UC Davis

Artificially drained agricultural systems can be a primary contributor of non-point source nitrogen (N) and phosphorus (P) pollution to ground and surface waters; therefore, it is imperative to understand the mechanisms responsible for nutrient transport from this land use. Here, 79 drainage nutrient-related studies were used to compile 1564 site-years of agricultural drainage N and P concentrations (annual mean flow-weighted and annual arithmetic average) into a major database to aid in assessment of factors leading to nutrient loss through drainage systems. This new Drain Concentration table in the MANAGE (Measured Annual loads from AGricultural Environments) database is a publicly available resource (https://www.ars.usda.gov/plains-area/temple-tx/grassland-soil-and-water-research-laboratory/docs/manage-nutrient-loss-database). Probability of exceedance values were calculated using a Weibull distribution, and across the database, there was a 69% probability a drainage site-year would exceed an annual nitrate-N (NO 3 -N) concentration of 10 mg/L, the US Environmental Protection Agency Maximum Contaminant Level for drinking water, but only a 27% probability a reported site-year would exceed 0.0375 mg/L dissolved reactive phosphorus, which is a critical total P concentration to avoid freshwater eutrophication. While a surprising number of reported annual means were arithmetic averages rather than flow-weighted averages (27 vs. 71%), reporting of arithmetic means in peer-reviewed literature has decreased over time. We encourage further contributions to and analysis with the MANAGE database, so it can remain a resource for new efforts to better understand and mitigate nutrient losses from agricultural systems.