Effects of LULCC on Streamflow and Nitrate-N Concentrations in Iowa and the Mississippi River Basin

Land use throughout the upper Mississippi River basin changed during the latter half of the 20th century with soybean production rapidly replacing perennial grasses and other sod-based crops in the agricultural landscape. In Iowa, combined with increasing corn production, total row crop area increased approximately 30-40 percent from 1940 to 2000. Improved soil conservation practices and expansion of tile drainage accompanied the land use change. Converting from perennial to seasonal cropping systems profoundly affected streamflow and water quality by reducing evapotranspiration, increasing groundwater recharge and increasing baseflow in many Midwestern rivers. Annual baseflow in Iowa increased by more than 20-30 percent over the last 60 years, with greatest seasonal increases in baseflow occurring during spring and fall when row crop fields are largely bare ground. Similar analysis extended to other rivers in the Mississippi River basin indicates that baseflow also has been increasing in response to increasing row crop production. In Iowa, the amount of row crop land in a watershed is not just a predictor of nitrate concentrations in streams, but now is a major factor in predicting the amount of baseflow in a river, as basins with expansive row crop land cover produce similar baseflow as basins underlain by fractured limestone. Since nitrate-nitrogen (nitrate) is primarily delivered to Iowa streams through groundwater discharge, either as baseflow or tile drainage, an increase in baseflow has contributed to increasing nitrate inputs to surface water. A two- and three-fold increase in nitrate concentrations observed in several Iowa rivers during the 1940-2000 period correlates with both increasing row crop land use and increasing baseflow. Study results indicate links among land use, baseflow and nitrate concentrations and loads that could be exploited for possible solutions to excessive nonpoint source pollution export.