A question about the unit in SDR and NDR model

Recently, I am confused about the unit in SDR and NDR model. The output files in SDR model include rkls_[Suffix].tif, sed_export_[Suffix].tif and usle_[Suffix].tif, and the unit of these raster files are all tons/pixel as the description in Interpreting Results. But the unit in the equations is tons/ha. So what is the difference and how is this unit transformed? If the pixel in data is 30m×30m, should the unit in output raster files be tons/900m2 as the description in Interpreting Results (tons/pixel)? And if the unit of the raster is tons/ha, how is sed_export in watershed_results_sdr_[Suffix].shp summed as tons/watershed. Same question exsits in NDR model.
I think this question is vital to many researchers study.
Thanks for your reply!

Hello @Prime -

As far as I know, the model translates the USLE and SDR equation values to a value of tons/pixel/year (which is output by the model for usle.tif, sed_export.tif and the watershed results), by adjusting for pixel size. @rich, @jdouglass, can one of you please verify this? Assuming that’s the case, I’ll update the User Guide with this clarification.

~ Stacie

Hello @swolny

Thank you for sovling my puzzle.My input data is 30m×30m (the area of pixel is 900m2), so the unit of sed_export.tif is tons/900m2/year according to your solution. Similarly, the unit of n§_export.tif in NDR model is kg/900m2/year. I can do my research better if this is verified again by two other experts (@rich and @jdouglass).

Thank you very much for your help!


Hello @Prime,

I have the same question than you, about the units of SDR. Because my output rasters have the pixel size 30m x30m, same as DEM, therefore,the units should be tons/pixel/year = tons/900m2/year.

But, when I convert this values to tons/ha/year (to be coherent with other papers) it gives me unrealistic values of soil loss, comparing it with the official JRC data, while the original outputs in tons/pixel/year has very close values to the official data (that is in tons/ha/year), making it easy to do a statistical comparison.

So, is it possible that in fact, the model converts automatically tons/pixel/year to tons/ha/year, for any pixel size? And, no further conversion is needed?

@swolny it would be great if we could confirm this, to better understand the model.

Thank you!

Really this is a critical question to resolve. I had assumed the values were tons/hectare, but now I’m not sure, and that makes me question all the results from these models.