On the thermal regime of water in a periglacial lake: the case of the Rutor marginal lake

The Rutor glacier is one of the most important glaciers in the north-west of Italy (Aosta Valley). Due to both climate change related phenomena and to natural cycles it is melting quite fast. Water coming from the melting of the main glacier tongue is converging through a seasonal stream to a lake, located in the periglacial area. The temperature and the water level of this seasonal lake has been measured during the summer period (mid-July to mid-September 2021) with a probe. The objective of the work is to create a model that can estimate the temperature of the lake based on the air temperature, incoming radiation, wind speed and relative humidity, measured in different meteorological stations located close to the lake area. A preliminary analysis on how air and water temperature series are correlated results in a delay of the response of water temperature to air temperature variations, with maximum shift occurring in the morning and afternoon. The model is based on an energy balance equation: the variation in time of the water temperature depends on the thermal flux between the lake water and the air temperature at the lake surface. In the total heat flux, shortwave radiation and longwave radiation heat fluxes are considered, as well as sensible and latent heat fluxes. The lakebed and the banks are assumed to be adiabatic. The obtained equation allows for the evaluation of the temperature variation of the lake water at the hourly time scale, due to the total thermal flux. The equation is solved by numerical integration, resulting in a time series of water temperatures of the lake, starting from a known initial condition. Results variability is investigated considering the contribution of single terms of the energy balance and weighting all the parameters with respect to the real series. The longwave and shortwave net heat fluxes are contributing more than the sensible and latent heat fluxes to the total heat flux. Some temperature fluctuations, wider in the estimated series than in the measured one, are anyway due to the less contributing terms and to neglected fluxes. Even though the resulting series is not perfectly reproducing the measured one, a realistic dynamic and a similar range of values is found. That means that the energy balance approach can be a good one to estimate the water temperature of a water body at this altitude, keeping in mind that further corrections are needed.