Water Quality Management - River Dynamics and Integrated River Management - page 594

more than 800 kg in 2000. In 17 provinces, per hectare chemical fertilizer application is higher than the
recommended level (225 kg) in the world (Wang, 2006).
Unfortunately, little is known on how a reduction of fertilizer and manure applications is translated
into a reduction in nutrients in surface water, ground water, or at some point in the receiving water
downstream from the field (Camacho, 1990). For example, in the Conestoga Headwaters Rural Clean Water
Project, it was found that reducing the nitrogen application by 50% resulted in approximately a 10%
reduction in nitrogen in runoff from a field with terraces. Similarly, Kanwar et al. (1987) found for field
plots of no-till corn, a reduction in fertilizer of 30% resulted in a 15% reduction in subsurface nitrogen
loss. Whitaker et al. (1987) studied the effectiveness of no-till in reducing total nitrogen in runoff and
found that the total nitrogen reduction efficiency was 40%-60% whether the fertilizer nitrogen application
was 7% or 166% of the recommended amount.
The other aspects of a nutrient-management plan also can be very important. For example, Shalit et al.
(1995) stated that for structured soils, where preferential flow can be substantial, the BMP for reduction
of dissolved constitutent loading to tile drains appears to be timing the fertilizer applications during drier
soil conditions and mixing the fertilizer into the upper soil layer (subsurface banding). Subsurface banding
of fertilizer is reported to reduce dissolved nitrogen and total phosphorus in surface runoff by 50 percent
for several different types of conservation tillage (Casman, 1990). However, Casman notes that these
results are more encouraging for phosphorus management than for nitrogen because most nitrogen is
transported from the field in subsurface flow. Finally, Casman reports that the gross effect of fall fertilization
on annual nutrient losses in runoff is to multiply losses observed from spring only fertilization by a factor
of 1.5-2.
The problem with nutrient management is that well-trained people and substantial time are required to
study the soils in each farm field and establish the optimum strategy for fertilization. For example, as of
June 1993, only about 80,000 ha of the total 800,000 ha of farmland in the Chesapeake Bay watershed had
nutrient managment plans despite several years of multi-State commitments to develop nutrient-management
plans throughout the watershed.
Best management systems —When properly applied, BMPs may reduce nutrients in runoff 30%-40%
relative to farming without BMPs, but proper application requires careful planning, implementation, and
maintenance. A non-point source pollution problem is seldom eliminated or mitigated by application of a
single BMP. Therefore, it should not be assumed that just because BMPs for soil conservation are
implemented, substantial water-quality benefits (nutrient reduction) are obtained. Usually, a combination
of BMPs that work together as a Best Management System (BMS) (Camacho 1993) is necessary to solve
non-point source pollution problems. In particular, BMSs that utilize nutrient management in combination
with agronomical BMPs such as strip cropping, conservation tillage, and winter cover crops (where
appropriate) have been found to be cost effective management alternatives for nutrient reduction and are
being implemented in the Chesapeake Bay, U.S., Nutrient Reduction Program (Camacho, 1993).
9.3.2.3 Watershed Scale Effectiveness of Agricultural BMPs
In the case of mixed land uses, a common assumption for non-point source pollution control is that
constituent loads at the watershed outlet can be proportionally reduced if non-point source loadings from
critical areas that substantially contribute to pollution are controlled. The effectiveness of BMPs applied
to critical areas typically has been estimated utilizing information from field-scale tests (summarized
previously) and/or non-point source pollution-model applications. It is not proven, however, that such
applications actually reduce non-point source loadings from watersheds with complex land-use patterns.
Non-point source pollution-model-based estimates of watershed-scale nutrient reduction can often be
grossly erroneous. For example, Garrison and Asplund (1993) compared model estimates with measured