Hello @atershy @dhooper and @Perrine -
NatCap science, software and analysts had a convening about NDR yesterday. The main thing that came out of it is the realization that we should be using export coefficients, not applied nutrients, for the n_load and p_load parameters.
This is because the model calculations currently do not take into account retention on the pixel itself, they only include retention downslope of each pixel. But in reality, if we apply nutrient to a pixel of cropland, the crop will use/retain some of the nutrient, and what remains will move downslope. Since this is not currently included in the model, we should be correcting for it when creating our n_load and p_load parameters. If you’ve read the previous posts, yes, that means that even much of NatCap’s own use of the model has used incorrect loading values, since we often used applied nutrient without correction.
So we’re doing two things to improve this situation:
- I have updated the User Guide to be correct and consistent with its guidance for how the model currently works, saying this:
Data sources may provide loading values as either the amount of applied nutrient (e.g. fertilizer, livestock waste, atmospheric deposition); or as “extensive” measures of contaminants, which are empirical values representing the contribution of a parcel to the nutrient budget (e.g. nutrient export running off urban areas, crops, etc.) In the case of having applied nutrient values, they should be corrected for the nutrient retention provided by the pixel itself, using the application rate and retention efficiency value (eff_n or eff_p) for that land cover type. For example, if the nitrogen application rate for an agricultural LULC class is 10 kg/ha/year, and the retention efficiency is 0.4, you should enter a value of 6.0 into the n_load column of the biophysical table. If you have “extensive”/nutrient export values, then you may use them directly in the biophysical table without correction.
- We will be updating the NDR model to allow users to enter either applied nutrient or export coefficient values for each LULC class in the biophysical table, with additional columns specifying whether each value represents applied nutrient or export coefficient. If the value is applied nutrient, the model will calculate (application rate x retention efficiency) as described above. If the value is export coefficient, then the model will use the value as is, without correction. The User Guide will also be updated.
I’m not sure how long the NDR update will take, but I will try to remember to post about it here when it is released.
Apologies for the confusion, and providing incorrect guidance. We’ve all learned from this one!
~ Stacie