18,420 kg/DM/ha, whereas the actual yield in 2015 was 11,480 kg DM/ha. In other words, the production of duckweed according to the model was 60% greater than reality. The reasons for this are the dip in production in the summer, the non-homogenous distribution of the duckweed over the pond, and the way in which the varying levels of solar radiation were included in the calculations. For a more detailed explanation, see Chapter 2. In other words, the model indicates that better management could increase production. Moreover, the growth of the duckweed in 2015 was reduced by limitations in temperature and photoperiod (see Figure below). 121 The results from the model and the actual measurements show that in January, February, March, October, November, and December the growth of the duckweed stagnated. This was caused primarily by the lack of light. The graphs also show that in November and December the water temperature was still relatively high. However, this did not result in growth. This is because in the late autumn, the short photoperiod per day (with low light intensity) limits the growth of the duckweed. Lessons After a year of intensive measuring and managing the cultivation of the duckweed, the process is much better understood. The most pressing modification to the current system of cultivation is to improve the lighting of the duckweed. The photoperiod, and therefore the amount of light available for the duckweed, is strongly limiting. The light transmission factor of the polycarbonate roof is 76%, compared to 90% in the case of the most modern greenhouse roofs. In addition, the ridge of the roof of the current construction casts a shadow over a significant part of the surface of the duckweed. The light yield could be increased from 70% to 90% if a different roof construction and material were to be used. Another option is to add a light source. The model shows that in ideal lighting conditions (and provided that other growth factors are not limiting), the production of duckweed could rise to as much as 50,000 kg dry matter per ha. Whether adding artificial lighting could be justified on economic grounds depends on the value of the duckweed. Also, the process of refreshing the water and that of fertilisation could be improved. There were areas in the pond where there was hardly any flow and where very little duckweed grew. Rounding off the corners could eliminate this problem. The use of a slowly rotating paddle wheel across the entire width would give a more homogenous flow, which would make the density of the duckweed more uniform. Figure: Daily growth of duckweed (picture on the left) and the value of the specific growth factors light and temperature (picture on the right) in 2015. A growth factor is limiting when its value is lower than 1. Pagina 128
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