Water Quality Management - River Dynamics and Integrated River Management - page 592

more nutrients may be lost in subsurface runoff than in surface runoff. Thus, nutrients in the water that
infiltrates must remain in the root zone long enough for plant uptake or fixation to soils by microbial
activity.
The effectiveness of BMPs is dependent on complex, site-specific factors such as soils, slopes, crops,
meteorology, and farmer diligence (Camacho, 1990). Therefore, available summaries of BMP efficiency
studies (Casman, 1990; Camacho, 1990) report broad ranges of the effectiveness of BMPs. These ranges
are presented here to provide information on the level of nutrient removal possible if the BMPs are
properly applied and maintained. The actual removal efficiency of BMPs depends on site-specific factors.
It should not be assumed that because a given BMP is applied that a favorable nutrient removal efficiency
automatically results. In general, application of BMPs results in reduced nutrient loads leaving farm fields;
however, examples where applications of BMPs resulted in increases in nutrient runoff are presented
later for consideration.
9.3.2.3 Field Scale Effectiveness of Selected Agricultural BMPs
Strip cropping and vegetative filter strips —Factors affecting the effectiveness of vegetative filter strips
(VFSs) include filter length, depth of flow, slope, cross slope, soil type, influent characteristics, clogging
of filter with repeated use, and hydraulic loading rate (Casman 1990). Casman (1990) reported effectiveness
ranges for nutrient removal utilizing VFSs based on several studies for near uniform, sheet inflow, as
follows. The removal efficiency of total nitrogen and total phosphorus in surface runoff ranged from
-17% to 71% and from 2% to 80%, respectively, whereas the removal efficiency of total nitrogen in all
runoff ranged from -20% to 92%. Casman (1990) also stated that with all the variation in experimental
design and results, it is clear that no single efficiency should be used to characterize VFS performance.
However, based on these results, Casman (1990) estimated that for planning purposes the removal
efficiencies for total nitrogen and total phosphorus in all runoff are approximately 30 percent and from
30 to 90 percent, respectively, for well designed and maintained VFSs.
The results of Dillaha et al. (1988) indicate that the nutrient removal efficiencies of VFSs determined
by Casman (1990) are probably unrealistic. They reported that most VFSs on farms are ineffective because
runoff tends to concentrate in natural drainageways before reaching the filter strip. They also found that
filters that received such concentrated flows were from 40% to 69% less effective for sediment removal,
from 70% to 95% less effective for phosphorus removal, and from 60% to 70% less effective for nitrogen
removal than VFSs that received nearly uniform sheet flow.
Maintenance of the VFSs is an important factor in attaining high nutrient removal efficiencies. The
sediment retention effectiveness of VFSs may decrease rapidly as the vegetation is covered with sediment.
Further, in the long term, the sediment bound nutrients filtered out by grasses tend to wash back into the
water (if allowed to build up); thus, decreasing removal effectiveness on a long-term basis. If the sediment
deposited on the filter strips is regularly removed from the filter and incorporated in the surrounding soil,
filter effectiveness may remain high.
Conservation tillage —The efficiency of tillage practices in reducing sediment and nutrient loads is
greatly affected by soil properties; surface slope; the previous crop; the amount of residue removed;
placement, type, quantity, and timing of fertilizer; harvesting practice; variety of crop; planter style;
orientation of contour; and meteorologic conditions (Casman, 1990). Casman (1990) summarized the results
of four studies on the removal effectiveness of conservation tillage for nitrogen. Subsurface losses of
nitrate were higher for conservation tillage than for conventional tillage (removal efficiencies from -70% to
38% were reported, with three of the four studies reporting negative efficiencies). Higher subsurface losses
of nitrate for conservation tillage compared to conventional tillage may result because plow tillage may
produce higher nitrogen mineralization from the soil than conservation tillage (Levanon et al. 1993).