Does a wetland lose as much, less or more water by evapotranspiration than a pond by evaporation? Experimental study in Limousin

Despite the rarity of serious studies based on solid scientific methodologies to estimate the quantity of water lost from French ponds by evaporation, the French authorities consider these water bodies as the main cause of water loss and therefore the summer low water levels of the hydrographic networks. Certainly, pond evaporation records its highest rates in summer, but previous studies had neglected the amount of water that can be lost through plant evapotranspiration that would replace the ponds if they were dried up. In this research the Authors adapt an approach based on possible over-evaporation that occurs in excess of the wetland evapotranspiration that existed or would exist in place of the ponds. Direct measurements of pond evaporation and the evapotranspiration of plants occupying the bottom of an old broken-up pond were taken during the warm semester of 2018 in Limousin (France). The results of this research show that the wetland has lost 1.37 times the amount of water lost from the nearby pond. Comparison between calculations from mathematical formulae and measurements taken in situ shows that the Penman-Monteith method does not consider the vegetative stage of plants and that it is more suitable for estimating evaporation from free water bodies. Aldomany’s formula gives close values with an average difference of 6.4 % of the measured evaporation. The mathematical methods used in this research can provide acceptable estimates of the actual evapotranspiration of of wet wetland if the crop coefficient calculated in this study (1.37) has been taken into consideration.