Evapotranspiration (ET) in constructed wetlands (CWs) represents a major factor affecting hydrodynamics and treatment performances. The presence of high ET was shown to improve global treatment performances, however ET is affected by a wide range of parameters including plant development and CWs age. Our study aimed at modelling the effect of plants and peat on ET in CWs; since we hypothesized peat could behave like the presence of accumulated organic matter in old CWs. Treatment performances, hydraulic behaviour, and ET rates were measured in eight 1？ m 2 CWs mesocosm (1 unplanted, 1 unplanted with peat, 2 planted with Phragmites australis, 2 planted with Typha latifolia and 2 planted with Phragmites australis with peat). Two models were built using first order kinetics to simulate COD and TKN removal with ET as an input. The effect of peat was positive on ET and was related to the better growth conditions it offered to macrophytes. Removal efficiency in pilot units with larger ET was higher for TKN. On average, results show for COD a k 2 0 value of 0.88 d ？ 1 and 0.36 d ？ 1 for TKN. We hypothesized that the main effect of ET was to concentrate effluent, thus enhancing degradation rates. 1. Introduction In constructed wetlands (CWs) for wastewater treatment, evapotranspiration (ET) may represent a major factor increasing the hydraulic residence time (HRT) during summer months in temperate countries. In horizontal subsurface flow constructed wetlands (HSSFCWs), ET ranges between 0 and 50 mm/d [1] and can reach up to 200？mm/d in favorable periods [2]. High ET may improve global treatment performances [3] and modify water flow [4]. ET follows a diurnal cycle and is affected by a wide range of parameters such as plant development, CW design (surface, subsurface or vertical flow), and CW age [5]. Simulations were used to predict the performance of CWs using a direct plug flow reactor model [1], a plug flow with axial dispersion model (PFD) [4, 6–8], a stirred tank in series model [9] or a combination of these models [10]. One of the most limiting factors in predicting CWs efficiency was identified as the hydraulic behavior governed by environmental conditions [11, 12]. Furthermore, in modeling or design equations, ET can be taken into account as an input to the model [8]. The aim of our work was first to determine ( 1 ) the effect of peat and of plant species on ET, since we hypothesized peat could behave like presence of accumulated organic matter in old CWs and ( 2 ) the importance of ET on treatment performances in an experimental constructed wetland
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