The water losses due to evapotranspiration (ET) should be considered for wetland designs in arid climates and can be a factor during the warm summer months in all locations. In the western United States where appropriative laws govern the use of water, it may be necessary to replace the volume of water lost to protect the rights of downstream water users. Evaporative water losses in the summer months decrease the water volume in the system; therefore, the concentration of pollutants remaining in the system tends to increase even though treatment is very effective on a mass removal basis. For design purposes, the evapotranspiration rate can be taken as being equal to 80% of the pan evaporation rate for the area. This in effect is equal to the lake evaporation rate. In the past, some controversy existed regarding the effect of plants on the evaporation rate. It is the current consensus that the shading effect of emergent or floating plants reduces direct evaporation from the water but the plants still transpire. The net effect is roughly the same rate whether plants are present or not. The first edition of this book indicated relatively high ET rates for some emergent plant species (Reed et al., 1988). These data were obtained from relatively small culture tanks and containers and are not representative of full-scale wetland systems.
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