The North American landscape has been profoundly altered to promote agriculturaldevelopment since European settlement (“settlement”) at the end ofthe nineteenth century. More than 98% of the North American prairie and vastareas of forest have been replaced with croplands. Bringing much of this landinto production under modern agricultural systems has been associated withextensive modifications to natural drainage networks. Extensive networks ofsurface ditches and subsurface drains (“tiles”) have been constructed to removeexcess water from the field soil surface or soil profile (Spaling & Smit,1995). By 1987, the most recent year for which survey data were collected,more than 17% of U.S. cropland (up to 30% in the Upper Midwest) had beenaltered by artificial surface or subsurface drainage (Pavelis, 1987).
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