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 Some of the lessons learned from RCWP include: a) a reduction in erosion may or may not
improve water quality, b) the effectiveness of structural BMPs (e.g. waste storage structures, sediment
basins, terraces, improved irrigation) may be enhanced by nutrient management, waste management
and conservation tillage c) practices that reduce surface runoff can increase the leaching potential for
nitrate and pesticides .
The National Water Quality Evaluation Project (1989) found that the level of participation by
farmers in a voluntary program is important for success. There is no single accepted relationship
between treatment strength (number of farmers participating) and pollutant reduction. Treatment of
75% of the critical area was sufficient for some RCWP projects and not for others. Nonpoint source
control was not demonstrated when: a) a water use impairment was not documented, b) the project
area was too large and contained varied types of pollution sources, c) where local interest was
inadequate, and 4) where monitoring capability was unable to detect trends.
Conservation Tillage. Conservation tillage, practice 329, is any tillage and planting system
that leaves at least 30% of the soil surface covered by residue after planting (SCS 1987). The soil is
tilled only to the extent needed to prepare a seedbed, incorporate chemicals, control weeds, and plant
the crop (Brach 1989). The purpose is to reduce soil erosion, help maintain good soil tilth and efficient
moisture use, improve water quality, and provide food and cover for wildlife. In general, conservation
tillage increases infiltration and decreases evapotranspiration resulting in reduced surface runoff but can
potentially increase percolation.
The average reduction in surface runoff due to conservation tillage may be on the order of 20-
25% (Baker and Laflen 1983; Wendt and Burwell 1985; Gilliam and Hoyt 1987).
Erosion control and the reduction of edge-of-field sediment loss is the primary attribute of
conservation tillage compared to conventional tillage with the moldboard plow. For every 9 to 16%
increase in residue cover, erosion is approximately halved (Baker and Laflen 1983). Conservation
tillage should therefore reduce erosion by 75 to 90% depending on the percentage residue cover over
that for conventional tillage (Dillaha et al. 1988).
Sediment-attached nutrient losses are generally reduced as more sediment remains in place.
With conservation tillage, it may be difficult to incorporate nutrients and maintain plant residue, and the
quantity of nutrients near the soil surface may increase. If the surface soil has a higher concentration of
nutrients then the nutrient content of eroded sediment is likely to be higher. Losses of sediment-
attached nutrients due to conservation tillage will be affected by the soil lost and its nutrient content
(Baker and Laflen 1983).
The nutrient content of surface runoff is also likely to be higher if fertilizer is not incorporated.
Runoff nutrient concentrations are directly proportional to soil nutrient concentrations (Baker and Laflen
1983). For conservation tillage fertilized with manure, losses of nutrients in surface runoff are greatly
4.2-10
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