Evolution of Superficial Lake Water Temperature Profile Under Diurnal Radiative Forcing

In lentic water bodies, such as lakes, the water temperature near the surface typicallyincreases during the day, and decreases during the night as a consequence of the diurnalradiative forcing (solar and infrared radiation). These temperature variations penetratevertically into the water, transported mainly by heat conduction enhanced by eddy diffusion,which may vary due to atmospheric conditions, surface wave breaking, and internaldynamics of the water body. These two processes can be described in terms of an effectivethermal diffusivity, which can be experimentally estimated. However, the transparency of thewater (depending on turbidity) also allows solar radiation to penetrate below the surface intothe water body, where it is locally absorbed (either by the water or by the deployed sensors).This process makes the estimation of effective thermal diffusivity from experimental watertemperature profiles more difficult. In this study, we analyze water temperature profiles in alake with the aim of showing that assessment of the role played by radiative forcing isnecessary to estimate the effective thermal diffusivity. To this end we investigate diurnalwater temperature fluctuations with depth. We try to quantify the effect of locally absorbedradiation and assess the impact of atmospheric conditions (wind speed, net radiation) on theestimation of the thermal diffusivity. The whole analysis is based on the results of fiber opticdistributed temperature sensing, which allows unprecedented high spatial resolutionmeasurements ( 4 mm) of the temperature profile in the water and near the water surface.