The soluble phosphorus model was based on runoff reduction only. The observations censored from (Dillaha et al., 1989) for the nitrates were also censored for soluble phosphorus. The soluble phosphorus model has the weakest relationship of all the nutrient models (R = 0.27), yet both the slope and intercept were significant (p = 0.01).
6:5.1.6
where is the dissolved phosphorus reduction (%); and is the runoff reduction (%). Like nitrate, there is a significant reduction in soluble phosphorus (29.3%) with no corresponding runoff reduction. Experimental observations of soluble phosphorus reduction at low runoff reductions are highly variable. Dillaha et al. (1989) found reductions in soluble phosphorus ranging from 43% to -31% with near zero runoff reduction. The minimum reduction predicted by equation 6 could be the result of mechanisms similar to those cited for the removal of nitrates, or simply an artifact of experimental variability.
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 (=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:
6:5.1.4
where is the nitrate nitrogen reduction (%); and 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.
In addition to the mechanisms by which sediment and runoff are captured, nutrients may be adsorbed onto vegetation, surface residues, or the soil surface (Barfield et al., 1998). For the sake of simplicity, nutrient reduction was considered to be a function of sediment or runoff reduction only. Only nitrogen and phosphorus were considered. All nutrient models were developed from measured VFS data; the current version of VFSMOD does not account for nutrients.
The total nitrogen model was based on sediment reduction only. Much of the nitrogen lost in runoff from agricultural fields travels with sediments. Harmel et al. (2006) found that approximately 75% of the nitrogen lost from conventional tilled fields was in particulate forms. They also found that dissolved nitrogen forms, such as nitrate, were more dominant in no-till treatments. The vast majority of VFS data derived from literature were designed to simulate higher erosion conditions where particulate forms would represent the majority of nitrogen losses.
The total nitrogen model was based on sediment reduction from 44 observations reported in the literature. Two trials were censored during the development of the model. These experiments from Magette et al. (1989) yielded significant increases in total nitrogen exiting the VFS. The authors attributed this phenomenon to flushing of fine particulates captured in the VFS from prior experimental trials. Both the slope and the intercept were significant (P < 0.01). The model is given below and shown in Figure 6:5-3.
6:5.1.3
where is the total nitrogen reduction (%); and is the sediment reduction (%). Although this model was developed from total nitrogen, which includes both soluble and particulate forms, it was applied only to particulate forms in the SWAT+ model.
The model for total phosphorus was based on sediment reduction. Although total phosphorus is composed of both soluble and particulate forms, particulate forms represent the bulk of phosphorus lost from conventionally cultivated fields. The total phosphorus model was developed from 63 observations; more data than any other nutrient model. The intercept was not significant. Sediment reduction was able to explain 43% of the variability. The model was applied to all particulate forms of phosphorus in the SWAT+ VFS submodel. The model is given below:
6:5.1.5
where is the total phosphorus reduction (%); and is the sediment reduction (%).