Iowa Stream Nitrate, Discharge and Precipitation: 30-Year Perspective.

We evaluated Iowa Department of Natural Resources nitrate (NO3-N) and US Geological Survey hydrological data from 1987 to 2016 in nine agricultural watersheds to assess how transport of this pollutant has changed in the US state of Iowa. When the first 15 years of the 30-year water-quality record is compared to the second 15 years (1987-2001 and 2002-2016), three different metrics used to quantify NO3-N transport all indicate levels of this pollutant are increasing. Yield of NO3-N (kg ha-1) averaged 18% higher in the second 15 years, while flow-weighted average concentrations (mg L-1) were 12% higher. We also introduced the new metric of NO3-N yield (g ha-1) per mm precipitation to assess differences between years and watersheds, which averaged 21 g NO3-N ha-1 per 1 mm of precipitation across all watersheds and was 13% higher during the second half of the record. These increases of NO3-N occurred within a backdrop of increasing wetness across Iowa, with precipitation and discharge levels 8 and 16% higher in the last half of the record, indicating how NO3-N transport is amplified by increasing precipitation levels. The implications of this are that in future climate scenarios where rainfall is more abundant, detaining water and increasing evapotranspiration within the cropping system will be necessary to control NO3-N losses. Land use changes that include use of cover crops, living mulches, and perennial plants should be expanded to improve water quality and affect the water balance within agricultural basins.