**3. Results and discussion**

The combination of different thematic maps of erosive factors with their data bases was a subdivision of these into 1315 homogeneous plots with a total annual amount of land loss of 60 million tones. These losses vary between 0 t/ha/year and 521 t/ha/year, with an annual average of 11.2 t/ha and a standard deviation of 18.6/t/ha/yr. Spatially (**Figure 2**), the resulting map shows that the rate of soil loss varies from one sector to another in the study area. In fact, the low to very low soil loss classes are mainly located in the middle of the study area.

Although, this sector is characterized by steep topography and relatively high soil vulnerability. Our results are in agreement with those of the work of [61]. These authors found that in Algeria, not only runoff, but also soil erosion, does not systematically increase with the topography, in particular the slope. In addition, we note that the erosion risk in this sector is generally very low, recording an average of around 3.5 t/ha/year (**Table 2**). This clearly explains why the plant cover factor, in particular the forest one, plays a protective role. Indeed, [62] show that factor

**Figure 2.** *Map of soil losses in the Wadi Mina basin.*

*Spatial Estimation of Soil Erosion Risk Using RUSLE/GIS Techniques and Practices… DOI: http://dx.doi.org/10.5772/intechopen.96190*

C decreases the risk of erosion to 0.01 under perennial crops with cover plants or meadows and to 0.001 under forests associated with mulched crops compared to a bare plot.

The high and dangerous soil loss classes are noted exclusively in the northern and northeastern part of the study area. These regions are subject to an interweaving of natural and anthropogenic factors every year. The nature of the soils and the superficial lithological formation resulting from mainly marly terrain shows great fragility to water erosion. This is all the more important since the land has been almost completely bare and cultivated. [63, 64] show that these areas, which form an important part of the wadi Mina basin (1000 km<sup>2</sup> ), are strongly affected by water erosion.

According to the soil loss map obtained and according to the classification described above, the distribution of soil loss classes in the wadi Mina watershed is shown in **Table 1**.

The results of soil losses show that approximately 50% of the study area is classified in category where the erosion risk is low to very low (< 7.4 t/ha/year). 13.9% of the study area are classified in category where soil losses are moderate (7.4 to 12 t/ha/year). Actually, 36.1% of the study area is considered to be located in high risk and dangerous regions where losses exceed a threshold of 20 t/ha/year. The average rate of soil loss estimated at 11.2 t/ha/year is in the moderate erosion risk category.

**Figure 3** highlights the following points (i) Sectors where soil loss exceeds the average of 11.2 t/ha/year, represent only 31.7% of the watershed. Their contribution to the overall soil loss is estimated at 92.5%. (ii) The sectors where the soil loss is lower than the average, occupy 68.3% of the surface of the basin. Their contribution represents only 7.5% of the global loss of soil**.**

The wadi Mina watershed has been the subject of several studies. These were carried out following the development of the Sidi Mhamed Benaouda dam and concerns raised by the scale of the erosive phenomenon and its consequences on the siltation


#### **Table 1.**

*Soil loss classes in the Wadi Mina catchment.*

**Figure 3.** *Distribution of soil loss compared to the average in the Mina basin.*

of the dam and the degradation of soil fertility from the 1990s. [65] established, from the classified parameters, the map of the sensitivity of marly lands to gullying in the western part of the watershed. This shows a predominance of land sensitive to linear erosion processes on the order of 57% of the territory. In fact, 25% of the land is strongly and very strongly sensitive to the incision and is mainly located on the right bank of wadi Mina, as well as in the downstream sector of the left bank of wadi Haddad. However, 18% of the basin surface is highly sensitive to solifluxion.

According to [62] and his collaborators observed that the different marly textures evolve by landslide and skin slide and those other environmental variables determine linear erosion namely: the slope, the vegetation cover and the morphology of the walls. The specific erosion of the wadi Mina basin estimated by the National Dams and Transfer Agency was on average around 3.26 t/ha/year, while the estimated soil losses in micro-watersheds with an area of 1000 km<sup>2</sup> located in the marly part can exceed a rate of 16 t/ha/year.

In 2001, [66] evaluated soil losses between 0.5 t/ha/year and 36 t/ha/year over the entire territory of the watershed. Most of this loss was recorded in the marl area with a rate exceeding 20 t/ha/year. However, [67] found that not only the marly areas participate in the production of sediments but the southern part of the basin of the wadi Mina can also participate with a significant contribution of sediments deposited in the lake of the Sidi Mhamed Benaouda dam.

In parallel, in 2004, the Algerian State under the supervision of Ministry of Agriculture and Rural Development launched a cooperation project with GTZ in order to develop a master plan for land use in the wadi Mina watershed. This is part of the conservation of soil and water strategy/planning. In 2006, under the supervision of the Ministry of Water Resources in collaboration with the Canadian consultancy firm (TECSULT), the Algerian State launched a study to identify and specify the measures to be undertaken to adequately fight against the siltation of reservoirs located in the Tellian hill including the wadi Mina basin which is one of these regions. The proposed developments have only been affected in areas classified as priorities A and B located in the marly region. According to this study, experts have shown that, if the improvements are made correctly in time, the lifespan of the Sidi Mhamed Benaouda dam will be increased twice as much as without it.

Before carrying out the development works according to the land use of our study area, it is first necessary to determine the average loss of each class as well as the degree of erosion risk. **Table 2** shows the sensitivity of the different types of land use to the risk of erosion. In fact, heavily vegetated areas, represented by vegetable crops and forests, are associated with low to very low soil losses, with 5.5 t/ha/year and 3.5 t/ha/year respectively. However, the higher and more dangerous ones correspond to bare soils with an average soil loss of around 29.8 t/ha/year. Soils used for agriculture, often protected during heavy spring showers, represent the type of vegetation cover most sensitive to erosion processes with an average soil loss of 16.1 t/ha/year. These last results are in agreement with those found by [68]. These authors have shown that cultivated fields can contribute significantly to sediment production. The formations based on scrub/scrubland, pasture and steppe produce moderate soil loss values with respective averages of 12 t/ha/year, 10.5 t/ha/year and 8.4 t/ha/year. This would be due to deforestation, overgrazing and bush fires which tend to substitute primitive formations for secondary cover of a different nature, such as savanna grassland.

The results of the evaluation of soil losses allowed us to deduce that an area of nearly 2400 km2 of the slopes of the study area (**Table 2**) will require intervention measures to counter soil erosion. However, in order to optimize the allocation of resources intended for the short-term reduction of the siltation of the Sidi Mhamed Benaouda dam, we propose that only priority areas receive special attention in terms


#### *Spatial Estimation of Soil Erosion Risk Using RUSLE/GIS Techniques and Practices… DOI: http://dx.doi.org/10.5772/intechopen.96190*

**Table 2.**

*Soil losses according to land use classes in the Wadi Mina basin.*

of anti-erosion measures, including those classified in the two categories A and B where the risk of erosion is moderate to dangerous (> 7.4 t/ha/year) (**Table 1**).

Bare soils and firebreaks covering 150 km2 and 75.4 km2 , respectively, are the main lithological occupations, and will require the most interventions in the watershed. These interventions are mainly intended to counter the gullying. These areas produce a significant amount of sediment estimated annually at an average of 29.8 t/ha and 19.9 t/ha respectively (**Table 2**). The protection strategy for these lands consists of installing torrential correction sills, constructing drains and outlets on slopes in order to avoid landslides with marly substrate, implanting dry stone lines and prohibiting their exploitation by livestock during the spring period when the soils must be covered.

The areas with agricultural activity adjacent to the Sidi Mhamed Benaouda dam and which come in second priority are responsible for a significant proportion of the siltation of this reservoir where soil losses exceed a threshold of 16.1 t/ha/year on an area of 1601.2 km2 , or 33.4% of the study area. The anti-erosion interventions recommended in these areas are the installation of arboriculture on terraces built along the contour lines, the establishment of living hedges, stone lines, drains and outlets along the road accesses as well as torrential correction thresholds in order to reduce the speed of runoff. In addition, it is important to ensure that farmers adhere to the principles of protection of arable slopes by mastering good mechanization which consists of working along the contour lines.

The scrubland and pastures with degraded soils and steppes are the third types of land use in terms of priority. Their erosion risk is between 7.4 t/ha/year and 12 t/ha/year requiring anti-erosion interventions. These interventions suggested over an area of approximately 600 km<sup>2</sup> include the establishment of torrential correction thresholds in the gullies, the planting of opuntia, revegetation and the installation of bulges, drains and outlets.
