BGR Bundesanstalt für Geowissenschaften und Rohstoffe

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TC Lake Chad Basin: Groundwater Management

Report of the project:

Watershed of the Lake ChadWatershed of the Lake Chad

Background:
The Lake Chad Basin, situated in the central part of the Sahel region at the southern edge of the Sahara desert, is one of the largest sedimentary groundwater basins in Africa. Although the Lake Chad Basin extends over an area of about 2,381,000 km².

The total population of the entire Lake Chad Basin is estimated at about 47 million in 2013. The main activities are agriculture, nomadic and semi-nomadic animal husbandry and fisheries.

Annual rainfall varies between 1,500 mm/a in the south of the basin to less than 100 mm/a in the north. The potential evapotranspiration exceeds 2,000 mm per year at the centre of the basin. For the Lake Chad, the most important influent rivers are the Chari and its tributary the Logone and the Komadougou-Yobe. The interminent El Beid and Yedseram play an important role during rain rich years, when their discharge is large enough to reach the lake.

Climate variability in the Southern Lake Chad Basin:
The southern basin of Lake Chad is climatically influenced by the West African monsoon, which is primarily expressed in a pronounced annual precipitation cycle. This variability is overlaid by a sinusoidal fluctuation of the annual precipitation sum, which had a period of about 60 years in the observation period 1901-2019. This multi-decadal variability is closely linked to the surface temperature of the tropical oceans and the Mediterranean Sea.

Variation of annual precipitation from 1900 to 2020Variation of annual precipitation

In the last century a humid period with a maximum in the 1950s and 1960s was followed by a pronounced dry period, which led to severe droughts, especially in the 1980s. It was found that a 10 % decrease in precipitation led to a reduction in runoff of at least 40 % in the Chari and Logone. As a result, Lake Chad shrank by 90 % from 18,000 km² to less than 2,000 km².

Since precipitation has returned to an upward trend, greening of the Sahel zone and an increase in runoff have been observed. However, regional climate models predict a decline in precipitation from the middle of this century onwards in the southern part of the Lake Chad Basin, which is particularly robust in the southern Charis river basin and along the Massénya groundwater recharge area. Over the same period, an increase in potential evapotranspiration is projected for the whole basin.

The predicted climate changes coupled with a strong population growth of 3 % per year will greatly increase the pressure on existing water resources. Technical solutions, such as artificial groundwater recharge, can help to minimise the effects of climate change and will therefore be tested on site within the project.

Animal husbandry in the flood plain of the Logone RiverAnimal husbandry

Logone floodplain pilot zone:
The large variability of river discharges together with the lack of surface water in most of the basin turns groundwater into the most important source of water supply. However, little is known about regionalisation and amount of groundwater recharge and even less about resources availability.

To palliate this lack of knowledge, the joint project of the Federal Institute for Geosciences and Natural Resources (BGR) and the Lake Chad Basin Commission (LCBC), in the second phase of the original project "Sustainable Water Management of Lake Chad Basin", has concentrated on the interaction between surface water and groundwater in the inundation plain of the Logone River that extends from the Lake Chad in the north to the Mandara Mountains in the south of Cameroon. The investigation area comprises the Yaéré plain with some 8,000 km² in Cameroon and the Naga plain with 4,500 km² in Chad and is very important for recessional agriculture, fishing, and animal breeding.

Flooding extent map of the Lower Logone floodplainFlooding extent map

Geology
The Chari-Logone floodplain lies at the edges of the quaternary plain of the Lake Chad Basin composed of recent and ancient quaternary formations. The thickness generally varies between 50 m and 70 m.

The type of deposits encountered in the study area mirror the depositional environment. The sandy layers are an indicator of past arid conditions, while the clayey layers either lacustrine or fluvial are the result of more humid period, when the size of the lake increased and the Logone and Chari riverbeds were much wider.

Mapping of the flooded areas
The extension of the flooded areas was mapped for the period 2000-2014 by means of remote sensing. The size of the inundated areas in km² varies considerably from year to year depending on a complex interaction of river discharge, precipitation patterns, and initial soil moisture. The largest inundated areas correspond to 2012 followed by 2001 and 2010. The smallest inundated area corresponds to 2009, where the Yaéré was barely under water. After application of statistic methods, a map of maximum, mean and minimum inundated area for the study period was produced.

Field work:
Bi-annual field campaigns have been performed aimed to measure the water table and to collect samples from surface and groundwater. A total of 83 water samples were taken, 76 concerning groundwater and 7 regarding surface water. Chemical and isotope analyses took place at the BGR laboratory in Germany. Analyses comprised complete anion and cation species, and trace elements. In-situ parameters (temperature, pH and electric conductivity) were measured by means of a multi-sensor set.

Groundwater contour lines (2013) in the flooding plane of the Logone RiverGroundwater contour lines

Results:
Groundwater dynamics
Contour lines in the study area show a distinct groundwater flow from the south to the north. Further, direct recharge from surface water at Bongor appears clearly. At this point, the Logone River discharges towards the Naga plane in Chad during high waters, whenever the discharge surpasses 1500 m³/s. The flood expands then towards the north spreading over the plane, where it mainly evaporates. Some discharge collects in a small river (Koulambou) to re-enter the Logone River at the Logone-gana station.

Fluoride
Elevated concentrations of fluoride are measured along the Logone River confirming the results obtained in the first phase of the BGR-LCBC project. Values above the limit were measured in three wells. The upwelling groundwater from the basement into the shallow aquifers is probably the cause of high fluoride here.

Nitrate
Nitrate concentration in the study area is generally low. Only 3 of the analysed wells present nitrate above the limit of 50 mg/l. The reason for this elevated concentration has to be further investigated.

Iron and Manganese
The presence of iron in groundwater is an indicator of anaerobic conditions in the aquifer. According to the World Health Organization (WHO), iron is not hazardous for human health. However, if it is present at elevated concentrations, turbidity and colour might develop in water making it non-acceptable. Furthermore, water containing iron leads often to coating of pipes. The highest concentrations are measured in the surroundings of Bongor and in the northern part of the Naga plain. Furthermore, elevated concentrations appear in the extreme north of the Yaéré plain in Cameroon.

Fluoride concentrations in the flooding plane of the Logone RiverFluoride concentrations

Nitrate concentrations measured in the flooding plane of the Logone RiverNitrate concentrations

Iron concentrations measured in the flooding plane of the Logone RiverIron concentrations

Manganese concentrations measured in the flooding plane of the Logone RiverManganese concentrations

Manganese at concentrations above 0.4 mg/l is hazardous for human health according WHO. However, at much lower concentrations (0.1 mg/l) water already turns unpalatable. Concentrations above the health hazardous value of 0.4 mg/l were measured in 5 samples taken in Cameroon and 13 in Chad. These extremely high concentrations should be reconfirmed by new sampling. If they remain high, measures should be taken to eliminate manganese before consuming the water.

Recharge estimation using environmental isotopes
Precipitation is one of the sources of groundwater recharge in the area, but also surface water in form of rivers, channels, and lakes.

Graph showing the measured δ2H and δ18O values of surface water from N'DjamenaEnvironmental isotopes

The blue line in the graph indicates the local water line for precipitation from N’Djamena and the red line corresponds to the water line of surface water. Surface water experiences evaporation that enriches its isotope composition compared to the precipitation water. This effect leads to a water line with a lower gradient (i.e. 4.5 compared to 6.3 from the precipitation water).

The water line produced with the boreholes located in the Yaéré plain plots parallel to the precipitation line. This means that recharge in the Yaéré plain is caused mainly by precipitation.

In the case of the Naga plain, the water line produced with the boreholes plots parallel to the surface water line. It can be concluded here that flooding water is the main source of recharge.

Using empirical formulas by Allison et al. (1983), it is possible to calculate the annual recharge volume based on the deviation of the groundwater line in relation to the water line of the recharge source. The results obtained applying the formula are:

Plain



Recharge
(mm/a)


Minimum
extent
(km²)

Minimum
recharge
volume
(m³/a)
Mean
extent
(km²)

Mean
recharge
volume
(m³/a)
Maximum
extent
(km²)

Maximum
recharge
volume
(m³/a)
Yaéré91,1079,963,0002,97926,811,0005,23047,070,000
Naga161,23819,808,0002,30836,928,0003,77660,416,000
Bongor162504,000,0004336,928,00085213,632,000
SUM33,771,00070,667,000121,118,000


Conclusions:
It is possible to delineate the flooded areas for each year in the period 2000-2014 depicting the low reflectance points from the SWRI images. The superposition of all calculated areas allows the estimation of maximum, mean and the minimum flooded area. This mapping shows clearly the area that receives flood water annually and help thus for planning, especially for recessional agriculture.

In general, groundwater is of drinking quality. However, high concentrations of iron and manganese are commonly encountered. Although iron is not health hazardous, manganese appears at concentrations that are considered harmful. Groundwater from these wells should not be used directly for human consumption. Measures like water mixing or filtering should be applied.

Groundwater is recharged within the Yaéré and Naga plains. The isotope analyses show different sources of recharge. While it is mainly precipitation that renews groundwater in the Yaéré plain, recharge in the Naga plain is caused by flooding water.

Although the minimum inundated areas for both plains are in the same range, much more groundwater is renewed in the Naga plain (16 mm/a compared to 9 mm/a in the Yaéré). The highest recharge at a rate of 25 mm/a takes place along the Logone River.

Recommendations:
The study has shown that an important quantity of groundwater is renewed annually from the flood water or the precipitation in the area. It is highly recommended that these plains are protected quantitatively and qualitatively to comply with safety and acceptability of water supply as set by the human right to water.


Literature:

Reports

Maps

Conference Contributions

  • BOSCH, K., HECKMANN, M., BRODA, S., SEEHAUSEN, L. & HASSANE TAHIROU, A. (2021): Mapping of aquifer productivity and assessment of groundwater vulnerability to pollution in the lower Chari-Logone River Basin, southern Lake Chad Basin. - Poster presented at the 48th IAH Congress, Brussels, Belgium.
  • GEERKEN, R., VASSOLO, S. & BILA, M. (2012): Impacts of climate variability and population pressure on water resources in the Lake Chad Basin. (PDF, 753 KB) In: BOGARDI, J., LEENTVAAR, J. & NACHTNEBEL, H-P. (eds.): River Basins and Change. - Contrib. to the intern. conference on "The Global Dimensions of Change in River Basins" organised within the Global Catchment Initiative of the Global Water System Project (GWSP), December 6 - 8, 2010, Bonn, Germany.
  • GEERKEN, R., VASSOLO, S. & SCHIMMER, R. (2012): Monitoring variations of Yaere Wetlands to understand effects of inter-annual climate variations. - Poster presented at the IWRM conference 2012, Karlsruhe, Germany.


Contact 1:

    
Dr.-Ing. Sara Ines Vassolo
Phone: +49-(0)511-643-2818

Contact 2:

    
M.Sc. Geowiss. Lilli Witt
Phone: +49-(0)511-643-2617

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