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

Abstract

Abstract. In 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 center 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 behavior in response to climate variability requires a better constraint of the factors that control its water and chemical balance. Based on the three-pool conceptualization of Lake Chad proposed by J. C. Bader, J. Lemoalle, and M. Leblanc (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 (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 kilometers 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 tenth of its size. According to our model, shrinking phases are efficient periods for salt evacuation from the lake towards the phreatic aquifer.

Insight into the stable isotopic composition of glacial lakes in a tropical alpine ecosystem: Chirripó, Costa Rica

“…The estimated slope of the LEL for Chirripó is consistent with the isotopic enrichment observed in δ 2 H versus δ 18 O space for evaporating water under 75% humidity, for which a slope of 5.2 has been reported (Clark & Fritz, ; Gonfiantini, ). The slope of the LEL in Chirripó is also in agreement with values reported for nonseasonal systems (i.e., with evaporation occurring consistently throughout the year with atmospheric moisture in equilibrium with precipitation), for which slope values are typically in the range 4 to 5 (Bouchez et al, ; Gibson et al, ). As the isotopic composition of atmospheric water vapour plots practically along the LMWL of Chirripó, it is likely that evaporation conditions at Chirripó are characterized by the input from precipitation that is in isotopic equilibrium with local water vapour (Gibson & Reid, ; Skrzypek et al, 2015).…”

Lake Chad Total Surface Water Area as Derived from Land Surface Temperature and Radar Remote Sensing Data

“…According to [3], Lake Chad reached 14,000 square kilometers in April 2013. This is within close to ten percent of our figure of 15,600 square kilometers for the same month.…”

“…In 2014, over two million people lived along the shoreline of Lake Chad [2], and by 2015, the lake provided a livelihood for an estimated thirteen million people [3]. The Lake Chad Basin (Figure 1), at approximately 2.5 million km 2 , is the world's largest endorheic basin, though much of the basin lies in the Sahara Desert and only about one-third of the basin (the southern portion) is hydrologically active [3]. Roughly ninety percent of the water reaching Lake Chad is provided by the Chari-Logone River [4].…”

Monitoring Recent Fluctuations of the Southern Pool of Lake Chad Using Multiple Remote Sensing Data: Implications for Water Balance Analysis

“…The results of LCBC [30] show that 96% of the water losses are consumed by evaporation. The research of Bouchez et al [47] indicates that the outflow of the whole Lake Chad through infiltration represents around 10% of the total water losses. However, in our research, evaporation only accounts for 16%, with the other 84% offset by outflow.…”

“…The research of Bouchez et al [47] shows that the lake level of Lake Chad has been monitored non-continuously from 1956 to 2008. However, the fact is that no such observations are available for us during the period under consideration .…”