The Birnin Lokoyo pond watershed is as sub-basin of the Dallol Maouri [14], which flows from the Ader heights in the north-east and descends to the east in the southwestern edge of the Niger sector of the larger Iullemmeden Basin (Figure 1). Located about 20 km north of Dogondoutchi (Figure 1(b)), it is confined within the latitudes 13.76707˚N - 13.83590˚N and longitudes 4.02905˚E - 4.08867˚E (decimal degrees).

Extending over an area of circa 43.7 km², the watershed has an elongated shape of an isosceles triangle whose base is materialized by the lateritic Matankari-Bagadji (Figure 2). The area belongs to the central southern phytogeographic sector of the Sahelian zone (600 - 300 mm per year) and experiences a rainfall regime that varies in time and space [21]. The rainy season lasts between five and six months between from May to October. The dry season occupies all the rest of the months of the year with its cold and hot variants. The temperatures are high and the prevailing winds are the harmattan (in the dry season) and the monsoon (in the rainy season) [18].

The Iullemmeden basin is characterized by a detritical infilling, mainly Paleozoic in age in its northern part, while Mesozoic and Cenozoic sediments occupy its southernmost part [19]. The study area consists of two main overlying formations represented by an upper unit (Oligocene to Miocene in age) of the Continental terminal 3 (Ct³) and Quaternary deposits. The Quaternary formations are composed mainly of sands of ancient erg, various colluvium and recent alluviums [20].

The geomorphological landscape is organized from the residual reliefs (witness mounds and tops of the Ct³ platforms) to the flat-bottomed basins and lowlands which constitute preferential areas for the accumulation of runoff water coming down the slopes of surrounding hills. The lowlands are also areas of high groundwater recharge [22], and it is along such landforms that water reservoirs (e.g. the Birnin Lokoyo pond) and other hydro-agricultural development works are constructed [14]. A moderate slope and a specific drop of 59.5 m characterize the framework of the study area relief [17].

The noch of the valley and the well-defined major forms which dominate it testify to a character inherited from a paleogenesis attributable to the humid beginning of the Quaternary [23]. The end of the intense runoff, and therefore the intense digging of the valleys, resulted in the establishment of dunes [24]. These dunes are attribuable to the dry period of the Ogolian (20,000 to 12,000 years BP) during which Aeolian veneers formed on the plateaus, against the sides of the hills, between the hills and the gorges [25]. In the study area, the most visible protusions of these veneers or even Aeolian reworking are those on the north (on the borders of Mazankwoila) and south (western of Natchira) slopes (Figure 2(b) and Figure 3).

The typical formation of the plateau tops of the Continental Terminal 3 (Ct³) is the tiger bush characterized by the alternation of covered areas and bare bands [26]. For this author, the bare bands play the role of an impluvium for the downstream wooded areas. In clayey sandstone slopes, vegetation cover is found along the koris that incise them [17]. It mainly consists of species such as

Combretum micrantum, Guiera senegalensis and grasses, as well as the tops of the plateaus. In glacis, generally short and steeply sloping [27], there is very sparse vegetation which is limited to a few species (Guiera senegalensis and combretaceae) along the flow axes. The vegetation of lowland areas is mainly characterized by Annona arenaria, Faidherbia albida, Guiera senegalensis and the herbaceous layer mainly grasses [17]. Feet of Hyphaene thebaica, in a sparse “green fascine” position in the minor bed of the main kori (i.e. wadi), still materialize the ancient expanse of Birnin Lokoyo pond. Understanding the dynamics that affect the lowland unit allows us to have an idea of stability or instability for better management of the watershed.

Three types of soils are commonly encountered in the municipality of Matankari, namely: 1) sandy soils, 2) clay soils and 3) loamy soils. According to [18], these soils are generally leached and hardly offer good agricultural yields. The population of the Birnin Lokoyo pond basin (Hausa, Peulh, Touareg and Zarma), always in search of fertile land, actively intervenes in the enhancement of wetlands, such as the shallows of the pond which constitute a palliative for climatic variability.

A graphical representation of precipitation, annual mean temperature and maximum annual standardized wind, derived from the compilation of data for the years 1960-2010, is presented in Figure 4. Since the 1960s, the 1980s were the least rainy in the Matankari Rural Commune (Figure 4(a)). After the 1990s, there was a sudden increase in rainfall. In addition, we observe that from 1960 to 2009, the last decade was the rainiest. The break observed in the 1990s is also characterized by an increase of +1.3˚C (Figure 4(b)) and the maximum annual linear trend of the wind (Figure 4(c)).

Satellite images from 1986 (Figure 5(a)) and 2016 (Figure 5(b)) show the evolution of soil, vegetation and surface water resources in the area under study. This allows to see roughly the dynamics of the watershed of the Birnin Lokoyo pond. By comparing the two images, one can see the existence of koris (i.e. wadis) in 2016 in the watershed, whereas in 1986 there were no koris but many gullies, and therefore vegetation that lives on the edge of running waters. These landscapes presented here are a good illustration of the drying up of the pond, hence the lack of gullies.

Google images and field observations in the Matankari and Bagadji watersheds show that most of the lineaments are aligned with the drainage network along the NW-SE, NE-SW and N-S directions (Figures 6-9). The sub-basins of the dallol Maouri are superimposed on NW-SE direction lineaments (Figure 2). The lineaments of NE-SW and N-S directions, still very active, are used by new secondary koris. On the southern borders, at Matankari and along the Angoal Kara basin, the lateritic plateaus present sinkholes and show a process of widening of the secondary basins according to the N-S and NE-SW fractures (Figure 7). To the East, towards the Goubey basin, a phenomenon of regressive erosion is in coupling with lineaments in NE-SW and N-S direction (Figure 8). In the north, the hydrosedimentary entities (sinkholes and slopes) sometimes form closed systems and the regressive erosion of the slopes leads to exchanges of sand with the lowlands downstream (Figure 9).

Interpolating data on climatic elements shows that the decade 2000-2010 is the wettest. This is why [17] estimates that the hydrodynamics in the municipality of Matankari are parallel of this increase in rainfall. These results confirm those of [3] who, after observing an increase in rainfall in the watershed of the kori of Mountséka (south central Niger), made the obvious but interesting observation that the flows increased, accentuating the influence of the phenomena of gullies, the stripping of loose soil, and therefore the availability of sand borrowings. In the same vein, [29] [30] and [31] went into more detail and found after a comparative analysis in the basins on basement and sediment cover a strong correlation between the improvement of rainfall and the intensification of climatic aggressiveness to admit that the increase in flows is respectively related to climate and man.

Thus, the large quantities of sand in the watershed of Birnin Lokoyo are the result of an evolution of the koris on the banks and specially of the great competence of the runoff water which transport them from their deflation or erosion towards the minor bed. In addition, the available enlisted surfaces and ferruginous rocks have increased with the decrease in vegetation cover on the southern edges of the basin. Decrease in land cover leads to increase closure of infiltration pores and therefore accelerated runoff [6] [27]. Furthermore, the increase in runoff remains dependent on the draining of materials on the banks by the population, which in addition to accentuating the creation of some gullies has also led to the extension of the drainage network in depth and space. In this sense, and from season to season, the stripping work in the banks makes koris the main geodynamic force which deteriorates the little that remains of “natural display parades” but also aging gabions. As a result, over time, many plants become bare upstream (Figure 10) and downstream buried and smothered (Figure 14). This observation is very close to those of [29] [32], as detailed in the methodological approach for building a database for monitoring hydrogeomorphological systems in the kori Dantiandou basin (Niamey square degree).

Analysis of the Google images (Figure 9 and Figure 15) revealed that the northeast of the pond experienced a significant increase in the rill erosion more concentrated around the slopes which lose soil after the stone ridges on the plateaus. This tendency to increase the availability of red sands must have led to the loss of market gardens to the east and today supports a bypass of the former expanse of the pond to the north. With the borrowing of pebbles on the bare slopes of Natchira, these red sands constitute the new constraints imposed on the western component of the pond at the foot of Matankari (Figure 9).

Under the confession of an operator of the Matankari perimeter, it is already remarkable that the last two years are the least flooded (Figure 16(a)). In this context, it is therefore to be fear that the lost of water from the pond will have a higher vertical component than the intense overflow of water at the level of the threshold of the protection dike to the west. The infiltration of water through the dike and in the downstream areas is justified by the intensity of the connection of the NE-SW and NW-SE fractures, and therefore their fortuitous and respective topographic position (Figure 9 and Figure 15(a)). These observations join those of [20], who showed that most these linear structures evolve to become permanent or temporary watercourses from which groundwater recharge takes place in the southwestern part of the Niger and the Iullemmeden basin.

Beyond the convergences of geodynamic factors of transformations that connect these different terrains, the other elements of kinship are linked to the evidence of the major role of powerful local decision-makers in the development of these processes. It is not a question here of denying the existence of a material reality of physical works and protection projects, but of questioning the theoretical foundations and representations that justify their context of execution. However, a number of criticisms can be leveled at their sources of unequivocal vulnerabilities. To paraphrase [7], the issue was not only ecological (e.g., stone cords on the plateaus to the north and east of the watershed; Figure 8). On the basis of fieldwork, with a very simplistic observation model, it is shown that the location of the protective structures, like that of the fenced perimeters on the southern banks, is not based on the dynamics of the basin and material realities of local knowledge (Figure 19 and Figure 20). The actions of the management of natural resources (MNR) visible on the ferruginous plateaus were more effective than the desirable correction of slopes of glacis and koris on the banks. This poor consideration of the species “parades” in the major bed explains that they are not only the causes and severity of land degradation that are observed, but also their extension and the ultimate solution.

Over the years, the degradation of the vegetation covers on the plateaus (for the sake of motorized machines!) has given way to crusting on the bare slopes and thus an increase in the speed of flows in the glacis and low terraces. The dominance effect of recovery work plays its full role (Figure 7 and Figure 8). To say that their presence certainly comes from the fact that wind deflation would be less [33] [34]. However, the jurisdiction of the land is already affected so that the stabilization of slopes and slopes will resist for sometime in the face of running water and local collapses (e.g. sinkholes; Figure 7 and Figure 9). These sinkholes, which are really mini basins, would be responsible for a certain loss of water to the benefit of the Angoal Kara basin to the south (Figure 7). In addition, an increase in runoff combined with an extreme superimposition of secondary koris, according to the lineaments of NE-SW and N-S directions, will aggravate the water lost to the south (Angoal Kara) and West (Matankari).

A significant regressive erosion trend, combined with NE-SW and N-S fractures, is also observed in the interfluve zones of the Goubey and Birnin Lokoyo basins (Figure 8). This highlights the decisive role of the southwestern arm of the Goubey basin in driving a flow of water bypassing the stone ridges in favor the eastern basin. In the outliers and escarps of Ct³ (Figure 6), the combination of the N-S and NE-SW fractures is accompagnied by stalls in favor of an increasingly pronounced plunge of the slope towards the Sud-East. This finding can thus be used as a structural argument and to invoke probable catches of water inflow (“exoreism”) to the east by contiguous watersheds. In short, the decrease in water volumes in the east and an increase in losses to the southern sinkholes could lead to the drying up of the Birnin Lokoyo watershed even more quickly and fairly early. These results confirmed the research carried out by [35] who found that, in the Iullemmeden basin, 85% of the ponds that have disappeared or tend to disappear are located in the river beds where the intensity of the erosive dynamics generates their siltation or fragmentation.

Finally, the difficulty of defining “good” indicators is all the more important since the defects in the protections of the surfaces concerned and the degradation processes and the ensalting at work also result from the shortcomings of the spatial positioning of the structures (e.g. paving) used as parades. To say that if the experimental effort has been insufficient or even destroyed upstream of the watershed, it is time to support downstream the nobles and valiant young people who value the land in the confines of the garden of the late Colonel Nouhou Bako (peace to his soul) in Matankari! In this, it is time to frame the problem of the cessation of flood margins in the watershed by planning and relying on deep drilling at high flows.