Original Research
Frontiers in Soil Science ·
Volume 6 - 2026 | https://doi.org/10.3389/fsoil.2026.1891092
Mapping soil vulnerability based on the soil erodibility factor (K) in the El Harrar Watershed (Eastern Pre-Rif, Morocco)
- Bahija Filal1*corresponding author
- Mohammed Abahrour1
- Mostafa Hmamouchi1
- Laboratory of Space History Dynamics and Sustainable Development, Department of Geography, Polydisciplinary Faculty of Taza, Sidi Mohamed Ben Abdellah University, Fez, Morocco
Abstract
Water erosion is a major cause of soil degradation, threatening soil sustainability and agricultural productivity, particularly in environmentally fragile regions. Its intensity is influenced by the interaction of natural and anthropogenic factors, including soil properties, topography, vegetation cover, climate variability, and land-use practices. This study aimed to assess the spatial variability of soil vulnerability to water erosion in the El Harrar watershed (Eastern Pre-Rif, Morocco) using the soil erodibility factor (K), by integrating soil physicochemical characteristics, GIS-based mapping, statistical analysis, and field observations. Previously established K-factor values were assigned to the mapped soil units according to soil-type correspondence to characterize the spatial distribution of soil erodibility across the watershed. The results indicate that 45.15% of the watershed is characterized by low soil erodibility (K < 0.25), mainly corresponding to calcimagnesic soils, whereas 23.78% exhibits very high erodibility (K > 0.45), primarily associated with Fersiallitic soils. Exploratory correlation analysis revealed negative relationships between soil erodibility and organic matter (R = −0.70) and clay content (R = −0.43), suggesting that higher contents of these soil properties are associated with greater soil structural stability and reduced erodibility. These findings highlight the importance of considering soil physicochemical characteristics in assessing the spatial variability of soil vulnerability to water erosion and provide a useful basis for identifying priority areas for soil conservation, sustainable land management, and territorial planning in the Eastern Pre-Rif of Morocco.
Keywords
1 Introduction
Soil is an essential natural resource for agriculture, ecosystem functioning and food security. But soil degradation caused by water erosion is one of the major environmental problems for Mediterranean countries, including Morocco (1). About 40% of Moroccan land is threatened by water erosion. Almost 20% of the land in Spain has already degraded and 1% is degraded (2). Such figures show the impact of soil degradation caused by water erosion and why there is need for good soil conservation in Mediterranean countries.
Morocco is particularly affected by water erosion, especially in the Rif Mountains where about 60% of sediment reaching reservoirs is coming from a region accounting for just 6% of the national area (3). According to the National Watershed Management Plan (4), annual soil losses are greater than 20 t ha−1 yr−1 in the Rif and between 10 and 20 t ha−1 yr−1 in the Pre-Rif. The evolution of water erosion is influenced by climatic, topographic, lithological, edaphic, vegetation, and anthropogenic factors. Slope gradient, rainfall erosivity, soil quality and vegetation cover are key natural controls (5, 6), while there are concerns that inappropriate agricultural practices, overgrazing and land use changes may accelerate soil degradation significantly (7–11).
Soil erodibility has been extensively studied in various environmental settings because soil erodibility represents the soil’s susceptibility to particle detachment and transport due to rainfall and surface runoff (12). Soil erodibility is not rainfall erosivity or topography but soil characteristics and is therefore one of the most important aspects to understand the spatial variation of water erosion. It is defined as the soil erodibility factor (K) with soil texture, organic matter content, soil structure, and permeability (13, 14). It has been shown in previous studies that the interactions between these physicochemical properties mainly contribute to soil erosion, but the effect varies by climatic conditions, parent material, topography, soil type, and land usage (12, 13, 15, 16).
The integration of the K factor with Geographic Information Systems (GIS) has become a popular approach to describe the spatial variation in soil erodibility and to identify priority areas for soil conservation. GIS-based assessment of soil properties can help to differentiate soil properties and erodibility within watersheds to facilitate the site-specific erosion control (6, 14, 16). In addition, several previous studies have shown that, in general, increasing organic matter content leads to a better overall stability of the aggregate, more bonding between particles, and better infiltration which reduces soil detachment and transport. Clay content can also be beneficial for cohesion and structural stability, but how it can contribute to the soil chemistry and interactions with other soil properties will be sensitive to the clay mineralogy (13, 15, 16). Therefore, K should be considered in the context of soil texture, organic matter, structure, and permeability and not in isolation. These ideas are consistent with the synthesis presented by Barchia et al. (6), which stressed the importance of spatial K-factor information in identifying priority areas for soil conservation.
Previous studies in Morocco have mainly focused on water erosion intensity and estimated soil loss based on empirical and spatial models such as RUSLE, PAP/CAR and EPM (8, 9, 17–19). While these studies have provided a very good understanding of erosion processes in the Rif and Pre-Rif regions, they generally focused on soil loss rates and overall erosion severity rather than the intrinsic spatial variability of soil erodibility, which is controlled by soil physicochemical properties in different parts of the country. Further, very few watershed studies have been conducted in northern Morocco that have also assessed the soil units, spatial distribution of K, the relationship of soil properties to the soil, and how the soil can be affected by these differences in soil properties and the evidence of erosion on the ground.
For this reason, this paper aims to address this issue by integrating soil physicochemical data, GIS-based spatial analysis, statistical relationships, and field observations on the spatial variability of soil erodibility in the El Harrar watershed. Unlike studies that focused on estimating soil-loss rates, this study combines published soil-erodibility data, soil-unit mapping, statistical analysis, and field observations to investigate the physicochemical controls that underlie the spatial variability of K. Our goal is to: (i) assign and map the previously known soil erodibility factor (K) values to the soil units in the El Harrar watershed; (ii) relate K to the major physicochemical soil components; and (iii) discuss the impact of these findings on soil conservation, sustainable land management, and territorial planning in the Eastern Pre-Rif of Morocco. The remainder of the paper details the study area and data sources, outlines the methodology, presents the spatial and statistical results, discusses our results and how they relate to previous studies and field observations, and concludes the paper.
Unlike previous works in northern Morocco, this study uses GIS-based mapping, statistical evaluation of soil physicochemical controls and field observations to explain the mechanisms of spatial variability of soil erodibility rather than just a K-factor map.
2 Materials and methods
2.1 Study area
The El Harrar watershed is located in the hills of the Eastern Pre-Rif (Figure 1) and occupies an area of 89 km2. The watershed has a heavily dissected relief, forming a hilly landscape. This area shows the relationship between the Jbel En nehir unit located to the North-Northwest and the Pre-Rif hill unit located to the South and Southeast of the watershed. The geological formations are primarily composed of clay and sandstone, with the majority of the land covered by calcimagnesic soils (45.15%) and vertisols (23.78%).
Furthermore, the watershed is characterized by a very limited extent of natural vegetation cover and a high intensity of human utilization of agricultural land, which is predominantly concentrated on hillside areas. This land-use pattern, combined with steep slopes and fragile soils, increases the susceptibility of the watershed to soil erosion and land degradation processes.
2.2 Data sources and soil database
Soil susceptibility to water erosion was evaluated in this study with soil cartographic information, physicochemical soil analysis, Geographic Information System (GIS) techniques, statistical analysis and field observations. The methodological framework adopted in this study is summarized in Figure 2. The soil map of the El Harrar watershed was derived from the 1:50,000 Taza Soil Map, from which different soil units were digitized and entered into a GIS environment using ArcGIS Pro. Seven major soil units were identified in the watershed: Lithosols, Regosols, weakly developed alluvial soils, weakly developed colluvial soils, Vertisols, calcimagnesic soils, and Fersiallitic soils.
The physicochemical database was built up from previous soil analytical data for the Oued Lahdar basin, particularly from Tribak (17) and the related studies. These data include clay content, silt plus very fine sand content, organic matter, soil structure, soil permeability, and previously known soil erodibility factor (K) values for the major soil types. Since the El Harrar watershed is part of the Oued Lahdar basin and contains the corresponding mapped soil units, the published physicochemical and K-factor values were associated with the equivalent soil classes identified on the 1:50,000 soil map. The matching procedure was based on soil-type correspondence and verified with field observations and high-resolution Google Earth Pro images. Although the data were taken from old studies, the selected variables are mostly stable intrinsic soil properties. However, their use gives a soil unit level analysis rather than a new sample-based laboratory data set. We considered this limitation when interpreting the statistical relationships.
2.3 Assignment of soil erodibility factor (K) values
The soil erodibility factor (K) is an intrinsic soil property expressing the susceptibility of soil to detachment and transport by rainfall and surface runoff (21). It is commonly estimated using the approach developed by Wischmeier et al. (20), which relates soil erodibility to soil texture, organic matter, soil structure, and permeability. The relationship is expressed by the following equation:
K=(2.110−4(12 −OM)M1.14+ 3.25(S −2) + 2.5*(P −3))100
where K is the soil erodibility factor, OM is the organic matter content (%), M is the soil texture parameter calculated as (% silt + % very fine sand) × (100 − % clay), S is the soil structure code, and P is the soil permeability code.
The soil structure and permeability codes reported in the published analytical database follow the USDA classification system. The corresponding classes are presented in Tables 1, 2. Representative values of clay, silt plus very fine sand, organic matter, soil structure, soil permeability, and the corresponding published K values were compiled for each mapped soil unit from the available soil analytical database.
Table 1. Soil structure classes and particle size (according to USDA).
| Code | Soil structure Class | Size |
|---|---|---|
| 1 | Very fine granular and very fine crumb structure | < 2 mm |
| 2 | Fine granular and fine crumb structure | 1–2 mm |
| 3 | Medium granular, medium crumb, and coarse granular structure | 2–5 mm |
| 4 | Platy, prismatic, and very coarse granular structure | 5–10 mm |
Table 2. Distribution of soil permeability classes and values (P) (according to USDA).
| Permeability code | Permeability class | Permeability value |
|---|---|---|
| 1 | Rapid to very rapid | > 60 mm/h |
| 2 | Rapid | 20–60 mm/h |
| 3 | Moderate | 5–20 mm/h |
| 4 | Slow to moderate | 2–5 mm/h |
| 5 | Slow | 1–2 mm/h |
| 6 | Very slow | < 1 mm/h |
In this work, K values were obtained from the analytical soil database for the Lahdar watershed by Tribak (17) and other related works. The soil physicochemical data were also compiled from the Société d’Études Techniques et Économiques (1996). These K values were originally derived using the Wischmeier et al. (20) method and assigned to the soil units in the El Harrar watershed based on the soil type correspondences. K values were then added to the GIS to create the map of soil erodibility. The physicochemical properties of each soil unit were then used to analyze the relationship between soil properties and the variation of the K factor.
2.4 GIS spatial analysis
The published K values were connected to their soil polygons in ArcGIS Pro to produce the spatial distribution map of soil erodibility. The soil types reported in the source database were assigned to the soil map at 1:50,000. The areas occupied by the erodibility class were then calculated and expressed in square kilometers and as percentages of the watershed area. In order to help interpret the K values, the erodibility classes were classified according to Manrique’s (22) criteria: very low (K < 0.15), low (0.15 < K < 0.25), moderate (0.25 < K < 0.35), high (0.35 < K < 0.45), and very high (K > 0.45). The thematic map was used to identify the spatial distribution of soil susceptibility to water erosion throughout the watershed.
2.5 Statistical analysis
To examine the relationship between soil physicochemical properties and soil erodibility, we calculated Pearson correlation coefficients between the published K values and the main soil variables such as the clay content, the silt content plus very fine sand content, and organic matter. The correlation was done using representative values for the seven mapped soil units and was completed in Python and MATLAB. In the limited number of soil units (n = 7), we used the correlation analysis as a means to investigate soil unit-level relationships and not as a means to provide evidence to show causality or statistical generalization. So, we interpreted the coefficients carefully and used them to support the physicochemical interpretation of the mapped erodibility patterns.
3 Results
3.1 Soil characteristics of the El Harrar watershed
Seven major soil classes have been identified in the El Harrar watershed (Figure 3). Calcimagnesic soils cover 45.15% of the watershed area, followed by Fersiallitic soils (23.78%), Lithosols (11.46%), Vertisols (9.20%), Regosols (4.18%), weakly developed colluvial soils (3.18%) and weakly developed alluvial soils (3.06%). This heterogeneous spatial distribution is a reflection of the geology and geomorphology of the watershed and indicates that the physicochemical properties of the soils are very diverse, and the soil erodibility is also very sensitive to the geomorphology and geology.
The surface profiles of the soils show the morphological differences among the mapped soils. Calcimagnesic soils show relatively deep clayey to silty-clay horizons with relatively higher organic matter contents, whereas Fersiallitic soils have reddish carbonate-free profiles and lower organic matter contents. Vertisols display deep clay-rich horizons, and Lithosols and Regosols are usually shallow and weakly developed because of the strong influence of the parent material. Weakly developed alluvial and colluvial soils show different textures that are strongly influenced by recent depositional processes. These field observations are consistent with the laboratory analyses and offer a morphological basis for interpreting the subsequent spatial variability of the soil erodibility factor (K).
3.2 Spatial variability of the soil erodibility factor (K)
The mapped soil erodibility factor (K) in the El Harrar watershed (Figure 4) has been shown to be highly variable throughout the spectrum of soils, with a K of 0.23 - 0.46 and the lowest being for calcimagnesic soils and the highest for Fersiallitic soils. Calcimagnesic soils are around 45.15% of watershed area and contain relatively high organic matter (2.01%) and clay (38.5%), leading to the lowest soil erodibility value (K = 0.23). Moderate soils (Colluvial soils and Lithosols) are about 14.64% of watershed area. Soils with high vulnerability (K = 0.35-0.45) are about 16.44% of watershed area and are Vertisols, Alluvial soils and Regosols. They are mostly distributed along stream and river banks and in the southern part of the watershed. Finally, soils with very high vulnerability (K > 0.45) made up 23.78% of the watershed area and consisted mainly of Fersiallitic soils on the slopes of Jbel En nehir with 0.95% organic matter and 28.4% clay content Tables 3, 4).
Table 3. Soil erodibility classes in the El Harrar watershed.
| Soil erodibility factor | Classes of soil erodibility | Area km² | Area % |
|---|---|---|---|
| <0.15 | Very Low Erodibility | 0.00 | 0.00 |
| 0.15<K<0.25 | Low Erodibility | 40.01 | 45.15 |
| 0.25<K< 0.35 | Moderate Erodibility | 12.97 | 14.64 |
| 0.35<K< 0.45 | High Erodibility | 14.57 | 16.44 |
| >0.45 | Very High Erodibility | 21.07 | 23.78 |
Table 4. Physicochemical properties and corresponding soil erodibility factor (K).
| Soil type | Clay (%) | Silt- very fine sand (%) | OM(%) | (S) | (P) | Factor (K) | Area (km²) | Area (%) |
|---|---|---|---|---|---|---|---|---|
| Vertisols | 50.20 | 47 | 1.63 | 4 | 5 | 0.36 | 8.15 | 9.20 |
| Lithosols | 22 | 76.2 | 1.66 | 2 | 1 | 0.32 | 10.15 | 11.45 |
| Regosols | 19.1 | 67.30 | 1.62 | 3 | 3 | 0.44 | 3.71 | 4.19 |
| Calcimagnesic | 38.5 | 56.3 | 2.01 | 3 | 3 | 0.23 | 40.01 | 45.15 |
| Alluvial | 39.5 | 32.1 | 0.5 | 2 | 2 | 0.39 | 2.71 | 3.06 |
| Colluvial | 43.2 | 50.2 | 2.37 | 2 | 3 | 0.27 | 2.82 | 3.18 |
| Fersiallitic | 28.4 | 44.4 | 0.95 | 2 | 2 | 0.46 | 21.07 | 23.78 |
Source: 17, 23, 24.
Based on the statistical analysis (Figure 5), we found that the relationship between K (soil erodibility factor) and the different soil properties was different, based on the correlation coefficients. Organic matter (OM) was the most negatively correlated with the K value (R = -0.70), which means that OM plays a strong role in soil structure stability and particle detachment, and so erosion becomes less difficult. Clay content is slightly less negatively correlated with K (R = -0.43) but this clay fraction plays a positive role in soil structure stability and erosion resistance. The proportion of very fine sand and sand has quite a weak negative correlation (R = -0.19).
Overall, these links showed that organic matter was the soil property most strongly related to variation in K among the seven mapped soil units. Clay also showed the second strongest relation while silt plus very fine sand was very weak. The correlation heatmap (Figure 6) serves as a complementary map to the scatter plots in Figure 5 in terms of the relationships between K factor and the soil properties studied. It illustrates the negative associations of K with organic matter and clay content, and weak ones with silt plus very fine sand, soil structure and permeability.
4 Discussion
The differences in soil erodibility factor (K) in El Harrar watershed reflect the mixing of soil physicochemical properties and geomorphological conditions. At least in Calcimagnesic soils the K level is lower because their organic matter content is high (2.01%) and clay content is high (38.5%), which helps soils to remain strong and less prone to particle detachment. The K values of Fersiallitic soils are the highest, mainly because their organic matter content (0.95%) and clay content (28.4%) are low and they are more susceptible to water erosion. These data show that the soil composition is a major factor affecting the spatial distribution of soil erodibility in the watershed.
These spatial patterns can also be verified from field observations (Figure 7), which demonstrate the presence of rill erosion, gully erosion, bank erosion and slope degradation in areas that are highly and very highly prone to erosion. The field evidence is thus in favor of the consistent spatial distribution of the K factor and the erosion features observed in the watershed. The K values in the El Harrar watershed (0.23-0.46) are within the range of other watersheds in northern Morocco. Similar values were reported by El Aroussi et al. (25) in the El Maleh watershed (0.00-0.58) and El Achari et al. (19) in the Ghrala watershed (0.27-0.46). However, the range observed in this study is narrower than those reported for the Amzaz watershed (0.10-0.82) (18) and the Asra watershed (0.11-0.82) (8). These differences may be a consequence of the different parent material, soil composition, topography and land cover, which are directly related to the spatial distribution of soil erodibility.
The statistical analysis also highlighted the importance of soil physicochemical properties in controlling soil erodibility. Organic matter has the highest negative correlation with K factor (R = -0.70) and contributes to soil stability and soil resistance to rainfall impact. Clay content also has a negative correlation (R = -0.43) that contributes to soil cohesion and erosion control. As a result, organic matter and clay are important for soil resistance to water erosion and must be kept or improved for soil erodibility to be reduced and water erosion to be minimized in the El Harrar watershed. The statistical findings should still be taken with caution because the analysis was based on representative values for only seven mapped soil units, rather than a more representative sample-level dataset. The observed coefficients indicate soil unit-level trends and should not be taken for causal connections. Additional field sampling and laboratory work would provide more accurate statistical evidence on physicochemical controls on soil erodibility.
5 Conclusion
In this study, we showed that soil erodibility in the El Harrar watershed is highly determined by the spatial variation in soil physicochemical properties. The mapped soil erodibility factor K ranged from 0.23 to 0.46, with considerable differences among the major soil types. Calcimagnesic soils exhibited the lowest erodibility due to their relatively high organic matter (2.01%) and clay (38.5%) content, while Fersiallitic soils showed the highest erodibility, mainly due to their lower organic matter (0.95%) and clay (28.4%) contents.
The exploratory statistical analysis revealed that organic matter (R = -0.70) and clay content (R = -0.43) were negatively correlated with K in the seven soil units surveyed and may thus be an important parameter affecting soil stability and erosion resistance. Aside from providing a spatial level of soil erodibility, this study is also a scientific basis for incorporating soil physicochemical information into erosion risk assessment and land management strategies in the Eastern Pre-Rif. The soil erodibility map is therefore a useful tool to identify the areas that require soil conservation and sustainable land management.
Since this assessment draws on previously published soil unit data, future analysis should improve the spatial data, with sample-level measurements and field-based validation. Based on these findings, conservation should emphasize organic amendments to increase soil organic matter, vegetation cover protection and restoration, and agricultural practices (such as terrace construction and maintenance on cultivated slopes) that should be implemented. Soil erodibility maps should also be incorporated into territorial planning and environmental decision-making to ensure sustainability of soils and to sustain natural resources in very sensitive Mediterranean areas.
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Footnotes
Edited by: José A. González-Pérez, Spanish National Research Council (CSIC), Spain
Reviewed by: Rayees Ali, Jamia Millia Islamia, India
Ahmed Abdllah Adam, Mizoram University, India
Statements
- Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
- Funding
The author(s) declared that financial support was not received for this work and/or its publication.
- Data availability
The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author/s.
- Author contributions
BF: Conceptualization, Formal analysis, Methodology, Data curation, Software, Writing – original draft, Visualization, Writing – review & editing, Validation, Investigation. MA: Writing – review & editing, Supervision, Validation. MH: Supervision, Writing – review & editing, Validation.
- Acknowledgements
The author would like to thank the National Center for Scientific and Technical Research (CNRST) for awarding the Excellence Scholarship. The author would also like to thank Dr. José A. González-Pérez for his support and encouragement.
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Citation
Filal, B., et al. (2026). Mapping soil vulnerability based on the soil erodibility factor (K) in the El Harrar Watershed (Eastern Pre-Rif, Morocco). Frontiers in Soil Science, Volume 6. https://doi.org/
- Received
- Accepted
- Published
- Volume
- 6 - 2026
Copyright
Copyright © 2026 Filal, Abahrour and Hmamouchi.
License: https://creativecommons.org/licenses/by/4.0/
*Correspondence: Bahija Filal
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