# Impact of Subsurface Drainage System Design on Nitrate Loss and Crop Production
## 3 Results and Discussion  
### Weather, Evaportanspiration, and Subsurface Drainage Flow  
The 20-year (2003-2022) average annual precipitation in the study area was 967.4 mm yr\({}^{-1}\). The annual precipitation records for the study period (2018-2022) show the long-term average precipitation in the year 2019 (1294.1 mm) and below-average precipitation in the year 2022 (907.8 mm). The 4-year average precipitation is 1102.8 mm yr\({}^{-1}\). The 4-year average evapotranspiration data show 526.7 mm yr\({}^{-1}\) (Figure S1, Supplementary Material). Compared to the 4-year average precipitation data, the rate is about 47.8%. The concept of a water year is important for considering the freezing and/or thawing effects of precipitation during the winter season [31]. Even if rainfall or snow occurs within a given year, the resulting flow may not occur until the following year due to the freezing effect. In Illinois, USA, it is more logical to include winter season precipitation from October to February within the same year. Consequently, we analyzed the monitoring data based on the water year, which spans from 15 October of the current year to October 14 th of the following year. The 4-year water precipitation, evapotranspiration, and drain water flow for each plot are summarized in Table 1. The drain flow removed an average of 41.6% of rainwater from the six plots over 4 years. The evapotranspiration rate is about 49% of the average annual precipitation. The drain water flow percentages of rainwater for the six sites (CS-1 to CS-6) were 64.3%, 34.5%, 57.6%, 38.6%, 39.1%, and 15.7%. These results are comparable and within the range (except CS-1 and CS-3) reported in other similar regional studies [20; 32; 33]. In a 4-year field study conducted in Iowa, ref. [32] found that the percentage distribution of total precipitation to drain water flow ranged between 18-40%. Ref. [20] reported that 8% to 20% of annual precipitation contributed to the average annual drain water flow in a 3-year field study in Indiana. In another study, ref. [5] found that drain water flow percentages of precipitation ranged from 15% to 22% in a 10-year study in Illinois. In the third year, surface runoff and percolation rates were higher than in other years due to flooding events. There were more flooding events in the third year compared to the other monitoring years. As a result, there was more surface runoff than in the second year, even though the second year had more rainfall than the third. We did not measure the runoff and percolation rates directly, which is a limitation of this research. Figure 3 shows the total drain water flow observed during all plot monitoring periods. Plot CS-1 had the highest percentage of drain water flow compared to the other plots, followed by plots CS-3, CS-5, CS-4, CS-2, and CS-6. The highest total drain flow of 273.0 cm was recorded in plot CS-1, and the lowest drain flow of 66.8 cm was recorded in plot CS-6 in a 4-year field study. CS-1 and CS-3 had a higher flow rate range than observed in the referenced studies. The higher percentage of drain water flow recorded in plots CS-1 and CS-3, associated with narrow drain spacing, resulted in an accelerated drainage discharge of rainwater compared to the other plots. Following an analysis of the topographic position index (TPI) results, a comparison of the average and median values across each field reveals that the CS-1 plot is at the lowest elevation, while CS-6 is at the highest elevation. This may contribute to surface runoff from other plots onto plot CS-1 and surface runoff loss from plot CS-6. CS-2, CS-4, and CS-5 have the same drainage coefficient, resulting in similar tile flow results. Although CS-6 has the same drainage coefficient as these fields, it shows lower tile flow results. This disparity can also be attributed to the topographical characteristics of plot CS-6, situated at the highest elevation within the fields (Figure 2).