The study area is located at the southwestern part of Niger in the Department of Tera, about 178 km from Niamey, the national capital (Figure 1) [19]. It extends from latitudes 13˚45'N to 14˚20'N of the Equator and from longitudes 0˚30' to 0˚50' East of the Greenwich Meridian (Figure 1). With a total surface area of 900 km² [17], the study area constitutes the central part of the Dargol river bassin, which is a tributary of the right bank of the Niger River (Figure 1).

The climate is Sahelian with alternating long dry season (October to May), and a short winter season from June to September. Based on meteorological data from 1999 to 2008, the extreme monthly temperature values range from 15˚C (January) to 43˚C (April-May) for an annual average of 29˚C. The most important variations in temperature are observed during the two periods, ‘‘cold’’ and hot, composing the dry season. The relative humidity of the air shows an annual variation of 18% (March) to 90% (August). The average annual rainfall at the Téra pluviometric station (Figure 1) is 409 mm (1999-2008), while the average annual potential evapotranspiration calculated using the Penman method at the same time is 2000 mm (i.e. five times the annual rainfall). Twenty-five per cent (25%) of the precipitation comes from high-intensity events [1] that produce rapid flows of the Hortonian type [20] [21]. The rainfall-related flow begins in June-July and ends three months later, for an average flow of 160,106 m³ per year (1964-1994). The shape, orientation and density of the drainage system vary considerably depending on the topography, the roofing material, the nature and the tectonic structure of the bedrock.

With dune cords inherited from the arid Quaternary periods and some isolated hills whose highest points are about 350 m, the refection of the study area is relatively flat. These morphological complexes are cut by koris (i.e. wadis) that are mostly dry which drain water towards depressions (valleys, ponds) during

the winter season. Degraded and sparse vegetation (savanna and steppe) occupies the plateaus, while lowlands and depressions give way to denser woody vegetation. The growth rate of the local population is about 3.9% per year over the period 2001-2012, and more than 95% of the population lives in rainfall agriculture or in extensive livestock farming [22].

The bedrock is made of Precambrian formations, predominantly comprising both alternating granitic intrusive massifs and metamorphic belts (Figure 1). The latter, oriented NNE-SSO, consists of green rocks (pyroxenites, amphibolites, epidotites, chloritoschists, metabasalts, metagabbros), greywackes, rhyolitic tuffs and breccia and various shales [23]. The granitoid intrusions, made up of Eburnean age granites, granodiorites and diorites, contain sometimes relicts of pegmatites and leptynites from the Archean [24]. The subterranean layers and the superficial cover, above the basement formations, include the deep weathering profiles (alterites and soils: 5 to 50 m thick), the Holocene alluvium and colluvium [25]. Associate major fracture families are mainly structural directions with N20˚ to 50˚, N60˚ to N90˚, N120˚ to N140˚ and N350˚ to N15˚ [17]. In crystalline and metamorphic environments, the aquifer zones are located at altered superficial levels (alterites and alluvium) and deep basement levels (cracks and/or fractures). These aquifers are, in many cases, superimposed and form the same aquifer system [26]. The upper aquifer, having a capacitive role, is consisted of semi-permeable materials. It overcomes the second level aquifer related to cracks (intermediate zone decompressed), with high permeability forming. Deeper and beneath the second aquifer, lies the third aquifer level in the healthy substratum affected by fractures of tectonic origin (faults). Although generally confined, the deep aquifers of cracked and fractured basements are depleted at the same time with respect to higher horizons.

Thus, the most fractured zones contribute to its essentially conductive function. In this zone, the depth of the water varies according to the topography, the type of aquifer and the seasons. On the other hand, the surface waters still organized around valleys (alluvium) and plateaus (alterites). Water depths range from 1 to 10 m in traditional wells (i.e. capturing alluvial surface water), while in wells capturing alterites (i.e. modern wells) the water depths are up to 15 or even 30 m below ground surface.

The prevailing view on the subsurface aquifers, and particularly in Liptako, is therefore that of recurrent problems of water scarcity resulting from their very reactive to rainfall and can dry up during the dry season fluctuations [15]. Deep basement plies are in principle more promising in terms of productivity and sustainability because of the role that fractures play in draining, storing and circulating groundwater. The depth of these holes capturing basement plies varies from 36 to 120 m with an average of 60 m and a standard deviation of 20 m. The best flow rates are generally between 30 and 65 m. This is because 76% of the water inflow is in the first 50 meters, which constitute the upper part of the basement [17]. Beyond this depth, the arrival of water becomes rare or almost absent. On granitic rocks, the depth limit for a good flow rarely exceeds 50 m, whereas it can reach 60 m and even be slightly exceeded in green rocks (shale). This configuration is believed to be generally explained by the nature and thickness of their weathering products [25] [26].

Lithological diversity and various tectonic phases influence the hydrogeological properties of basement aquifers and of course their productivities. Analysis of the drilling data reveals that the exploitable flows are generally low, ranging from 0.5 to 5 m³ per hour with most of the drilling being limited to values of less than 2.5 m³ per hour. The main finding in such data is that green rocks present higher exploitable flows (average of 2.72 m³ per hour against 1.91 m³ per hour in the granitoids). The greater flows (i.e. more than 10 m³ per hour) are obtained in the drilling carried out at the level of a network of kilometer fractures forming a shearing lane.

In this study, data from various sources are used. First, emphasis was laid on obtaining topographic data included two maps (1:200,000; [17] ) and the leaves of Sebba and Téra. Then, preferred sampling geological data were Tillabéry and Niamey leaves and geological maps (1:2,000,000; [23] ). Piezometric levels data and two complementary scenes of satellite images (Aster, Landsat ETM+) were also been selected. The spatial resolution of these images is 30 meters, Digital Shuttle Radar Topography Mission (SRTM).

Pretreatment consisted of georeferencing and mosaicking maps, and images to cover the entire study area. The ENVI 4.3 and PCI Geomatica 9.1 software were used in the processing of satellite images and ArcGis 9.2 for the implementation of the Geographic Information System (GIS) [27]. Additional data, independent of the development of the chosen themes, were included to validate the maps developed. These data concern dry and high-flow boreholes, but also tritium and nitrate contents of boreholes.

The processing of primary data made it possible to generate derived information including slope maps, fracturing densities (lineaments, major fractures) and drainage, then piezometric levels, superficial and deep lithology. The combination of the maps will lead us to the development of potential recharge maps and the potential productivity index.

The methodology adopted for the mapping of potential recharge areas and the potential productivity index of basement aquifers is based on a multi-criteria approach. This process consists of four steps: definition of criteria, classification of criteria, weighting of criteria and combination of criteria.

Both the criteria selected for their influence on the potential recharge zones and potential productivity indices of the aquifers are shown as synthetic flowchart in Figure 2. This list of criteria, particularly important to discriminate between event itself and its consequences, is not necessarily exhaustive. However, given the available data and the hydrogeological and hydroclimatic specificities

of the area, the criteria mentioned appear to us to be essential. As a reminder, a multi-criterion methodology is always affected by a certain empiricism and a certain subjectivity [10] [27].

The objective is to define the realistic criteria used to map the recharge zones and the potential aquifer productivity index in the Dargol basin. The six (6) criteria presented here (Figure 2) are the slope, the drainage and fracturing densities, the piezometric levels, and the shallow and deep lithology.

To facilitate interpretation, each identified criterion is sub-divided into a class ranging from very low to high. The importance of each class varies according to its influence on the subject studied. Thus, all the criteria selected are classified into four classes except the criterion “deep lithology” which has only two classes. We limited the number of classes to two at this criterion, because the study of the rock and productivity of the structures did not allow us to distinguish the role of each facies in the productivity of the structures [17]. This constraint is due to the small number of structures and their concentration at the village level.

The spatial evaluation of the criteria depends on the theme studied. The achievement of each theme (Potential Recharge Zones and Potential Productivity Index) uses a combination of several criteria (Table 1).

As part of this study, the theme “potential recharge areas” brings together criteria such as slope, shallow lithology, fracturing and drainage densities. For the “potential productivity index” theme, the criteria are the potential recharge zones, the fracturing density, the deep lithology and the piezometric levels.

Multicriteria analysis requires the weighting of the elements according to the importance of each in the realization of the theme. Assigning a weight to a criterion is the most delicate and empirical part of a multi-criteria analysis. It must be done with the overall context of the study in mind. For example, for the realization of the theme “potential recharge zones”, the most important “weights” could be assigned to the criteria “fracturing density” and “superficial lithology” by some actors. While for others, the more important are the slope and drainage density criteria. Indeed, they show the very subjective nature of the multi-criteria analysis approach and the role played by the actors in the design and production of realistic maps.

The different themes of validation are carried out using reference data from field measurements carried out in the catchment works. The data used for this evaluation must comply with the principle of independence [2]. This principle requires that these data have not participated in the development of the map submitted for

evaluation. In the case in point, the data used to validate two topics are the hydrogeochemical and isotopic data for the map of potential recharge areas and drilling parameters (operating flow, dry boreholes) for the potential productivity index map.

The data mosaic shows that the study area has four potential recharge areas (Figure 5(a)), namely the very low, low, medium and high potential recharge. Areas of high recharge are located around the city of Téra (Figure 1) and in its northeastern part, the regions of Doumba, Diagorou, Toumbindé, Zindigori, Chatoumane and Sékomé. All these regions are marked not only by a high fracturing density but also by low slopes. Given the interdunal valleys and the ridge of the very wet marshy valleys during the rainy season, the Fonéko area at the northern end belongs to the high potential recharge class. At rocky outcrops and encrusted areas forming the glacis, which are highly developed in both northern and eastern parts, areas of low to very low recharge are observed. These regions are indeed marked by the presence of steep-sided dunes (e.g. North East Sékomé, South Zindigori, East Téra) and granitic outcrops or even green rocks.

In addition to the potential recharge areas, Figure 5 also shows zones of potential productivity index (Figure 5(b)) at sites where both piezometric levels, deep lithology and fracturing density were delineated. Fortunately, map of potential recharge areas provides information on sites where groundwater could be renewed under normal rainfall conditions. They can also be used to prevent the risk of pollution to better manage groundwater resources [17].

The net result from integrating around potential recharge areas is a possible four potential productivity index zones (Figure 5(b)). The areas that possesses

the outcrops of granites and points of basic rocks fall within the very low productivity index zone, and matched with very low permeability where infiltration is very difficult. The low potential productivity index areas are located in the North of the region and in the South-West near the Tondigoungou sector. The moderate potential productivity index zone is well developed in the central part, Southwest and Northwest. Generally located on the granite massif topped by sand dunes, those areas depend more on medium fracturing density.

A high potential productivity index zones are concentrated in the southeastern part, particularly the regions of Téra, Doumba and Diagorou. There are also some pockets of these classes in the area of Zindigori, Toumbindé and Sekomé. This class is observed mainly on green rocks, but also at the level of the contact zones. It is also localized on the granitoïdes in particular in the zone of Sékomé and its North-East part. Because they are highly charged, these areas are high index of potential productivity. Fractures are unequivocal zones with very strong permeability favoring infiltration and drainage of groundwater. Therefore, the mapping of these areas is very important in the choice of future geophysical exploration sites [10].

The discontinuous nature and the variability of the hydrodynamic properties make the hydrogeological study of the basement medium very difficult. The probability of success and having a significant amount of water for drilling is determined by several factors, the most dominant of which is fracking (e.g. megafractures and the nodes that they form; [25] ).

In this type of context, the validation test of overlaying field reference data with thematic maps developed by multi-criteria analysis is very difficult. The difficulty of this test is explained by the fact that this environment always reserves surprise, because next to a high-volume drilling can be a low-flow or dry drilling. It follows that local characteristics (e.g. the presence of sinkholes in the basement [19] or the nature of the veins and their alteration quality [25] ) are important. Despite the highly discontinuous nature of this environment, we proceeded to validate by superimposing the tritium contents, nitrates, operating flows and dry boreholes on potential recharge and potential productivity index maps. In doing so, the criterion of independence is respected. Regarding the map of potential recharge areas, the data used relate to tritium and nitrate contents (Figure 6).

The results of the study carried out in this zone have shown that the presence of a certain content of these parameters is synonymous with recent waters, hence the current recharge [17]. Therefore, recent waters will have a tritium content close to, or substantially equal to that of current precipitation (5 UT). The superimposition of tritium contents on the potential recharge chart shows that on 14 boreholes with a high tritium content (>3 UT), 11 drill holes are superimposed on the high recharge classes, i.e. a correspondence rate of 79% (Figure 6(a)). For low tritium drilling, only Fonéko drilling out of the four is superimposed on the very low recharge class. Yanga drilling, which has a tritium content corresponding to the detection limit < 0.9 UT, is located in the high recharge zone. However, the very limited number of low tritium drills does not allow a conclusion on the correspondence within the low recharge class. On the other hand, the correspondence of more than 79% observed on high to medium sensitivity classes could validate this potential recharge map.

A second validation test was conducted using nitrate levels (Figure 6(b)). It can be seen that of the 22 boreholes with a nitrate content > 50 mg/l, fifteen (15) are located in areas of high or medium recharge, corresponding to a rate of about 70%. The significant degrees of correspondence obtained for these two tests show that the recharge card established by multi-criteria analysis is reliable.

For the validation of the potential productivity index map, we used the drilling operating flow data and the dry boreholes (Figure 7). Indeed, the choice of operating rate for this test is very realistic because it expresses the productivity of the catchment works.

Figure 7(a) shows that the different classes of flow are superimposed to a large extent on the corresponding zones. As a result, 82% of high-volume structures are superimposed on areas of high and medium productivity. For low flow rates, they can be found in both high and low productivity areas. This is due to the fact that the high productivity of structures in basement areas is very dependent on major fractures. Low-flow drilling is found to be isolated or to secondary fractures. The influence of fractures (fracturing density) on groundwater flow and the productivity of structures has been proven by several authors [6] [29] [30] [31].

Another validity test led us to superimpose dry boreholes on the potential productivity map (Figure 7(b)). At all, 62% of the works are located in areas with low or very low productivity, and of the remaining 38%, none of them are located in the area with a high productivity index. These observations further confirm the effectiveness of the approach taken in this work.