Environmental Engineering Reference
In-Depth Information
surfaces (soils and wet vegetation) and by transpiration (
T
) from plants through
stomata in the plant leaves (Fig.
18.1
). Evaporation is the physical process by which
water is transformed into water vapour and is removed from the evaporating sur-
face by mass transfer. Transpiration is the process by which water in plant tissues
is transformed into water vapour and removed towards the atmosphere. Important
weather parameters affecting
ET
are radiation, air temperature, humidity and wind
speed (see further). Another important factor is the amount of water available at the
evaporating surface (e.g., the soil surface), or in the soil for uptake by plant roots.
The available water is determined by such factors as soil type (texture), depth to
ground water, irrigation and management practices. Root water uptake is negatively
influenced by water logging (i.e., when the soil is completely or almost completely
saturated), and pore water salinity. Crop characteristics such as crop type, variety,
development stage, crop height, crop roughness, reflection, ground cover and crop
rooting characteristics all influence the resistance to transpiration, and hence
ET
.
To remove the effects due to soil type, management and crop factors on calcu-
lations of the evaporative demand of the atmosphere, the evapotranspiration rate is
generally calculated for a reference surface not short of water. This
ET
is called
the reference evapotranspiration,
ET
o
, and usually calculated following guidelines
of the FAO56 paper by Allen et al. (
1998
) using the Penman-Monteith equation
(Monteith
1965
) (see Section
18.2.6
). The reference surface is a hypothetical ref-
erence crop (grass) with an assumed height of 0.12 m, a fixed surface resistance of
70 s m
−
1
and an albedo of 0.23.
ET
o
thus only depends on climatic parameters and
can be considered as a climatic parameter expressing the evaporating power of the
atmosphere at a specific location and time of the year (Allen et al.
1998
).
Since actual ground cover, canopy properties and aerodynamic resistance of the
crop are different from those used for calculating
ET
o
, the evapotranspiration rate
under standard conditions (i.e., of a large field under excellent agronomic and pore
water conditions) for a specific crop,
ET
c
, is required for specific applications.
ET
c
can be obtained either by using specific crop parameters (e.g., albedo, aero-
dynamic and canopy surface resistances) in the Penman-Monteith equation, or by
multiplying
ET
o
with a crop coefficient
K
c
. The crop coefficient incorporates four
primary characteristics that distinguish a specific crop from the reference crop (crop
height, albedo, canopy resistance, and evaporation from the soil) (Allen et al.
1998
)
and is determined by crop type, climate, soil evaporation and crop growth stages.
Consequently,
K
c
coefficients change during a growing season.
Actual evapotranspiration under field conditions,
ET
a
, takes into account non-
ideal (non-standard) conditions such as water or salinity stress. Under dry soil
conditions, water flow in a soil can be too slow to satisfy the evaporative demand.
Similarly as for transpiration, very dry or very wet soil conditions, or high salt con-
centrations, impose water and salinity stresses and reduce root water uptake. Allen
et al. (
1998
) related
ET
a
to
ET
o
by means of a water stress coefficient and/or by
adjusting
K
c
for all kinds of stresses. In this chapter, a mechanistic approach to
determine
ET
a
is described using water flow in the soil and water uptake by plant
roots, the latter incorporating empirical water stress reduction functions (see Section
18.2.5
).
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