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 slope aspect, vegetative cover coefficient, decimal fraction of daily cloud cover, clear sky daily solar
radiation, maximum daily temperature, mean daily wind speed, mean daily dew point and daily rainfall.
Snowmelt. (M) as given in GWLF can also be estimated from the degree-day equation given
by Haith (1985) from daily precipitation depth and mean daily temperature. While this approach is not
expected to provide the most reliable results, some snowmelt estimation is required for many northern
temperate regions. Precipitation is assumed to be snow when:
Tt ≤ 0 oC
(1.1.5)
Snowmelt on day t may be estimated by the degree day equation proposed by Haith (1985). For daily
mean air temperature, T, (oC) greater than zero, then
M = 0.45 Tt for T > 0
(1.1.6)
A degree-day model is based solely on average daily temperature effects on snowmelt. Thus, a
degree day equation may not adequately represent the day to day variability in snowmelt.
1.1.1.6 Functional Relationship of Loads and Concentrations
For the purpose of watershed management, pollutant loads in kilograms per time period will be
used to evaluate the contribution that each source makes to the receiving water body. Ranking
of pollutant sources based on total load contribution is useful for assessment and targeting
source areas for treatment.
For eutrophication modeling and assessing pollutant impact on the ecosystem, the most useful
quantification of pollutant flux to the water body is the nutrient concentration. The stream and lake
models (Section 4.0) require average period of assessment pollutant concentrations.
1.1.1.7 Hydrologic Budget for the Loading Function
The GWLF model (Haith and Shoemaker, 1987) features a simple, daily time step hydrologic budget
for the lumped parameter model. The daily water budget is summarized in Figure 1.1.3.
1.1-8
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