Nutrient Delivery Ratio Model

I am running the Nutrient Delivery Ratio (NDR) Model.
I have a question regarding one paramter, the Threshold Flow Accumulation. It is not clear to me how to set up this parameter. Can you provide me some further information or literature?


  • swolnyswolny Member, NatCap Staff
    Hi Daniel -

    The Threshold Flow Accumulation (TFA) is used to create streams from the DEM. If you've used GIS hydrology tools, you'll be familiar with "flow accumulation", which is a function that calculates where streams form, based on the direction of flow over the DEM. Flow accumulation is essentially adding up how many upstream pixels flow into a given pixel; the more upstream pixels that flow into that pixel, the higher the flow accumulation value.

    TFA is used to say "X number of upstream pixels must flow into a given pixel before the model considers it part of a stream." Higher TFA values mean that higher numbers of pixels (thus a larger contributing area) must flow in, lower TFA values mean that a smaller contributing area must flow in before a stream is created. So, higher TFA values create coarser stream networks, which have fewer tributaries, and lower TFA values create more tributaries.

    The correct TFA value for your area is the one that causes the model to create a stream layer (output in intermediate/stream.tif) that most closely matches a known correct stream map. The known correct map can be from any source (not necessarily digital, but optimally a shapefile of streams that you can overlay), as long as you can look at it and compare the model-generated streams with what's correct. If the model is making too many tributaries, increase the TFA, if it isn't making enough, lower the TFA. It is important to tune this value to your area, as the model only routes nutrient until it hits the nearest stream.

    One way of doing this more quickly is to use the InVEST RouteDEM tool, which allows you to create a bunch of stream maps from the DEM, given a range of TFA values, that you can use for comparison.

    ~ Stacie
  • Hi Stacie,
    this was very useful answer. Thanks for this.

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