Paired-field evaluation of a saturated buffer reveals significant water-quality benefits through upstream weir management

Past studies evaluating the conservation drainage practice of saturated buffers (SB) have used the water flowing over the upstream weir as a control, assuming that upstream weir management does not reduce flow and nitrate load. The objectives are to (1) investigate whether the upstream weir functions as controlled drainage (CD) to provide water-quality benefits beyond those from the buffer flow (BF, defined as water diverted into the buffer), and (2) evaluate the performance of the SB system under high (shallow) and low (deep) weir management levels. From June 2019 to September 2024, we conducted a paired-field study at an on-farm site in Michigan, USA, comparing an SB system to a free drainage control field. Using this method, we separated the two components of the SB system into CD and BF. Results revealed a significant reduction in nitrate-N load (54.5% and 11.1 kg/ha annually, p-value < 0.01) of the SB system compared to free drainage. The CD component emerged as the dominant contributor to nitrate-N load removal (48.0% and 9.6 kg/ha annually), confirming the crucial water-quality role of upstream weir management. Studies that ignored the water-quality effect of the CD component likely underestimated the performance of SB. The BF component contributed to only 6.4% of nitrate-N load reduction. High-level weir setting significantly removed more nitrate-N (0.076 kg/ha daily) than low-level setting (0.036 kg/ha daily). In conclusion, this study shows the significant water-quality contribution of the CD component and underscores the importance of weir management for enhancing nitrate load removal.