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Discussion papers | Copyright
https://doi.org/10.5194/hess-2018-86
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Apr 2018

Research article | 05 Apr 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Unraveling the hydrological budget of isolated and seasonally contrasted sub-tropical lakes

Chloé Poulin1, Bruno Hamelin1, Christine Vallet-Coulomb1, Guinbe Amngar3, Bichara Loukman3, Jean François Cretaux4, Jean-Claude Doumnang3, Abdallah Mahamat Nour1,3, Guillemette Menot1,2, Florence Sylvestre1, and Pierre Deschamps1 Chloé Poulin et al.
  • 1Aix-Marseille Université, CNRS, IRD, Collège de France, CEREGE, Europole de l Arbois 13545 Aix-en-Provence, France
  • 2Univ. Lyon, Ens de Lyon, Université Lyon 1, CNRS, UMR 5276 LGL-TPE, 69342 Lyon, France
  • 3Département de Géologie, Faculté des sciences Exactes et Appliquées, Université de NDjaména, NDjaména, Tchad
  • 4Legos, UMR5566, 14 Avenue Edouard Belin, 31400 Toulouse, France

Abstract. Complete understanding of the hydrological functioning of large scale intertropical watersheds like the Lake Chad basin, which become a high priority in the prospect of near future climate change and increasing demographic pressure, require integrated studies of all surface and groundwater reservoirs and their quite complex interconnections. In this respect, detailed hydrological studies of secondary peripheral lakes of these large basins may provide us with interesting small scale analogs of the major waterbodies, which can help disentangling the multiple influences of various forcing factors of the water cycle and its evolution.

We present here a simple method for estimating the annual mean water balance of sub-sahelian lakes subject to high seasonal contrast, and located in isolated regions with no road access during the rain season, precluding continuous monitoring of in-situ hydrological data.

The approach is illustrated by the study of the two lakes Iro and Fitri in the eastern basin of lake Chad, so far unstudied, and also tested on lake Ihotry (Madagascar), extensively studied previously by our group. We combine the isotopic data (δ18O; δ2H) that we measured during the dry season with altimetry data from the SARAL satellite mission, in order to model the seasonal variation of lake volume and isotopic composition. The annual water budget is then estimated from mass balance equations using the Craig and Gordon's model for evaporation. We show that the closed-system behavior (precipitation equal to evaporation) can be confirmed for lake Ihotry, whereas we calculate E/I ratios of 0.6±0.3 and 0.4±0.2 for Iro and Fitri, respectively, in both cases compatible with water fluxes estimated from nearby gauging stations. In the case of Fitri the estimated output flux is contributing to the groundwater recharge, since the lake has no identified surface outlet.

Finally, we use our data to discuss possible inferences about the hydro-climatic budget of the catchment basins of those two lakes. We show that the average rainfall isotopic composition monitored by IAEA at NDjamena is slightly offset from the two distinct Local Evaporation Lines that we obtain on the two lake-aquifer systems, and that this slight difference may reflect the impact of vegetation transpiration on the basin water budget. We conclude that, while being broadly consistent with transpiration being on the same order of magnitude as evaporation in those basins, we cannot derive a more precise estimate of the partition between these two fluxes, owing to the large uncertainties of the different end-members in the budget equations.

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Short summary
This study investigates the water budget of two inter-tropical lake systems in absence of long-term hydrological monitoring. By coupling dry season isotopic data with satellite imagery, we were able to provide quantitative constrains on the hydrological balance and show that these two lake systems can be considered as miniature analogs of the lake Chad, making them important targets in future set up of any large scale program of the hydro-climatic evolution in the Sahel region.
This study investigates the water budget of two inter-tropical lake systems in absence of...
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