Agricultural Chemical Movement through a Field-Size Watershed in Iowa:  Surface Hydrology and Nitrate Losses in Discharge

Nonpoint source pollution of surface water by nitrate from agricultural activities is a national problem. An agricultural watershed in the Iowa Loess Hills with a 23-year history of annual corn production with average N fertilization is studied. Headcut seepage is transported through a natural riparian zone and observed as weir baseflow; surface runoff is measured separately. Nitrate runoff graphs illustrate the importance of high-frequency sampling of each event to permit quantitative estimation of chemical loss. The concentration of nitrate carried from the field in basin drainage steadily increased from 20 mg L-1 in 1991. The rate of cumulative increase in the amount of applied N is greater than the rate of removal by the crop. Over the 23-year record, 23% of the mean annual application of N remains stored and available for leaching or chemical conversion by soil microbes. Nitrate removal during early spring snowmelt surface runoff shows a diurnal pattern that corresponds to the daily freezing and thawing of the surface soil in early March. Contribution to the load of nitrate deposited on the soil surface by rainfall is very small in comparison to the amount applied by fertilizer application. Measurable changes in water quality within various hydrogeologic compartments are seldom observed in just a few years of monitoring. Therefore, these results emphasize the importance of long-term data sets incorporating temporal variability when evaluating the impact of agricultural practices on surface water resources.