WATERSHEDS - Water Quality Engineering in Natural Systems - page 237

Figure 6.24. Nutrient management.

Figure 6.25. Terraces. Source : Natural Resources Conserva-
tion Service (2005b).

applies contact herbicide only to those weeds that grow
above the soybean canopy. Complete studies on the
effectiveness of IPM show mixed results.

6.3.3.3 Nutrient Management. Nutrient management
is a series of practices designed to decrease the avail-
ability of excess nutrients through improvements in
timing, application rates, and location selection for
fertilizer application. Current nutrient management is
based on the limiting-nutrient concept that the fertilizer
application rate should be based on the nutrient most
needed by the plant for optimum growth, usually nitro-
gen. This practice is especially effective in controlling
the soluble phases of nutrients. An example of nutrient
management is shown in Figure 6.24, where a nitrogen
fertilizer is being applied to a field at levels that are
compatible a nearby housing development, which means
limiting nitrate levels in the underlying groundwater
and minimizing the impact on nutrient levels in nearby
drainage canals.

Figure 6.26. Drop structure. Source : Natural Resources Con-
servation Service (2005b).

6.3.3.5 Critical Area Treatment. Grade stabilization
structures and critical-area planting are both consid-
ered critical area treatment. A grade stabilization
structure is used to control the grade in natural or
artificial channels. These structures reduce water
velocity, thus preventing additional sediment detach-
ment and decreasing the transport capacity of water.
Structures can be constructed of metal, wood, rocks,
concrete, or earth, and the area surrounding the struc-
ture must be stabilized. A drop structure is a type
of grade stabilization structure that is sometimes built
at the outlet of a grassed waterway to stabilize the
waterway and allow runoff to leave the waterway
without causing gully erosion. A typical drop structure
is shown in Figure 6.26. Critical area planting involves
planting suitable vegetation (such as grass, trees, or
shrubs) on highly unstable sites. Intensive methods,

6.3.3.4 Terraces and Diversions. A terrace is an
earthen embankment, channel, or a combination ridge
and channel constructed across the slope to intercept
runoff. Terraces reduce the slope effect on erosion by
dividing fields into segments with less steep or nearly
horizontal slopes. Soil particles and adsorbed pollutants
are thus not transported from the field, with terraces
retaining all water applied to the fields, hence reducing
the loss of dissolved pollutants. hillside vineyard ter-
races in Sonoma County, California, are shown in Figure
6.25. Level terraces can remove up to 95% of the sedi-
ment, up to 90% of its associated adsorbed nutrients,
and between 30% and 70% of dissolved nutrients.
Diversions can reduce sediment movement by 30-60%
and adsorbed nutrients by 20-45%.