I am running the NDR model for nitrogen retention in Denmark, which is basically flat. An hydrologist here told me that for that reason almost 100% of all the nitrogen export happens through subsurface flow. On top of that, the soil here has different layers that have different effects on retention (due to oxigen availability). That means each area has a different subsurface retention, independent of LULC, which would be responsible for the retention only until the root zone.
Ok, that said, the data I had access here is a subsurface retention map for 15km2 basins for the whole country. Seems like a very good work. I also have access to data on leaching through the root zone for different crops. So I have a retention map for subsurface flow that does not relate to LULC, no data for surface flow and data for what gets to the root zone for each crop.
But the problem is that there is no way to differentiate the subsurface retention in the model, there is only one overall retention value for it. And the NDR equations are different for the different flows, so I can't just use the subsurface as the surface and set the subsurface proportion to 0.
The only solution I can think of is to use the leaching until the root zone as the load and then the subsurface retention as the surface retention? But would that work?