Le centre thermique, nouvel outil de compréhension du bilan thermique et de l’évolution spatio-temporelle de la température d’un étang ?

Abstract : Many studies have focused on the thermal impact of waterbodies on downstream waterways. With respect to headwaters, interest has tended to focus on streams, neglecting the internal dynamics of waterbodies. For ponds, defined as discontinuous polymictic waterbodies with a maximum depth of less than 6 m (19.7 ft) and an area of less than 100 ha (250 acres), the mixing and stratification stages frequently alternate. Therefore, knowledge of a waterbody’s dynamics is key to understanding the upstream–downstream temperature trends within a drainage basin. This study is based on a limnosystemic approach in order to study the annual thermal dynamics of two ponds located on headwaters. The first, Étang de La Ramade (La Ramade Pond), in the French département of Puy-de-Dôme in Auvergne, is a pond of 62 ha (153 acres), with a mean depth of 1,1 m (3.6 ft) and a maximum depth of 3,5 m (11.5 ft) (figure 1). Its morphology lends itself to significant spatial variations in temperature. The second pond is the Étang de Rilhac (Rilhac Pond), located close to Limoges in the French Department of Haute-Vienne (Limousin). This is a 0.4‑ha (1‑acre) pond, with a maximum depth of 4,2 m (13.8 ft) and a mean depth of 2 m (6.6 ft). Rilhac Pond is representative of small headwater ponds, damming a small valley. In order to measure the temperature within the pond, the methodology implemented uses a number of chains hung on a flotation device, with data loggers to measure the temperature every hour and every 25 cm (10 in.) from the surface to the bottom, at six different locations. The heat-content approach, which multiplies the temperature recorded by the volume of each layer of a column of water, takes account of all factors – internal and external – that influence the waterbody temperature. The temperature can vary vertically by up to 13 °C (23 °F) between the surface and the bottom, with differences of up to 7 °C (13 °F) between one measuring location and another. Adopting a geographical approach to water temperature makes it possible to highlight certain new indicators such as the heat center. Studying changes in the heat center is useful in gaining an overall comprehension of the spatial and temporal temperature trends within the waterbody. Heat-center variations can be significant, and depend on a number of factors, such as the stratification and mixing of the water column and the temperature distribution within the waterbody. Latitudinal, longitudinal and depth variations in the heat center can be analyzed either separately – so as to obtain specific data for each – or all together, for a comprehensive overview. The variations in the dynamics of a pond’s different sub-basins in comparison with the central part of the pond can be expressed through the heat center. This new indicator may have a number of benefits in the field of pond management – allowing, for example, outflow locations to be modified or correctly selected, and surrounding areas to be easily be adapted in order to reduce a pond’s effect on a stream system.
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Quentin Choffel, Laurent Touchart, Pascal Bartout, Mohammad Aldomany. Le centre thermique, nouvel outil de compréhension du bilan thermique et de l’évolution spatio-temporelle de la température d’un étang ?. Norois, Presses universitaires de Rennes, 2018, Eaux, marais, lac, étang, pêche, pp.57-73. ⟨http://journals.openedition.org/norois/6350 ; DOI : 10.4000/norois.6350⟩. ⟨10.4000/norois.6350⟩. ⟨hal-02078489⟩

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