**4. Rainfall-runoff relationships**

The relationships between rainfall and runoff processes that result in erosion at a given location are generally complex (Bedford & Small, 2008). Prediction of runoff and soil loss is important for assessing soil erosion hazard and for determining suitable land uses and soil conservation measures for a catchment (Bedford & Small, 2008). Soil erosion by water occurs as a result of the detachment of soil particles by raindrops and runoff (Kim & Gilley, 2008). The most known and widely used parameter to predict the erosive potential of raindrop impact and to reflect the amount and rate of runoff generated by erosive storms is the rainfall erosion index, also known as rainfall erosivity or R-Factor of the Universal Soil Loss Equation or USLE (Wischmeier & Smith, 1978).

The smaller amount of rainfall in semi-arid areas compared with that in humid climates does not mean a corresponding low level of soil erosion by water. Indeed rainfall erosion can be higher in semi-arid areas than in any other climatic zone. This is also because there is poor protective vegetative cover, especially at the beginning of the rainy season. Some of the soils common in semi-arid areas are particularly vulnerable, either because they have poor resistance to erosion or because of their chemical and physical properties (Schlesinger & Pilmanis, 1998).

Spatial patterns of soil properties are linked to patchy vegetation in arid and semi-arid landscapes. The patterns of soil properties are generally assumed to be associated to the ecohydrological functioning of patchy dryland vegetation ecosystems (Bedford & Small; 2008).

Researchers, such as Ares et al. (2003) and Bautista et al. (2007), have found an inverse relationship between water runoff and the scale of heterogeneity of a vegetation mosaic. They have suggested that the landscape scale characteristics, such as topography, constrain the plant spatial pattern at finer scales. More runoff is produced as the vegetation cover is more disperse or bare areas increase, since their soils are susceptible to develop physical and biological crusts (Rango et al., 2006).

Amount of runoff per event in four native species and a control (bare surface), as evaluated in USLE-type plots is presented in Figure 6. Runoff decreases considerable when a greater vegetation cover is present. Bare or nearly bared surfaces always produced greater amounts of runoff.

In a more specific analysis (Figure 7), an inverse relationship between runoff and leaf area index, both canopy and ground, was found for the studied species and their fertility island. The results demonstrate the positive effect of native vegetation by diminishing the runoff processes in a considerable way.

The contribution of different layers of vegetation (canopy, understory, and lower vegetation), is significant, by diminishing the kinetic energy of rainfall against soil and intercepting precipitation, promoting infiltration. Runoff decreased from 87 to 98% from the total rainfall amount due to canopy cover (Vásquez-Méndez et al., 2010).

Sealing soils occur when soil surface is unprotected against precipitation effects and wetting and drying occurs quickly. Rainfall impacts consolidation the surface, resulting in pressure and erosion by splash of soil particles (Ries & Hirt, 2008). The consequences of sealing and crusting soil are the reduction in hydraulic conductivity, increased in horizontal flow (runoff), the inability of soil gas exchange (Li et al., 2005; Ries & Hirt, 2008). When these conditions exist, depending on the vegetation cover area, runoff coefficients fluctuate from 8 to 60% (Al-Qurashi et al., 2008), and total runoff can increased up to 80 and 90% in sealed soils (Ries & Hirt, 2008, Vásquez-Méndez et al., 2010). Bare surfaces decrease infiltration, for sealing soils (Ruan et al., 2001; Chen et al., 2008).

Fig. 6. Box plot of runoff by native vegetation in a semiarid environment of Central Mexico (Vásquez-Méndez et al., 2010).

Fig. 7. Typical behavior between runoff and leaf area index of native vegetation in a semiarid environment of Central Mexico.

#### **5. Soil erosion**

32 Soil Erosion Studies

The relationships between rainfall and runoff processes that result in erosion at a given location are generally complex (Bedford & Small, 2008). Prediction of runoff and soil loss is important for assessing soil erosion hazard and for determining suitable land uses and soil conservation measures for a catchment (Bedford & Small, 2008). Soil erosion by water occurs as a result of the detachment of soil particles by raindrops and runoff (Kim & Gilley, 2008). The most known and widely used parameter to predict the erosive potential of raindrop impact and to reflect the amount and rate of runoff generated by erosive storms is the rainfall erosion index, also known as rainfall erosivity or R-Factor of the Universal Soil Loss

The smaller amount of rainfall in semi-arid areas compared with that in humid climates does not mean a corresponding low level of soil erosion by water. Indeed rainfall erosion can be higher in semi-arid areas than in any other climatic zone. This is also because there is poor protective vegetative cover, especially at the beginning of the rainy season. Some of the soils common in semi-arid areas are particularly vulnerable, either because they have poor resistance to erosion or because of their chemical and physical properties (Schlesinger &

Spatial patterns of soil properties are linked to patchy vegetation in arid and semi-arid landscapes. The patterns of soil properties are generally assumed to be associated to the ecohydrological functioning of patchy dryland vegetation ecosystems (Bedford & Small; 2008). Researchers, such as Ares et al. (2003) and Bautista et al. (2007), have found an inverse relationship between water runoff and the scale of heterogeneity of a vegetation mosaic. They have suggested that the landscape scale characteristics, such as topography, constrain the plant spatial pattern at finer scales. More runoff is produced as the vegetation cover is more disperse or bare areas increase, since their soils are susceptible to develop physical and

Amount of runoff per event in four native species and a control (bare surface), as evaluated in USLE-type plots is presented in Figure 6. Runoff decreases considerable when a greater vegetation cover is present. Bare or nearly bared surfaces always produced greater amounts

In a more specific analysis (Figure 7), an inverse relationship between runoff and leaf area index, both canopy and ground, was found for the studied species and their fertility island. The results demonstrate the positive effect of native vegetation by diminishing the runoff

The contribution of different layers of vegetation (canopy, understory, and lower vegetation), is significant, by diminishing the kinetic energy of rainfall against soil and intercepting precipitation, promoting infiltration. Runoff decreased from 87 to 98% from the

Sealing soils occur when soil surface is unprotected against precipitation effects and wetting and drying occurs quickly. Rainfall impacts consolidation the surface, resulting in pressure and erosion by splash of soil particles (Ries & Hirt, 2008). The consequences of sealing and crusting soil are the reduction in hydraulic conductivity, increased in horizontal flow (runoff), the inability of soil gas exchange (Li et al., 2005; Ries & Hirt, 2008). When these conditions exist, depending on the vegetation cover area, runoff coefficients fluctuate from 8 to 60% (Al-Qurashi et al., 2008), and total runoff can increased up to 80 and 90% in sealed soils (Ries & Hirt, 2008, Vásquez-Méndez et al., 2010). Bare surfaces decrease infiltration, for

total rainfall amount due to canopy cover (Vásquez-Méndez et al., 2010).

**4. Rainfall-runoff relationships** 

Equation or USLE (Wischmeier & Smith, 1978).

biological crusts (Rango et al., 2006).

processes in a considerable way.

sealing soils (Ruan et al., 2001; Chen et al., 2008).

Pilmanis, 1998).

of runoff.

Erosion, the detachment of particles of soil and surficial sediments and rocks, occurs by hydrological (fluvial) processes of sheet erosion, rill and gully erosion, and through mass

Soil Erosion Processes in Semiarid Areas: The Importance of Native Vegetation 35

Fig. 8. Box plot of soil loss by native vegetation in semiarid environment (Vásquez-Méndez

It appears that on average interception can amount to 10-50% of the precipitation (Calder, 1990; Gerrits et al., 2007; Wang et al., 2007). Therefore, knowledge about the process of interception is important (Gerrits et al., 2007). The canopy interception process, which is a basic process controlling interactions of precipitation with plant canopies, plays an important role in the water resources cycle of forest watersheds and isolated environments.

Interception, a process affecting the availability of water in the hydrological cycle, is often considered as the trapping, storage and disposition of materials on the vegetative surface of plants, or as the process of aerial redistribution of precipitation by vegetation. Rainfall interception includes the processes that result from the temporary storage of precipitation by the tree canopy. Water can either evaporate directly to the atmosphere, absorbed by the canopy surfaces, or ultimately transmitted to the ground surface (Xiao et al., 2000). Thus, interception can be described as the difference between gross rainfall (PG) and throughfall (T). There are two principal ways in which the physical and physiological characteristics of plants can influence the hydrological cycle. These effects can be separated broadly into the effects vegetation has on water delivered as precipitation on the quantity and distribution of that water to the soil, and on the amount and distribution of water that is removed from soil and subsoils (Wang et al., 2005). The first influence is to a large extent a physical one, whereas physiological characteristics determine the way plants remove water from the soil. The interception processes determine water losses from vegetation canopies wetted by rain

Rainfall interception processes is considerable in the tree or shrub canopies at semiarid and arid environments. Plant communities have a positive response to intercept rainfall water through their leaves, branches and trunks. Mastachi-Loza et al. (2010) and Návar & Bryan (1994) registered that canopy of native vegetation (Prosopis and Acacia trees) can intercept

This rainfall distribution affects the runoff generation and flow concentration.

et al., 2010).

(Wang et al., 2005).

up to 20 to 30% of the rainfall.

wasting and the action of wind. Erosion, both fluvial and eolian (wind) is generally greatest in arid and semi-arid regions, where soil is poorly developed and vegetation provides relatively little protection. Where land use causes soil disturbance, erosion may increase greatly above natural rates.

Soil erosion, continues to be a primary cause of soil degradation throughout the world, and has become an issue of significant and severe societal and environmental concern (Wei et al., 2007). About 80% of the world´s agricultural land suffers moderate to severe erosion, and 10% suffers slight to moderate erosion (Pimentel et al., 1995). Erosion by water and wind adversely affects soil quality and productivity by reducing infiltration rates, water-holging capacity, nutrients, infiltration rates, water-holding capacity, organic matter, soil biota and soil depth.

The removal of vegetation is the main cause of soil degradation in semiarid areas (Castillo et al., 1997). Changes in soil properties induced by vegetation removal modified the runoff and soil erosion response in a semiarid area of Spain. Total runoff was significantly greater when vegetation was removed (48.8 mm) as compared to undisturbed conditions (34.9 mm). Runoff ratios between the disturbed and natural plots increased with time from 1.4:1 in 1990 to 2.5:1 in 1993. Vegetation removal increased the soil losses by 127% compared the undisturbed conditions. The annual soil loss ratio between the disturbed and natural plots increased from 1.6:1 in 1989 to 4.2:1 in 1993. The observed increase in surface runoff and soil loss was attributed to a progressive deterioration of soil physical properties. Bulk density increased by 8.4% (from 1.55 to 1.68 Mg m-3), total organic carbon was reduced from 4.0 to 2.6% and the percentage of stable aggregates decreased from 81.6 to 56.3% in the disturbed area. There was no evidence of vegetation recovery, suggesting that reduced vegetal cover might lead to irreversible soil degradation in semiarid areas.

The relationship between erosion and vegetation cover have been shown from various researchers (Stocking & Elwell, 1976; Evans, 1980) that erosion declines exponentially as vegetation in cover increases. One particularly important interaction is how, during rainstorms, patches of vegetation serve as surface obstructions that slow and trap runoff, sediments, and nutrients from open interpatch areas (Schlesinger et al. 2000) due to their sufficient stem and biomass densities.

Soil loss from vegetation patches of *P. laevigata*, *A. farnesiana* and *O. sp*, was significantly smaller as compare to a bare surface area and an area with *O. imbricata*, with low vegetation cover.

The maximum values of soil loss were 1275, 1366, 120, 130 and 21 kg ha-1, while the soil loss cumulative correspond to values of 3520, 3913, 240, 177 and 38 kg ha-1 for bare surface, *Opuntia imbricate*, *Prosopis laevigata*, *Acacia farnesiana* and *Opuntia* sp., respectively (see Figure 8). Corresponding values of soil loss were 97%, 93% and 99% with respect to the bare surface of vegetal species of *Acacia farnesiana, Prosopis laevigata and Opuntia* sp.

#### **6. Interception**

Interception is defined in three different ways: i) Interception storage (L), is considered the amount of rainfall which is temporarily stored on the land and evaporated shortly after and during the rainfall event. ii) Interception flux is considered the amount of intercepted water, which is evaporated in a certain time (LT-1). iii) Interception process (I) (LT-1) is considered as the part of the rainfall flux which is intercepted on the wetted surface after which it is fed back to the atmosphere. The interception process is equals to the sum of the change of interception storage and the evaporation from this stock (Gerrits et al., 2007).

wasting and the action of wind. Erosion, both fluvial and eolian (wind) is generally greatest in arid and semi-arid regions, where soil is poorly developed and vegetation provides relatively little protection. Where land use causes soil disturbance, erosion may increase

Soil erosion, continues to be a primary cause of soil degradation throughout the world, and has become an issue of significant and severe societal and environmental concern (Wei et al., 2007). About 80% of the world´s agricultural land suffers moderate to severe erosion, and 10% suffers slight to moderate erosion (Pimentel et al., 1995). Erosion by water and wind adversely affects soil quality and productivity by reducing infiltration rates, water-holging capacity, nutrients, infiltration rates, water-holding capacity, organic matter, soil biota and soil depth. The removal of vegetation is the main cause of soil degradation in semiarid areas (Castillo et al., 1997). Changes in soil properties induced by vegetation removal modified the runoff and soil erosion response in a semiarid area of Spain. Total runoff was significantly greater when vegetation was removed (48.8 mm) as compared to undisturbed conditions (34.9 mm). Runoff ratios between the disturbed and natural plots increased with time from 1.4:1 in 1990 to 2.5:1 in 1993. Vegetation removal increased the soil losses by 127% compared the undisturbed conditions. The annual soil loss ratio between the disturbed and natural plots increased from 1.6:1 in 1989 to 4.2:1 in 1993. The observed increase in surface runoff and soil loss was attributed to a progressive deterioration of soil physical properties. Bulk density increased by 8.4% (from 1.55 to 1.68 Mg m-3), total organic carbon was reduced from 4.0 to 2.6% and the percentage of stable aggregates decreased from 81.6 to 56.3% in the disturbed area. There was no evidence of vegetation recovery, suggesting that reduced vegetal cover

The relationship between erosion and vegetation cover have been shown from various researchers (Stocking & Elwell, 1976; Evans, 1980) that erosion declines exponentially as vegetation in cover increases. One particularly important interaction is how, during rainstorms, patches of vegetation serve as surface obstructions that slow and trap runoff, sediments, and nutrients from open interpatch areas (Schlesinger et al. 2000) due to their

Soil loss from vegetation patches of *P. laevigata*, *A. farnesiana* and *O. sp*, was significantly smaller as compare to a bare surface area and an area with *O. imbricata*, with low vegetation

The maximum values of soil loss were 1275, 1366, 120, 130 and 21 kg ha-1, while the soil loss cumulative correspond to values of 3520, 3913, 240, 177 and 38 kg ha-1 for bare surface, *Opuntia imbricate*, *Prosopis laevigata*, *Acacia farnesiana* and *Opuntia* sp., respectively (see Figure 8). Corresponding values of soil loss were 97%, 93% and 99% with respect to the bare

Interception is defined in three different ways: i) Interception storage (L), is considered the amount of rainfall which is temporarily stored on the land and evaporated shortly after and during the rainfall event. ii) Interception flux is considered the amount of intercepted water, which is evaporated in a certain time (LT-1). iii) Interception process (I) (LT-1) is considered as the part of the rainfall flux which is intercepted on the wetted surface after which it is fed back to the atmosphere. The interception process is equals to the sum of the change of

surface of vegetal species of *Acacia farnesiana, Prosopis laevigata and Opuntia* sp.

interception storage and the evaporation from this stock (Gerrits et al., 2007).

might lead to irreversible soil degradation in semiarid areas.

sufficient stem and biomass densities.

cover.

**6. Interception** 

greatly above natural rates.

Fig. 8. Box plot of soil loss by native vegetation in semiarid environment (Vásquez-Méndez et al., 2010).

It appears that on average interception can amount to 10-50% of the precipitation (Calder, 1990; Gerrits et al., 2007; Wang et al., 2007). Therefore, knowledge about the process of interception is important (Gerrits et al., 2007). The canopy interception process, which is a basic process controlling interactions of precipitation with plant canopies, plays an important role in the water resources cycle of forest watersheds and isolated environments. This rainfall distribution affects the runoff generation and flow concentration.

Interception, a process affecting the availability of water in the hydrological cycle, is often considered as the trapping, storage and disposition of materials on the vegetative surface of plants, or as the process of aerial redistribution of precipitation by vegetation. Rainfall interception includes the processes that result from the temporary storage of precipitation by the tree canopy. Water can either evaporate directly to the atmosphere, absorbed by the canopy surfaces, or ultimately transmitted to the ground surface (Xiao et al., 2000). Thus, interception can be described as the difference between gross rainfall (PG) and throughfall (T).

There are two principal ways in which the physical and physiological characteristics of plants can influence the hydrological cycle. These effects can be separated broadly into the effects vegetation has on water delivered as precipitation on the quantity and distribution of that water to the soil, and on the amount and distribution of water that is removed from soil and subsoils (Wang et al., 2005). The first influence is to a large extent a physical one, whereas physiological characteristics determine the way plants remove water from the soil. The interception processes determine water losses from vegetation canopies wetted by rain (Wang et al., 2005).

Rainfall interception processes is considerable in the tree or shrub canopies at semiarid and arid environments. Plant communities have a positive response to intercept rainfall water through their leaves, branches and trunks. Mastachi-Loza et al. (2010) and Návar & Bryan (1994) registered that canopy of native vegetation (Prosopis and Acacia trees) can intercept up to 20 to 30% of the rainfall.

Soil Erosion Processes in Semiarid Areas: The Importance of Native Vegetation 37

For some insects and epiphytes, trees are the minimum habitat unit and can be considered islands, because, like islands, there are discrete ecological units with fixed borders surrounded by different environment (Flores-Palacios & García-Franco, 2006). In

Some phreatophytes, especially in drier regions, have extensive, subsurface lateral roots to take advantage of any light rainfall that might occur. Such subsurface root systems help to stabilize the soil. The aerial system too has a role in reducing wind erosion and ameliorating the microclimate although information on changes in humidity and temperature is generally

The indirect contribution of native species to soil fertility is two-fold. First, there is a contribution through nitrogen is probably minimal since the foliage on the ground probably undergoes two periods of rapid degeneration (Wickens, 1995). Along with the improvement in soil fertility, soil physical and micro-climate conditions, the trees play an important role in soil binding processes and the reduction in the eroding action of both water and wind

The amount and seasonality of rainfall is also reflected in the amount of vegetation cover within the increase of vegetation (surface and canopy). The annual distribution of vegetation has a tendency to increase shortly after the first rains, as in all arid and semiarid environments. However, there was a greater contribution of shrubs or beneath the surface

Aguiar, M.R. & Sala, O.E. (1999). Patch structure, dynamics and implications for the

Álvarez-Yépiz, J.C. ; Martínez-Yrizar, A. ; Búrquez, A. & Lindquist, C. (2008). Variation in

Bautista, S. ; Mayor, A.G. ; Bourakhouadar, J. & Bellot, J. (2007). Plant spatial pattern

Bedford, D.R. & Small, E.E. (2008). Spatial patterns of ecohydrologic propoperties on a hillslope-alluvial fan transect, central New Mexico. *Catena*, 73(2008), 182-191. Belsky, A.J. ; Amundson, R.G. ; Duxbury, J.M. ; Riha, S.J. ; Ali, A.R. & Mwonga, S.M. (1989).

Castillo, V.M. ; Martínez-Mena, M. & Albaladejo, J. (1997). Runoff and soil loss response to

Chen, H. ; Shao, M. & Lo, Y. (2008). The characteristics of soil water cycle and water balance

semi-arid Savanna in Kenya. *Journal of applied ecology*, 26(3), 1005-1024. Bisigato, A.J . ; Villagra, P.E. ; Ares, J.O. & Rossi, B.E. (2009). Vegetation heterogeneity in

vegetation structure and soil properties related to land use history of old-growth and secondary tropical dry forest in northwertern Mexico. *Forest Ecology and* 

predicts hillslope runoff and erosion in a semiarid Mediterranean landscape.

The effects of trees on their physical, chemical and biological environments in a

Monte Desert ecosystems: A multi-scale approach linking patterns and processes.

vegetation removal in a semiarid environment*. Soil science society of America Journal*,

on steep grassland under natural and simulated rainfall conditions in the Loess

pastureland trees are often isolated and distant from other trees.

patches of vegetation with a greater coverage area of vegetation.

functioning of arid ecosystems. *Tree*, 14(7), 273-277.

*Management,* 256, 355-366.

*Ecosystems*, 10(2007), 987-998.

61(4), 1116-1121-

*Journal of arid environments,* 73, 182-191.

Plateau of China*. Journal of hydrology,* 360, 242-251.

lacking (Wickens, 1995).

(Pimentel et al., 1995).

**9. References** 

Infiltration rates in arid and semiarid depend on factors like vegetation cover, rainfall amount, intensity, and duration, soil type and moisture, slope degree, and land use (Salas, 2000; Wilcox et al., 2003). Soil water infiltration rates are greater under canopies as a result of the soil protection from raindrop impact and compaction by the addition of organic matter from plants, improving soil crumb structure (Salas, 2000; Rango et al., 2006). Trees are a principal factor to increase organic material levels, to moderate soil temperature and to improve soil moisture, resulting in a higher infiltration rate beneath the canopy cover (Gutiérrez, 2001; Zehe, 2008).
