Geoscience Reference
In-Depth Information
Input of basic meteorological data
Input of reference evapotranspiration
Apply Penman-Monteith
with actual crop data
Apply Penman-Monteith with
reference crop data and crop factor
Apply crop factor
Evapotranspiration of dry and wet uniform canopy and of wet soil
Divide over soil and crop using either leaf area index or soil cover
Interception
Potential transpiration
T
p
Potential soil evaporation
E
soil
,
p
Water stress
Reduce to maximum soil water flux
If selected, in addition reduce with empirical
soil evaporation method
Salinity stress
Actual soil evaporation
E
soil, a
Actual transpiration
T
a
Figure 9.5
Method used in SWAP to calculate actual plant transpiration and soil
evaporation of partly covered soils from basic meteorological input data.
the canopy resistance is set to zero and only the aerodynamic resistance applies. In
the case of a dry crop with optimal water supply in the soil, the canopy resistance is
equal to its minimum value and varies between 30 s m
-1
for arable crops to 150 s m
-1
for trees. For a dry and wet crop, the actual crop heights are used, while for bare soil
'crop height' is zero. The relection coeficient in case of a wet or dry crop equals
0.23, while for a bare soil the value 0.15 (-) is assumed.
Figure 9.5
shows an overview of the top boundary procedure followed in SWAP.
Both the use of the Penman-Monteith method and the use of a reference evapotrans-
piration rate with crop factors are allowed. After calculation of the evapotranspiration
lux of the dry and wet canopy and the wet soil, the potential plant transpiration
T
p
and soil evaporation
E
soil, p
luxes are derived by taking into account the amounts of
rainfall interception and the leaf area index (LAI) or soil cover. In the case of agri-
cultural crops interception is calculated with the methods of Von Hoyningen-Hüne
(
1983
) and Braden (
1985
) and in the case of forests with the method of Gash (
1979
,
1995
) (
Chapter 6
).
Subsequently,
T
p
in combination with the root length distribution over the root
zone, is used to derive the maximum root water extraction rates at various depths.
The actual root water extraction rates are calculated taking into account reductions
due to oxygen deiciency, water deiciency or salinity excess. For oxygen and water
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