Assessment of the Iowa River's South Fork watershed: Part 1. Water quality

Abstract

Iowa's South Fork watershed is dominated by corn (Zea mays L.) and soybean [Glycine max L. (Merr.)] rotations, and animal feeding operations are common. Artificial subsurface (tile) drainage is extensive; hydric soils cover 54% of the watershed. During spring and early summer, NO₃-N concentrations in tile and stream discharge often exceed 20 mg L^-1. Total N loads during 2002 to 2005 ranged from 16 to 26 kg NO₃-N ha^-1 y^-1 (14 to 23 lb ac^-1 yr^-1). Nitrate concentrations increased linearly with log baseflow, effectively a surrogate measure of tile discharge. Phosphorus loads were only 0.4 to 0.7 kg P ha^-1 y^-1 (0.4 to 0.6 lb ac^-1 yr^-1), but concentrations commonly exceeded 0.1 mg L^-1, a eutrophication-risk threshold. Mean E. coli populations in the stream exceeded 500 cells 100 ml^-1 during summer. Statistical comparison of actual nitrate records with independent records generated using regression equations provided modeling efficiencies of 0.91 or less, suggesting performance targets for watershed model validation. Tile drainage is more important in transport of nitrate and dissolved phosphorus than E. coli. Variations in nitrate, phosphorus, and E. coli are uniquely timed, highlighting the complexity of integrated water quality assessments.