S415 Soil Science Building & Zoom
Precipitation and landforms as the primary drivers of river baseflow
Nitrogen losses from agricultural lands to the Gulf of Mexico are mainly through subsurface drain tiles in the Midwest United States. The carrier for this nitrogen is the water that infiltrates and percolates through the soil to drain tiles; a major component of the river baseflow. Factors controlling infiltration and redistribution are the landforms; surface and subsurface topography, soil type and sub-surface geology. However, tile drainage rather than landform features have been blamed for the changed river flow characteristics in the Midwest. In this study, we analyzed streamflow data from 23 Iowa and 3 Minnesota rivers covering six landforms with varying degrees of tile drainage to characterize precipitation, landform, and tile drainage effects on several flow metrics. The flow metrics were the streamflow, baseflow, baseflow ratio, flow duration curves, peak discharge, flashiness index, recession coefficient and Master Recession Curve (MRC). Consistent with the water balance principle, both streamflow and baseflow varied exponentially with annual precipitation indicating a dominating effect of the precipitation. Consistent with the infiltration principle, baseflow-precipitation function and baseflow ratio were higher for the low relief depression dominated Des Moines Lobe landscape than from steeper landscapes in Southern Iowa Drift Plain and other landforms. Except for watersheds where there have been extensive implementation of conservation practices promoting infiltration, baseflow ratios were nearly constant over time irrespective of the extent of tile drainage. Variations in flow duration curves, recession coefficients, and MRCs between the non-tiled and tiled watersheds were similar. Again, consistent with the landscape features of low relief and abundant depressions, the peak discharge was lower and the flashiness index was higher for the Des Moines Lobe rivers. These observations suggest that the landform and not tile drainage controls baseflow to the rivers. These results also indicate that the flat, depression dominated Des Moines Lobe landscape in the Upper Midwest is conducive to higher infiltration and baseflow and thus any conservation practices that promote infiltration in this landscape will further lead to increased baseflow and thus higher N losses.
Dr. Satish Gupta, Professor Emeritus