Hi @gabi89 ,
Thanks for writing in with your question.
One approach we’ve used with the raster results is to subtract export from intermediate_outputs\modified_load , so:
-
N_retention = modified_load_n - n_total_export -
P_retention = modified_load_p - p_surface_export
I’m using the terms “retention”, “trapping”, and “avoided export” interchangeably here. But be aware that this simplified approach is a bit of a workaround, because SDR includes a deposition/trapping component, which is what is retained from upslope pixels, whereas NDR does not. The nutrient load results do not consider upslope pixels, only the user-provided parameter values from the biophysical table input. Here is another way my colleague, @adrianvogl , explained it:
“there is no way to calculate nitrogen retention on a pixel, only that pixel’s reduced export as a function of downstream pixels. The equation that Jesse suggested (modified load - export) works at a watershed scale, but at a pixel scale it only tells you how much N is retained between that pixel and the stream, not how is much is retained on that pixel.”
Another option is to run multiple scenarios and then compare results, like creating an all bare soil scenario to get at the biophysical “retention supply” provided by the existing land cover. This would be similar to what the authors of this study did (“Including Additional Pollutants into an Integrated Assessment Model for Estimating Nonmarket Benefits from Water Quality”, Griffin, R. et al. 2020, Land Economics, https ://doi.org/10.3368/wple.96.4.457 ).
You’re not the only one trying to solve this. Perhaps others could provide better insight @elonsdorf , @lons0011 , or @swolny .
-Jesse