NDR 3.7.0 Export vs. Retention

Hi there,

If I wanted to calculate total retention over my watershed, would I take the total load minus total export? In the new version (3.7.0) I do not get a total retention output like in the sediment model.


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I’m also very interested in the same question. After reading through the equations in the user manual, there doesn’t look to be an easy way to quantify nutrient retention on a cell-by-cell basis, is that correct?

You could of course subtract N or P exports from loads. But that would give you the amount of N or P from a given cell that gets absorbed somewhere between that grid cell and the stream, not the amount absorbed by each grid cell. Is there any way to quantify the latter using the NDR model? Or is the assumption that there’s too much uncertainty in the underlying biophysical processes to accurately quantify this without using more complex models?

Many thanks!

Hi @audiae -

The retention output in the sediment model does not actually calculate the total amount of sediment retained, it just calculates the difference in erosion between the current land cover map, and a hypothetical one where all land cover types were turned to bare ground. This does not include any deposition from upslope, only values the erosion difference due to current vegetation. I believe that the software team is working on an actual sediment retention function, that will include retention from upslope pixels, but that’s not ready yet, and I’m not sure if it is also being planned for NDR or not. @rich, can you comment?

To value the service, we usually take the difference in nutrient export between the current landscape and a scenario. I don’t see an obvious reason why you couldn’t take load minus export as a proxy for retention, although I’d be careful presenting absolute values for this, as Ken notes.

~ Stacie

Thanks Stacie. Just to be 100% clear, is it correct then that the NDR model cannot calculate total retention by a given grid cell?

If that’s the case, then potentially useful nutrient retention calculations could be the amount of a given grid cell’s nutrient load that is retained somewhere downstream (i.e., grid cell-based load - export), or the same summed e.g., at watershed scale, correct?


Hi @swolny, yes, we’ll have an SDR model on the next release that calculates per-pixel sediment deposition. I don’t have anything planned for NDR, but nobody’s asked me either. :slight_smile:

And yes, @kjbagstad, if I understand your question correctly, you can always calculate per-watershed nutrient retention with the “sum of total load - sum of export” calculation.

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Thanks @rich . FYI, per-pixel nutrient retention would indeed be useful for ecosystem accounting, if the opportunity arises to tweak the NDR model to enable that calculation in the future.