Predicting the effect of drain depth on profitability and hydrology of subsurface drainage systems across the eastern USA

The choice of drain depth in subsurface drainage design affects system performance. Previous studies have focused on investigating the hydrology of shallow drains at discrete locations. However, to our knowledge, no study has investigated the profitability of shallow drains across a wide range of soil and climatic conditions. The main objective of this study was to comparatively evaluate the effects of 75-cm shallow drains versus 125-cm deep drains on profitability and hydrology of subsurface drainage systems across the eastern USA. We conducted DRAINMOD simulations under continuous corn production (Zea mays L.) using 30 years of weather data (1990–2019) for combinations of three factors: two drain depths, four soils, and seven locations. Simulations were optimized for crop production that maximized annual economic return on investment. The results showed that corn yield for shallow drains went from 1.6% increase to –1.3% decrease with increasing growing-season rainfall from northeast to southeast USA. Shallow drains provided the benefit of reduced year-to-year corn yield variability across the region. Corn yield for coarse-textured soil benefited more from shallow drains than fine-textured soil across the region. Even though shallow drains increased annual benefit from higher corn yield under certain soil and weather conditions, they were less profitable than deep drains (lower benefit-cost ratio and higher payback period). The benefit of drainage discharge reduction under shallow drains generally intensified with increasing annual precipitation from northeast to southeast USA. Results were used to develop an empirical equation for estimating the benefit of drainage discharge reduction with shallow drains in the eastern USA. In conclusion, even though shallow drains were less profitable than deep drains, they provide benefits that may be of interest to farmers and policymakers.