**1. Introduction**

24 Soil Erosion Studies

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Arid and semiarid zones cover approximately 40% of the land surface, with a continuous increase in the area by desertification processes, induced mainly by anthropogenic activities and/or climatic change (IPCC, 2008). The process of desertification affects the world at ecological, economic and social levels (Salih, 1998).

Maestre et al. (2006) considers that desertification produces biophysics and socioeconomics consequences, with the later including household debt increasing, loss of traditional knowledge and local traditions, people migration, reduction of food production, costs of living increases, poor supplies in quality and quantity, increase in poverty, and political instability (Sivakumar, 2007). The UNCCD (2004), considers that over 250 000 million of people are directly affected by desertification, mostly in regions with significant poverty and marginalization in the world. In the case of Mexico, experts estimate that each year, between 700 000 and 900 000 people are forced to leave Mexico´s rural dry land areas in search of better livelihood in urban areas, including foreign countries.

On the other hand, biophysics consequences involve the loss of soil and vegetation cover, reduction in soil fertility and biodiversity, reduction in rainfall infiltration rates, and modification of local climate (Maestre et al., 2006). Soils of arid and semiarid zones are very susceptible of water erosion (Cornelis, 2006) mostly due to the scarce vegetation cover, low organic matter content and the small resistance to the erosion forces. The magnitude of water erosion also depends on their texture, water content, evaporation, percolation and lixiviation. These soil characteristics are not favorable to the resistance of the soil to water erosion (D´Odorico & Porporato, 2006). In terms of soil orders, typical arid and semiarid zone soils around the world include Aridosol, Alfisols, Entisols, Molisols, and Vertisols (Dregne, 1976).

In arid and semiarid areas, soils with little or no vegetation cover are exposed to torrential precipitation events, which are characterized by short durations and high intensities, and are prompt to the occurrence of physical and chemical processes that change the surface layer conditions, such as surface sealing and crusting. When the surface is dry, a hard layer is formed (crust). Crusting soils are typical of these dry areas, where soil degradation is

Soil Erosion Processes in Semiarid Areas: The Importance of Native Vegetation 27

Fig. 1. Histogram distribution by year, with different characteristics in amount of year in

Cadereyta de Montes, Queretaro, Mexico.

induced soils by diminishing infiltration rates and increasing runoff and erosion rates (Ries & Hirt, 2008).

Arid and semiarid areas are considered fragile environments where vegetation cover is scarce and where soil erosion processes occur rapidly and severely after rainfall events fall in these areas. However, even under those conditions, the importance of native vegetation is very significant in the regulation of surface hydrological processes.

The objective of this chapter is to discuss through a study case from a semiarid area in Querétaro, Mexico, the importance of native vegetation on the regulation of soil erosion processes. An extensive work on the quantification of vegetation changes in space and time (Chen et al., 2006; Moro et al., 2007) has been done for arid and semiarid areas. However, only few efforts have been directed towards a better understanding of the hydrological functioning and the native vegetation-soil relationships (Huxman et al., 2004, Bautista et al., 2007; Vásquez-Méndez et al., 2010) compared with other environmental variables.
