Total phosphorus export from Iowa agricultural watersheds: Quantifying the scope and scale of a regional condition

Total phosphorus (TP) export in rivers contributes to development of hypoxic conditions in receiving water-bodies and is typically sourced to agricultural nonpoint sources. Systematic quantification of TP loads and yields at a regional scale is not often reported but TP yields are known to vary considerably. In this study, TP yields from 46 Iowa watersheds were evaluated for the period from 2000 to 2017 to evaluate long-term patterns of TP export and quantify the contribution of Iowa-sourced TP loads to the Gulf of Mexico. Using two common load estimation programs, annual mean and median TP yields ranged from 0.44 to 7.71 kg/ha and 0.32 to 3.75 kg/ha, respectively. Highest median yields were found in western and southern Iowa. The weighted average TP yield for Iowa was 1.70 kg/ha and, on average, Iowa's TP export contributes approximately 15% of the TP load to the Gulf of Mexico. Reducing TP export will require improved quantification and differentiation of sources of TP loads in Midwestern rivers and a commitment to find strategies that reduce both TP and NO3-N loss from agricultural lands in a complementary and additive approach.