Dynamics and Vulnerability of Hydrogeomorphological Units to the Degradation of Climatic Conditions in the Watershed of the Birnin Lokoyo Pond (Iullemmeden Basin, Southwestern Niger)

The present study concerns the lowlands of the Birnin Lokoyo watershed located in the Matankari Rural Commune (Southwestern borders of the large Iullemmeden basin in the Niger). Dominated by the Birnin Lokoyo pond, this watershed faced with the continuous silting up of the minor bed, dissipates further upstream in the east, so that it moves downstream to the west for the benefit of the populations of Matankari city. The main objective of this work is to analyze the representative elements of the new climatic and environmental situation as well as the various factors determining the surface conditions in the watershed. The methodological approach is based on documentary research, field observations, individual surveys and interviews with targeted groups. The results show that the silting up of the lowlands is the result of the concomitant degradation of the plant cover and the soils, which, as a result, triggered the development of crusting surfaces and area erosion on the slopes. The study underlined the importance of endogenous knowledge in the choice of remedies against the silting up of lowlands. In addition, rational land management practices on plateaus and structures (within the watershed) do not seem to be in phase with topographical factors and the tectonic framework. This work is part of the vision of better management of glacis on the banks, but also of sills subject to regressive erosion upstream and in order to guide the policies of resilience of the populations. How to cite this paper: Hantchi, K.D., Hassane, B., Ousmane, H., Balla, F.I.S., Konaté, M. and Garba, Z. (2022) Dynamics and Vulnerability of Hydrogeomorphological Units to the Degradation of Climatic Conditions in the Watershed of the Birnin Lokoyo Pond (Iullemmeden Basin, Southwestern Niger). Journal of Environmental Protection, 13, 233-260. https://doi.org/10.4236/jep.2022.132015 Received: December 16, 2021 Accepted: February 22, 2022 Published: February 25, 2022 Copyright © 2022 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/


Introduction
In the Sahelian environment and particularly in a context of strong anthropization, uncertainties now relate to the extension of the phenomenon of runoff on the slopes [1]. These flows make it a dynamic hydro-sedimentary system, where changes in surface conditions play a major role in the gullying, widening or even deepening and silting up of watercourses [2] [3]. Numerous studies in these regions have linked this dynamic to changes in land use and to developments previously carried out by humans [4]- [9].
In Niger, [10] and [11] confirm the links established between the increase in runoff and the cultivation of land by showing that this leads to the crushing of soils. Due to the already difficult climatic conditions and/or changes, the problem of soils erosion arises acutely to become a major environment issue which consequently affects agricultural practices [12]. Thus, throughout the century, food security should be affected in its four aspects, namely: access, stability, availability and quality [13]. Therefore, responding to adapt to the reality of climate change is one of the strategic priorities of all communities and Nations. This is why farmers have taken a great interest in lowland lands since the drought of 1984-1985 [4]. What characterizes these wetlands is the vital role they play in preserving biodiversity, and the fight against poverty and food insecurity [14]. However, these ecosystems are in the process of silting up significantly, in other words undergoing extensive degradation, resulting in a drop in their production potential and a persistence of various conflicts [15] [16]. This is the case of Birnin Lokoyo pond where the hydrogeomorphological units of the watershed undergo quantitative transformations of surface states, as well as the influence of qualitative and consubstantial processes of climatic and environmental deterioration [17] [18].
The propose of the present work is to investigate and evaluate the main representative elements and determining factors of surface conditions. More specifically, it is about: 1) identifying the weight of the demands of use and development in a physical context subjected to severe tests, 2) present the different opportunities and challenges for the community, and 3) propose perspectives allowing to lay the foundations of a reflection framework on sustainable management of the Birnin Lokoyo watershed.

Area of Study
The Birnin Lokoyo pond watershed is as sub-basin of the Dallol Maouri [14], (decimal degrees). Extending over an area of circa 43.7 km 2 , 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 (Ct 3 ) 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 Ct 3 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 (Ct 3 ) 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   [19], modified and localization of study area (b) within Iullemmeden basin [20]. Journal of Environmental Protection  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. Ultima forsan, land use maps taken from [18], in addition to giving a fairly representative overview of the diachronic evolution (1986-2016) of the mineral landscapes of the municipality, made it possible to compile the climatic risks incurred in the study area. In short, this phase, in addition to taking stock of existing results, has the advantage of making it possible to assess the relationship between climatic variables and pressure from use. Basically, this involved an immediate site visit during the rainy season (July-August 2020), then another data collection in the dry season (December 2020).

Field Investigation
It was about collecting data directly in the field. Two approaches were used to identify the contours of the surface conditions and to know how the populations perceive and integrate the climatic deteriorations in the study area. The first was to make direct and systemic observations. Thus, visits were made throughout the

Hydroclimatic and Morphodynamic Trend of the Landscape
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).

Human Activities on the Banks and in the Major Bed of the Watershed
The populations of the surrounding villages dig on the banks to find mud and gravel ( Figure 10). This extraction of local materials on the banks partly clears the beds of torrents and accelerates the phenomenon of water erosion, in addition to promoting wind deflation in the dry season. Each year, field work as well as human traffic promotes the destruction and weakening of soils, as well as the disruption of the hydrological regime in the southern part of the watershed ( Figure 11).
Water erosion has also created encrusted surfaces at the foot of the banks of Natchira and Barébari ( Figure 12). We can therefore observe the existence of     ferruginous rocks which are left by runoff water on large bare surfaces ( Figure   12(a)). In addition, the formation of encrusted surfaces in the banks left imprints which, in terms of water competence, could thus contribute to gullying ( Figure 12(b)). This would increase the risk of the secondary beds being silted up. Thus, on the southern banks, near Barébari and Natchira, there are branches which bring sand to the minor bed of the pond (Figure 13).
The silting up (Figure 14), despite the population's attempts to prioritize water retention, caused the pond to recede, in addition to the displacement of its water towards the town of Matankari. According to the population of Barébari, the dike ( Figure 15) that they built to prevent the flow of water is the second.     to the north (towards Mazarkwoila) and improvisations of fences drawn from thorny species (Figure 16(b)).

Environmental Management Methods and Impacts on the Degradation Process
In order to achieve good agricultural performance, gardeners use fertilizers and amendments. The different types listed in the field include urea and NPK (15-15-15), but specially compost and organic manure (Figure 16(c) and Figure   16(d)). As shown in Figure 17), Matankari growers (at Colonel Nouhou Bako's site) use self-prepared insecticides from ripe Neem seeds (Azadirechta indica).
However, the survey shows that phytosanitary products, distributed by the SWISSAID and Lux Dev projects, are also used.
Under these conditions of water runoff from the pond and its advance towards Matankari, silting is gaining ground upstream and becomes a daily and permanent concern for the populations of the eastern component upstream of the basin. Among the other consequences of the reduction in the duration of the flow of the pond, one can point out that the dike protecting against the advance of water and certain works to fight against water erosion, such as lifting, does not prevent the appearance of gullies ( Figure 18).
The reality on the ground shows that the gullies are widening, damaging more and more the displays and threatening the houses and the mosque, the road and paths, schools and fields ( Figure 19).
According to the population, some houses in Birnin Lokoyo fell by the runoff of water from tributaries of the pond so that landowners could be moved. To stop the sand advancing from the banks towards the pond, upstream in the minor bed of the main kori and the northern banks (towards Birnin Lokoyo), the species Sida rhombifolia was planted by making a curtain of living hedge ( Figure  20). Wherever there is this species (i.e. Sida rhombifolia), alone or in association with Hyphaene thebaica (Figure 20(c)-NP2 and Figure 19(b)-NP1), it is more difficult for the sand to pass through it.

Concentration, Effects and Adaptations of Herbaceous and Woody Formations
The main species found at the study site that, over the years, have declined the most on both sides of the pond are Faidherbia albida, Eucalyptus pauciflora, Piliostigma reticulatum, Mitragyna inermis, Accacia macrostachya and Cassia mimosoides. At the height of Natchira and Barébari (Figure 3), it is the species naturally present on the banks that block the sand and slow the wind speed. These include species such as Guiera senegalensis, Combretum micranthum et Andropogon gayanus (Figure 21).
In the minor bed of the pond, the species Vetiveria nigritana, naturally present, also retains the sand (Figure 22). By its roots, it fixes the water, which allows the clay to remain. This species is very important to humans and animals.
Thus, the local populations use it to make the clay bricks compact, while it con-

Discussion
The

Triggers of Pond Silting up Problems
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).

Amplifying Factors of the Silting up Problems of the Watershed
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  (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.

Impact of the Changing Surface Conditions of the Plateaus on the Productive Potential of the Birnin Lokoyo Lowlands
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 addi-Journal of Environmental Protection tion, 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 Ct 3 (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

Conclusion
This study has made it possible to identify the vulnerabilities and environmental (villages of Barébari, Birnin Lokoyo and Natchira) find themselves without activity because it is the pond that allowed them to satisfy many needs. Nevertheless, the advance of the pond towards the West is an asset for the inhabitants of the town of Matankari.