tl;dr - if I’m using a retention efficiency (
ret_n) of 0.4 for an LULC, and I know the N application rate is 10 kg/ha/yr (as per the example in the user guide, and if the value for
load_n that goes in the biophysical parameters table is the load of that LULC pixel on the one downslope as opposed to the loading on that pixel, why is the logic not to say “if my LULC is using 0.4, then it can export 1-0.4, that is 0.6, and so the value for the
load_n entry for that LULC should be 0.6”?
Firstly, I’d like to acknowledge this great thread on this topic. I’m glad I’m not the only one who’s gone round in a circle (or two) on this. I’m sure I must have read an old version of the guide initially because I was sure I was doing the right thing by just directly using applied N values for
load_n instead of transforming it using the retention efficiency
ret_n. Thanks to @swolny 's update, I see that the definitive right thing to do by the software is multiply your N application rate by the retention efficiency. I just can’t get it straight in my head why it is this way round.
I read and reread this section:
Loads are the sources of nutrients associated with each pixel of the landscape. Consistent with the export coefficient literature (California Regional Water Quality Control Board Central Coast Region, 2013; Reckhow et al., 1980), load values for each LULC class are derived from empirical measures of nutrient export (e.g. nutrient export running off urban areas, crops, etc.). Alternately, if information is available on the amount of nutrient applied (e.g. fertilizer, livestock waste, atmospheric deposition), it is possible to use it by estimating the on-pixel nutrient use, and applying this correction factor to obtain the load parameters.
Distilling this down and paraphrasing, I get: “loads - (
load_n) - are derived from empirical measures of nutrient export (from a pixel to downslope) and if actual application rate data is available, one can use it (actual application rate) by estimating on-pixel nutrient use (reflected in the retention efficiency -
ret_n) and applying this correction factor (the retention efficiency) to obtain the load parameters”. Here I’ve inserted my understanding of what is being referenced by pronouns etc for clarity. Someone shout if they think anything is incorrect.
I find this confusing for a number of reasons
- The use of “associated with” feels vague - is it the source of nutrients to a pixel or from a pixel?
- I interpret this to be saying
load_nis not equivalent to nutrient exported from a pixel to downslope
- I interpret this to be saying the two are linked via
ret_n, the retention efficiency
- It does not clarify at this point what that relationship is, although you may attempt to intuit it
- If, indeed, this is saying that (N) “loads” are quantified via the
load_ncolumn in the biophysical parameters table, it suggests to me that the correct value in that table should be the applied N, and if your input is actually the N exported from a pixel then you should back calculate the applied N (load) using the retention efficiency when creating your table
Nomenclature aside, if I’m thinking of loadings in terms of the source of nutrients associated with each pixel to downslope, and retention efficiency as being the fraction of nutrient utilized by a pixel, and I know 10 kg/N/ha is the application rate for a given LULC (to use the example in the user guide), and I’ve decided my LULC uses 40% of applied nutrient, why do I enter 4, (10 * 0.4), for the value of
load_n for that LULC, which is actually the loading of that pixel on the one downslope, and not 6, (10 * (1-0.4))?
In other words, the above quoted passage still makes me feel that the value you should use in the biophysical parameters table is the actual N application rate (at best it’s not clear), and those people using the model coming at it with their input values being the N exported from a pixel to downslope should be the ones to convert their export values to loadings. On the other hand, if the parameter is indeed actually the loading of a LULC pixel to downslope, and we define
app_N as the N application rate and
ret_n as the fraction utilized by a pixel, why is the value entered in the biophysical parameters table
ret_n and not
app_n * (1 -