## 2 Materials and Methods  
### Study Area  
The Merguellil watershed was selected as a study area based on several criteria, including geographic relevance, environmental significance, human impact, data availability, and management relevance. Indeed, the Merguellil watershed represents a particular ecosystem with its size, location, or unique characteristics. It is particularly vulnerable to land use and land cover changes. Also, the Merguellil watershed has experienced significant human interventions, such as agricultural activities, urbanization, and industrial development. The availability of relevant and reliable data for the Merguellil watershed is crucial for conducting research or analysis. Finally, it could be strategically chosen based on its importance for water resource management, land use planning, or conservation efforts. The Merguellil catchment area (1175 km\({}^{2}\)), situated in Central Tunisia, exemplifies the environmental degradation challenges common to semi-arid zones, including high erosion rates, low land cover, and limited economic income (Figure 1). This area encompasses 1175 km\({}^{2}\) and consists of cropland (35%), rangelands (45%), and forests (20%). The Merguellil watershed belongs to a climatic transition zone between the north of the country, under Mediterranean influence, and the pre-Saharan zone to the south, and it is hot and arid. During the observation period from 1980 to 2020, the average annual rainfall in the area was approximately 300 mm, with variations ranging from 265 mm in the plain to 515 mm in the highlands. The average annual temperature was 19.5 \({}^{\circ}\)C, with average daily temperatures less than 12 \({}^{\circ}\)C in January and more than 28 \({}^{\circ}\)C in July-August. These climatic conditions play a significant role in shaping the hydrological and ecological dynamics of the region, influencing factors such as water availability, vegetation growth, and erosion patterns. The Merguellil catchment area is demarcated by the El Haouareb Dam, constructed in 1989. The catchment area encompasses a hilly terrain with diverse geological, morphological, vegetation, and land use conditions. The elevation within the area varies from 310 m.a.s.l. at El Haouareb Dam to 1226 m.a.s.l. at djebel Barbrou. The slope direction predominantly follows a north-west/southeast axis, which aligns with the prevailing wind direction. This topographical configuration can have significant implications for water flow, erosion patterns, and ecological dynamics. The geology of the Merguellil watershed is complex, primarily consisting of limestone, marl-calcareous, and marly sedimentary formations from the Cretaceous and Eocene periods. The oldest formation in this area dates from the Triassic era [27]. The soils are mainly developed in situ from dissolution and weathering of the bedrock. The content of humus and clay-humic complexes is low, about the low density of the land cover. Exposed to very strong erosion, they are weakly developed and shallow. Based on the French classification system (_Commission de Pedologie et de Cartographie des Sols_, 1967), the major soil types in the study area are defined as follows \"_Sols Calcinagnesiaques_ and _Sols Isohumiques_\". According to the international classification system (International Union of Soil Sciences Working Group--World Reference Base, 2022), they could be named, respectively, \"Leptosols or Cambisols, Calcisols or Cambisols, and Kastanozems or Cambisols\". Figure 1: Location of the Merguellil watershed in Central Tunisia. Additionally, over the past four decades, extensive conservation measures have been undertaken in the upstream catchment areas to mitigate erosion and prevent the siltation of the El Haouareb Dam. These widespread efforts focus on reducing soil erosion and sediment deposition, thus ensuring the dam's lifetime and maintaining its functionality for the surrounding environment and communities. In fact, works mainly consisted of bench terraces and small dams to create hill lakes.