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Highlights

Reductions of nitrogen due to drainage water management were synthesized from 290 plot-years.

Abstract. Drainage water management (DWM) is the practice of seasonally adjusting the drainage outlet elevation with a control structure and is implemented to decrease nutrient loss in drained agricultural landscapes. In this study, we review the effect of DWM on annual nitrogen (N) and phosphorus (P) loads compared to free drainage and examine its cost-effectiveness using data from around the world published between 1979 and 2022. Studies included in the review were limited to row-crop agriculture without irrigation, with controlled drainage and free drainage compared during the same year. A total of 290 plot-years for N were compiled and analyzed using a mixed effects linear model to assess differences in flow, load, and concentrations and account for within and among site variability. Few studies examined dissolved reactive P (n=7) or total P (n=3); thus, only DRP results from these studies were statistically evaluated using Kruskal-Wallis tests. Analysis of N data showed that DWM reduced N loss on average by 13.3 kg/ha/yr (95% confidence interval: 9.4&-17.3 kg/ha/yr), which corresponds to a 46% load removal efficiency (95% confidence interval: 37.4%-54.5%). Drainage water management decreased annual DRP and TP load compared to free drainage by an average of 0.04 kg/ha/yr (mean=34%; range=-35% to 80%) and 0.06 kg/ha/yr (mean=36%; range=7% to 72%), respectively. Economic analysis showed that the cost of N reduction would range from $1 to $11 per kg N removed assuming no yield benefit, while a 3% yield increase would lead to a likely net economic benefit to the producer. Findings indicate that DWM can be an effective practice for decreasing nutrient loading from drained landscapes, however, losses through unmeasured flow paths such as seepage and surface runoff may lead to overestimating the effectiveness of DWM. Additionally, complex interactions among soil organic matter, soil nutrient levels, and water table depths need to be accounted for to ensure both N and P removal is achieved. Recommendations for monitoring discharge and nutrient loads in future DWM studies are provided to better understand the processes influencing nutrient load reductions and/or increases and further quantify the effectiveness of DWM implementation in drained landscapes.