**3. Rainfall energy effects on soil erosion and runoff generation in semi-arid forested lands**

The effects of fire and torrential rainfall, two important factors in Mediterranean semiarid environments, on the soil erosion and hydrology, are analyzed in this section.

#### **3.1 Methods**

Experiments were carried out using a portable sprinkler-based rainfall simulator with different nozzles and pressures. Two levels of rainfall energy (12,6 J m-2 mm-1 and 24,7 J m-2 mm-1) and similar intensity (85±8 mm h-1) reproduce torrential rainfall characteristics. Rainfall simulations were conducted immediately after Aleppo pine (*Pinus halepensis L.*) litter covering soil surface was burned. The experiments were conducted on calcareous, loamy clay soils classified as Calcaric Regosol, frequent in the semi-arid Central Ebro Valley (NE-Spain). Paired burned areas were established in nine micro-plots and compared with paired not burned control areas (2-soil status x 2 rainfall energy x 9 plots or replicates). In each rainfall simulation the soil loss, soil infiltration (calculated by Horton model), wetting front, runoff coefficient and runoff quality (EC and pH) were measured (Badía & Martí, 2008).

#### **3.2 Results and discussion**

The results indicate that when litter cover was burned rainfall significantly increases the sediment yield. Burned plots generated 18.5 times more sediment than unburned plots (control) for fine rainfall energy (Raindrop D50 = 1,0 mm) and 33.6 times more for coarse rainfall energy (Raindrop D50 = 1,4 mm) (Fig. 4).

The return period for a single storm with I30 (rainfall intensity for 30 min) ranged from 5 to 200 years from the East Mediterranean areas to semiarid Central Ebro Valley, respectively (Fomento, 2001). These rainfalls of high intensity are especially frequent in autumn, just after summer, the season of maximum risk of wildfires. Sediment loss was given as solutes dissolved in overland flow, mainly in unburned plots, and as particles in suspension, mainly in burned plots. However, soil erosion rates were without significant differences between both rainfalls. Fire increased runoff quantity (about 1,6 times) and decreased its quality temporarily by increasing significantly both pH and specially EC in relation to unburned plots (Fig. 5).

The results indicate that when vegetation and litter cover of soils were burned, the first rainfalls duplicate runoff, soil infiltration decreases significally and soil erosion increases 20 – 30 times in relation to unburned plots, especially with the highest rainfall energy. Because of the dynamic climate of the area that we discussed, a restoration strategy for a short-term response would be an interesting response, especially in conditions where plant succession is very slow, e.g., high slopes and soils of low permeability materials, thus highly erodible.

In Figure 3, there are also some turning points in the vegetation cover and erosion evolution related to environmental conditions. Thus, in spring vegetation the cover rate is accelerated then decreases during the summer period mainly dominated by therophytes plants; the

It must be taken into account that in burned kermes evergreen oak shrubland, the average erosion is 7 times higher than in unburned plots; in Aleppo pinewood soil loss can be as much as 36 times higher in the early years (Rodríguez et al., 2000). This is due to the

**3. Rainfall energy effects on soil erosion and runoff generation in semi-arid** 

The effects of fire and torrential rainfall, two important factors in Mediterranean semiarid

Experiments were carried out using a portable sprinkler-based rainfall simulator with different nozzles and pressures. Two levels of rainfall energy (12,6 J m-2 mm-1 and 24,7 J m-2 mm-1) and similar intensity (85±8 mm h-1) reproduce torrential rainfall characteristics. Rainfall simulations were conducted immediately after Aleppo pine (*Pinus halepensis L.*) litter covering soil surface was burned. The experiments were conducted on calcareous, loamy clay soils classified as Calcaric Regosol, frequent in the semi-arid Central Ebro Valley (NE-Spain). Paired burned areas were established in nine micro-plots and compared with paired not burned control areas (2-soil status x 2 rainfall energy x 9 plots or replicates). In each rainfall simulation the soil loss, soil infiltration (calculated by Horton model), wetting front, runoff coefficient and

The results indicate that when litter cover was burned rainfall significantly increases the sediment yield. Burned plots generated 18.5 times more sediment than unburned plots (control) for fine rainfall energy (Raindrop D50 = 1,0 mm) and 33.6 times more for coarse

The return period for a single storm with I30 (rainfall intensity for 30 min) ranged from 5 to 200 years from the East Mediterranean areas to semiarid Central Ebro Valley, respectively (Fomento, 2001). These rainfalls of high intensity are especially frequent in autumn, just after summer, the season of maximum risk of wildfires. Sediment loss was given as solutes dissolved in overland flow, mainly in unburned plots, and as particles in suspension, mainly in burned plots. However, soil erosion rates were without significant differences between both rainfalls. Fire increased runoff quantity (about 1,6 times) and decreased its quality temporarily by increasing significantly both pH and specially EC in relation to

The results indicate that when vegetation and litter cover of soils were burned, the first rainfalls duplicate runoff, soil infiltration decreases significally and soil erosion increases 20 – 30 times in relation to unburned plots, especially with the highest rainfall energy. Because of the dynamic climate of the area that we discussed, a restoration strategy for a short-term response would be an interesting response, especially in conditions where plant succession is very slow, e.g., high slopes and soils of low permeability materials, thus highly erodible.

pulses in the soil erosion are related to the greatest intensity of rainfall.

differential evolution of vegetation to cover the soil surface after the fire.

environments, on the soil erosion and hydrology, are analyzed in this section.

runoff quality (EC and pH) were measured (Badía & Martí, 2008).

rainfall energy (Raindrop D50 = 1,4 mm) (Fig. 4).

**forested lands** 

**3.1 Methods** 

**3.2 Results and discussion** 

unburned plots (Fig. 5).

Fig. 4. Fire and drop size effects in the soil erosion (g m-2 h-1) with rainfall simulator method.

Fig. 5. Evolution of electrical conductivity of runoff (EC, dS m-1) in burned and control plots with rainfall simulator method (in a Calcaric Regosol).

SAS

Particle density (Mg m-3)

Water available (%)

CaCO3

pH 1:2,5

Total N

C/N

CEC

P-Olsen

K+-exc

ECe

Basal

Biomass C

OM

Soil Erosion and Conservations Measures in Semiarid Ecosystems Affected by Wildfires 95

**Treatment G25 G150 G250 G250A G500 C25 C150 C250 C250A C500** 

(%) 75b 72b 48d 45d 0.8e 86a 83a 65c 63c 0.9e

(mg kg-1) 96b 95b 103b 122b 136b 308a 319a 320a 326a 328a

(H2O) 7,8d 7,7de 7,3f 7,5e 8,8b 8,2c 8,1c 7,6e 7,8d 9,4a

(g kg-1) 27d 28d 22e 25d 3f 42a 42a 32c 35b 4f

(g kg-1) 1,5b 1,6b 1,5b 1,5b 0,5c 2,7a 2,6a 2,5a 2,3a 0,7c

ratio 10,4a 10,1a 8,5a 9,7a 3,5d 9,0ab 9,4a 7,4c 8,8b 3,3d

(cmolc kg-1) 12,7c 13,9c 12,4c 12,1c 9,9d 27,2a 28,8a 22,0b 21,7b 14,4c

(mg kg-1) 0,4g 0,4g 1,2f 1,3f 1,7e 2,3d 2,7d 9,1c 9,8b 11,5a

(mg kg-1) 46,0f 36,5f 48,0f 102,0e 186,0d 176,0d 154,0d 204,0c 286,0b 528,0a

(dS m-1) 2,6e 5,6b 8,5a 8,7a 3,3d 0,9f 1,2f 3,4d 3,7c 2,4e

respiration 100.1e 89.2f 86.9f 98.3e 29.2g 153.7d 198.6c 254.8b 277.0a 33.4g

(mg C kg-1) 635c 532d 387e 426e 217f 737b 742b 812a 821a 353e

Table 2. Changes induced by simulated fire at different temperatures and ash addition (A) in the physical, chemical and biological properties of gypsiferous (G) and calcareous (C) soils in the semi-arid Central Ebro Basin. In each row, treatment data with the same letters

Among the results it should be noted that heating soil to 250°C caused a decrease in pH and an increase in electrolytic conductivity (ECe) and soluble Ca. Heating soil to 500°C caused an increase in pH and a decrease in ECe and soluble Ca. Increasing heat intensity increased organic matter loss by combustion, which was accompanied by an increase of available nutrients content. Total N content decreased at temperatures greater than 250°C, with about one-third being volatilized. Cation exchange capacity (CEC) was reduced for gypsiferous soil heated to 500°C and to 250°C for calcareous soil. As for physical soil properties, heating increased the quantity of sand-sized particles by fusion of clay with

are not significantly different (LSD test, P>0.05). Basal respiration (mg CO2 soil kg-1 day-1)

**Gypsiferous Soil, G Calcareous Soil, C** 

2.45c 2.44c 2.48c 2.49c 2.68a 2.55b 2.57b 2.58b 2.57b 2.65a

22.6b 23.9b 24.4b 24.8b 27.8a 12.6d 12.6d 13.1d 12.7d 18.4c
