Nitrate Nitrogen

The nitrate nitrogen model was developed from 42 observations. Four observations from Dillaha et al. (1989) had negative runoff reduction values due to additional runoff generated in the VFS. Because the VFS SWAT+ sub model is not allowed to generate additional loads, these observations were censored. All nutrient models initially included both runoff and sediment reductions as independent variables, but the nitrate nitrogen model was the only model where both were significant ($\alpha$=0.05). Nitrate is soluble and should not be associated with sediments, yet they were statistically correlated in the measured data. It is likely that the relationship between nitrate and sediment is an artifact of cross- correlation between sediment and runoff reductions (as demonstrated by Equation (2)). The nitrate model was based only on runoff reduction; both the slope and the intercept were significant (p<0.01). The nitrate nitrogen model is given below:

$NN_R=39.4+0.584R_R$ 6:5.1.4

where $NN_R$ is the nitrate nitrogen reduction (%); and $R_R$ is the runoff reduction (%). Because both the slope and the intercept were significant, there is a minimum reduction of 39.4% in nitrate, even if there is no reduction in runoff due to the VFS. This outcome may be unexpected, but it is supported by the measured data. Dillaha et al. (1989) observed nitrate reductions of 52% and 32% with only 0% and 7% reductions in runoff volume. Lee et al. (2000) also found significant reductions in nitrate (61%) with low runoff reductions (7%). One possible explanation is that sufficient runoff can be generated in the VFS such that there is little net reduction in runoff, but significant infiltration may still occur. Another possibility is foliar uptake of nitrates by vegetation within the strip.