Nutrient Retention: Question about water purification threshold table

I am having difficult understanding exactly what is being asked for in the threshold table. Many of the water quality standards that I have researched are reported in mg/liter, however the module is asking for kg/yr. Furthermore, because the module asks for thresholds per watershed, I am led to think that that number represents the total number of allowable kilograms in a watershed in one year--which is a figure I'm having difficulty finding at all. The sample data that comes with the module lists "2" and "1" for threshold N and P values, which seem low if that represents the total number of kilograms of N, for example, that are allowed in the watershed in one year's time. Any guidance would be much appreciated!


  • PerrinePerrine Moderator, NatCap Staff
    Hi Matthew, 

    You are right, standards are often reported as a concentration, especially when they relate to health issues. For ecological reasons, it is possible to find requirements expressed as a load (e.g. maximum nutrient load reaching a bay or a lake, to mitigate eutrophication).

    The conversion is rather straightforward when you have the total flow, although it does involve a number of assumptions about the flow regime (e.g. concentrations may fluctuate above or below the standard, but the total load may be below the annual acceptable load). As a rule of thumb, multiplying the concentration (in kg/m3 by the total flow in m3/yr will give you the threshold load in kg).

    This answers your second question: the calculation is done for each watershed indeed.

    I am not sure about the source of the sample data, but they do seem very low (obviously these numbers depend on the watershed area). As a guide, you could have a look to the work by Terrado et al. (see ref below) where they provide these numbers the Llobregat watershed in Spain.



    Terrado, M., Acuña, V., Ennaanay, D., Tallis, H., & Sabater, S. (2014). Impact of climate extremes on hydrological ecosystem services in a heavily humanized Mediterranean basin. Ecological Indicators, 37, 199–209. doi:10.1016/j.ecolind.2013.01.016
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