi-arid systems around the world are well understood (Puigdefábregas, 2005). Semi-arid zones have ‘patchy’ ground cover at a range of spatial scales, and this patchiness largely determines how water and other resources, such as sediment, nutrients and organic matter, are redistribu- ted following a rainfall event (Tongway and Ludwig, 1994, 2005; Wilcox et al., 2003). In turn, this redistribution of resources provides a positive feedback effect which influences the persistence and occurrence of ground cover (Wilcox et al., 2003). There are numerous references in the literature to the ‘two-phase’ mosaic of vegetation at micro (<1m 2 ) to broad (ha) scales in these environments, where runoff (source or ‘inter-patch’) zones are juxtapositioned with runon (sink or ‘patch’) zones (Schlesinger et al., 1990; Ludwig et al., 1999; Puigdefábregas, 2005; Strauss and Schickhoff, 2007; Mayor et al., 2009; Wiesmeier et al., 2009). Inter-patches are typically defined as having minimal ground cover, low microrelief, and other soil properties that limit infil- tration, while patches have greater total amounts of ground cover, microrelief and other soil properties that increase infiltra- tion (Tongway and Ludwig, 2005; Michaelides et al., 2009). However, terms such as ‘minimal’ ground cover, or ‘limited’ infiltration are problematic to define. A common approach is to define these terms relatively within a system, and identify areas within the system that have disparate indicators of re- source retention (such as areas that have little or no herbaceous vegetation compared with areas that are well vegetated). This has led to many studies, at several spatial scales, in which a two-phase mosaic of patch and inter-patch has been investi- gated (Ludwig et al., 1999, 2002; Cammeraat, 2002; Bartley et al., 2006). Recently, a combination of ground cover and soil-related variables have been identified within both fine-scale patches and inter-patches that are related to hydrological responses (Eldridge and Koen, 1993; Mayor et al., 2009; Muñoz-Robles EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms 37, 620–632 (2012) Copyright © 2012 John Wiley & Sons, Ltd. Published online 2 January 2012 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/esp.3191