Geoscience Reference
In-Depth Information
saturation vapour pressure is the
vapour pressure deficit
(vpd). The vpd is a measure of how much extra water
vapour the atmosphere could hold assuming a
constant temperature and pressure. The higher
the vpd the more water can be absorbed from an
evaporative surface.
Atmospheric mixing is a general term meaning
how well a parcel of air is able to diffuse into the
atmosphere surrounding it. The best indicator of
atmospheric mixing is the wind speed at different
heights above an evaporating surface. If the wind
speed is zero the parcel of air will not move away
from the evaporative surface and will 'fill' with water
vapour. As the wind speed increases, the parcel of
air will be moved quickly on to be replaced by
another, possibly drier, parcel ready to absorb more
water vapour. If the evaporative surface is large (e.g.
a lake) it is important that the parcel of air moves
up into the atmosphere, rather than directly along
at the same level, so that there is drier air replacing
it. This occurs through turbulent diffusion of the
air. There is a greater turbulence associated with
air passing over a rough surface than a smooth one,
something that will be returned to in the discussion
of evaporation estimation.
One way of thinking about evaporation is in
terms of a washing line. The best conditions for dry-
ing your washing outside are on a warm, dry and
windy day. Under these circumstances the evapora-
tion from your washing (the available water) is high
due to the available energy being high (it is a warm
day), and the receiving atmosphere mixes well (it
is windy) and is able to absorb much water vapour
(the air is dry). On a warm and still day, or a warm
and humid day washing does not dry as well (i.e. the
evaporation rate is low). Understanding evaporation
in these terms allows us to think about what the
evaporation rate might be for particular atmospheric
conditions.
Evaporation above a vegetation
canopy
Where there is a vegetation canopy the evaporation
above this surface will be a mixture of transpiration,
evaporation from the soil and evaporation from
wet leaves
(canopy interception or interception loss or wet
leaf evaporation)
. The relative importance of these
three evaporation sources will depend on the degree
of vegetation cover and the climate at the site. In
tropical rain forests transpiration is the dominant
water loss but where there is a seasonal soil water
deficit the influence of canopy interception loss
becomes more important. This is illustrated by the
data in Table 3.1 which contrasts the water balance
for two Pinus radiata forests at different locations
in New Zealand (with different climates).
Transpiration by a plant leads to evaporation from
leaves through small holes (stomata) in the leaf. This
is sometimes referred to as dry leaf evaporation. The
influence of stomata on the transpiration rate is an
interesting plant physiological phenomenon. Some
Table 3.1
Estimated evaporation losses from two
Pinus radiata
sites in New Zealand
Puruki (Central North Island,
Balmoral (Central South Island,
NZ) (% annual rainfall in
NZ) (% annual rainfall in
brackets)
brackets)
Annual rainfall
1,405 mm
870 mm
Annual interception loss
370 mm
(26)
220 mm
(25)
Annual transpiration
705 mm
(50)
255 mm
(29)
Annual soil evaporation
95 mm
(7)
210 mm
(24)
Remainder (runoff + percolation)
235 mm
(17)
185 mm
(21)
Source
: Data adapted from Kelliher and Jackson (2001)
