# Impact of Subsurface Drainage System Design on Nitrate Loss and Crop Production
## 3 Results and Discussion
### Weather, Evaportanspiration, and Subsurface Drainage Flow
When segmenting the study fields into the east and west parts, CS-2 is located in the highest position within the west part, whereas CS-6 is situated in the highest position within the east part. Consequently, based on the findings from the surface flow line analysis, even though the elevation of plot CS-2 is lower than that of plots CS-4, CS-5, and CS-6, its total tile flow \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline  & & & \multicolumn{3}{p{113.8 pt}}{**Surface Runoff**} & \multicolumn{3}{p{113.8 pt}}{**Subsurface Drainage Flow**} & \multicolumn{3}{p{113.8 pt}}{**C5-4**} & \multicolumn{3}{p{113.8 pt}}{**CS-5**} & \multicolumn{3}{p{113.8 pt}}{**CS-6**} \\
**Water Year** & **Precipitation (mm)** & **ET (mm)** & **+ Percolation (mm)** & **Ave. (12.2 m \(\times\) 1.1 m)** & **(24.4 m \(\times\) 1.1 m)** & **(12.2 m \(\times\) 1.1 m)** & **(24.4 m \(\times\) 1.1 m)** & **(18.3 m)** & **(18.3 m)** \\ \hline Year 1 (October 2018 -September 2019) & 1077.2 & 575.7 & 93.9 (53.4) a & 408.7 (8.7) & 408.5 (37.9) & 697.3 (64.7) & 366.5 (34.0) & 607.4 (33.3) & 358.6 (25.5) & 274.7 (13.6) \\ \hline Year 2 (October 2019 -September 2020) & 1141.5 & 527.7 (46.2) & 57.4 (50.0) & 557.6 (48.8) & 557.6 (66.1) & 574.3 (50.6) & 577.3 (65.2) & 521.6 (45.7) & 479.2 (23.5) \\ \hline Year 3 (October 2020 -September 2021) & 1085.3 & 506.4 (46.7) & 210.0 (46.7) & 369.4 (34.1) & 369.6 (66.4) & 270.6 (24.9) & 450.5 (41.5) & 281.6 (255.9) & 389.0 (35.8) & 1055.5 (9.7) \\ \hline Year 4 (October 2022 -September 2022) & 930.4 & 474.7 (51.0) & 28.3 (3.0) & 427.4 (45.9) & 557.9 (60.0) & 264.1 (28.4) & 626.8 (67.4) & 474.3 (51.0) & 493.5 (53.0) & 147.7 (15.9) \\ \hline Average (October 2018 -September 2022) & 1058.6 & 521.1 (49.2) & 97.4 (49.2) & 440.1 (9.2) & 440.1 (41.6) & 682.5 (41.6) & 369.6 (34.5) & 607.4 (38.6) & 404.9 (39.1) & 167.0 (15.7) \\ \hline \hline \end{tabular}  
* a: Ratio to precipitation (%). Data sources: Precipitation-Champaign weather station (Illinois Water survey) and observed data (WatchDog); ET-MODIS16A2 (NASA DAAC); Subsurface drainage flow: Observed data; Surface runoff + percolation: Precipitation-ET-Subsurface drainage flow. Water year: from 1 October this year to 30 September next year.  
\end{table} 
Table 1: Summary of water balance according to water year from 2018 to 2022.  
is slightly lower compared to the tile flow results observed in plots CS-4 and CS-5. Thus, spatial differences in topography and soil types were also reflected in these results through the TPI analysis and surface flow line results. Throughout the study period, we observed four growing seasons (from mid-May to mid-October) and four non-growing seasons (from mid-October to mid-May). An average annual precipitation of 463.0 mm yr\({}^{-1}\) was received during the growing season, whereas, during non-growing seasons, an average annual precipitation of 562.5 mm yr\({}^{-1}\) was received. Drain flow occurred sporadically throughout the growing seasons, typically at the beginning or during heavy rainfall events. During the non-growing seasons (mainly between November and January), most plots had very small tile flow due to low precipitation or frozen soil conditions (Figure S2). The results of the precipitation and drain water flow observed during growing and non-growing seasons are presented in Table 2. During the crop growing season, the percentage of drain flow to precipitation was recorded in the range of 0.5% (CS-6) to 66.8% (CS-1). One reason why the higher percentage of drain water flow during the fourth non-growing season is associated with flooding and the backwater effect is that it causes the percentage of rainwater in the drain flow to exceed 100% (CS-3). We conducted an ANOVA (analysis of variance) single-factor test to compare the mean daily discharge between the plots.