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In 2006 five constructed wetlands with Reed were created in the Netherlands. One is an open system and the others are hydrological closed systems. Quantifying the effectiveness E of wetlands in reducing nutrient loads requires assessment of the nutrient balance of the wetland system, E = 100 * (Nin - Nout) / Nin %, where Nin and Nout are incoming and outgoing nutrient loads, respectively. In general most of the nutrients flow in and out of these systems via the surface water, but sometimes seepage occurs. To check this, a water balance has to be calculated, requiring a good estimate for evapotranspiration. Constructed wetlands tend to be designed in such a way that evapotranspiration is small compared to in and outgoing water flow, but in more natural systems it may become a relevant part of the water balance.
For the quantification of evapotranspiration two clear benchmarks are set: open water or pan evaporation and a cut grass sward, well provided with water. Wetlands clearly are a much more complicated system, partly open water, but with a higher surface roughness/wind profile, and partly vegetation that is actively transpiring during growing season, but not during fall and winter. Varying maintenance of the wetland is an extra complicating factor. Reed for instance may be cut in September for maximum phosphorus retention, but may also be left until spring for maximum nitrogen retention.
We suggest a combined literature, field experiment and modeling study focused on the evapotranspiration of these constructed wetlands. Depending on the study period, some measurements may be possible in the constructed wetland systems that are now under study in the Netherlands (Provincie Limburg). Measuring data of the past few years are available from these systems.
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