 |
| Titel |
Hydrological budget of Lake Chad: assessment of lake-groundwater interaction by coupling Bayesian approach and chemical budget |
| VerfasserIn |
Camille Bouchez, Julio Gonçalvès, Pierre Deschamps, Jean-Luc Seidel, Jean-Claude Doumnang, Florence Sylvestre |
| Konferenz |
EGU General Assembly 2014
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250097469
|
| Publikation (Nr.) |
 EGU/EGU2014-13057.pdf |
|
|
|
|
|
| Zusammenfassung |
| Estimation of lake-groundwater interactions is a crucial step to constrain water balance of
lacustrine and aquifer systems. Located in the Sahel, the Lake Chad is at the center of
an endorheic basin of 2,5.106 km2. One of the most remarkable features of this
terminal lake is that, despite the semi-arid context and high evaporation rates of
the area, its waters are fresh. It is proposed in the literature that the solutes are
evacuated in the underlying quaternary aquifer bearing witness to the importance of
surface water and groundwater exchanges for the chemical regulation of the lake. The
water balance of this system is still not fully understood. The respective roles of
evaporation versus infiltration into the quaternary aquifer are particularly under
constrained.
To assess lake-groundwater flows, we used the previous conceptual hydrological model of
the lake Chad proposed by Bader et al. (Hydrological Sciences Journal, 2011). This model
involves six parameters including infiltration rate. A probabilistic inversion of parameters,
based on an exploration of the parameters space through a Metropolis algorithm (a Monte
Carlo Markov Chain method), allows the construction of an a posteriori Probability Density
Function of each parameter yielding to the best fits between observed lake levels and
simulated. Then, a chemical budget of a conservative element, such as chloride, is introduced
in the water balance model using the optimal parameters resulting from the Bayesian inverse
approach.
The model simulates lake level and chloride concentration variations of lake
Chad from 1956 up to 2008. Simulated lake levels are in overall agreement with the
observations, with a Nash-Sutcliffe efficiency coefficient above 0.94 for all sets of
parameters retained. The infiltration value, obtained by such probabilistic inversion
approach, accounts for 120±20mm/yr, representing 5% of the total outputs of the lake.
However, simulated chloride concentrations are overestimated in comparison to the
scarce measurements available over that period. As an example, the mean chloride
concentration measured in the southern pool on a basis of our synthesis of existing chemical
data since the 1970’s is approximately three time lower than the computed mean
concentration. This may be due to either the non-representativeness of our chemical
dataset or overestimation of the evaporation rate that is fixed to 2000mm/yr in our
model.
This study tackles the quantification of the lake water flows to the quaternary aquifer
system and the associated uncertainties from a probabilistic point of view. This is an essential
step to improve predictions of groundwater resources in the Lake Chad Basin under climate
change. |
|
|
|
|
|
|