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Poplar and willow short-rotation coppice have been shown to be suitable to improve wastewater quality in vegetation filters. Nevertheless, water requirements of these species are still uncertain. The aim of this work was to estimate crop evapotranspiration (ETc) and crop coefficient (kc) of these species grown under high fertilised (HF) and low fertilised (LF) conditions through the water balance of lysimeters. This paper deals with the results of the first growing season after coppicing of the second rotation (2006) in comparison with those of the first season of the first rotation (2004). During the 2006 growing season (around 132 days), ETc was 319 mm (LF) and 719 mm (HF) in poplar and 607 mm (LF) and 919 mm (HF) in willow, with maximum daily values of 11.55 mm in HF poplar and 15.09 mm in HF willow. Ten-day ETc rates were in general higher than those found in the 2004 growing season. Maximum kc, reached in August and September, ranged between 1.2 and 2.9 in LF treatment and between 2.5 and 4.0 in HF ones, and were higher than maximum kc values reached during the 2004 growing season. In both growing seasons evapotranspiration and kc seemed to be related more to the availability of nutrients than to differences between the two species.

The trial allowed us to investigate ETc and kc of poplar and willow SRC under field conditions. The results can be useful to predict with more accuracy the volumes of wastewater to be provided to a vegetation filter grown under Mediterranean climatic conditions, reducing the risk of nutrients losses to the environment and the costs for distributing the wastewaters. ETc seemed to be dependent more on availability of nutrients than on the species. This, theoretically, enables both species to be used for such purposes and allows a greater choice among the clones, to spread these multifunctional bioenergy systems also in Mediterranean regions. Moreover, it could be interesting to better understand some further aspects, such as water use efficiency of the two species and the amount of nutrients retained by the system (i.e. in litter, soil organic matter and plant tissues).