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 assumes all watershed precipitation exists as evapotranspiration or stream flow (Haith and Shoemaker,
1987):
Dt = s St
(1.1.12)
Where:
Dt = deep seepage (cm)
s = rate constant for deep seepage (dimensionless)
St = soil moisture of the shallow saturated zone (cm)
1.1.18 Nutrient Budget for the Loading Function
Screening-level methods for estimating source area nutrient loads include the export coefficient
(unit area load) and the loading function. Reckhow et al. (1980) developed a procedure for use of
nutrient export coefficients with an empirical lake model for phosphorus. The procedure included an
analysis of pollutant loading uncertainty expressed as a high and low pollutant export range associated
with the most likely export. Reckhow et al. 1980 lists phosphorus and nitrogen export coefficients from
well-designed runoff studies for the major land uses, along with size of study area, annual precipitation
and soil characteristics. The following nonpoint nutrient sources are not included in the discussion, but
references are noted for each source:
Nutrient Sources
References
atmospheric inputs
Reckhow and Chapra, 1983
septic tank inputs
nutrient regeneration from
water body sediments
waterfowl fecal matter
Manny et al., 1975
To estimate the total watershed pollutant load we suggest the use of the nutrient budget.
Reckhow et al. (1988) provide guidance on nutrient budget estimation for state-level applications. The
methods provided in this methodology include loading functions, export coefficients and the references
provided herein.
Both annual and seasonal variability in pollutant loading are important to the assessment and
they should be quantified. Annual variability is generally high so that several modeling years may be
required to achieve a satisfactory estimate of the mean annual pollutant export. Seasonal land use
changes within an agricultural watershed may be modeled using the seasonal period of assessment with
monthly loading estimates.
The generalized watershed loading function (GWLF) model developed by Haith and
Shoemaker (1987) is the basis for the following nutrient and sediment assessment. The GWLF
1.1-11
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