Earth observation from space is becoming an increasingly indispensable tool in the development of a multitude of scientific disciplines. This technique makes it possible to track the dynamic changes and transformations undergone by the Earth through diachronic analyses [1][2]. Indeed, thanks to new information and communication technologies, the last two decades have seen an unprecedented boom in scientific work in Earth monitoring.

Lower Casamance, located in southern Senegal in the Ziguinchor region, covers an area of 7339 km² (Figure 1). In terms of climate, it belongs to the South Sudanese coastal domain [3]. It is considered one of the rainiest regions in the country with very rich agricultural land. In addition, Lower Casamance is crisscrossed by numerous rivers and valleys (the Casamance River and its tributaries) with a concentration of surface and underground water flows, thus favoring the cultivation of these depressions. It has long been self-sufficient in rice. However, for several decades, the region has been subject to soil salinization and acidification, exacerbated by climate variability, marked since 1968 by a significant decrease in rainfall and a change in the water regime, as well as a reduction in the flow of the Casamance River and its tributaries [4][5]. Some of its valleys are also marked by a profound invasion of seawater inland due to the low slopes. According to the National Institute of Pedology [6], 73% of Senegal’s saline soils are located in the alluvial valleys of the Casamance and Senegal rivers. However, the Diola society of Casamance, the main player in lowland rice cultivation, demonstrates remarkable expertise in the development of rice fields compared to other rice-growing societies [7][8]. This has affected agricultural activities, which are the main sources of income and food for the population. To address this phenomenon, water management is seen as a strategy for securing rice production, resulting in the development of small, traditionally designed hydro-agricultural structures. Numerous small anti-salt structures have been installed throughout the Ziguinchor region. The purpose of these hydraulic structures is to recover and make profitable some of these valleys affected by salinization and rainfall deficit in order to intensify rice production. The salinization of lowlands and valleys and the construction of small anti-salt structures in Lower Casamance have attracted the interest of many researchers since the 1960s, including [9]-[14]. However, given the extent of salinization and acidification of agricultural land, the impact of small, traditionally designed hydro-agricultural structures appeared to be limited.

This led to the construction of more modernized anti-salt micro-dams in the area starting in the 1990s, in the hope of substantially slowing down the salinization of rice-growing land. However, the proliferation of these types of structures has led to changes in ecosystems, which in turn have led to changes in land use around the structures. Numerous studies have been conducted in the area to explain the phenomenon of salinization and identify the different types of saline soils [15]-[17] and the dynamics of agricultural landscapes on agricultural activities [18][19]. There has been little research into changes in land occupation and use around anti-salt structures in the area. Today, the planning and monitoring of rice-growing areas is a constant concern for those involved in water resource management and for producers for whom rice plays a major role in their diet and in certain cultural rituals. In order to understand the dynamics of land use change around anti-salt structures and analyze their impact on rice farming, we undertook this study in Lower Casamance, an area characterized by a predominance of rice cultivation (more than 80%). Rice farming, the main activity in the area, is influenced by climatic hazards that cause significant changes. Space-based remote sensing offers opportunities for continuous observation of the Earth’s surface. Thanks to remote sensing, it is possible to create databases that provide information on the state of natural ecosystems and their evolution. In Sudanian regions undergoing rapid socio-environmental change [20][21], monitoring land use is essential for establishing a diagnosis and better understanding the causes of vegetation change and its consequences.

It is a suitable tool for understanding spatio-temporal changes in land use [22]. The availability of Landsat satellite images provides an opportunity to characterize the evolution of the soil in this sub-watershed. However, the heavy reliance on spectral characteristics is one of the main reasons for the unreliability of the classified map [23]. To overcome this constraint specific to remote sensing, we used field data to improve the accuracy of classification and the reliability of land cover maps obtained from Landsat data from 1984, 2000, and 2025. This topic has become essential in most cartographic inventories and environmental monitoring [24]. Unable to study changes in land use and occupation around saltwater barriers across all production systems in Lower Casamance, this study focuses on analyzing changes in occupation and the impacts caused by hydro-agricultural developments. It centers on the Kabaline sub-watershed.

The high cartographic accuracy achieved in this study can be explained by the good knowledge of the study area, but also by the small number of land cover classes used and the definition of homogeneous plots when selecting training sites [34]. However, the confusion noted between slope fields and tannes and between tannes and mudflats did not significantly affect the reliability of the classification, as it was only around 8.46% and 7.36%. The confusion between sloping fields and tannes could be explained by the fact that fields from which agricultural products are harvested appear as bare areas [29].

The evolution of land use over the last forty years shows significant reforestation overall in the Kabaline sub-watershed in Lower Casamance, with an increase of 313.27% over forty years. This increase in forest areas in the study area has also been observed in central-western Burkina Faso [35], Benin [36], and Brazil [37]. This increase in forest area in the Kabaline sub-watershed could therefore be explained by the fact that dense dry forests have undergone fewer changes and conversions to other types of land use. Thus, several land use units have been converted back to dense dry forests. This is the case for wooded savannas, which have lost 768.59 hectares of their area in forty years to dense dry forests. The same is true for plateau fields, 6.53 hectares of which were converted to dense dry forests in 2024. In addition, this noticeable reforestation in the sub-basin could also be considered a positive impact of the construction of small anti-salt structures, which have helped to retain rainwater and reduce soil salinity. The protection of forest resources based on traditional social rules surrounding the “sacred” could also explain the reforestation of the sub-watershed. In fact, in the study area, the concept of sacredness goes beyond prohibitions based on legal laws. It carries the weight of a strict ban on cutting wood and even gathering it, which all citizens are required to respect, failing which they will face severe penalties. Contrary to our results, studies conducted by [34][38][39] and [40] have shown a regressive spatio-temporal dynamic of forest resources in the Sahelian and Sudanian zones of West Africa. Indeed, [34] showed drastic deforestation in the municipalities of Bantè, Glazoué, and Ouèssè in the Sudano-Guinean zone of Benin, while [38] in the Pama partial wildlife reserve and its surroundings (southeastern Burkina Faso) showed that vegetation has declined in favor of bare soil and rocky outcrops, while [39] demonstrated deforestation in favor of savannas in the natural grazing areas of transhumant herds in the municipalities of Banikoara and Karimama in Benin, and [40], whose study in the Agbo 1 classified forest in Côte d’Ivoire revealed a decline in forest cover in favor of agricultural areas.

Unlike dense dry forests, wooded savannas are the land cover type that has undergone the most change in the sub-watershed studied. They have experienced a decline in the area. This decline could be the result of several factors. Anthropization is one of the factors behind this phenomenon. Between 1984 and 2024, 40.81 hectares of wooded savannas were converted into residential areas. Similarly, 54.68 hectares of wooded savannas were converted into plateau fields and 6.84 hectares into slope fields in 2024. Land clearing to increase the area of agricultural fields is one of the factors explaining the decline in tree savanna vegetation cover. These results are similar to those of [35]. Indeed, several authors have reported that changes in landscape composition in the Sahelo-Sudanese, Sudanese, Sudano-Guinean, and Guinean zones are the result of rapid and progressive anthropization manifested by unsustainable agro-silvo-pastoral practices [41]-[43]. Furthermore, our results contradict those of [44] and [45], who observed savannization in the Sudanese zone of Côte d’Ivoire. At the same time, the areas of plateaus and slopes also declined between 1984 and 2024, by 72.09% and 55.73% respectively, representing an annual rate of decline of 1.80% and 1.39%. These results corroborate those of [46], who found that in the rural commune of Kaour from 1990 to 2024 (Sédhiou/Casamance region, Senegal), there was a 45.57% decline in agricultural land between 1990 and 2006. There are two possible reasons for the decline in plateau and hillside cultivation. The first is related to the salinization and acidification of agricultural land prior to the construction of anti-salt structures in the sub-watershed. This situation led to the abandonment of agricultural activities in these farming areas, resulting in their savannization and even reforestation. In fact, 101.76 hectares of plateau fields have been converted into wooded savannahs and 6.53 hectares into dense dry forests. The second reason is the change in land use that took place in the hillside fields between 1984 and 2024, with 81.76 hectares of land being converted into lowland fields. These results confirm our hypothesis that the installation of anti-salt structures in the Kabaline sub-watershed has sparked interest among farmers, particularly rice farmers who cultivate rice in low-lying areas, as evidenced by the increase in the area of low-lying crops. However, the 30 m resolution of the Landsat data used may be a limitation in the spatial analysis of rice-growing areas. Rice cultivation in the area is traditional, with ridges measuring a few centimeters in height that are often submerged. Under these conditions, these features are not easily detectable by Landsat images. Looking ahead, the overall impact of anti-salt micro-dams and land use dynamics on rice cultivation in the Kabaline sub-watershed is an urgent need for the region and must be studied and analyzed accurately using appropriate tools and techniques, such as drone images or very high spatial resolution images (like Pléiades).

Monitoring land use in the developed sub-watershed is essential for policymakers, development actors, and scientists alike. This detailed study of the sub-watershed developed by anti-salt micro-dams aimed to analyze changes in land use and the impacts of hydro-agricultural developments in the Kabaline sub-watershed. Several lessons can be learned about landscape change. Water retention by the structures has become a reality in the sub-basin. Although water is not available on a permanent basis, it has enabled the resumption of agricultural activities, particularly rice cultivation in the lowlands. Rational management of this water will therefore offer opportunities to intensify and diversify agricultural production. Land use changed considerably between 1984 and 2024. The most striking observation is the decline in agricultural fields on the plateau. On the other hand, the most encouraging observation is the restoration of vegetation cover, particularly dense dry forests. Changes in land use showed significant variations from one class to another. The most significant changes are those observed in shrub savanna formations. More than half of the tree savanna areas have undergone changes, the most significant of which has been to the benefit of dense dry forests. The sub-watershed studied is an area that needs to be well understood in order to better exploit its agricultural potential.

The results obtained in this research, particularly those concerning changes in land use through remote sensing, are of great importance for understanding the most decisive factors. This study is a first step in the process of identifying land use patterns in the sub-basins of Lower Casamance. The practical aspect of this study also opens up prospects for research on the analysis of changes in rice fields, interactions between farmers’ organizations, and climate impacts. Several avenues should therefore be explored to better enhance the value of hydro-agricultural structures in the Kabaline sub-basin. Therefore, we strongly recommend that a policy be put in place to preserve land that is not contaminated by salt. In addition, local populations should be involved from the design phase through to project completion, drawing on their knowledge of micro-dam salt management. This approach will make it possible to move beyond a sectoral and vertical approach, empower local populations in the management of structures after projects, and ensure the long-term maintenance and effectiveness of anti-salt structures.

This research is the product of the work of three researchers from the Geomatics Research and Teaching Laboratory affiliated with the Institute of Environmental Sciences and the Ecole Polytechnique de Ties (Senegal). We would like to express our sincere gratitude to the anonymous reviewers and the editor-in-chief of the journal for their constructive and valuable comments.