**2. Precipitation**

Precipitation is, without doubt, the most important component of the land-atmospheric system accountable for shaping the climatic state and variability of water in the soil and atmosphere (Wang et al., 2006). Precipitation is a major factor controlling the hydrological cycle of a region and, thereby, also the ecological and geomorphologic processes. This meteorological phenomenon has many characteristics, and the relative amount of rain and snow, their seasonal timing, and the volumes and intensities of rainfall events affect ecological developments and water management activities (Jong & Jetten, 2007).

Prediction of mean annual values is useful in areas where annual rainfall tends to follow a normal probability distribution (Figure 1). In arid and semiarid zones, annual rainfall distributions typically show a pronounced skew and correspond better to lognormal, extreme value or Pearson Type III probability density functions. In contrast to temperate regions, semiarid ecosystems are also more impacted by extreme events, such as severe storms, floods and droughts (Mannaerts & Gabriels, 2000).

In semiarid zones, the distribution of precipitation through the year is not uniform, and usually they are intermittent. In particular, in arid and hiperarid areas, it is possible that in some years precipitation does not occur at all (Salas, 2000). In general, the precipitation in these areas is characterized by torrential events, occurring in short time spans and at high rainfall intensities (Rango et al., 2006; Wei et al., 2007). It has been observed that the precipitation pulses promote biological activities in plants, soil and the ecosystems as a whole. Furthermore, these events are important drivers for soil physical processes (Huxman et al., 2004; Dekker et al., 2007). However, the influence of rainfall characteristics on the vegetation-soil relationship has been poor studied compared with other environmental variables (Huxman et al., 2004).

Rainfall precipitation is high variable in semiarid areas. Events in the semiarid area of Cadereyta, Querétaro, Mexico, for a 3 years period of registration showed that rainfall depth varied from 1 to 74 mm. The time span of occurrence varied from 5 minutes to 2 days. Also, 60% to rainfall events have a rainfall depth smaller than 5 mm (Figure 1). This percentage increased to 75% when considering events smaller than 10 mm. On average, only 10 rainfall events greater than 20 mm occurred per year, with a low probability of occurrence but with high potential consequences due to their erosivity (Figure 2).

induced soils by diminishing infiltration rates and increasing runoff and erosion rates (Ries

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

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.,

Precipitation is, without doubt, the most important component of the land-atmospheric system accountable for shaping the climatic state and variability of water in the soil and atmosphere (Wang et al., 2006). Precipitation is a major factor controlling the hydrological cycle of a region and, thereby, also the ecological and geomorphologic processes. This meteorological phenomenon has many characteristics, and the relative amount of rain and snow, their seasonal timing, and the volumes and intensities of rainfall events affect

Prediction of mean annual values is useful in areas where annual rainfall tends to follow a normal probability distribution (Figure 1). In arid and semiarid zones, annual rainfall distributions typically show a pronounced skew and correspond better to lognormal, extreme value or Pearson Type III probability density functions. In contrast to temperate regions, semiarid ecosystems are also more impacted by extreme events, such as severe

In semiarid zones, the distribution of precipitation through the year is not uniform, and usually they are intermittent. In particular, in arid and hiperarid areas, it is possible that in some years precipitation does not occur at all (Salas, 2000). In general, the precipitation in these areas is characterized by torrential events, occurring in short time spans and at high rainfall intensities (Rango et al., 2006; Wei et al., 2007). It has been observed that the precipitation pulses promote biological activities in plants, soil and the ecosystems as a whole. Furthermore, these events are important drivers for soil physical processes (Huxman et al., 2004; Dekker et al., 2007). However, the influence of rainfall characteristics on the vegetation-soil relationship has been poor studied compared with other environmental

Rainfall precipitation is high variable in semiarid areas. Events in the semiarid area of Cadereyta, Querétaro, Mexico, for a 3 years period of registration showed that rainfall depth varied from 1 to 74 mm. The time span of occurrence varied from 5 minutes to 2 days. Also, 60% to rainfall events have a rainfall depth smaller than 5 mm (Figure 1). This percentage increased to 75% when considering events smaller than 10 mm. On average, only 10 rainfall events greater than 20 mm occurred per year, with a low probability of occurrence but with

2007; Vásquez-Méndez et al., 2010) compared with other environmental variables.

ecological developments and water management activities (Jong & Jetten, 2007).

storms, floods and droughts (Mannaerts & Gabriels, 2000).

high potential consequences due to their erosivity (Figure 2).

variables (Huxman et al., 2004).

very significant in the regulation of surface hydrological processes.

& Hirt, 2008).

**2. Precipitation** 

Fig. 1. Histogram distribution by year, with different characteristics in amount of year in Cadereyta de Montes, Queretaro, Mexico.

Soil Erosion Processes in Semiarid Areas: The Importance of Native Vegetation 29

rainfall intensity in semiarid areas is comparable with the intensities from tropical climates (Hoyos et al., 2005). EI30 it is an important parameter for compute erosivity of a rainfall event. The mean value of I30 was 15 mm h-1, while maxim values were from 40 to 60 mm h-1, typical values registered in this arid and semiarid areas (Hussein et al., 1994; Wei et al., 2007;

Vegetation patterns in arid and semiarid ecosystems are characterized by the size and spatial distribution of the higher plant-cover patches. These patches have its main distribution in spots or bands distributed as a mosaic (Aguiar & Sala, 1999). The spots are generally isodiametric or with irregular circular shapes, and the bands range from 1 to less than 100 m (Aguiar & Sala, 1999). Investigators have documented the presence of "islands of fertility" or "resource islands" in arid and semiarid ecosystems at individual plant scale (Moro et al., 1997; Aguiar & Salas 1999; Facelli & Temby, 2002; Rango et al.,

These areas can also be considered as hydrologic islands, and they are observed at the individual plant scale as well as in large area patterns of banded vegetation, where playettes and beaded drainage networks run-on and water infiltration stimulates vegetation growth

Fig. 4. Typical distribution of native vegetation in a semiarid environment of Central

Stone et al., 2008).

**3. Vegetation** 

2006).

Mexico.

(Rango et al., 2006).

Fig. 2. Precipitation histogram for the years 2006, 2007 and 2008 at Cadereyta, Queretaro, Mexico.

One of the main factors related to the erosion potential of rain is the rainfall frequency and different magnitudes of events in combination with the soil geomorphic thresholds (Bisigato et al., 2009). High intensities of rainfall are the main factor driving the runoff processes (Renard et al., 1974).

Mean, maximum and maximum intensity in 30 minutes are shown in Figure 3 for the Cadereyta area. Overall, the values of intensities varied from 0.71 to 88.2 mm h-1. Maximum

Fig. 3. Box plot of rainfall intensity by events measured in the semiarid of Mexico.

rainfall intensity in semiarid areas is comparable with the intensities from tropical climates (Hoyos et al., 2005). EI30 it is an important parameter for compute erosivity of a rainfall event. The mean value of I30 was 15 mm h-1, while maxim values were from 40 to 60 mm h-1, typical values registered in this arid and semiarid areas (Hussein et al., 1994; Wei et al., 2007; Stone et al., 2008).
