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 Evapotranspiration. (Et) for day t may be estimated by the following equation:
Et = Cvt Pet
(1.1.9)
Where:
Et = evapotranspiration (cm)
Cvt = monthly cover coefficient (dimensionless)
Literature sources for monthly cropland cover coefficients are Chow (1964) and Davis and
Sorensen (1969).
Potential evapotranspiration is estimated in the GWLF model by the Hamon (1961) equation.
The model calculates saturated vapor pressure. The model user is required to supply monthly average
daylight hours (Mills et al., 1985) and mean daily temperature.
Percolation. Percolation (Pct) into the shallow saturated zone occurs when unsaturated zone
soil moisture exceeds field capacity. Percolation is calculated by normalizing the unsaturated zone to
represent the soil moisture content at field capacity. For the unsaturated zone at field capacity Ut = 0
(Haith and Shoemaker, 1987). The following mass balance equation estimates percolation for day t:
Pct
= Max (O; Ut + Rt + Mt - Qt - Et)
(1.1.10)
Ground water discharge (Gt) (Figure 1.1.3) to the stream channel and deep seepage movement
of water from the shallow saturated zone to the deep saturated zone is modeled by a simple linear
reservoir (Haan, 1972):
Gt = r St
(1.1.11)
Where:
r = ground water recession constant (dimensionless)
St = soil moisture of shallow saturated zone (cm)
The user must supply a ground water recession constant r, which may be estimated from the
hydrograph separation procedure (Chow, 1964). From a partial stream flow record of the planning
watershed a hydrograph is plotted on semilogarithmic paper to determine the separation between
surface and subsurface runoff.
Deep Seepage . Deep seepage (Dt) may be estimated from the residual of ground water
discharge. A conservative approach assumes deep seepage is negligible. The conservative approach
assumes deep seepage is negligible. The conservative mass balance, allowing no deep seepage,
1.1-10
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