Effect of variable annual precipitation and nutrient input on nitrogen and phosphorus transport from two Midwestern agricultural watersheds

Authors: KALKHOFF, STEPHEN - Us Geological Survey (USGS); HUBBARD, LAURA - Us Geological Survey (USGS); Tomer, Mark; James, David

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2016
Publication Date: 7/15/2016
Publication URL: https://handle.nal.usda.gov/10113/5321263
Citation: Kalkhoff, S.J., Hubbard, L.E., Tomer, M.D., James, D.E. 2016. Effect of variable annual precipitation and nutrient input on nitrogen and phosphorus transport from two Midwestern agricultural watersheds. Science of the Total Environment. 559:53-62. doi: 10.1016/j.scitotenv.2016.03.127.

Interpretive Summary: Nitrogen and phosphorus loads from agricultural watersheds contribute to Gulf of Mexico hypoxia and affect local aquatic ecosystems. Artificially drained watersheds contribute substantially to these nutrient loads. This study compared nutrient loads from the Little Cobb River in southern Minnesota and the South Fork Iowa River in northern Iowa during 1996-2007. This period could provide a reasonable period against which to judge impacts of recent nutrient reduction intitiatives. Both watersheds, while being dominated by cropland, showed substantial differences in the timing and amounts of nutrients discharged in streamflow. Nutrient loads peaked during snowmelt in the Little Cobb, whereas peak load from the South Fork River occuring during late spring and early summer rains. Average annual nutrient loads were substantially greater from the South Fork than the Little Cobb for both nutients, by nearly 50% for nitrate-N and by more than 100% for total P, when considered on a unit area basis. This was related to proportionally more acreage in continuous corn and a larger number of livestock feeding operations in the SFIR. Strategies aimed to reduce nutrient loads should address vulnerability during seasonal peak flows, which differed between these two watersheds separated by only about 150 km (100 mi). These results are of interest to a range of agricultural and conservation stakeholders interested in greater agricultural efficiency and water quality protection.

Technical Abstract: Precipitation patterns and nutrient inputs impact transport of nitrate (NO3-N) and phosphorus (TP) from Midwest watersheds. Nutrient concentrations and yields from two subsurface-drained watersheds, the Little Cobb River (LCR) in southern Minnesota and the South Fork Iowa River (SFIR) in northern Iowa, were evaluated during 1996-2007 to document relative differences in timings and amounts of nutrients transported. Both watersheds are located in the prairie pothole region, but the SFIR exhibits a longer growing season and more livestock production. The SFIR yielded significantly more NO3-N than the LCR watershed (31.2 versus 21.3 kg NO3-N ha-1 y-1). The SFIR watershed also yielded more TP than the LCR watershed (1.13 versus 0.51 kg TP ha-1yr-1), despite greater TP concentrations in the LCR. About 65% of NO3-N and 50% of TP loads were transported during April through June, and <20% of the annual loads were transported later in the growing season. Monthly NO3-N and TP loads peaked in April from the LCR but peaked in June from the SFIR; this difference was attributed to greater snowmelt runoff in the LCR. The annual NO3-N yield increased with increasing annual runoff at a similar rate in both watersheds, but the LCR watershed yielded less annual NO3-N than the SFIR for a similar annual runoff. These two watersheds are within 150 km of one another and have similar dominant agricultural systems, but differences in climate and cropping inputs impacted amounts and timing of nutrient transport.