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deiciency, the reduction function of Feddes et al. (
1978
) is used. For saline conditions,
the reduction function of Maas and Hoffman (
1977
) is employed. In the case of simul-
taneous water and salt stress, SWAP multiplies both reduction factors. Integration
over the depth of actual root water extraction rates yields the actual transpiration rate
(
Chapter 6
).
Reduction of
E
soil,p
for dry soil conditions occurs in two ways. SWAP calculates
the maximum upward soil water lux near the soil surface, using Darcy's equation,
the prevailing soil hydraulic functions and the actual soil water status. In addition, we
employ empirical reduction functions based on Black (
1969
) or Boesten and Stroos-
nijder (
1986
). Although from a physical point of view the maximum soil water lux
based on Darcy should sufice, the resulting lux generally overestimates the evap-
oration rates of dry soils. Probably the soil hydraulic functions change close to the
soil surface because of splashing rain, crust formation and plant residues. Therefore
SWAP determines the actual evaporation rate as the minimum of
E
soil,p
, the maximum
Darcy lux and a selected empirical reduction function.
Question 9.3:
The top boundary conditions for rainfall and evapotranspiration described
in the preceding text can be applied at time intervals of days or shorter. For which pur-
pose would you prefer shorter time intervals?
9.1.4 Bottom Boundary Condition Hydrology
The following options are offered to prescribe the bottom boundary condition:
1. Specify the groundwater level or soil water pressure head as function of time.
2. Specify the bottom lux as function of time.
3. Specify the bottom lux as function of groundwater level.
Measurements of groundwater levels are relatively easy and are often used during
model calibration with experimental data. However, when alternative scenarios have
to be simulated, groundwater levels may change, and therefore cannot be prescribed
anymore. Prescribed bottom luxes to simulate experiments are an attractive option,
as ixed bottom luxes may increase the accuracy of simulated soil moisture proiles
and solute leaching. Unfortunately, despite considerable efforts no reliable and practi-
cal soil water lux measurement devices have been developed until now. Nevertheless,
situations in which the bottom lux can be prescribed occur when a soil layer with a
low permeability is present in the subsoil, or when the seepage lux is more or less
constant and known.
For scenario analysis a more general boundary condition should be used, such as
soil water lux as function of groundwater level or bottom pressure head. Such rela-
tions in general require calibration with ield data.
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