# Impact of Subsurface Drainage System Design on Nitrate Loss and Crop Production
## 3 Results and Discussion
### Weather, Evaportanspiration, and Subsurface Drainage Flow
The analysis results showed that one or more plots have a significantly different mean daily discharge, with a \(p\)-value of 2 \(\times\)\(10^{-16}\) at a 95% confidence interval (\(p\) < 0.05). Among the various ANOVA test methods, we used [PERSON]'s HSD test to identify which plot group has a significantly different mean discharge of daily flow. The [PERSON]'s test results in Figure 4 indicate that the mean drainage discharge in plots CS-1 and CS-3 significantly differs from that of CS-2, CS-4, CS-5, and CS-6 (\(p\) < 0.05). Additionally, there is no significant difference in flow observed among plots CS-2, CS-4, and CS-5. However, plot CS-6 shows significant differences from all other fields. This result also draws a very similar conclusion to the total flow analysis. The result suggests that plots with narrower drain spacing have higher mean discharge than plots with wider drain spacing. Figure 3: Cumulative daily tile flow at each site from 2018 to 2022. The daily nitrate-nitrogen (NO\({}_{3}\)-N) load (kg N ha\({}^{-1}\)), NO\({}_{3}\)-N concentration (mg L\({}^{-1}\)), and mean NO\({}_{3}\)-N concentration for all six plots from 2018 to 2022 are presented in Figure S3. The NO\({}_{3}\)-N concentration in subsurface water samples fluctuated from almost zero to 2018, is shown in Figure S4. The NO\({}_{3}\)-N concentration in subsurface water samples fluctuated from almost zero to 2018, is shown in Figure S5. The NO\({}_{3}\)-N concentration in subsurface water samples fluctuated from almost zero to 2018, is shown in Figure S6. The NO\({}_{3}\)-N concentration in subsurface water samples fluctuated from almost zero to 2018, is shown in Figure S7. The NO\({}_{3}\)-N concentration in subsurface water samples fluctuated from almost zero to 2018, is shown in Figure S8. The NO\({}_{3}\)-N concentration in subsurface water samples fluctuated from almost zero to 2018, is shown in Figure S8. 73.6 mg L\({}^{-1}\) for sites CS-1 to CS-6. During the study period, the highest mean NO\({}_{3}\)-N concentration of 20.3 mg L\({}^{-1}\) was observed in plot CS-6, and the minimum mean NO\({}_{3}\)-N concentration of 7.5 mg L\({}^{-1}\) was observed in plot CS-2. In CS-1 and CS-2, the daily NO\({}_{3}\)-N concentration in drainage flows was below USEPA's drinking water standard of 10 mg L\({}^{-1}\) most of the time (Figure S3). However, the NO\({}_{3}\)-N concentration in drain flow from plots CS-3, CS-4, CS-5, and CS-6 often exceeded the USEPA's limit (10 mg L\({}^{-1}\)). The concentration of NO\({}_{3}\)-N in the drain flow in each plot varied widely from month to month. Peak concentration was observed at the beginning of the corn planting season (mainly in May each year) and late fall after harvesting the crops. This variation in the NO\({}_{3}\)-N is associated with pre-planting and late fall N applications. For annual nitrate loss, plot CS-3 had the highest loss compared to other plots (Table 3). Even though CS-1 had higher tile flow compared to CS-3, CS-3 had relatively higher tile flow and nitrate concentration from 2018 to 2020. Another noteworthy observation is that CS-4 shows greater total nitrate loss compared to CS-1. According to Figure S3, CS-4 kept higher levels of NO\({}_{3}\)-N effluent concentration spanning from 2018 to the middle of 2020. Consequently, despite the comparatively less volume of water flow from the field within the confines of CS-4, in comparison to the drainage plots of CS-1 designed by narrower drain spacing, the amount of nitrate loss, as quantified by the product of water volume and effluent concentration, had a substantial result. Upon conducting a comparative analysis of the outcomes over various years, it was found that Year 2, characterized by higher precipitation levels (the wettest year), showed the most pronounced NO\({}_{3}\)-N loss. Year 3, not Year 4, which had the least rainfall, showed the lowest nitrogen loss. This result can be attributed to the fact that, in Year 3, soybeans were cultivated and there was a relatively reduced use of fertilizers.