## 2 Study Catchment  
The Wadi Haly catchment is situated within the Asir Mountain region along the Red Sea coast in the southwestern part of the Saudi Arabia, primarily composed of basement rock units. It spans the latitudes 18\({}^{\circ}\)13\({}^{\prime}\)45\({}^{\prime\prime}\) to 19\({}^{\circ}\)02\({}^{\prime}\)56\({}^{\prime\prime}\) N and the longitudes 41\({}^{\circ}\)21\({}^{\prime}\)18\({}^{\prime\prime}\) to 42\({}^{\circ}\)29\({}^{\prime}\)47\({}^{\prime\prime}\) E, covering approximately 5225 km\({}^{2}\) and extending about 160 km (Figure 1). The landscape of the study catchment features the prominent mountains of Jabal Sawda and Jabal Thirban. The key residential areas within the region include Haly, positioned downstream along the coast of the Red Sea, and MuHayil City, which is centrally located within the catchment area (see Figure 1). Furthermore, the Wadi Haly catchment ranks among the largest wadis on the southern Thiama Plain in Saudi Arabia. The predominant geological features of the Red Sea Hills in the southwestern part of Saudi Arabia consist of Neoproterozoic volcanic-sedimentary formations associated with the Arabian-Nubian Shield [36]. Geologically, a thick succession of sedimentary strata, spanning from the Cambrian to the recent periods, unconformably overlies the crystalline basement (Figure 2). Figure 2 shows the distribution of the geological units covering the study catchment. The Proterozoic crystalline rocks of the Wadi Haly catchment are characterized by numerous fractures and joints, resulting in several trending lineaments. In these arid to semi-arid regions, rainfall is inconsistent in both time and location, and its intensity can vary significantly. As a result, this can lead to sporadic flash floods that pose significant risks to dams, property, human life, and transportation [37].  
The Red Sea coast of Saudi Arabia is home to the Wadi Haly catchment, which has a varied land cover primarily composed of arid desert landscapes with little vegetation, especially in low-lying places and close to ephemeral streams. Only a small portion of the land is used for agriculture, mostly in the upstream areas, and is sustained by irrigation or seasonal rainfall. In addition, the catchment is characterized by alluvial plains below and steep topography in its higher reaches, which have an impact on the distribution of vegetation and human activity. Significant rates of erosion have been seen in regions with steep slopes and delicate soils in earlier research. For instance, the authors in Ref. [38] emphasized how deforestation, excessive grazing, and poor land management techniques have made soil degradation worse in southern Saudi Arabia, including areas like Wadi Haly. Furthermore, in Wadi Bin Abdullah, a nearby catchment with similar geomorphological and climatic circumstances, the authors in Ref. [39] recorded substantial rates of soil erosion and sediment movement. Their results highlight how vulnerable these catchments are to erosion during flash floods, which are common in the coastal lowlands of the Red Sea. The discussion made in these studies underscored the importance of adopting sustainable land use practices into the catchments to lower the risk of erosion and advance ecological stability. By taking these viewpoints into account, in this study, not only is the present  
Figure 3a-d illustrate the general morphology of the study basin. The highest elevations are concentrated along the narrow eastern zones of the Wadi Haly basin. The overall slope decreases from the high elevations in the eastern parts to the lower elevations towards the Red Sea in the western parts at the basin's mouth. Most of the Wadi Haly basin consists of mountains with high reliefs and dissected topography, divided terrain, and steep slopes. In contrast, the lower portion of the basin features a valley floor with low to moderate slopes. In summary, the slopes and various trends of the study basin are clearly depicted in Figure 3.