Quantification of water erosion using the RUSLE method in the Mazer watershed (Berrechid region, Morocco)

. The losses of soil due to water erosion pose a major problem exacerbating land degradation in many regions worldwide, including Morocco. Soil degradation has detrimental consequences for agriculture, ecosystems, and the environment in general. In this context, the main objective of this study is to assess the risk of soil erosion in the Mazer watershed, located at the heart of a significant agricultural region in the Settat province of Morocco. To achieve this goal, we have employed an advanced methodology that combines the use of the Revised Universal Soil Loss Equation (RUSLE) with the advantages offered by the Geographic Information System (GIS) and remote sensing. This approach allows us to analyze five key factors contributing to soil losses through water erosion: rainfall erosivity (R), soil erodibility (K), vegetation cover (C), topography (LS), and anti-erosion practices (P). By overlaying these five elements, we have developed a quantitative map of soil losses for the Mazer watershed. The results show an average soil degradation of 77.2 tons per hectare per year, with variations ranging from 0.6 to 885.2 tons per hectare per year. This diversity highlights the significant impact of environmental and human factors. This precise methodology enables the identification of major contributors to erosion, facilitating the implementation of tailored protective measures. Understanding the causes of water erosion is essential for promoting sustainable agricultural practices, soil conservation, and environmental policies aimed at preserving the ecosystem and local natural resources, thereby mitigating soil erosion and promoting sustainability.


Introduction
Water erosion represents a major environmental challenge with negative consequences on a local and national scale, accentuated by demographic growth and climate change.In Morocco and other regions, this problem threatens infrastructure, disrupts crucial agricultural activities and compromises water quality.Notable researchers, including El Garouani (2008), Markhi et al. (2015), Briak et al. (2016), Ouadjane (2021), Mazouzi (2021) El Brahimi (2022) and El Assaoui (2023), have studied water erosion in Morocco, highlighting its specific features and proposing mitigation strategies.The economic repercussions are substantial, with considerable annual losses affecting Moroccan dams.The study aims to assess the risk of erosion in the Oued Mazer basin, using the RUSLE of Renard et al. (1997), with remote sensing techniques and GIS.This approach will contribute to an in-depth understanding of water erosion, providing recommendations for sustainable management of soil and water resources.The study thus aspires to play a significant role in preserving the environment, protecting infrastructure, and building resilience to environmental challenges in Morocco.

Study area
The Mazer basin, located in the western part of the Settat-Ben Ahmed plateau in the province of Settat, covers an area of around 180 km² (Fig. 1).It plays a crucial role in the recharge of the Berrechid aquifer, an important regional underground aquifer, particularly during flood periods (El Assaoui et al., 2023).The river's flow is marked by prolonged periods of low flow, or no flow at all, as well as occasional short-lived but violent floods.These occur mainly in November, December and February.(El houssine et al., 2014) Climatically, the region has a Mediterranean climate with temperate winters, although it is subject to a semi-arid regime.

Erosivity factor « R »
The rainfall erosivity factor R was estimated using the formula proposed by Rango and Arnoldus (1987), for the Mazer area.
Where: Pi represents the mean monthly precipitation in millimeters, while P denotes the average annual precipitation in millimeters.
The results indicate a range of R factor values from 192 to 216.The greatest values are observed in proximity to the downstream and upstream extremities of the basin, whereas the lowest values are located near the central and southwest ends of the basin.

Soil erodibility factor « K»
The formula used to determine the erodibility factor is as follows (William et al 1985): Where: AN, SIL and CLA are abbreviations for the sand fraction, silt fraction and clay fraction (in percent) of the soil.The symbol C represents the carbon content of the surface soil (in percent).SN1=1-SAN/100.Preliminary erodibility values reveal (Table 1) that the 0.44 value, which includes erodible soils (Regosols), represents 92.08% of the surface area.The results obtained in the Table also indicate that 5.18% of the watershed's surface area has a medium erodibility (Luvisol) of 0.52, 0.92% of the encrusted red soils and 0.85% of the soils with low erodibility (0.37).

The topographical factor « LS »
The LS factor map is based on the ASTER digital elevation model.The L and S factors were calculated using the following equation (Weschmeier and Smith): Where: L represents the slope length in meters, S denotes the slope angle expressed as a percentage, and m is a constant that depends on the slope value: 0.5 for slope angles greater than 5%, 0.4 for slopes ranging from 3% to 5%, 0.3 for slopes between 1% and 3%, and 0.2 for slopes less than 1%. (3) According to Table 2's results, the majority of the LS factor values are roughly average.They occupy 32% of the surface area surrounding the major river system, which is a sizable portion of the basin.The Quaternary alluvial terraces have the lowest values.With: =2, =1 To adjust C-factor values to the specific characteristics of the Mazer watershed, we used data from the land-use map developed from previous research (see Table 3).
In the Mazer watershed, cultivated agricultural land (0.6) has the highest values of the five land uses identified.Conversely, small grains (0.26) represent the lowest value of the total basin area.

The factor of anti-erosion practices « P »
The P factor reflects the impact of agricultural practices aimed at reducing the quantity and velocity of runoff water, thereby reducing soil erosion.Given the lack of information on erosion control measures, the P factor has been assigned a value of 1 for the entire watershed.

Soil loss assessment « A »
The soil loss map (Fig. 3) obtained after applying the RUSLE equation by multiplying the factors (LS-C-R-k) enabled us to estimate erosion of up to 885 t/ha/year.The regions experiencing minimal soil losses are primarily situated in proximity to the wadis, featuring flat terrain with very low slopes of 14.75% (Table 4).Additionally, irrigated areas with dense vegetation, providing substantial resistance to erosion, contribute to reduced soil loss.Nevertheless, certain areas within the watershed exhibit elevated erosion rates, particularly along river banks with a slope of 48.30%, attributed to friable materials and irregular terrain lacking vegetation.

Conclusion
The study presents the results of a mapping exercise based on the overlay of maps of the main factors of the Revised Universal Soil Loss Equation (RUSLE) using a Geographic Information System (GIS).This approach enabled the creation of a water erosion risk map, highlighting the presence of this risk along the river banks in the Wadi Mazer basin.The results show a coherent spatial distribution, strongly influenced by factors such as relief and soil type.In downstream areas of the basin, hills and slopes are more exposed to erosion, while alluvial valleys show less erosion.This soil degradation has visible environmental repercussions and negative socio-economic consequences in the Oued Mazer basin.

Tab 1 :
Distribution of K-factor classes in the Mazer basin k

Tab 2 : 4
LS factor class distribution in the Mazer basin The vegetation cover factor « C » Vegetation cover was mapped using the formula of Van der Knijff et al. (2000).