Precipitation Drives Nitrogen Load Variability in Three Iowa Rivers

Abstract

Study region: Iowa, a leading agricultural state in the United States is a major source of nitrogen (N) to the Mississippi River. Study Focus: The study evaluated the impact of changes in precipitation, land cover (area under soybean production, Sb), and N fertilizer application (Napp) on variations in streamflow (SF), baseflow (BF) and N-load in the Des Moines, the Iowa, and the Raccoon rivers. New hydrological insights for the region: Ln(SF) and Ln(BF) for all three rivers were influenced by the current year (P1) and previous year (P2) precipitations. Presence of P2 in the regression suggested a deficit or excess soil water that influenced fillable pore space causing decreased or increased river flows. The Ln(N-load) for the Raccoon River was also related to P1 and P2 but the Ln(N-load) for the Des Moines and the Iowa rivers were related to P1 with p-values of P2 slightly above α = 0.05 for significance. Presence of P2 in Ln(N-load) regression reflected changes in both fillable porosity and residual soil N. Contributions of Sb and Napp in explaining variations in predictor variables were none to minimal. We conclude that wet climate and not changes in Sb or Napp is the primary reason for recent higher N-loads in these rivers. Under current cropping systems, we suggest directing efforts on treating river water by expanding floodplain interactions to reduce downstream N-loads.

Keywords

• River N loads
• Streamflow
• Baseflow
• Precipitation
• Land cover
• Fertilizer use