Drylands (arid, semi-arid and dry sub-humid areas) cover approximately 41 % of the Earth's surface and approximately 10 to 20 % of these regions are experiencing degradation processes (Deichmann and Eklundh, 1991; Reynolds et al., 2007), resulting in a decline in agricultural productivity, loss of biodiversity and the breakdown of ecosystems. According to the United Nations Conference to Combat Desertification (UNCCD), when land degradation happens in the world's drylands it often creates desert-like conditions. Land degradation occurs everywhere but is defined as desertification when it occurs in the drylands, resulting from various factors, including climatic variations and human activities (UN, 1979; UNCCD, 2012). The vegetation is composed of scrublands patches (high plant cover) interspersed with herbaceous patches (low plant cover)(Aguiar and Sala, 1999). This heterogeneity is induced by overgrazing, one of the main causes of the increase of bare soil that facilitates water and wind erosion and accelerates the desertification process (Cerdà and Lavee, 1999; Kröpfl et al., 2013; Pulido-Fernández et al., 2013; Ziadat and Taimeh, 2013).

Forty-four percent of global agricultural areas and almost 2 billion people are located over the drylands, and the majority (90 %) are in developing countries (D'Odorico et al., 2013). Overexploitation of natural resources in extremely vulnerable regions can accelerate land degradation and desertification process, affecting ecosystem functions and decreasing productivity, biodiversity and landscape heterogeneity, and represents a major threat to the environment and human welfare (Mainguet, 1994; Reynolds and Stafford Smith, 2002; Montanarella, 2007; Salvati and Zitti, 2008; Cerdà et al., 2010; Santini et al., 2010; Kashaigili and Majaliwa, 2013; Pulido-Fernández et al., 2013; Bisaro et al., 2014).

In South America, the United Nations Convention to Combat Desertification report (ONU, 1997) concluded that, until 2025, one-fifth of the productive land could be affected by the desertification process. The most susceptible areas are located in Argentina, Bolivia, Chile, Mexico, Peru and Brazil (Arellano-Sota et al., 1996). In Brazil, the most critical desertification hot spots are located in the semi-arid northeast. In this region the climate is one of the factors that control the desertification process. Soil type, geology, landscape, vegetation, socioeconomic factors and land management also are considered important aspects of this process (IBGE, 2004). The main causes of desertification in this region are (i) deforestation to produce fuel wood and explore clay deposits; (ii) intensive land use employing poor agricultural methods, such as slash and burn, harvesting and land clearing; (iii) salinization; and (iv) extensive herding and overgrazing (Nimer, 1988).

Considering that the Brazilian semi-arid region is the world's most populous dry land region (Marengo, 2008), with more than 53 million inhabitants and a human population density of approximately 34 inhabitants per km2 (IBGE, 2010), and that global climate change scenarios indicate that the region will be affected by increased aridity in the next century, this area is seen as one of the world's most vulnerable regions to climatic change (IPCC, 2007).

The UNCCD recognizes desertification as an environmental problem with huge human, social and economic costs (Hulme and Kelly, 1993).

The most accepted definition currently states that desertification is land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors, including climatic variations and human activities (UN, 1979). Due to the complex social interactions and the biophysical processes, the identification and assessment of the desertification areas have been addressed through a multidisciplinary framework across different spatial and temporal scales (e.g., Prince et al., 1998; Diouf and Lambin, 2001; Thornes, 2004; Santini et al., 2010).

Several methods have been successfully applied for desertification analysis based on indicators and indices (Kepner et al., 2006; Sommer et al., 2011). For instance, the MEDALUS methodology, developed for the European Mediterranean environment, is widely used because of its simplicity and flexibility. The MEDALUS methodology is based on the environmentally sensitive area index (ESAI; Parvari et al., 2011; Salvati et al., 2011; Izzo et al., 2013; Jafari and Bakhshandehmeh, 2013). In order to identify areas potentially affected by land degradation, the method analyzes four main variables: climate, soil, vegetation and land management (Kosmas et al., 1999, 2006; Lavado Contador et al., 2009). It has been validated on regional and local scales (Basso et al., 2000; Brandt et al., 2003; Salvati and Bajocco, 2011) and was applied to quantify the impact of mitigation policies against desertification (Basso et al., 2012).

Symeonakis et al. (2014) estimated the environmental sensitivity areas on the island of Lesvos (Greece) through a modified ESAI, which included 10 additional parameters related to soil erosion, groundwater quality, demographic and grazing pressure, for two dates (1990 and 2000). This study identified areas that are critically sensitive on the eastern side of the island mainly due to human-related factors that were not previously identified.

Although several studies have been conducted to detect desertification or to identify the drivers (indicators) of the process in critical hot spots in the Brazilian northeast (Matallo Júnior, 2001; Lemos, 2001; Sampaio et al., 2003; Aquino and Oliveira, 2012), there have been no studies addressing the entire region.

Crepani et al. (1996) developed a methodology based on the concept of the eco-dynamic principles, proposed by Tricart (1977), and on the relationship between morphogenesis and pedogenesis to identify areas that are susceptible to soil erosion. The author provided an integrated view of the physical environment and the conceptual basis for developing human–nature relationships. However, this study did not include socioeconomic and management indicators as parameters that can influence soil loss.

Therefore, this paper presents a novel approach which integrates the MEDALUS project and the methodology developed by Crepani et al., 1996 to identify areas that are susceptible to desertification in the northeastern region of Brazil and the northern regions of the states of Minas Gerais and Espírito Santo by combining social, economic and environmental indices. This study was conducted considering two reference periods: early 2000s and 2010. The results will be useful for providing basic information for the diagnosis and prognosis of desertification in the region and providing subsidies for the technical support for mitigation and adaptation actions.

The study area is located in the equatorial zone (1–21∘ S, 32–49∘ W), totaling an area of 1 797 123 km2, which corresponds to 20 % of the Brazilian territory (Fig. 1).

The climatology of the northeast of Brazil includes three different rainfall regimes: (i) in the south-southwest area, the rainy season occurs from October through February, which is associated with the displacement of cold fronts coming from the south; (ii) in the north of the region, rainfall occurs from February to May, which is associated with the southward movement of the Intertropical Convergence Zone; and finally, (iii) in a narrow area that is close to the coast at the east, the rainy season occurs from April through August, triggered by temperature differences between the oceans and the sea shore (Kousky, 1979; Marengo, 2008). The evaporation rate in the region is very high and can reach 1000 mm yr-1 in the coastal region and up to 2000 mm yr-1 in the interior (IICA, 2001), based on 11 stations distributed in the semi-arid region and on historical series (Molle, 1989). Annual evaporation average is 2700 to 3300 mm, with the highest values occurs from October to December and the lowest from April to June.

Because of the high evaporation rates and the short duration of the wet season, most of the rivers are temporary, and flash floods occur only during the rainy season (MMA-IBAMA, 2010).

In the northeast region of Brazil, natural vegetation includes rainforests, riparian forests, savannas and montane forests, among others (Foury, 1972). However, the natural vegetation that dominates 62 % of Brazilian semi-arid region is caatinga (MMA, 2007). Caatinga vegetation is composed of shrubs and small trees, usually thorny and deciduous, that lose their leaves in the early dry season. Caatinga is a highly dynamic ecosystem that responds quickly to climatic conditions. The dominant factor that controls the structure and distribution of vegetation is the precipitation, with an annual mean of 500–800 mm and high spatial and temporal variability (Hastenrath and Heller, 1977; Oliveira et al., 2006). Caatinga, in comparison with other xeric areas in South America, presents climatic distinctiveness that resulted in numerous important morphological and physiological adaptations to aridity by many species of plants (Mares et al., 1985). Nowadays, more than 10 % of the semi-arid area has already undergone a very high degree of environmental degradation, being susceptible to desertification (Oyama and Nobre, 2004).

To identify areas susceptible to desertification, we evaluated 11 indicators of susceptibility to desertification (Table 1) based on previous studies of the area (Vasconcelos Sobrinho, 1978; Ferreira et al., 1994; Matallo Júnior, 2001; Lemos, 2001). From Table 1, each indicator was sub-divided into various uniform classes. Each class received a weight factor, related to the potential influence on desertification process, that ranged between 1 (low susceptibility) and 2 (high susceptibility), producing 11 susceptibility maps (SM). The weight factors were assigned based on previous analyses of the literature (Crepani et al., 1996, Torres et al., 2003; Alves, 2006; Santini et al., 2010; Symeonakis et al., 2013). These indicators were grouped into two groups as described below.

This work presents the first effort to identify the areas that are most susceptible to desertification in the semi-arid region of Brazil through a system that enables continuous and integrated analysis of the factors that provide the best explanation of the desertification processes.

The weight factors assigned to each indicator are described in Tables 3, 4, 5 and 6.

Analyses from 11 indicators stress that areas with predominantly humid and sub-humid climate are potentially susceptible to desertification due to inadequate soil management, which is a key factor for adaptation and mitigation of climate change (IPCC, 2007).

On the MEDALUS methodology, variables like HDI and conservation units were not included. However, these two indicators were considered important in the semi-arid region Brazil based on the fact that the region has relatively low development indexes and several inadequate land uses practices, and previous studies in other regions of Brazil (Trancoso et al., 2010) have shown that conservation enforcement in protected areas is crucial for avoiding degradation.

The environmentally sensitive area index calculated in the present study allowed a better understanding of the degradation/desertification process in the Brazilian semi-arid region. The study showed that desertification susceptibility ranges from moderate to high in the Brazilian semi-arid region.

From a climatic point of view, the humid and sub-humid areas have low vulnerability. However, when management issues associated with land use are taken into consideration, these areas become potentially susceptible to degradation.

The northwestern part of the study area is highly susceptible to land degradation due to inadequate soil management associated with intensive agricultural land expansion. In the last 50 years, the area received millions of migrants looking for better opportunities created by agriculture expansion.

This study is the first effort to produce a comprehensive diagnosis of the desertification processes for the entire region and combines the existing experience from previous studies in the region with a consolidated methodology. Additionally, new indicators were included in the methodology of this study, such as HDI (social indicator) and conservation units (management indicator), because previous knowledge indicated that they would be relevant in the study area.

In addition, it was possible to obtain a database with biophysical and social information on the same scale and resolution, which allowed the integrated analysis of the desertification indicators.

One of the major issues facing humanity today is the development of knowledge in regards to the occupation of land in regions affected by desertification in a sustainable way. Then it becomes critical to define adaptation alternatives for living in semi-arid regions. Furthermore, it can be applied in multi-scale studies, showing the magnitude of the risk in different areas and the factors that may contribute to triggering the process. The approach was based on the use of indicators that are routinely surveyed in the area, allowing for continuous monitoring of the desertification processes. The proposed methodology proved to be a useful, timely and cost-effective tool to identify areas that are susceptible to degradation/desertification.