The relationship of nitrate concentrations in streams to row crop land use in Iowa

The relationship between row crop land use and nitrate N concentrations in surface water was evaluated for 15 Iowa watersheds ranging from 1002 to 2774 km(2) and 10 smaller watersheds ranging from 47 to 775 km(2) for the period 1996 to 1998. The percentage of land in row crop varied from 23 to >87% in the 15 large watersheds, and mean annual NO3-N concentrations ranged from 0.5 to 10.8 mg/L. In the small watersheds, row crop percentage varied from 28 to 87% and mean annual NO3-N concentrations ranged from 3.0 to 10.5 mg/L. Ln both cases, nitrate N concentrations were directly related to the percent age of row crop in the watershed (p < 0.0003), Linear regression showed similar slope for both sets of watersheds (0.11) suggesting that average annual surface water nitrate concentrations in Iowa, and possibly similar agricultural areas in the midwestern USA, ran be approximated by multiplying a watershed's row crop percentage by 0.1. Comparing the Iowa watershed data with similar data collected at a subwatershed scale in Iowa (0.1 to 8.1 km(2)) and a larger midcontinent scale (7300 to 237 100 km(2)) suggests that watershed scale affects the relationship of nitrate concentration and land use. The slope of nitrate concentration versus ram crop percentage decreases with increasing watershed size.