Agriculture, Erosion, and Sediment Yields
James C. Knox
Agricultural land use normally accelerates rates and magnitudes of natural erosion and sedimentation processes because the stability of the land surface is disrupted when the protective benefits of natural vegetation are depleted through overgrazing or during crop cultivation. Accelerated erosion and sedimentation mostly result from processes related to the actions of running water and/or wind. Although erosion and sedimentation from running water are accelerated nearly everywhere when the vegetation cover is depleted, wind erosion and sedimentation are most common in arid and semiarid environments and in areas of sandy soils. The general objective of this chapter is to review examples that illustrate the influence of agricultural activities on erosion and sedimentation and on the degradation of soil resources for selected regions of North America.
It is evident from the stratigraphic record that Native American agriculture had at least local and sometimes regional impact on accelerating erosion and sedimentation. However, the arrival of European agriculture in North America accelerated erosion and sedimentation rates to magnitudes that are unparalleled in the preceding postglacial record. For Canada and the United States, the great impact of agriculture generally begins in the 1600s in the eastern sector but dates only from the 1800s in the central and much of the western sector (Happ et al., 1940; Daniels, 1966; Wolman, 1967; Knox, 1972, 1977, 1987; Butzer, 1974;
Trimble, 1974, 1975, 1983; Costa, 1975; Cooke and Reeves, 1976; Davis, 1976; Meade, 1982; Sparrow, 1984; Trimble and Lund, 1982; Magilligan, 1985; Jacobson and Coleman, 1986; Barnhardt, 1988; Williams, 1989; Balling and Wells, 1990; Ashmore, 1993; Miller et al., 1993, Phillips, 1993, 1997; Orme and Orme, 1998). However, in Mexico and the southwestern United States, European agricultural influences, including cropping and livestock raising, occurred in the 1500s (Butzer, 1992a,b).
Erosion and Sedimentation
Most agricultural land uses increase the surface runoff fraction of total runoff from a watershed, resulting in accelerated flooding and soil erosion. Increases in the surface runoff fraction result from direct and indirect responses of reducing vegetation cover, and from destruction of the natural soil structure associated with cultivation and compaction by heavy farm equipment. Changes in the character of vegetation can be expected to have particularly important hydrologic influences because vegetation cover prevents direct raindrop impact on soil particles, greatly increases the hydraulic roughness at the soil surface, and