EVAPORATION - Fundamentals of Hydrology - page 52

1.0 then the plants are exerting stomatal control on
the transpiration rate.

Q G is soil heat flux density (MJ/m 2 /day)
T

is mean daily air temperature at 2 m height
(°C)

u is wind speed at 2 m height (m/s)

Simple estimation of E t from PE
and soil moisture

e

is the saturation vapour pressure deficit (kPa)

is the slope of the vapour pressure curve
(kPa/°C)

Where there is no stomatal control exerted by plants
(e.g. in a pasture) the relationship between actual
evaporation ( E t ) and potential evaporation ( PE ) is
by and large driven by the availability of water. Over
a land surface the availability of water can be esti-
mated from the soil moisture content (see Chapter
4). At a simple level it is possible to estimate the
relationship between potential and actual evap-
oration using soil moisture content as a measured
variable (see Figure 3.9). In Figure 3.9 a value of
1 on the y-axis corresponds to actual precipitation
equalling potential evaporation (i.e. available water
is not a limiting factor on the evaporation rate).
The exact position where this occurs will be depend-
ent on the type of soil and plants on the land
surface, hence the lack of units shown on the x-axis
and the two different curves drawn. This type of
simple relationship has been effective in deter-
mining actual evaporation rates in a model of soil
water budgeting (e.g. Davie et al ., 2001) but can-
not be relied on for accurate modelling studies. It
provides a very crude estimate of actual evaporation
from knowledge of soil moisture and potential
evaporation.

is the psychrometric constant (kPa/°C)

The reference evapotranspiration, provides a
standard to which evapotranspiration at different
periods of the year or in other regions can be
compared and evapotranspiration of other crops can
be related (Allen et al ., 1998). Scotter and Heng
(2003) have investigated the sensitivity of the
different inputs to the reference evaporation equa-
tion in order to show what accuracy of measurement
is required.
Table 3.4 outlines some crop coefficients as set out
by FAO (Allen et al ., 1998). At the simplest level
the evapotranspiration for a particular crop can be
estimated by multiplying the crop coefficient with
the reference evapotranspiration although there are
more complex procedures outlined in Allen et al .
(1998) which account for growth throughout a
season and climatic variability. Where the crop
coefficient values shown in Table 3.4 are higher than
1.0 it is likely that the aerodynamic roughness
of the canopy makes for higher evaporation rates
than for short grass. Where the values are less than

Table 3.4 Crop coefficients for calculating evapotranspiration from reference evapotranspiration

Crop type Crop coefficient (K c ) Comment

Beans and peas 1.05 Sometimes grown on stalks reaching 1.5 to 2 metres in

height. In such cases, increased K c values need to be
taken.

Cotton 1.15-1.20

Wheat 1.15

Maize 1.15

Sugar Cane 1.25

Grapes 0.7

Conifer forests 1.0 Confers exhibit substantial stomatal control. The K c can

easily reduce below the values presented, which
represent well-watered conditions for large forests.

Coffee 0.95