@swolny How do I calculate nutrient retention in InVEST? The modified load raster is expressed in kg/year, while the export raster is in kg/ha/ year.

Hi " @taherunnesa . The formula is: Modified. load - N export . you can find modified.load in the “intermediate output” folder.

But the units are not the same. When I converted the modified load from kg/yr to kg/ha/yr by dividing by 0.09, the values became quite large. Then I subtracted the nitrogen export from the modified load to calculate retention. However, the resulting retention map shows unexpectedly higher retention …

The nitrogen and phosphorus export results in the “watershed_results_ndr.gpkg” shapefile are reported in kg/year, representing the sum of kg/ha/year values across the entire watershed. Although InVEST version 3.16 and later states that export values are calculated in kg/ha/year, the raster outputs a…

@Taherunnesa and @Eshetu , I believe the modified load raster is already in units kg/ha/yr. The load values included in the biophysical table input are expected to be kg/ha/yr, and the model no longer does any pixel area conversions when creating raster outputs based on those values. I believe our m…

Reading your comments, I subtracted the export raster from the modified load, keeping the unit as it is. Still, the retention shows higher in urban areas than in wetlands. Do you know what the possible reasons are? @dave @swolny

" @dave Thank you for your explanation. However, after running the NDR model, I noticed that the N and P export values reported in the watershed_results_ndr.gpkg shapefile were much lower than the values I calculated using zonal statistics by watershed. This led me to suspect that the units of the N …

Thank you, @Eshetu , I think we are actually in agreement. In order to calculate the totals per watershed, the model sums the per-hectare value from all the pixels in the watershed using zonal stats. Then we apply a conversion to that sum so that it ends up in kg/yr. This sounds like the same conver…

@dave thank you so much for sharing this related thread.

NDR Retention Raster & units of load rasters

Science, Research & Data

dave October 29, 2025, 7:07pm 10

@Taherunnesa , @Eshetu , as far as the method for calculating retention. Please see this related thread: Using NDR outputs to map nutrient trapping - #2 by jesseG

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NDR Retention Raster & units of load rasters

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