Assessing the impacts of tile drainage on hydrological cycle and crop growth in the U.S. Midwest agroecosystems

Tile drainage is an essential agricultural management practice to improve field drainage conditions and increase crop productivity in the U.S. Midwest. Previous studies have shown that tile drainage significantly changes hydrological and biogeochemical cycles by lowering the water table and reducing residence time of subsurface water. However, these impacts of tile drainage still have large uncertainties, and how tile drainage further impacts crop growth remains poorly understood. Here we used an advanced process-based agroecosystem model, ecosys, to quantify the impacts of tile drainage on hydrological and biogeochemical cycles and crop growth at corn-soybean rotation fields in the U.S. Midwest. Tiles are represented as a water sink connected to the atmosphere, characterized by tile depth and tile spacing. Water flow from saturated soil layers to tiles is governed by the lateral hydraulic gradient defined by the water table depth in the field, tile depth, and tile density. Water flow to tile drainage only occurred in the soil layers above the tile water surface. Soil water movement in saturated soil is estimated with Darcy's equation. Model results show that tile drainage decreases soil water content, increases subsurface discharge, and reduces surface runoff. The results also reveal that tile drainage increases soil oxygen concentration, reduces plant oxygen stress, and further increases the Gross Primary Productivity (GPP). The GPP benefit due to the tile drainage increases with more precipitation in both corn and soybean years. This study provides a comprehensive assessment of the function of tile drainage to hydrology, biogeochemical, and crop productivity, demonstrating the ability of the ecosys model to represent the complex impacts of tile drainage in agroecosystems.