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 1.1.5 NONPOINT SOURCE SIMULATION MODELS
1.1.5.1 Estimating On-Site and Off-Site Impacts
On-site benefits are highly desirable, yet unless the needed off-site benefits are derived from the
collective implementation of management measures and practices across the watershed,
Table 1.1.13 Total Nitrogen and Total Phosphorus
Concentration in Wastewater Treatment Plant Effluent
Mean Nitrogen
Mean Phosphorus
Treatment Type
(mg/l)
(mg/l)
Activated Sludge
15.8
5.91
Tricking Filter
17.9
7.25
Phosphorus Removal
4.0
2.85
Primary Settling and
23.8
8.71
Digestion
Oxidation Pond
17.1
6.42
Sand Filter
--
5.02
(after Gakstatter et al. 1978 and Reckhow and Chapra 1983)
then implementation has not been fully successful. It is important to estimate the collective impacts of all
management activities in the watershed to gage whether water quality goals will be achieved. In
watersheds with easily characterized problems (e.g., bacterial contamination is due to a few obviously
polluting animal operations in a watershed that has no other identifiable sources of pathogens) it may be
very easy to project that water quality benefits will be achieved through implementation of the
management measures for nutrient management, erosion and sediment control, and facility wastewater
and runoff, for example. However, in a watershed with multiple land uses where agriculture is
considered to contribute about one-third or so of the pollutants, it is more complicated to estimate the
combined impacts of a variety of management measures and practices on a fairly large number of
diverse farming operations. In this type of situation, computer modeling may be needed.
A variety of models exist to help assess the benefits of implementing practices at the farm level,
some of which could also be used on small watersheds.
These include the following:
GLEMS (Knisel et al., 1991) simulates the effects of management practices and irrigation
options on edge of field surface runoff, sediment, and dissolved and sediment attached nitrogen,
1.1-37
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