Pour une évaluation plus géographique de l'impact environnemental des plans d'eau, le cas de la température de l'eau du Grand Étang de Cieux (Haute-Vienne)

Abstract

The goal of the paper is to assess the temperature effect of a large (40 ha) pond with a surface weir, damming a confluence zone of four streams in Limousin (French Massif Central). The research is based on 536,552 water temperature measurements, upstream, downstream and inside the pond. Compared to the most frequent calculation method, which consists in measuring the difference between the inlet and the outlet, two new methods are used. The first consists in weighting the temperature values of each of the four tributaries, in order to construct an overall inlet temperature. The second considers the distance existing between the inlets and the outlet. After reconstitution of the former hydrographic network, the method applies a temperature gradient from upstream to downstream mixing temperatures to each confluence. According to the method of weighting the inlets, the Great Pond Cieux warms the watercourse by 1.8°C for a full year and by more than 4°C from June to September. It also modifies the thermal regime of the watercourse, by reducing its diurnal temperature variation. According to the temperature gradient method, the significant warming during the summer months would be due one third to the natural evolution of the streams from upstream to downstream and two thirds to the pond. During some weather types, in particular periods of forced convection by stormy winds, the responsibility of the pond would even be much less.

Keywords

• pond
• Limousin
• water temperature
• standing water body
• thermal effect
• cumulative impact