You would have to know the "watershed area" for the runoff. If you are assuming that the runoff is generated from a specific field, you would use area (in hectares) for that field. You also have the time dimension to consider. Perhaps converting your mass loss to a per year basis (kg / ha / yr).

As suggested watershed or catchment area is important. If the field is not a self contained catchment, but included in a larger hydrologic drainage, part of the runoff may be field generated, and part of runoff from outside of field. Also storm runoff hydrograph is not likely to have a constant nitrate concentration. And nitrate sources can be varied from fertilization, plant decay, wildlife, animal or other inputs. For various reasons, individual storms would Likely have storm nitrate runoff variability in amount and concentration. With just one grab sample of one runoff event, and not knowing about the amount of in-field runoff in comparison to out-field runoff contribution, the need to try to convert units may be Ineffective in producing meaningful results. Defined and properly instrumented experimental catchments can be a powerful tool in water and nutrient cycling, if you have the funding and time to develop this type study.

Ok Dear William F. Hansen !

The system is designed so that the collected runoff is generated only by plot surface.

Sabi Kidirou Gbedourorou , that's a good set-up. ... Following the comment by William F. Hansen , are you using grab samples ? Or do you have a method to collect all runoff from the plot ? Or perhaps integrating many samples with an auto-sampler ?

An oversimplification example. Let’s say your self contained plot is 1 hectare, and you collect all the runoff from each runoff event. There were 10 runoff events last year with various amounts collected in each event. After each event, you measure the number of liters and the average concentration from that event. Each storm, you empty the collection area and start over. So at the end of the year, you add up each storm runoff in liters times the mg/l to give the number of mg of nitrate from that storm. You do that for each storm runoff event. At the end of the year there was a total of 5,000 liters collected and 3,000,000 mg accumulated from the ten events. That’s 3 kg/ha/yr for that one year (10 storm runoff events) from your plot. Because the nitrate concentration can vary within each event, this above example where all the runoff is collected and the mean concentration is then analyzed when you mix the collected runoff.