THERMOCLINES IN PONDS: A NEW TYPOLOGY BY THE STUDY OF WATER TEMPERATURE CONTINUOUS MEASUREMENTS

Laurent Touchart, Pascal Bartout, Université d’Orléans, France

Abstract

The management of ponds and the choice of equipments aimed to reduce the impact on the rivers depend on pond water evacuation systems. The depth at which the pond water is drawn off determines temperature, oxygen saturation and chemical composition of the emissary.  The study of stratification within ponds is necessary to understand the efficiency of monks and deep sluice gates.
The discontinuity layer in four ponds and two reservoirs of Limousin region (France) has been followed up by means of recording thermometers. According to Brönsted and Wesenberg-Lund, we regard thermocline as the point of maximum temperature gradient. But we modernize the definition by calculating the most negative value of the directional derivative of the temperature profile.
Depth and duration of summer thermoclines allow to describe three types. (i) The surface thermocline is several hours long. The temperature profile of this discontinuity layer is concave, so that the thermocline is located at a depth of 0 m. (ii) The thermocline located at a depth of about 1 m is several weeks long. It goes down during the night and the temperature gradient lowers at the same time. The profile of this discontinuity layer is convexo-concave and the thermocline forms the change of direction of it. The thermocline located at a depth of about 5 m remains stable during four or five months, without modifying temperature gradient according diurnal cycle. The temperature profile of this discontinuity layer is convex, so that the thermocline is located on the bottom of the pond.
Meteorological data and calculations on pond stability by Wedderburn Number are used to make assumptions about thermoclines development and running. (i) The surface thermocline takes the exact profile of the penetration of the solar radiation through pond water during the hottest hours of a calm and sunny day. It disappears by nocturnal convection. (ii) The intermediate thermocline weakens but resists during the night, when it becomes deeper because of the thickening of the mixed layer. It is destroyed only when atmospheric disturbances with cold front or thunderstorms cause strong winds and temperature drop. Most of ponds, which are 1.5 to 4 m deep, are entirely mixed and stirred during these homothermies by a combination of convection mixing and wind-induced turbulent stirring. (iii) The deepest ponds present a parent-thermocline. They encroach on lake metalimnion, as it is proved by our measurements within two neighbouring reservoirs.