how to determine the hydraulic retention time (HRT) of a constructed wetland?
- 1 decade agoFavorite Answer
HRT= volume / net influx or volume /net outflux. In a state of equilibrium, influx and outflux should be roughly the same. Otherwise the wetland would quickly become a lake, or it would cease to be wet.
The influx is the sum of the inflow of water through rivers or streams, influx of groundwater (if there is any), the amount of precipitation, minus evaporation.
The wetlands volume (for water, that is), is the area-size of the wetland, times the depth of the water-permeable depth (possibly the depth until the first layer of clay or rock) times the permeability of the soil. You might want to correct for the presence of water bodies, which have a permeability of 1. But then again, these water bodies are going to have much shorter retention times, so my guess is to substract them altogether.
If you are dealing with a coastal wetland and tides, this whole thing doesn't go of course. Also, if there's a lot of seasonal variability, the math is going to get more complex. Any textbook on hydrology should give you my information, and more, in a much more detailed and argumented fashion.
- 1 decade ago
Brandy Hunt's thesis from 1998 desribes the method in detail, including the specific choice of dye. The thesis is freely available from the link below.