water budget will vary on a case-by-case basis. For example, if application efficiencies lead to less water being withdrawn from an aquifer, this would leave more water in long-term groundwater storage for future use. On the other hand, if lower water withdrawals from a stream only serve to make additional water available for junior water rights holders, the net effect on the regional water budget might be negligible.
As pointed out in the previous chapter, soil erosion can impair the water quality of streams and rivers and also contribute to nutrient pollution. Surface cover, especially in conjunction with conservation buffers, is crucial in reducing sediment in runoff and limiting soil erosion (Figure 4-1). Farmers can employ a number of conservation tillage techniques that leave some portion of crop residues on the soil surface. In “no-till” systems, as the name implies, crops are simply planted into the previous year’s crop residues. In “strip-till” systems, less than full-width tillage is conducted, leaving a relatively high amount of crop residue between rows. For corn, the stalks and cobs left in the field after the grain has been harvested—called the corn stover—can potentially be converted to cellulosic biofuel, but leaving them on the fields can greatly reduce soil erosion.
The effects of crop residue management on soil erosion can be represented by the “cover-management factor” (C) in the U.S. Department of Agriculture’s Revised Universal Soil Loss Equation. Because soil loss varies directly with C, a lower value corresponds to lower erosion estimates. In Table 4-1, the C-factor is estimated to be 0.02 for perennial grass, 0.04 for continuous corn when 100 percent of the corn stover is left in the field, and 0.55 for continuous corn when 95 percent of the residue is removed. Thus, from the standpoint of water quality with regard to erosion, sediment, N loss, P loss, and pesticide loss, it is clear that perennial grasses or polyculture (a form of agriculture in which one raises multiple species of crops at the same time and place) would have a great advantage over continuous corn, especially if most of the stover is removed.
Overall, conservation tillage appears to have had a positive effect on erosion. For example, in 1985, incentives were put in place to encourage adoption of conservation tillage practices. According to data from the National Resources Inventory (NRI), maintained by the U.S. Department of Agriculture (USDA) Natural Resources Conservation Service, overall annual cropland erosion fell from 3.06 billion tons in 1982 to about 1.75