Hydrological, chemical, and isotopic budgets of Lake Chad: a quantitative assessment of evaporation, transpiration and infiltration fluxes

Abstract

International audienceIn the Sahelian belt, Lake Chad is a key water body for 13 million people, who live on its resources. It experiences, however, substantial and frequent surface changes. Located at the centre of one of the largest endorheic basins in the world, its waters remain surprisingly fresh. Its low salinity has been attributed to a low infiltration flow whose value remains poorly constrained. Understanding the lake's hydrological behaviour in response to climate variability requires a better constraint of the factors that control its water and chemical balance. Based on the three-pool concep-tualization of Lake Chad proposed by Bader et al. (2011), this study aims to quantify the total water outflow from the lake, the respective proportions of evaporation (E), transpiration (T), and infiltration (I), and the associated uncertainties. A Bayesian inversion method based on lake-level data was used, leading to total water loss estimates in each pool (E + T + I = ETI). Sodium and stable isotope mass balances were then used to separate total water losses into E, T, and I components. Despite the scarcity of representative data available on the lake, the combination of these two geochemical tracers is relevant to assess the relative contribution of these three outflows involved in the control of the hydrological budget. Mean evapotranspiration rates were estimated at 2070 +/- 100 and 2270 +/- 100 mm yr(-1) for the southern and northern pools, respectively. Infiltration represents between 100 and 300 mm yr(-1) but most of the water is evapotranspirated in the first few kilometres from the shorelines and does not efficiently recharge the Quaternary aquifer. Transpiration is shown to be significant, around 300 mm yr(-1) and reaches 500 mm yr(-1) in the vegetated zone of the archipelagos. Hydrological and chemical simulations reproduce the marked hydrological change between the normal lake state that occurred before 1972 and the small lake state after 1972 when the lake surface shrunk to a one-tenth of its size. According to our model, shrinking phases are efficient periods for salt evacuation from the lake towards the phreatic aquifer

Response to M. Coenders - Gerrits, Referee

“…The inflow (I ) is then between 17 and 31 m 3 s −1 (after subtraction of the minor contribution of direct rainfall on the lake surface), and the outflow (Q) is between 6 and 11 m 3 s −1 . These figures are thus coherent with the average flow of 30 m 3 s −1 measured on the Bahr Azoum at the Am Timan station between 1953 and 1975. For Lake Fitri, using the evaporation of about 2 m yr −1 , similar to that calculated on the southern pool of Lake Chad under the same climatic condition (Bouchez et al 2016), we obtain a flux on the order of 16.10 8 m 3 yr −1 and an inflow between 43 and 110 m 3 s −1 . The maximum flow rate recorded at the Ati station between 1956 and 1993 was 66 m 3 s −1 , corresponding to the maximum rainfall of 571 mm yr −1 over this watershed in 1962 (DREM).…”

“…average value, and the green polygon is the associated uncertainty. δa is the mean annual value (Jasechko et al, 2013) weighted by evaporation flux (DREM data) and δ I is the Chari-Logone rivers' mean annual value weighted by flow rate (Bouchez et al, 2016). The red point represents the δ L−closed and E/I values for δ I taken at the intersection between Iro Lake's LEL and the GMWL.…”

“…(11) for the three lakes studied. Other data from Lake Chad come from (Bouchez et al, 2016), and data from Lake Ihotry are from Vallet-Coulomb et al (2008). .…”

Unraveling the hydrological budget of isolated and seasonally contrasted subtropical lakes

“…Besides their importance for local sedentary and nomadic populations, the study of their hydrological functioning may provide pertinent small-scale analogs of Lake Chad itself and help test the respective influence of the different forcing parameters and processes, thus constituting potential sentinel systems for future evolutions. Indeed, understanding the hydrology of Lake Chad and the origin of its strong surface variability has been the focus of a large number of studies (Fontes et al, 1970;Carmouze, 1969;Olivry et al, 1996;Bouchez et al, 2016;Bader et al, 2011), but predicting its future behavior in response to climate change remains a challenge owing to the complexities of its hydrography and to the extremely diverse characteristics of the various compartments of its catchment (Lemoalle et al, 2012).…”

The Lake CHAd Deep DRILLing project (CHADRILL) – targeting ∼ 10 million years of environmental and climate change in Africa

“…Formation of accommodation space started during the Cenozoic (Burke, 1976) with nearly continuous deposition of terrestrial/lacustrine sediments since ∼ 10 Myr (Lebatard et al, 2008;Schuster et al, 2009). Today's Lake Chad is a terminal and highly variable shallow freshwater lake (∼ 3 m deep, measured in 2012) with a strong S-to-N conductivity gradient (50 to 700 µS cm −1 ; Bouchez et al, 2016). The annual invigoration of the northern African monsoon system and migration of the intertropical convergence zone (ITCZ) results in a short rainy season from June to October and a pronounced dry season for the rest of the year.…”