The prevision of heat budget and temperature variations on ponds: a key to manage a pond in a river system
Résumé
Temperature is a key factor to understand the water body dynamic. It has direct and indirect effects on the physic and chemistry of the water and especially on oxygen (anoxia in hypolimnion). In polymictic water bodies, as ponds, the thermal stratification can stay a few days or even a few weeks during summer, reliant on weather conditions. The knowledge of the strength and the duration of the stratification and the spatial variation of temperature is important in this system. The European Water Framework Directive requirement on stream and water bodies is on a difficult application at the local scale. Temperature and oxygen data loggers are implemented within the pond, in several chains, and every 25cm in depth. A meteorological station is place close to the pond in order to have high-precision data. Stability results, 3D maps and heat budget are the main purpose of this work to achieve a complete thermal analysis. With an environmental policy focused on ecosystem and rivers in Western Europe, the ponds localized on streams system are considered as a negative impact. The outflow temperature and oxygen from pond is on a particular importance and is seen as pollution for the river. To maintain the river in “good status”, the first proposition is usually to remove the pond, while a lot of minor adjustment can be realizes in order to diminish the pond’s outflow temperature impact. One of the propositions could be to adjust the outflow vertical position depending on homothermy or stratification, the outflow localization according to the prevailing wind or in a shadow area and so to preserve pond’s biodiversity and utility. According to the study of three ponds of various morphologies, we can start estimating the dynamic of a water body according to meteorological conditions and propose a new tool for owners to better manage their ponds.