# Impact of Subsurface Drainage System Design on Nitrate Loss and Crop Production
## 1 Introduction  
Subsurface (or tile) drainage is a practical and advantageous system for enhancing crop yields by controlling planting timing, managing efficient fertilizer use, and facilitating the removal of excess seasonal water from the soil [1, 2]. Because of these agronomic benefits, the subsurface drainage system is strategically applied in regions with poorly drained but highly productive soils [3, 4], resulting in its widespread adoption. However, alongside its widespread use, there have also been water quality concerns regarding this functional drainage system all over the world, including the United States [5], Canada [6], Sweden [7], and the U.K. [8], among others. The primary cause of these water quality concerns lies in the fact that the subsurface drainage system may create preferential pathways for the rapid transfer of contaminated water directly from agricultural fields to water bodies [9, 10]. Moreover, nitrate, being a soluble substance, can easily leach into tile-drained water without undergoing soil processes that typically reduce nitrate concentrations [11]. Subsurface drainage has adverse consequences as a major contributor to nutrient loss in the aquatic environments of the Midwestern regions of the United States. According to [12], approximately 25% of total phosphorus (P) and 52% of total nitrogen (N) delivered to the Gulf of Mexico can be attributed to the corn and soybean fields of those regions. Many researchers are also concerned that the prevalence of elevated nitrate levels has also been linked to a range of environmental issues, including hypoxia in the Gulf of Mexico [13]. In particular, ref. [12] estimated that Illinois contributes around 17% of annual nitrogen flux to the Gulf, predominantly due to the widespread adoption of subsurface drainage systems. Therefore, strategic agricultural water management practices to reduce nitrate-N transport and concentration levels through subsurface drainage systems emerge as crucial and pressing environmental objectives. The Illinois Nutrient Loss Reduction Strategy (NLRS) calls for actions to decrease the state's nutrient loads entering the Mississippi River by 15% to 25% within the next decade [14]. While the overall conclusions are clear--that broad measures may be needed across most corn-soybean regions to meet these ambitious reduction targets--many uncertainties remain at the individual farm level. The scientific complexities of nutrient losses need to be better understood, and many farmers are being urged to alter their practices and reduce nutrient inputs without a clear understanding of how specific recommendations will affect outcomes. Given the increasing need for proactive efforts, the coming years offer a critical opportunity to engage the agricultural community in scientific projects and outreach initiatives that will ultimately determine the success of the NLRS. Among the various practices contributing to this effort, one important practice is the efficient management of subsurface drainage, including the design of these systems. The spacing and depth of subsurface drainage systems, along with soil and field characteristics, control the amount of water drained from cropland, directly impacting nitrate-nitrogen (NO\({}_{3}\)-N or nitrate-N) losses, crop yields, and water availability for crops. Concerning the aspect of subsurface drainage design and crop yield, ref. [15] conducted a comprehensive 6-year field study in Indiana, USA, employing tile drains spaced at intervals of 10, 20, and 30 m. Their findings reported that 20 m tile spacing maximized corn yield. Ref. [16] undertook a decade-long investigation in Indiana, USA, examining the impact of three tile spacings (5, 10, and 20 m) on corn yield. Their results indicated that corn yield was inversely correlated to tile spacing. When it comes to the results of [17] in Iowa, USA, their modeling study indicated that employing a shallower drainage system could potentially heighten water stress on crop production, thereby leading to relatively diminished yields.