Introduction
The severe drought that affected the Sahel in the 1970s–1980s is one of the
major signs of climate variability recorded on a global scale during the 20th
century . The surface of Lake Chad decreased from
21 000 km2 in 1960 to 2500 km2 in 1987
, illustrating the consequences of climatic changes in
the Sahelian belt . Lake surface area
oscillations have major socio-economic impacts on populations established
along the lakeshores who depend on its fresh water and halieutic resources
for their livelihood . It was
initially thought that withdrawals of water, mainly for irrigation,
contributed to the lake decline observed during the 80s and 90s
, but more recent studies have shown that withdrawals were
very likely overestimated and argue that extraction for irrigation is
negligible in the hydrological budget of the lake . Since
2000, the lake surface area has increased, reaching 14 000 km2 in
April 2013 due to more favourable rainfall in the western Sahel
. This recent evolution of Lake Chad clearly indicates that
its variability is essentially driven by variations in the monsoon regime
over its southern drainage basin, combined with a very short residence time
of the water in this system.
Although it is the terminal lake of an endorheic basin dominated by
evaporation (∼2000 mmyr-1), Lake Chad waters remain
surprisingly fresh . It has been suggested that
the chemical regulation of Lake Chad is controlled both by geochemical
precipitation and clay neoformation in the lake
and more significantly by infiltration of the lake water into the underlying
Quaternary aquifer . The aquifer connected with the lake is
the major source of water for domestic use but its waters in some areas show
total dissolved salt concentrations above the recommended values
.
A thorough estimate of the hydrological and chemical fluxes of Lake Chad is
crucial to understand its variability and chemical regulation as well as to
constrain the relationship between the lake and the Quaternary aquifer.
Despite considerable efforts to evaluate evaporation and infiltration out of
this lacustrine system, much remains to be done in order to better constrain
the hydrological and salinity budget of Lake Chad, especially in its present
state, characterized by seasonal or perennial pools. For instance, the impact
of transpiration, which might be a significant process impacting the
hydrological budget owing to dense vegetation on the lake
, has so far never been taken into account. Different
lake water balance models were previously implemented to simulate
fluctuations in the level of Lake Chad . An
integrated lake–catchment approach, coupling the distributed surface water
balance calculated over the basin (Integrated Biosphere Model, IBIS) with runoff transported across the
land surface (hydrological routing algorithm, HYDRA) described using a digital elevation model (DEM) approach
yielded a good representation of the river discharge of the basin and the
lake-levels in the eastern part . Nevertheless, this model was
unable to distinguish the different states and pools of the lake and
groundwater–surface water interactions were not accounted for.
Infiltration of lake waters into the Quaternary aquifer was evaluated by
isotopic studies or by
quantitative approaches based on simple water or salt budgets
, direct estimates of seepage
velocities or hydrogeological models
. However, they lead to a large range of
infiltration estimates of between 2 and 32 % of the total inputs of
the lake. The accurate quantification of leakage from lake waters is
a challenging issue especially in semi-arid to arid environments
, as the estimates are highly dependent on the methods
selected . This suggests that a combination of different
approaches is necessary to accurately estimate all the fluxes of the lake
water balance. Chemical modelling using conservative elements has proved to be
relevant to quantify groundwater outflows
. Isotopic budgets have also been
widely used to constrain lake water balances
.
However, the combination of the two approaches has seldom been undertaken.
The objective of the present study is to make the best use of all available
information on Lake Chad to combine for the first time hydrological,
chemical,
and isotopic mass balance approaches over the 1955–2011 period. This aims to
(i) refine the range of the global total water losses of the Lake Chad
system; (ii) determine the respective contributions of evaporation,
infiltration, and transpiration in the system not completely explored so far;
and (iii) assess the influence of infiltration on both the chemical
regulation of the lake and the recharge and chemistry of the underlying
aquifer.
(a) Location of Lake Chad basin in
Africa. Lake Chad (light blue) and its tributaries (blue) are figured in the
African hydrological network (blue lines). The climatic zones in North Africa
are presented. The elevation colour bar corresponds to the altitude and the
black line is the boundary of Lake Chad basin. (b) Presentation of
Lake Chad in its normal state. The three pools are depicted and the double
line between the southern and the northern pools is the Great Barrier. The
tributaries are shown in blue and mean rainfall rates in red. The piezometric
map around the lake and its particular features are presented in grey lines,
the piezometric domes: I – Kanem, II – Harr and depressions,
III – Chari Baguirmi, IV – Bornu, V – Kadzell. The dotted
black line represents the Bahr el Ghazal, a temporary outlet of the lake.
General description of the study site
Lake catchment settings
The Lake Chad basin (LCB) is a 2.5 billion km2 hydrologically
closed drainage basin located in central Africa, between Chad, Cameroon,
Nigeria, Niger, and the Central African Republic and extending from 7 to
24∘ E and from 5 to 25∘ N. It is bordered by high
mountains, the Tibesti and the Hoggar in the north, the Adamaoua in the
south, the Air in the west and the Ennedi in the east. The drainage basin is
characterized by a south–north climatic gradient. The northern part lies
in the Saharan climatic zone, dominated by low rainfall and high evaporation
rates. The southern Sudanese part is under a monsoon regime and is the only
hydrologically active part of the basin (about 610 000 km2). The
internal part of the LCB is a flat sedimentary depression, with a mean
topographic slope of 1.3 % and Lake Chad is
located in a small depression in the central Sahelian part
(Fig. ).
Lake morphology
Lake Chad is subdivided into three areas, the southern and the northern
pools, separated by an east–west vegetation-covered sand barrier named the
Great Barrier, and an island area in the east named the archipelagos
(Fig. ). These zones are either connected or disconnected
depending on the lake level. Different states and morphologies of the lake
have been described as a function of lake levels . During
the period on which this study focuses, Lake Chad oscillated between its
normal state, corresponding to a surface area of about 20 000 km2
occurring between 1950 and 1972 and its small state, below
15 000 km2 since 1972. During its normal
state, the mean water depth is about 3 m in the southern pool and
6 m in the northern pool. Lake levels occasionally exceed a threshold
located in the eastern part of the southern pool, at 282.3 m, leading
to overflows in the Bahr-el-Ghazal channel, which drains water northward to
the Bodele depression (Fig. ). During the Holocene, the
Bodele depression was filled and the lake Mega-Chad covered
350 000 km2.
Hydrological and chemical features of Lake Chad
The Chari–Logone River flows from the Central African Republic along the
Cameroon border into the southern shore of Lake Chad. It is the major
tributary of the lake and accounts for almost 80–90 % of the water
inflow. Other inputs are the Komodougou Yobe River, which flows from Nigeria
and Niger (5 %) into the northern pool and direct rainfall at the
lake surface (5–10 %, . Lake Chad
waters are subject to a high evaporation rate of about
2000 mmyr-1, which is the major water loss of the lake, while
infiltration remains small. In the normal state, river discharge is around
40 km3yr-1 and rainfall at the surface of the lake around
250 mmyr-1 (4 km3yr-1) for a lake volume of
about 72 km3. Consequently, under the rough assumption of a steady
state, the residence time of the waters in the lake is very short, about
1–2 years. With previous estimates of the annual infiltration into
the Quaternary aquifer of nearly 5 % of the lake volume, the
residence time of a conservative element in the normal state is around
20–40 years. Lake Chad hydrological budget is thus controlled by the
balance between rainfall in the southern part of the basin and evaporation
over the lake surface. This balance directly governs not only on very short timescales the volume but also the lake surface because of the flat topography of
the basin responsible for the lake shallowness.
The regional Quaternary aquifer
The lake is hydraulically connected to an unconfined aquifer of
500 000 km2 . It is composed by an
alternation of silts, sands and clays resulting from extended and restricted
lacustrine phases during the Quaternary. The hydraulic gradient and thus
groundwater flow around the lake is oriented from the lake towards the
aquifer except at the north-eastern end of the lake (Fig. ).
The water table is characterized by three major piezometric depressions
around the lake, Chari-Baguirmi, Kadzell, and Bornu and two piezometric domes,
Harr and Kanem (Fig. ). The origin of such features, also
described in other phreatic aquifers in the Sahelian belt
, is still debated. The main assumption
of a high and localized evapotranspiration rate associated with low hydraulic
conductivities was confirmed in the LCB by a steady-state regional flow model
of the Quaternary aquifer . Isotopic studies suggest that
the main recharge occurred during past humid periods
. Infiltration from both lake and rivers may
also contribute locally to the recharge of this unconfined aquifer but its
quantification remains an issue .
General approach: combination of water, chemical and isotopic mass balances
In the case of a shallow closed lake where aquatic vegetation has
a non-negligible role through transpiration fluxes, the hydrological,
chemical, and stable isotopic mass balances are expressed as
follows:
ΔVLΔt=SL×[P-(E+T+I)]+Qint-ΔtΔ(VL⋅CL)Δt=SL×[P⋅CP-I⋅CL]+Qin⋅Cint-ΔtΔ(VL⋅δL)Δt=SL×[P⋅δP-(I+T)⋅δL-E⋅δE]+Qin⋅δint-Δt
where VL is the volume of the lake (m3),
SL the surface (m2), P the rainfall
(m day-1), E the evaporation (m day-1), I the
infiltration (m day-1), T the transpiration
(m day-1), Qin the river discharge
(m3day-1), CL, δL, CP,
δP, Cin, and δin the
concentrations (mg L-1) and isotopic compositions (‰) of the
lake, the rainfall and the rivers, and δE the isotopic
composition of the evaporated moisture. This set of balance equations assumes
a perfect mixing at each time step. Considering a steady state of vegetation
growth cycle in the lake, transpiration has no effect on salt balance but has
the same role as infiltration for the isotopic mass balance since it does not
induce any fractionation at stationary state . Therefore, in
the chemical mass balance, infiltration is the only process exporting salts
while in the stable isotope mass balance, evaporation is the only
fractionating pathway.
According to , the isotopic composition of the evaporated
moisture above a lake can be calculated from the measured relative humidity
(h), the measured isotopic composition of the regional vapour (δA),
the isotopic composition of the lake (δL), the total
fractionation coefficient (ε*), and the kinetic fractionation
(εk) coefficient as follows:
δE=(δL-ε*)/α-h⋅δA-εk1-h+εk,ε*=(α-1)εk=(1-h)⋅θ⋅n⋅CD,
where n, θ, and CD are turbulence parameters such that n=1/2
for an average turbulent flow , CD was
determined experimentally as CD18O = 28.5 ‰,
CD2H = 25.1 ‰ . Few studies
have focused on the determination of the θ parameter. Its value is
generally lower than 1 for a water body whose strong evaporation flux perturbs
the atmospheric boundary layer , with a value of 0.88
estimated for the Great Lakes , and commonly used elsewhere.
Nevertheless, a lower value (θ=0.5) has been estimated for the
eastern Mediterranean , attributed to the high contrast
between the air column above the sea surface and the advected air masses.
Thus, since low values may be expected when humidity is not measured near the
lake surface , and because the Lake Chad evaporative
conditions are closer to the conditions of the eastern Mediterranean Sea than
to those of the Great Lakes, we chose a value of 0.5. The choice of this
value is further discussed based on the slope of the evaporation line.
Recycling of evaporated moisture is able to influence the local atmosphere
above the lake . The impact of Lake Chad
(normal size) and of its desiccation on regional climate has been
investigated, using a mesoscale regional atmospheric model coupled to a
soil–vegetation–atmosphere transfer model , showing that
whatever the size of the lake, lake evaporation does not affect significantly
the atmospheric hydrological cycle and the total precipitation amounts. The
authors calculate a moisture recycling ratio of less than 7 % in the
total studied area, that remains unchanged regarding the size of the lake.
Even during episodes of lake Mega-Chad, covering 340 000 km2, which is
over a hundred times its current surface area, the contribution of Lake Chad
to regional moisture remains low . In addition, the
humidity around Lake Chad, documented both from global data and local studies
is low (around 0.4 in average), arguing for a small influence of local vapour
recycling on the simulation of lake water isotopic composition. Therefore,
although available data are not enough for investigating the effect of local
evaporation on the isotopic composition of regional atmosphere, we believe
that we can reasonably neglect this effect and that the associated
uncertainty is likely lower than the uncertainty associated with the isotopic
composition of regional atmosphere.
According to the system of Eq. (), E, T, and I are
undistinguishable in the water mass balance but they are separated in the
chemical and stable isotope mass balances. Denoting ETI the sum of
evaporation, transpiration, and infiltration, the exporting fraction of salts
is defined as FI=I/ ETI and the fractionating fraction is
FE=E/ ETI. The equation can thus be rewritten as follows:
ΔVLΔt=SL×[P-ETI]+Qint-ΔtΔ(VL⋅CL)Δt=SL×[P⋅CP-FI⋅ETI⋅CL]+Qin⋅Cint-ΔtΔ(VL⋅δL)Δt=SL×P⋅δP-(1-FE⋅ETI)⋅δL-FE⋅ETI⋅δE+Qin⋅δint-Δt.
According to the system of Eq. (), ETI can be determined from
the water mass balance, the exporting salt fraction (FI), i.e.
infiltration from the chemical mass balance, and the fractionating fraction
(FE), i.e. evaporation from the stable isotope mass balance.
Transpiration can be deduced from 1-(FI+FE). Therefore, our
approach combining water, salt, and isotope mass balances is a way to formally
separate evaporation, infiltration, and transpiration occurring at the surface
of the lake.
Based on a deterministic calibration of the lake model,
quantified infiltration out of Lake Chad using a forcing value of evaporation
corresponding to measurements. However, the estimated infiltration lies
within the range of evaporation uncertainty (10 %). Hence, a small
error on the evaporation value introduced in the model will have a major
impact on the calibrated infiltration. Moreover, despite the density of the
vegetation cover in some areas of Lake Chad, the contribution of plant
transpiration to the water and salt balances of the lake was suggested
but never quantified to our knowledge. Our alternative
approach proposes the following improvements:
An evaluation of the uncertainties associated with input variables and modelling,
an important issue widely discussed in the literature .
An integration of water, chemical, and isotopic mass balances in order to better
constrain poorly known terms of the lake water balance such as the respective
contributions of evaporation, transpiration, and infiltration to the lake water budget.
We thus propose to go one step further by using all the geochemical data
available since 1950 in our approach combining water, salt, and isotope mass
balances.
For these reasons, the approach detailed above for a single pool was applied
to the three connected pools conceptualization of Lake Chad
. Water balances were calculated at a daily time step and
total outflows (ETI) were obtained from a Bayesian inversion of lake levels.
Chemical and isotopic mass balances were then calculated at a daily time step
using the parameters resulting from the inversion. The FI and
FE parameters were subsequently calibrated at each pool from the
comparison between observed and simulated chemical and isotopic values.
Details are presented in Sect. . According to
Eqs. ()–(), this modelling approach requires
some input data (Qin, Cin, δin,
P, CP, δP, δA, h) as well as some
calibration–validation data for comparison with simulation outputs
(hL, CL, δL). The model is
built at a daily timescale because it is the appropriate time step to avoid
numerical oscillations of the water balance. However the results of the
water, isotope, and chemical mass balances are further evaluated and discussed
at seasonal and annual timescales.
Hydrological, climatic, and geochemical data
Our study is based on available hydrological, chemical, and isotopic data and
some additional data acquired in this study on the LCB. References and
details on the data used are provided in Supplement. A particular focus of
the study was the assessment of the reliability and the quantification of
uncertainties of this heterogeneous data set.
Ground-based and altimetric lake-level data
The lake level was monitored non-continuously from 1956 to 2008 at three gauges,
in Bol (archipelagos), Kalom and Kirenowa (southern pool), and Nguigmi
(northern pool); see Fig. for locations
. The Bol station, located in the
archipelagos, provides the most complete record on a daily timescale. The
other stations provide sparse observations. The data set comprises 885
lake-level data measured in the archipelagos, 150 in the southern pool and 97
in the northern pool over the 1956–2008 period. In addition, lake surface
estimates in the northern pool were obtained from remote sensing
and, in this study, converted
into lake levels using the surface-level relationships provided by
.
Sodium concentration and δ18O
isotopic composition of Lake Chad water over the 1968–1978 time period in
(1) the southern pool, (2) the northern pool, (3) the archipelagos. Red dots
are mean values with their 1σ uncertainties (calculated from spatially
distributed sampling campaigns) and cross-symbols are local measurements,
whose localization is shown by a same-coloured cross-symbol on the Lake Chad
map on the right.
Lake Chad chemical and isotopic data
Data from
, , ,
, , , ,
, , , and
were collected and completed by new samplings performed between 2008 and
2012. Due to its endorheism and climate characteristics, the lake is expected
to be a concentrated basin of dissolved salts. However, the water salinity
is low and relatively time constant. Exhaustive studies carried out by
on the whole lake during its normal state, between
1968 and 1971, provide a description of its geochemical features. These data
show a global trend of increasing concentrations from the Chari–Logone delta
(σ=50 µS cm-1) to the northern pool (σ=1000 µS cm-1) and to a lower extent to the
archipelagos (σ=600 µS cm-1). The Lake Chad
waters are dominated by HCO3- while Cl- and SO42- accounts for
only 2 % of the anionic balance with chlorine concentrations below
1 mgL-1. Calcium is the most abundant cation near the
Chari–Logone mouth, while sodium becomes dominant in more concentrated waters
because it does not react with the lake substratum or the vegetation
. Therefore, sodium and chlorine can be considered
conservative elements except in the northern margin where limited deposits of
natron (Na2CO3) are described. However, because of the low chlorine
content in the waters, very few accurate chlorine measurements are available.
Hence, only sodium is considered a conservative element in this study. Sodium
concentrations in the lake during the normal state were about
0.3 mmolL-1 in the southern pool, 2 mmolL-1 in the
northern pool and 0.5 mmolL-1 in the archipelagos
(Fig. ).
Four extensive spatial studies were carried out in 1969 for stable isotopes
. They confirmed the spatial distribution shown by the
chemical concentrations: waters are more enriched as they flow away from the
Chari mouth (δ18O ∼ 2 ‰) to the northern pool
(δ18O ∼ 10 ‰). The oxygen and hydrogen isotopic
composition of all available lake water measurements plot along an
evaporation line of δ2H = 5.2×δ18O +1
and δ2H =4.4(±0.2)×δ18O +2.82(±1.12) . Apart from
extensive spatial studies, the seasonal monitoring of sodium concentrations
and oxygen isotope data at local stations illustrate the spatial and temporal
variability of geochemical data in the lake (Fig. )
. Most studies have been carried out on Lake
Chad in its normal state. Although punctual in space and time, a few recent
studies (, this study) provide valuable data
on Lake Chad in its small state.
River data
The Chari–Logone discharges were reconstructed from river height measurements
at the gauging station of N'Djamena. The Komadougou Yobe and El Beid
discharges were calculated from observations and correlations with the
Chari–Logone discharge . These discharges were considered
for a period between 1956 and 2011. Uncertainties, which could depend on
installations, reading, gauging station calibration curve, and reconstructions
of the data set, were evaluated at a reasonable value of 10–15 %
.
The sodium concentrations and the isotopic compositions of the Chari–Logone
and Komadougou Yobe rivers were measured near their mouth between 1956 and
2009
. We
completed these data sets by new data, from samples collected between 2008
and 2013.
River input variables (a) sodium
concentrations in the Chari–Logone and Komadougou Yobe (KY): measured sodium
concentrations in the Komadougou Yobe (triangles); mean annual scenario of
sodium concentration reconstructed for the KY (black line with plain
triangles); measured concentrations in the Chari–Logone (grey diamonds); mean
annual scenario of sodium concentration reconstructed for the Chari–Logone
(black line with plain diamonds). (b) Oxygen-18 composition in the
Chari–Logone and Komadougou Yobe (KY): measured δ18O in the
Komadougou Yobe (triangles); mean annual scenario of δ18O
reconstructed for the KY (black line with plain triangles); measured
δ18O in the Chari–Logone (grey diamonds); mean annual scenario of
δ18O reconstructed for the Chari–Logone (black line with plain
diamonds).
The Komadougou Yobe River is characteristic of more arid climatic conditions
with concentrations in Na+ between 0.2 and 0.5 mmolL-1
against 0.1–0.2 mmolL-1 in the Chari–Logone and oxygen isotopic
compositions between -4 and +8 ‰ in the Komadougou Yobe
against -6 and +3 ‰ in the Chari–Logone. Both [Na+] and
δ18O evidenced a high seasonality for the two rivers with minima
recorded during high-water levels at the end of the humid season (September)
and maxima recorded during the dry season (April–May). The long-term-weighted average isotopic composition of the Chari Logone is
-3 ‰ for δ18O and -20 ‰ for
δ2H. The interannual variability of concentrations and isotopic
compositions in rivers is low and there is no significant change in the water
composition of the rivers between the normal and small states of the lake
(Fig. ).
Rainfall data
In this study, we used the rainfall data set reconstructed by
from precipitation measurements at five stations around
Lake Chad. Sodium concentrations in rainfall were measured at a monthly time
step by in 1969. They range between 0.05 and
0.24 mmolL-1. Concentrations around 1 mmolL-1 were
measured in rainfall in Nigeria but are higher than the
concentrations of river and lake samples. These questionable values were thus
not considered in this study.
In all, 86 non-continuous measurements of deuterium and oxygen isotopes in
precipitation (Fig. ) were carried out between 1964
and 1995 at N'Djamena by the GNIP (Global Network for Isotopes in
Precipitation; http://www.iaea.org/water). The measurements at N'Djamena
define the local meteoritic line:
δ2H=6.3×δ18O + 4.3 (r2=0.95) with long-term-weighted averages and standard deviations of -3.8±1.7 ‰ for
δ18O and -18±10 ‰ for δ2H. The low
slope of the local meteoritic line is in agreement with measurements
throughout the Sahelian band and indicates that enriched precipitations are
probably affected by evaporation in the atmosphere
.
Atmospheric vapour isotopic data, air temperature, and relative humidity
The isotopic composition of atmospheric vapour is rarely measured and the
usual assumption of an isotopic equilibrium with rainfall is highly
questionable in climates with long dry periods . A few
vapour measurements were performed in 1969 using the cryogenic trapping method
, but because of their scarcity, and of the sampling
conditions, they can only be considered as a rough indication of the order of
magnitude (Fig. ). Recently, a detailed annual record
of δ18O of water vapour was performed by using laser spectrometry at
Niamey, Niger (Fig. ). It is the
only available record in the Sahelian band, and we assume that it can be used
as a record of the seasonal isotopic variations in the Lake Chad region,
since both areas are under the same climatic conditions. This assumption will
be further discussed through sensitivity analysis.
Isotopic composition of precipitations
(black diamonds, data from GNIP, IAEA), and regional atmospheric vapour
measured by , triangles, and by , black
crosses. The grey lines represent the mean annual scenarios.
Mean monthly data of air temperature and relative humidity were obtained from
the Climatic Research Unit
0.5∘×0.5∘ data set. Air temperature reaches a maximum
in May (30 ∘C) and a minimum in January
(20 ∘C) and relative humidity reaches a maximum in August
(70 %) and a minimum in March (20 %). The amplitude of
variation and the low values of humidity are consistent with the relative
humidity measurements at N'Djamena . We assume that the lake
temperature is similar to air temperature because the lake is shallow, well
mixed by the wind and seasonal variations of air temperature in central Sahel
remain low.
Evaporation
Evaporation above Lake Chad was estimated by several methods. Measurements
from lysimeters provide the most reliable data while measurements by
evaporimeter or evaporation pans lead to different estimations
. Lysimeter measurements at Bol
(archipelagos) between 1965 and 1977 gave an average annual evaporation rate
of 2170 mmyr-1, a maximum during the dry season in April
(229 mmmonth-1), another one in October
(199 mmmonth-1), and a minimum in January
(136 mmmonth-1). The potential evapotranspiration rates obtained
from reanalysis by the Climatic Research Unit (CRU) from 1900 to 2012 lead to a total annual
evaporation of 2022 mmyr-1 in the southern pool,
1952 mmyr-1 in the archipelagos, and 2365 mmyr-1 in
the northern pool.
The different estimates remain within a 10 % error range,
consistently with other studies , and the
seasonality, the interannual variability, and the spatial variability were
found to be similar whatever the method used. The seasonality shown by the
lysimeter measurements is significant and reveals the same pattern as the CRU
data. The interannual variability of the total evaporation is less than
10 % and the standard deviation on the CRU
evaporation over the whole period is less than 2 % for all pools
(with a maximum in 2009 of 2100 mmyr-1 in the southern pool and
a minimum in 1961 of 1941 mmyr-1). Moreover, no significant
change during the climatic transition period was observed. Finally, both
methods show that evaporation rates are almost similar in the southern pool
and in the archipelagos and 10 % higher in the northern pool.
Discussion
Evapotranspiration rates
The calibrated value of ET in the southern pool of 2070 mmyr-1
is slightly lower than local evaporation measurements
(2170 mmyr-1) but within the 10 % uncertainty range.
The calibrated value of ET in the northern pool of 2270 mmyr-1
is 1.1× ET in the southern pool. This 10 % increase between
south and north of Lake Chad is consistent with evaporation measurements and
CRU data.
Why is ET much lower in the archipelagos than in the southern pool?
Low ET values are required in the archipelagos to reproduce the low isotopic
enrichments and sodium concentrations. The value obtained (1530±150) is
significantly below the ET value found in the southern pool (2070±200)
while similar climatic conditions are observed. Several reasons for this
apparent ET underestimation can be considered. A first cause could be the
uncertainty on the surface–volume curve, in the case of an overestimation of
the evaporative surfaces. Indeed, in the archipelagos, it is difficult to
determine the free water surface because of its discontinuity and we assumed
that it represents one-fourth of the southern pool surface as proposed by
. However, an error on lake surface would also affect ETI
and not only ET. Secondly, the geochemical concentration of water entering
the archipelagos from the southern pool could be incorrectly estimated. In
the model, we assumed that it corresponds to the average concentration of the
southern pool, but the water entering the archipelagos could be less
concentrated and isotopically enriched than the mean value, since the
connection between the two pools is not far from the mouth of the
Chari–Logone river. Using the values of the Chari–Logone as inputs leads to
a better estimate of the isotopic composition with a value in the normal
state of 6 ‰ but sodium concentrations are still largely
overestimated, the mean calculated concentration in the normal state being
1.3 mmolL-1 against the 0.5 mmolL-1 measured. This
assumption is thus not sufficient to explain a potential underestimation of
ET by the geochemical model. A third explanation could be related to the role
of the vegetation and its potential influence on the chemical regulation
during the transition period, as already suggested by .
We made the assumption of a constant FT ratio and no salt exportation
associated to transpiration. However, this relies on a constant vegetation
cover with a steady-state turnover, corresponding to the natural vegetation
cycle with no human exportation.
We made the assumption that evaporation and transpiration do not export
salts. This assumption is robust for E but for T it relies on a steady
state of the aquatic vegetation with a turnover corresponding to the
vegetation cycle and no human exportation. Such a steady state is not valid
since the lake surface has shrunk and the vegetation cover of the
archipelagos has considerably increased, potentially storing Na. Considering
that the land–water contact surface is much more extensive in the
archipelagos than in the other pools, transpiration on islands could also
attract water under the islands, trapping salts similarly to the Okavango
Delta . Thus, if we assume that there is no climatic
reason for a different ET in the archipelagos as compared to the southern
pool, the missing evapotranspiration is 550 mmyr-1 and the
infiltration would be only 300 mmyr-1. This missing flux can be
attributed to transpiration as it has no influence on δ18O (not
fractioning).
It is the first time that transpiration is accounted for in the Lake Chad
budget. Our isotopic and chemical budgets estimate transpiration to be around
300 mmyr-1 in the southern and northern pools and probably up to
550 mmyr-1 in the archipelagos. This represents around
15 % of the total water evaporation in the southern and northern
pool and almost 40 % in the archipelagos. This result is based on
sparse isotopic measurements but is supported by the sensitivity analysis.
Chemical regulation of the lake
Our modelling of salt stocks shows that salt outputs exceed salt inputs during
shrinking phases (Fig. ), due to a combination of two
factors: when the volume decreases (i) the waters are more concentrated and
(ii) the ratio of infiltration over the lake volume is greater. Therefore,
the drying episodes of the lake correspond to efficient periods of salt
evacuation and they play a major role in the preservation of the freshness of
Lake Chad both at seasonal and decadal timescales. The northern pool plays
an important role in the chemical regulation of the lake since 95 %
of a conservative element such as Na is concentrated in the northern pool
characterized by a higher volume and greater evaporation rates. Sodium
precipitation simulated in the archipelagos during the 1980s accounts for
less than 1 % of the total evacuated sodium
(Fig. ). In our modelling, precipitates of natron
definitively take Na out of the lake. If this is consistent with previous
assertions of a rapid re-dissolution of salts by rainfall and infiltration
into the Quaternary aquifer or of exportation by wind since the Harmattan is
very strong in this region, these salts may also be dissolved during
subsequent water level rising periods and thus return to the lake. In view of
the low stock of precipitate salts in our simulations, this would induce weak
uncertainties on the sodium balance.
Infiltration rates and impact on the recharge of the regional aquifer
In this study, the estimated total amount of water flowing out of the lake
(infiltration) in its normal state is 6×109 m3yr-1,
twice the 2.6×109 m3yr-1 value estimated by
. It is higher than previous estimates based on a steady-state chemical balance yielding between 2
and 4×109 m3yr-1 but lower than measured values of
infiltration by of 10×109 m3yr-1.
Our estimate, under the assumption of mean composition waters flowing out of
the whole lakebed, is a maximum since the waters mostly move to the aquifer
at the lake shorelines with higher concentrations. Net recharge from the lake
was estimated from hydrogeological modelling of the phreatic aquifer at 3 to
10×107 m3yr-1 by and 28×107 m3yr-1 by . The difference by almost
2 orders of magnitude between infiltration outflow from the lake and
effective recharge of the phreatic aquifer can be explained by substantial
evapotranspiration of infiltrated waters in the aquifer close to the
shoreline. showed that in arid Sahelian environments,
evaporation may reach down to 100 m depth into the ground and
increases dramatically for a shallow water table (up to
200 mmyr-1 at 0.5 m depth). This calculation is
a minimum value of the groundwater uptake, since it does not take into
account plant transpiration. To explain the difference between aquifer
recharge and infiltration from the lake, 98 % of the infiltration
outflow from the lake must be evaporated, i.e. 5.9×109 m3yr-1.
All geochemical tracers indicate a restricted zone of lacustrine water
influence in the phreatic aquifer. Stable isotopes are relevant tracers as
the lake water is much more enriched (>5 ‰) than rainfall
(weighted-average of -3 ‰). A regional cross section of the
phreatic groundwater isotopic composition in the southern part of the lake
points to an area no larger than 50 km from the lake of waters
showing a lake-like isotopic signature . This result was
also evidenced in the northern part of the lake . Chemical
facies of the phreatic groundwaters also indicate lake-like waters up to
∼20 km from the shoreline . We can thus
reasonably consider that the evaporation of the infiltrated lake waters
occurs in a band of between 20 and 50 km away from the lake.
Therefore, the ratio of evaporative loss (5.9×109 m3yr-1) to potential evaporation surface leads to mean
evaporation rates of between 362 and 134 mmyr-1. This estimate
is in the upper range of the values estimated by but is
plausible because of (i) dense vegetation on the lake shorelines, and
(ii) deep tree roots in the Sahelian band .
In conclusion, although crucial for the chemical balance of the lake as it is
the only outflow of salts, the amount of lake waters effectively recharging
the aquifer is small because of the shallowness of the water table in the
first few kilometres away from the lake shorelines, which leads to
substantial evapotranspiration. However, it can be assumed that during the
African humid period, when climatic conditions were less evaporative and the
lake Mega-Chad covered a surface area of 340 000 km2, the lake
recharge to the Quaternary aquifer may have been larger.
Conclusions
This study represents a first effort at coupling hydrological, chemical, and
isotopic budget models of Lake Chad. It allows a complete quantification of
the total water losses, i.e. infiltration, evaporation, and transpiration in
each pool and their associated uncertainties. Despite a sparse set of
climatic, chemical, and isotopic data especially over recent time periods, we
show that it is possible to constrain hydrological and chemical flows,
leading to a better understanding of the hydrochemical response the lake to
climate and environment forcings. Evapotranspiration is found to be 2070 and
2270 mmyr-1 following a south–north gradient. Infiltration,
assumed to be a constant and homogeneous rate over each pool bed, is
estimated between 100 and 300 mmyr-1, which represents around
10 % of the total outflows. A low isotopic and chemical signature in
the archipelagos supports the influence of transpiration by plants accounting
for up to 40 % of total outflows while transpiration represents
about 15 % in the other two pools.
We have confirmed that the surprisingly fresh waters of Lake Chad are
explained by small infiltration flows but we also show that efficient
evacuation of salts occurs during shrinking phases. The southern pool always
remains fresh and the progressive evaporation in the other pools allows for the
infiltration of more concentrated waters, which export salts towards the
aquifer.
In the present-day context of huge climatic variability and demographic
changes in this semi-arid area, there is a need to explore hydrological
responses in vulnerable hydrosystems such as Lake Chad. Our coupled approach
is one of the methods that can help understanding the hydrological behaviour
of a lake at different timescales and in poorly instrumented areas. However,
a coupled lake hydrogeological modelling would be necessary to fully tackle
the groundwater–surface water dynamic issue.