Water budget of tropical volcanic lakes in center-north Cameroon: Reconciling the stable isotope and chloride mass balance

The Cameroon volcanic line (CVL) hosts numerous volcanic lakes whose internal processes and hydrological functioning remain poorly documented. A detailed understanding of these hydro-systems is however essential, both for the consideration of these lakes as sentinels of the regional hydro-climatic changes and for the calibration of palaeoenvironmental proxies. Here, we present a hydrological and geochemical investigation of five of these lakes (Mbalang, Tabere, Tizon, Gegouba and Baledjam) around Ngaoundere on the Adamawa Plateau, based on repeated sampling of water profiles and monthly monitoring of rain and lake water samples over two seasonal cycles. We show that each of these throughflow lakes bears a distinct geochemical and isotopic signature, despite quite similar morphometric characteristics and a common climatic regime, due to varying contribution of the watersheds to the water-mass balance and different partitioning between evaporation (E) and outflow (Inflow minus E). We use these differences as a benchmark for a sensitivity analysis of the classical budget equations of conservative tracers. The results demonstrate that to reconcile chloride and stable isotope data with the standard single-box steady state model would require unusual values of the physical parameters of Craig and Gordon's equation such as the n.theta term that would have to be significantly lower than its usual value (n.theta = 0.5). We also show that the data can be simulated more easily by including the inflow from the watershed while assuming that transpiration exceeds evaporation for this compartment. Using this conceptualization of the throughflow lakes, we were able to constrain the different fluxes. Transpiration from the watershed and evaporation from the lake are on the same order of magnitude, or slightly in favour of transpiration. While providing evaporation and transpiration rates which are in general in the right order of magnitude, the system of equations remains underdetermined at this stage. Only direct measurements of the isotopic composition of the atmosphere, possible by laser mass spectrometry, would allow to reduce the range of under-determination of this problem.

Automated summary

This study investigates the hydrological and geochemical characteristics of volcanic lakes in the Cameroon volcanic line. The results show that each lake has a distinct geochemical and isotopic signature, which is influenced by the contribution of watersheds and the partitioning between evaporation and outflow. Sensitivity analysis of classical budget equations suggests that the standard model needs to be modified to explain the data. A conceptual model is proposed to better constrain the different fluxes in these lakes.