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 agricultur al 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 th rough a natural riparian zone and observed as weir baseflow; surface r unoff is measured separately. Nitrate runoff graphs illustrate the imp ortance of high-frequency sampling of each event to permit quantitativ e estimation of chemical loss. The concentration of nitrate carried fr om the field in basin drainage steadily increased from <1 mg L-1 in 19 69 to >20 mg L-1 in 1991. The rate of cumulative increase in the amoun t of applied N is greater than the rate of removal by the crop. Over t he 23-year record, 23% of the mean annual application of N remains sto red 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 toad of nitrate d eposited on the soil surface by rainfall is very small in comparison t o the amount applied by fertilizer application. Measurable changes in water quality within various hydrogeologic compartments are seldom obs erved in just a few years of monitoring. Therefore, these results emph asize the importance of long-term data sets incorporating temporal var iability when evaluating the impact of agricultural practices on surfa ce water resources.