Environmental Engineering Reference
In-Depth Information
evaporation of the intercepted water (
E
p,I
), potential soil evaporation (
E
p,s
) and
potential transpiration (
T
p
) using procedures outlined below.
Daily values of the actual soil evaporation rate,
E
a,t
[L], and actual transpiration
rate,
T
a,t
[L], may be calculated using HYDRUS-1D (Šimunek et al.
2005
). The
input variables for HYDRUS-1D are time series of daily values for the through-
fall (e.g., precipitation or irrigation reaching the soil surface),
T
, potential soil
evaporation,
E
p,s
, and potential transpiration,
T
p
.
18.2.7 Root Water Uptake
The sink term
S
in Eq. (
18.8
) is defined as the volume of water extracted from a unit
volume of soil per unit time by the roots. The potential
root water uptake rate S
p
(z)
is often obtained by multiplying a normalized water uptake distribution
b
(
z
)[L
−
1
]
with the potential transpiration rate
T
p
[LT
−
1
] as follows:
S
p
(
z
)
=
b
(
z
)
T
p
(18.19)
The function
b
(
z
) may be obtained from the root distribution with depth:
b
(
z
)
b
(
z
)
=
(18.20)
b
(
z
)
L
R
where
b
(
z
) is the root distribution function and
L
R
is the soil root zone. Note that
b'
(
z
) can be of any form.
T
p
depends on climate conditions and vegetation (leaf area
index, crop coefficients
see further). The actual root water uptake rate
S
(z) may
be obtained by multiplying
S
p
(
z
) with a root water stress response function (e.g.,
Feddes et al.
1978
; Van Genuchten
1987
) to account for a possible reduction in root
water uptake due to water stress conditions in the soil profile:
−
S
(
h
,
z
)
=
α
(
h
)
S
p
=
α
(
h
)
b
(
z
)
T
p
(18.21)
where
(
h
) is the water stress response function as a function of the pressure head.
To obtain the actual transpiration rate
T
a
of the vegetation, the actual root water
uptake
S
(
h
,z) (Eq. (
18.21
)) is integrated over the rooting depth:
α
T
p
L
R
T
a
=
α
(
h
,
z
)
b
(
z
)
dz
(18.22)
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