Whether or not the "effective_retention_x.tif "in the intermediate_outputs folder is actually the Nutrient Retention Services result we want

As the title states,Whether or not the "effective_retention_x.tif "in the intermediate_outputs folder is the Nutrient Retention Services (or Water purification service) result we want?
What puzzles me is that there is mention in the User’s Guide that "To calculate per pixel nitrogen retention services within a single scenario, we recommend subtracting n_total_export.tif from the modified_load_n.tif result located in the intermediate output folder. ".But actually, I don’t think the result of this subtraction is representative of nutrient retention services. For example, if the nutrient load of grassland or wetland is small, the total_export is very small, and the result after subtraction is also small, but such result may not represent the water purification service of grassland or wetland.
Therefore, if the result of this subtraction is divided by the nutrient load, is it more representative? Or there are other better calculation methods.
Also, as the title says, has the model already calculated the results of the nutrient retention service, that is "effective_retention_x.tif "in the intermediate_outputs folder?
Looking forward to getting your answers.

Welcome to the forum @Allen!

As noted in the User Guide Interpreting Results section, effective_retention.tif is calculated using equation 48. This equation takes into account the efficiency for each LULC type given in the biophysical table, adjusting it based on whether there is more of the same LULC type downslope, and the critical length parameter for that LULC type, also in the biophysical table.

But it does not take into account how much nutrient is actually moving downslope to be retained by each pixel. What we’d probably want to quantify as the nutrient retention “service” is the actual amount of nutrient retained by the pixel, based on what runs off from upslope plus the efficiency of the LULC type. But the model does not calculate this. (I’m not sure why, it’s definitely something that should be improved, especially since the SDR model is similar and has explicit service outputs).

As for the calculation of retention services, you might be interested in this forum thread that contains a lot of detail about the NDR model, and toward the end they talk about retention, where the model designer really just confirms the recommendation in the User Guide.

I’d be interested to hear from people who have used this model more than I have!

~ Stacie

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