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Study of Vulnerable and Water Erosion Risk Areas in Sareg Catchment (Central Tunisia) Using Remote Sensing, GIS and P.A.P/R.A.C. Qualitative Approach

The study of water erosion has always been a major concern for human societies. Efforts to reduce resulting forms of degradation and to understand their extent have not ceased. This article aims to identify vulnerable and water-erosion-prone areas in the study area. This is achieved through a predictive and descriptive approach leading to the integration of main physical parameters from various sources with direct observation data. The use of remote sensing, GIS and direct observation according to PAP/RAC guidelines has made it possible to prioritize areas at risk of erosion. The basin's lands show a general predisposition to water erosion, with 80% of them experiencing significant, high or very high erosion, including 20% for the last two categories. This situation is linked to a high erodibility of the geological outcrops of the basin (81%), with a low to very low degree of soil protection through vegetation cover (80% of the surface). The descriptive phase shows eroded surfaces of about 18%, a proliferation of ravines (12% of the surfaces) of which 6% are hierarchical ravines and bad lands. The integration phase, while confirming most of the previous data, provides an overview of the trend towards the extension of gullies observed during the descriptive phase to neighboring lands that were not initially subject to erosion. This study shows that a good integration of descriptive and predictive methods can lead to a good understanding of water erosion, which will facilitate solutions to control this phenomenon. It also shows the importance of GIS, remote sensing and their integration with direct field observation in order to produce a document that can be used, at a low cost, as a decision support tool.


Study Area
Study area corresponds to Sareg watershed, a northern tributary of the Leben Wadi. Vast of 157 km2 (15700 ha), this basin extends between 9°28' to 9°38' East and 34°42' to 34°52' North ( Fig. 1). Altitudes range from 215 to 742m (Fig. 2). The catchment is bounded on west by the eastern end of Meloussi Mountain, on north by Boudinar, on east by Gouleb, Méhiri and Jebs. However, most of land corresponds to plain of Hachena in north and Remilia in south. While Meloussi and Bou Dinar reliefs are formed by dolomitic limestones of Cretaceous, those of Méhiri, Jebs and Gouleb are composed of Triassic or Eocene evaporite rocks (Fig. 3). Foothills are composed of Miopliocene clays, silts and sands. Rest of land corresponds to alluvial and fine Conservation of water and soil resources is one of the major challenges for all the world's nations. Faced to soil erosion and its losses to human communities, an effective and sustainable strategy is more than necessary. Catching runoff water and retaining its solid charges, which are the soils, must begin with a full understanding of erosion processes and with accurate and clear-sighted diagnoses. Water erosion is the main factor in soil degradation in Tunisia. The annual loss of useful land is well documented (Andersson 2010;Cormary et al.1964;Jebari 2009;Jebari et al. 2010;Masson 1971;Zante et al. 2003;. More recent work using modern assessment techniques confirms these trends (Gaubi et al. 2017

Materials and Methods
Erosion assessment methodology used here is that of PAP/RAC model (Priority Action Program of Regional Activity Centre 1998), which is part of Mediterranean Action Plan (MAP), which is itself part of United Nations Development Program (UNDP) (Grisbach et al., 1998, Fig. 5 (Fernandez et al. 2016) (Ousmana et al. 2017). We have also followed this choice. In this step we used classification categories given by guidelines (Table  1, 5, 7). Second step is to integrate, using integration matrices provided in guidelines, the four base maps two by two (  (Fernandez et al. 2016) (Ousmana et al. 2017). We have also followed this choice. In this step we used classification categories given by guidelines (Table 1, 3, 5, 7) Second step is to integrate, using integration matrices provided in guidelines, the four base maps two by two (Table 5, 11, 13). The first two ones (slopes and lithofacies) produced soil erodibility map. The other two ones (land use and vegetation density) were used to obtain soil protection map. This integration was carried out using GIS software (ArcGIS) with the following steps (tools): Union -dissolves for each two polygon maps. Last step consists in integrating the two maps of second step to have final map of erosive states.

-descriptive approach:
It consists in producing erosion forms map by direct observation via a satellite image navigator and field verification. Erosion categories of forms and symbols used are those provided by PAP/RAC guidelines. Erosion forms polygons were obtained by direct digitization and then transferred to GIS software.

-integration approach:
It consists of superimposing erosion forms map on erosion states map. It allows consolidation of erosion data by confirming or not the existence of areas subject to erosion.

Soil Erodibility Lithofacies predispose lands to erosion:
Lithofacies map was made from geological map of Tunisia. It shows that most of land (78%) is in soft and loose rock category (fig. 6/ Table 2). Coherent and compact rocks represent only 22%. This predisposes basin land to water erosion.

Steep slopes characterize a significant part of the basin:
Slope map was based on a digital elevation model (DEM). It shows that steep, very steep and extreme slopes represent 30% of the total and are confined to the mountainous areas bordering the catchment (Fig. 5) ( Table 4). Moderate slopes represent 58% and low slopes 8%. They also represent most of land in basin.

Erodibility map confirms high soil vulnerability:
Soil erodibility map is the result of lithofacies and slopes combination. Its examination shows that 81% have a medium, high or very high erodibility (Fig. 8) (Table 6). Only 19% have moderate or low soil erodibility. As expected, spatial distribution situates highly erodible terrain in soft rocks located on steep slopes of mountainous areas. It also shows that majority of the basin's low-slope terrain with moderate to moderate erodibility often correspond to quaternary and Neogene deposits occupying the largest part of surface area.

Land cover is limited to irrigated crops in the western part:
Land cover map was produced from Sentinel-2 10m resolution satellite image classification and interpretation. It shows that most of land in the basin (63%) is bare soil or bare rock (Fig. 6) (Table 8). Fruit trees, particularly olive trees planted in rows, represent 17.5% of total, while irrigated crops represent the same percentage (18%).

Low density of vegetation cover except in intensive areas:
Land cover map was produced from Sentinel-2 10m resolution satellite image classification and interpretation. It shows that most of land in the basin (63%) is bare soil or bare rock (Fig 8/Table 8). Fruit trees, particularly olive trees planted in rows, represent 17.5% of total, while irrigated crops represent the same percentage (18%). Table 9 Land cover density classes according to PAP/RAC guidelines.

Classes
Degree of vegetation cover 1.

A Very limited protection by vegetation cover:
Soil protection map is the result of the land use combined with the vegetation density map. Its examination shows that 80% of land has low or very low soil protection (Fig. 9). Only 20% therefore have high or very high protection. The relatively protected areas are those where islands of natural vegetation still cover the ground, but they are mainly linked to intensive, particularly irrigated, crops that effectively occupy soil surface. Major part of the basin remains without significant protection.

Integration of physical parameters output:
Final result of qualitative study is erosive states map. The latter shows that 80% of basin's land suffers from significant to very high erosion, with 66% of land suffering from high to very high erosion (Fig. 14) ( Table  14). (20%) suffer from low to very low erosion. Spatial distribution shows location of lands highly prone to erosion in areas of steep to very steep slopes (between 12 and 35%) on mountain hillsides surrounding the basin.
They correspond mainly to tender outcrops of Triassic, Miocene and marl-clayey-sandish intercalation of Cretaceous. Lands experiencing significant erosion are more widespread in basin where Miopliocene and Quaternary unconsolidated formations outcrop in surface. Low and very low erosion areas are characteristic of sectors protected by natural vegetation or intensively cultivated (irrigated), often located on gentle slopes or hard rocks that minimize erosion and loss of material.

Direct observation reveals lands invaded by gullying:
Erosion form map represents field observations that are therefore current and observable facts of land degradation and have been mapped using satellite images. (Fig. 12-13, Table 15). Table 15. It emerges that gully forms represent the most notable phenomenon among the observed erosion forms. Gullying of different densities represents 13% of basin, more than 4% of which is in badlands. The gully mainly characterizes the clay-marly outcrops of the eastern and north-eastern mountainous areas. Sheet erosion represents 6% and mainly characterizes foothills with a slight slope, often without gullies. The total area of eroded land represents 2837 ha or 18% of the basin. Some areas have been stabilized by using benches (6%). Others periodically experience alluvial landings (6%) or are occasionally flooded (6.5%). Stable or alluvial soils represent 18 of totals. It shows that about 1/5 of basin's land is subject to erosion, which is sometimes very serious or even irreversible. These lands are not subject to any type of conservation or protection. Flooded or alluvium land prevents any agricultural activity and can be considered as sterilized in its current state

Integration of qualitative and descriptive data confirms trends towards gully expansion:
This is an integration map of map of erosive states and map of erosion forms. Superimposition of the two maps allows to confirm or not erosive state suggested by the final PAP/RAC map (Fig. 14). Badland areas have invaded lands that were initially not prone to intense erosion, as shown on erosive states map. This indicates a trend of expansion and a contagion effect and proliferation of gullies in these areas to irreversible state. Sheet erosion areas are almost always sectors of significant erosion, high to very high. They therefore confirm their status as erosion sectors by stripping soil. Stabilized areas are located in red areas of high vulnerability. They explain interventions that have been made to prevent erosion. Dominant gully zones dangerously surround Bad Lands, indicating a contagion and multiplication of ravines on clay slopes in absence of a protective plant cover or a water and soil conservation measure. On silty or clayey foothills, individualized ravines frequently appear, especially on those north of Meloussi Mountain, whose head retreat has reached areas that are not very vulnerable to erosion. This informs us of trend to expansion of these gullies in absence of a control strategy. Frequently alluviated or flooded areas of the northwestern plain of the basin correspond to areas of significant to very high erosion. They appear on the field as lands representing a vast bed of undecided riverbed and without clear borders. Gullies are almost always absent and crops are impossible because of significant and frequent alluvial inputs that disturb vegetation. This part of the basin is an accumulation zone that may experience a loss of materials if the flow of water exceptionally increases.

Conclusion
The purpose of this study was to assess water erosion in the Sareg catchment. This was done using a multi-source predictive approach, a descriptive approach and an integration phase for the two previous ones. The result is that majority of basin land is soft and/or loose (78%), that steep slopes are frequent (30%) or moderate (58%) that most of surfaces are bare (63%). Integration of soil erodibility parameters reveals that 82% of land has moderate, high or very high erodibility with 20 for the last two categories. Similarly, soil degree of protection by vegetation cover is also significant with 80% of the surfaces having a low to very low level of protection. Erosion map shows a general predisposition of land to significant, high or very high erosion (80%), of which 20% for the last two categories. Descriptive phase shows eroded surfaces of about 18%, a proliferation of gullies (12% of surfaces) including 6% for hierarchical gullies and Bad Lands. These are located at steep slopes of mountainous reliefs taking advantage of clay-marly intercalations but which extend dangerously on surrounding slopes. Localized gullies often dot foothills. Similarly, 12% of land area has often been flooded or alluviated, making it unusable for farming. Finally, integration phase, while confirming most of previous data, gives ideas mainly on trend towards gullies extension observed during descriptive phase to neighboring lands that were not initially predisposed to erosion.