Geoscience Reference
In-Depth Information
This numerical scheme is mass conservative and stable. To solve accurately evapo-
ration and iniltration luxes at the soil surface, the compartment thickness should be
maximum 1 cm. Application of Eq. (
9.5
) to each compartment results in a set of
n
equations with
n
unknown pressure heads, which can be solved eficiently. The time
step is based on the number of iterations required to solve the set of equations. At a
large number of iterations, the time step is decreased; at a small number of iterations,
the time step is increased.
9.1.3 Top Boundary Condition Hydrology
Measurement of reliable evapotranspiration luxes is far from trivial and strongly
varies with the local hydrological conditions. Therefore SWAP simulates evapotrans-
piration luxes from basic weather data with the Penman-Monteith equation (
Chap-
ter 7
) or from reference evapotranspiration data. Application of the Penman-Monteith
equation requires daily values of air temperature, net radiation, wind speed and air
humidity. In case these data are not available, popular alternative evapotranspiration
formulas can be used, such as Priestly-Taylor (
1972
), Makkink (Makkink,
1957
; Fed-
des,
1987
) and Hargreaves et al. (
1985
). The Priestly-Taylor and Makkink meth-
ods require only air temperature and solar radiation data. The method of Hargreaves
requires solely air temperature data. These alternative methods calculate a reference
evapotranspiration lux that is generally deined for a hypothetical grass cover of
12 cm high, with an albedo of 0.23 and a canopy resistance of 70 s m
-1
. To derive the
luxes for the actual crop, so-called crop and soil factors are used (
Chapter 8
).
In general the daily water luxes passing through a canopy are large compared to
the amounts of water stored in the canopy itself (
Chapter 6
). On a daily basis we may
assume soil water extraction by roots to be equal to plant transpiration. Whereas root
water extraction occurs throughout the root zone, soil evaporation occurs at the soil
surface. Owing to the steep gradient of water contents and pressure heads near the
soil surface, during drying conditions evaporation luxes decline more rapidly than
transpiration luxes. Once water has iniltrated into the soil and the soil surface has
become dry, soil evaporation luxes become small. Water harvesting, in which ields
are left fallow during one or several seasons, is based on this phenomenon. Because of
the different physical behaviour of the transpiration and evaporation process, SWAP
simulates these processes separately.
SWAP calculates three quantities with the Penman-Monteith equation:
•
Evapotranspiration rate of a wet canopy, completely covering the soil
•
Evapotranspiration rate of a dry canopy, completely covering the soil
•
Evaporation rate of a wet, bare soil
These quantities are obtained by using the appropriate values for canopy resistance,
crop height and relection coeficient. In the case of a wet canopy or a bare wet soil,
Search WWH ::
Custom Search