Heat and mass fluxes monitoring of El Chichón crater lake

El Chichon crater lake is characterized by important variations in volume (40, 000 m 3 to 230, 000 m(3)) and in chemical composition alternating between acid-sulfate and acid-chloride-sulfate composition (Cl-/SO42- = 0-79 molar ratio). These variations in volume can occur very fast within less than a few weeks, and are not always directly correlated with the precipitation rate; the seepage rate of lake water is also an important parameter to consider in the lake mass balance. In this study, we present for the first time continuous physical data (temperature, depth, precipitation, wind velocity, solar radiation) of the crater lake registered by a meteorological station and two dataloggers. A heat and mass balance approach is proposed to estimate the heat and mass fluxes injected into the lake by the sublacustrine fumaroles and springs. Tracing the evolution of such fluxes can be helpful to understand this highly dynamic lake and offers an efficient way of monitoring the volcanic activity. During the observation period, the hydrothermal heat flux was estimated to be 17-22 MW, and the mass flux 10-12 kg/s (error on both values of +/-15%). These fluxes are mainly counterbalanced by the loss of heat and mass by evaporation, respectively of 20-24 MW and 8-10 kg/s. Furthermore, the seepage rate of the lake waters was estimated and shown to be a highly variable parameter (12-42 kg/s), depending on the lake surface. This new data set constitutes a baseline to monitor the future activity of El Chichon volcano. In case of volcanic activity renewal, one of the first precursor signals would probably be the full evaporation of the lake.