Groundwater level changes since 1978 in an African city on basement rocks: the case of the CIEH borehole in Ouagadougou (Burkina Faso)

The CIEH piezometer, located in the center of Ouagadougou city presents a water level record spanning the West African Drought which peaked during the 80s and 90s. Its water level is investigated as a potential proxy for groundwater water resources in West African basement rock aquifers submitted to climate changes. 23 boreholes and wells in various land uses and within a 2 km radius around the CIEH piezometer were monitored during the 2013-2014 hydrologic year. The minimum water level occurred in May, at the end of the dry season, while the maximum took place in October, one month after the end of the rainy season. The mean water level amplitude is 3 m, the minimum amplitude being reached at the CIEH piezometer (0.76 m). Moreover, the CIEH piezometer is located in a 2 m amplitude water table depression either in May or in October. Simplified 2d modeling using a general basement aquifer structure shows that (i) the water level in the piezometer is under ongoing influence of the spillway raise of the nearby dam#3 lake in 2002, (ii) the whole 1978-2004 period cannot be modelled with constant parameters. A 3% decrease of water uptake is adopted after 1985, presumably resulting from land use changes in the Ouagadougou city. The water table at the CIEH piezometer is presently at its 1978 level, which can considered as a pre-drought value. However this includes a 1.5 m contribution of the two abovementioned anthropic effects Further quantitative interpretations of the CIEH piezometer record will require additional geophysical and hydrological investigations. How to cite this paper: Mouhouyouddine, A.H., Yameogo, S., Genthon, P., Paturel, J.E. and Guilliod, M. (2017) Groundwater Level Changes since 1978 in an African City on Basement Rocks: The Case of the CIEH Borehole in Ouagadougou (Burkina Faso). Journal of Water Resource and Protection, 9, 1097-1118. https://doi.org/10.4236/jwarp.2017.910072 Received: June 27, 2017 Accepted: September 12, 2017 Published: September 15, 2017 Copyright © 2017 by authors 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
Situated in the center of the Ouagadougou main city (Figure 1), the CIEH (Inter-African Comity for Hydraulic studies) piezometer presents a mostly continuous record starting in 1978. At our knowledge, this record is unique at the scale of the whole West Africa and therefore it deserves throughout analysis.
This record spans a large part of the African Drought, which peaked between the 80s and the 90s in the Sudanian and Sahelian zones of West and Central Africa, and is now considered as one of the most significant drought events worldwide [1] [2]. Moreover, Ouagadougou is situated on basement rocks where aquifers are fully included in the few tens of meter thick weathering profile [3] [4] [5]. Therefore, the available groundwater resource is limited when compared to sedimentary aquifers [6] [7]. Moreover, basement aquifers are sensitive to climate events at any time scale larger than a few months [8]. 40% of African people rely on basement aquifers for groundwater. Fast development of African cities-more than 7% annual population growth in Ouagadougou [9]-results in increasing water demand and serious risks of water shortage [10]. In Ouagadougou, tap water comes from a series of dams, the main one-the Ziga dam-being located 50 km away from the city. However, due to combined silting and evaporation and due to the growth of the city, water shortage is planned in the few coming years and a complementary resource will be sought from boreholes. The water level record at the CIEH piezometer (P-CIEH), since it spans the duration of the African Drought, may help to assess the effect of climate changes on groundwater resources in Ouagadougou, and provide some general indications for African cities on basement rocks.
In order to better define the specificities of P-CIEH, a piezometric network was set up on different land use sites around this borehole ( Figure 1) and mo-

Study Area
Ouagadougou is located near the center of Burkina Faso and the study area lies in the eastern part of the city center ( Figure 1). It includes (i) an old urban area: the Zogona neighborhood, (ii) a part of the Ouagadougou University built from 1974, (iii) a protected forested recreational area, the Bangr Weogo Park (iv) a vegetable gardening area. This corresponds to contrasted land uses within the study area. This area is bordered to the north by a series of three reservoirs built in 1963 in the Nabaouli riverbed and that drain into the Bangr Weogo wetland.
A large part of rainfall and sewage waters from the city of Ouagadougou are drained toward the Bangr Weogo wetland trough the Central and University concreted channels. The University channel was only an earth channel until 2004. Ouagadougou is located on Proterozoic granites and granodiorites [11] which are basement rocks and present specific properties.

The Weathering Profile of Granitic Rocks and Its Relationship with Groundwater Resources
Recent works allowed to define the relationship between weathering and aquifer properties [3] [15]. The hydraulic conductivity of saprolites is low, near 10 −6 m/s, their specific yield amounts to 1 -15% [16]. A specific yield ranging between 1% and 5% is proposed in the central Burkina Faso [17], a 0.5% -5% range results from aquifer tests on the nearby Sanon site [18], while a 5% mean value is proposed in the Ouagadougou city [15] [19]. In some instances, the base of the saprolitic layer includes a few meters thick arenic layer of high hydraulic conductivity [3]. However this layer is absent or it contains a high proportion of clay in Ouagadougou [20]. Below the saprolitic layer, granite presents horizontal fissures with an extension ranging from 3 to 40 m which produce a hydraulic conductivity of the order of magnitude of 10 −5 m/s and allow to drain water included in the saprolitic layer [21] [22].
Within this layer, both the density of horizontal fissures and their aperture decrease with depth. Recognition of this fissured layer as part of the weathering profile [16] and of its role in the basement aquifer drainage constitute a significant and recent advance for water resource management of basement rocks aquifers [5].

Climatic Data
Rainfall data at the Ouagadougou airport are available for the 1953-2010 period.
After 2010, we use data from the IRD raingauge after having checked on overlapping periods that rainfall measured there was close to that of the Ouagadougou airport. Daily temperatures, potential evaporation rates and relative humidity for the 1978-2009 period were measured at the Ouagadougou airport.

The Piezometric Network
One of the longest piezometric record in West Africa is available for the CIEH  as a reference. Topography is gently dipping toward the east-west trending low zone constituted by the three reservoirs and the Bangr Weogo wetland. Level curves of this map were digitized and the elevation of wells and boreholes of the network were interpolated from these data. Since the contouring interval is 5 m, only a few meters accuracy can be expected. Leveling data with an accuracy of 5 cm were also available for boreholes in the University area and the level gauge of the dam#3 lake as well as a topographic map resulting from optical leveling for the Bangr Weogo park [24]. A constant elevation offset was considered between the IGB 1984 map and these local data.

Water Level at the Dam#3 Lake
Three reservoirs were built in 1963 in the Nabaouli riverbed to provide drinking water for the Ouagadougou city. Only the third one-the dam#3 lake-is pre-

Groundwater Models
The GARDENIA 1d three reservoirs model [25] and the MOFLOW96 2d model [26] [27] are used in the present study.

Rainfall
Rainfall is governed by the African monsoon. October to April are almost dry

Temperature, Potential Evapotranspiration (PE), Humidity
These data are provided in Figure 4 as monthly averaged values. Temperature is controlled by the season and by rainfall. Minimum temperature occurs in January. It increases until April, when the maximum daily temperature currently exceeds 40˚C, is buffered by evaporation during the rainy season, and increases again temporarily after rain stops in October. Relative humidity is controlled by

Water Level Fluctuations during the 2013-2014 Year
In 2013, major rainfalls began at the end of May just after the setup of the network ( Figure 5). The gardening area was partly flooded and several wells (not  It was found that at the scale of the whole crystalline Burkina Faso, the amplitude of water level fluctuations was decreasing with increasing mean water depth, which corresponds to damping of the yearly signal during its downward propagation [28]. Such a study is not possible with the present dataset due to the limited duration of the record. As a simplified approach, we explore the relation between the minimum water level, considered as being reached in May 2013, and the difference with the maximum water level in October, considered as an approximation for the water level amplitude.

Water Table Maps
Two maps (  is provided by tap water corresponds to the lowest depletion, while the Bonheur well corresponds to the largest amplitude due to strong depletion and to strong recharge due to its proximity with an area flooded during the rainy season.
P-CIEH and the Triangle well lie well below the straight line defined in Figure 7, i.e. they correspond to both a lower minimum level and amplitude. We propose that they both lie in a low hydraulic conductivity area which prevents recharge either by rainwater or by lateral flow from nearby areas.
Recharge by infiltration from the dam#3 lake was questioned by computing the characteristic distance [30] to which a perturbation of pulsation ω arising  from seasonal lake level changes could propagate inside an aquifer of transmissivity T and specific yield φ , which is: The yearly recharge corresponds to ω = 2 × 10 −7 s −1 . Then, assuming that transmissivity is controlled by a 10 m thick fissured layer of hydraulic conduc- where the water table is table is shallow and mainly controlled by the water level at the dam#3 lake, both the water level amplitude and water table depth seem to be controlled by local differences in a given land use rather than systematic differences in the hydrologic cycle between the different land uses.

Water Level Fluctuations at the Dam#3 Lake
Water level in this reservoir ( Figure 8) is governed by rainfall and evaporation.
The reservoir is filled up to the spillway level at the beginning of the rainy season and empties during the dry season due to evaporation. The water level drop stops when a significant rain event occurs during the dry season, which mainly  which mimic the heightening of the spillway. This allows extrapolating the mean water level for missing years, before 1979 and after 2006. As seen in the previous paragraph, the piezometric level in the study area will be only sensitive to the mean water level outside a narrow band near the reservoir.

The CIEH Piezometric Record
This record (Figure 9) shows annual variations, with amplitude commensurate with annual rainfall, superposed to a pluriannual trend associated to the pluriannual rainfall trend. The water level decreases between 1978 and 1986, displays a complete oscillation between 1986 and 2002 and increases again after 2002.

Correlation Analysis
In order to assess the relationship between the water level at P-CIEH and annual rainfall, a correlation analysis was carried out, starting from both the annual amplitude and the mean annual piezometric level. Due to the large amount of missing data the mean piezometric level is not computed but estimated visually.   The cross-correlogram between rainfall and the mean water level as well as the correlograms of these two variables were computed in order to detect a delayed response of the piezometric level to rainfall. There is strong correlation between water depth and past rainfall, reaching a maximum of 0.6 for a time lag of −4 yrs ( Figure 11(a)). However, this result must be considered with care, since the autocorellation of the water level exhibits also strong correlation with past values (Figure 11(b)), reaching −0.8 for a time lag of −7 yrs which reflects the oscillatory component near the same period of 7 yrs visible on Figure 9. The rainfall Figure 11. Correlation analysis of the relation between the mean annual water level at P-CIEH and annual rainfall.  (Figure 11(b)) shows that there is complete decorrelation between rainfalls of successive years. There might be secondary correlation peaks near 5 -6 yrs time lag, they will not be discussed here. The conclusion of this section is that due to the oscillatory component in the water level signal, the correlation analysis cannot be used to assess its long term response to rain. The rainfall-water level relationship is therefore explored by numerical modeling in the next section.

Numerical Modeling of Water Level Change in P-CIEH
The models are designed in order to assess the different mechanisms controlling only can be compared with data. The specific yield is a free parameter within the 1% -15% range proposed for basement aquifers [16]. The value of 5% proposed for the Ouagadougou region [15] is adopted here. In Modflow, the water level change is the ratio between the amount of infiltrated water and the specific yield. be assessed by cancelling both the infiltration and the water uptake terms and starting with a flat water level between the two boundary conditions: the water level rises by 0.55 m during the modeled period and will continue to rise during the next 15 yrs. As indicated by the P-CIEH record the water level in the study area is not only controlled by climate, by also but by anthropic effects which was already noted in urban [32] or rural context [33].

Discussion
The

Conclusions
Groundwater is available at a few meters depth in the investigated area. The Bangr Weogo park, which is a natural area could be explored for drinking water. constant abstraction, presumably due to ET. However, the model requires a decrease of 3% of water abstraction after the 80s, which could be attributed to building construction in the University area. Without this 3% abstraction decrease, the present water level at P-CIEH would lie 1 m meter below the present one. Moreover a 0.55 m increase of the water level at P-CIEH results from the 1.2 m heightening of the spillway of the dam#3 lake and it is expected that with constant climatic conditions, the water level will still increase during the next 15 yrs. Therefore, without the two anthropic effects discussed above, the P-CIEH water level would lie nearly 1.5 m below its 1978 level, which implies that groundwater resource would have not completely recovered from the drought effects since 1978.
Results of the present study suggest that P-CIEH is mostly controlled by climate with minor anthropic effects. Therefore, this piezometer could constitute a proxy for climate change impacts on the water resource in basement aquifers of