# Organic N in Surface Runoff

Organic N attached to soil particles may be transported by surface runoff to the main channel. This form of nitrogen is associated with the sediment loading from the HRU and changes in sediment loading will be reflected in the organic nitrogen loading. The amount of organic nitrogen transported with sediment to the stream is calculated with a loading function developed by McElroy et al. (1976) and modified by Williams and Hann (1978).

            $$orgN\_{surf}=0.001*conc\_{orgN}*\frac{sed}{area\_{hru}}\*\varepsilon\_{N:sed}$$                                   4:2.2.1

where $$orgN\_{surf}$$ is the amount of organic nitrogen transported to the main channel in surface runoff (kg N/ha), $$conc\_{orgN}$$ is the concentration of organic nitrogen in the top 10 mm (g N/ metric ton soil), $$sed$$ is the sediment yield on a given day (metric tons), $$area\_{hru}$$ is the HRU area (ha), and $$\varepsilon\_{N:sed}$$ is the nitrogen enrichment ratio.

      The concentration of organic nitrogen in the soil surface layer, $$conc\_{orgN}$$, is calculated:

          $$conc\_{orgN}=100\*\frac{(orgN\_{frsh,surf}+orgN\_{sta,surf}+orgN\_{act,surf})}{\rho\_b\*depth\_{surf}}$$                       4:2.2.2

      where $$orgN\_{frsh,surf}$$ is nitrogen in the fresh organic pool in the top 10mm (kg N/ha), $$orgN\_{sta,surf}$$ is nitrogen in the stable organic pool (kg N/ha), $$orgN\_{act,surf}$$ is nitrogen in the active organic pool in the top 10 mm (kg N/ha), $$\rho\_b$$ is the bulk density of the first soil layer (Mg/m$$^3$$), and $$depth\_{surf}$$ is the depth of the soil surface layer (10 mm).


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