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Verifiable metamodels for nitrate losses to drains and groundwater in the Corn Belt, USA

January 1, 2012

Nitrate leaching in the unsaturated zone poses a risk to groundwater, whereas nitrate in tile drainage is conveyed directly to streams. We developed metamodels (MMs) consisting of artificial neural networks to simplify and upscale mechanistic fate and transport models for prediction of nitrate losses by drains and leaching in the Corn Belt, USA. The two final MMs predicted nitrate concentration and flux, respectively, in the shallow subsurface. Because each MM considered both tile drainage and leaching, they represent an integrated approach to vulnerability assessment. The MMs used readily available data comprising farm fertilizer nitrogen (N), weather data, and soil properties as inputs; therefore, they were well suited for regional extrapolation. The MMs effectively related the outputs of the underlying mechanistic model (Root Zone Water Quality Model) to the inputs (R2 = 0.986 for the nitrate concentration MM). Predicted nitrate concentration was compared with measured nitrate in 38 samples of recently recharged groundwater, yielding a Pearson’s r of 0.466 (p = 0.003). Predicted nitrate generally was higher than that measured in groundwater, possibly as a result of the time-lag for modern recharge to reach well screens, denitrification in groundwater, or interception of recharge by tile drains. In a qualitative comparison, predicted nitrate concentration also compared favorably with results from a previous regression model that predicted total N in streams.

Publication Year 2012
Title Verifiable metamodels for nitrate losses to drains and groundwater in the Corn Belt, USA
DOI 10.1021/es202875e
Authors Bernard T. Nolan, Robert W. Malone, Jo Ann M. Gronberg, K.R. Thorp, Liwang Ma
Publication Type Article
Publication Subtype Journal Article
Series Title Environmental Science & Technology
Index ID 70032658
Record Source USGS Publications Warehouse
USGS Organization National Water Quality Assessment Program