MODELLING THE IMPACTS OF WILDFIRE ON ASH THICKNESS IN A SHORT-TERM PERIOD P. PEREIRA 1 *, A. CERDÀ 2 , X. ÚBEDA 3 , J. MATAIX-SOLERA 4 , V. ARCENEGUI 4 AND L. M. ZAVALA 5 1 Department of Environmental Policy, Mykolas Romeris University, Ateities g. 20, LT-08303 Vilnius, Lithuania 2 Department of Geography, University of Valencia, Blasco Ibañez 28. 46010 Valencia, Spain 3 GRAM (Mediterranean Environmental Research Group), Department of Physical Geography and Regional Geographic Analysis, University of Barcelona, Montalegre, 6. 08001 Barcelona, Spain 4 Environmental Soil Science Group, Department of Agrochemistry and Environment, Miguel Hernández University, Elche, Alicante, Spain 5 MED_Soil Research Group, University of Sevilla, Spain Received: 2 July 2012; Revised: 20 November 2012; Accepted: 21 November 2012 ABSTRACT Ash can provide valuable soil protection. However, ash is also very mobile, and soil protection patterns can be quickly changed, favouring the protection of some areas and exposing others with implications for soil erosion. In this research, the effects of a high severity wildfire on ash thickness were studied, 1 and 15 days after the fire. For this evaluation, several interpolation methods were tested to identify the best spatial predictor of ash distribution. The results showed that 1 day after the fire, ash was thinner in high severity areas. Fifteen days after the fire, ash thickness decreased, and the spatial pattern changed. This implies that evaluation of fire severity based on ash thickness must take a place immediately after the fire because it is affected by (re)distribution. There was an increase in the spatial autocorrelation, and ash distribution corresponded to a specific spatial pattern, because of wind (re)distribution. One day after the fire, the most accurate predictor was Inverse to a Weight 3 (IDW3) that detected easily the small-scale variability of ash thickness, and after 15 days, ordinary kriging identified a specific pat- tern of ash distribution. Copyright © 2012 John Wiley & Sons, Ltd. keywords: ash; soil protection; thickness; fire severity; spatial predictor; spatial autocorrelation; ash mobility INTRODUCTION Summer wildland fires are a pervasive and common problem in Europe, mainly in the Mediterranean environment. Every year, thousands of hectares burn with important implications for ecosystems, but also in social and economic realms. Presently, 95 per cent of the fires are attributed directly or indirectly to human activities. Nevertheless, there are enormous differences between northern and southern European countries. In the southern countries, the fire incidences are mainly intentional, contrary to the northern countries, where the fire occurrences are related to negli- gence (Catry et al., 2010). Summer high severity wildfires in the Mediterranean environment are a cause of land degradation. High fire severity and frequency seriously damage ecosystems, the soil physical and chemical status, species composition and vegetation structure. These impacts have strong implications on biodiversity loss and soil erosion (Perez-Cabello et al., 2009; Bajocco et al., 2010). After wildland fires, the removal of vegetation and ground cover have important consequences for soil protection, especially on hillslopes, where the hydro-geomorphological dynamic is more active, mainly in the immediate period following fire (Sheridan et al., 2007; Cerdà and Doerr, 2008; Mataix-Solera et al., 2011; Fernandez et al., 2012; Neary et al., 2012). In these areas, the sediment and nutrient depletion is more intense as observed in some studies (Lasanta and Cerdà, 2005; Moody and Martin, 2009; Shakesby, 2011). The ash remaining on the soil surface after fire is of major importance because of the protection it provides against soil erosion agents (Cerdà and Doerr, 2008; Woods and Balfour, 2011), the input of nutrients into the soil profile and the increased capacity for plant cover (Pereira et al., 2010b, 2012). Ash also gives important information about fire impacts on the ecosystem. Very often, ash is used as an indirect estimator of fire severity (Goforth et al., 2005; Roy et al., 2010; Pereira et al., 2012). Ash properties and capacity to cover soil is of major importance because it can reduce the amount of runoff and soil erosion (Cerdà and Doerr, 2008). In recent years, an important number of studies have been carried out on ash chemistry (Ulery et al., 1993; Pereira et al., 2011, 2012) and the effect of ash from prescribed fires and wildfires on soil erosion (Cerdà and Doerr, 2008; Gabet and Sternberg, 2008; Onda et al., 2008; Larsen et al., 2009; Zavala et al., 2009; Moody et al., 2009; Woods and Balfour, 2011), soil water retention (Stoof et al., 2010), hydropho- bicity (Bodi et al., 2011, 2012; Dlapa et al., 2012) and water quality (Smith et al., 2011). Some of these studies (Cerdà and Doerr, 2008; Woods and Balfour, 2011) evaluated the ash thickness after the fire but did not linked it with the fire severity. A preliminary study was carried out by Robichaud *Correspondence to: P. Pereira, Department of Environmental Policy, Mykolas Romeris University, Ateities g. 20, LT-08303 Vilnius, Lithuania. E-mail: pereiraub@gmail.com Copyright © 2012 John Wiley & Sons, Ltd. land degradation & development Land Degrad. Develop. (2013) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/ldr.2195