Daily Estimates of Evaporation - Terrestrial Hydrometeorology - page 355

The restriction on evaporation rate is then parameterized in terms of the
ambient volumetric soil moisture content of the soil sample, q s . Often a simple
moisture stress function f ( q s ) with the general form shown in Fig. 23.3 is assumed,
with values of soil moisture defined when the soil is saturated, q s sat , when it is at
'field capacity' (after draining from saturation), q s fc , and at a 'wilting point, q s wilt .
At wilting point, it is assumed the stomata are closed and evaporation ceases.
In Fig. 23.3, typically the value of ( q s sat / q s fc ) is 0.5 to 0.8. If the evaporation estimate
is made using a crop factor approach, the value of the ambient water-stressed crop
factor, K c , at any point in time is assumed to be given by:

Kf K ¢ q = ()

(23.39)

c s c

If the evaporation estimation is parameterized in terms of the surface resistance of
the crop, ( r s ) c , the value of the ambient water-stressed surface resistance, ( r s ) c ', a t
any point in time is given by:

() r ¢ = () r f ( ) q

sc sc s

(23.40)

In such models, the daily running water balance takes the general form:

()

MMPE D

n

=

n − 1

+

n

−

n

q

n

−

n

s s s

(23.41)

where M s n and M s n−1 are the depth of water in mm in the soil store on days n and
( n −1), respectively, and P n , E n (q s n ) and D n are the precipitation, evaporation, and
drainage on day n , with E n (q s n ) either calculated from Equation (23.12) using the
soil moisture weighted crop factor K c ′

given by Equation (23.39), or from Equation
(23.30) or Equation (23.37) with the soil moisture weighted surface resistance
given by Equation (23.40).

Important points in this chapter

Daily average meteorological variables : daily estimates of evaporation are
usually made from meteorological variables that are themselves estimates of
daily average or daily total values derived from the limited measurements taken
at agro-meteorological climate stations using the procedures given in the text:
- for temperature, humidity and wind speed, by Equations (23.1) to (23.7)
with calculation illustrated by example in Table 23.1;
- for net radiation, using the equations defined in the relevant section with
calculation illustrated by example in Table 23.2.

●

Use of the Penman-Monteith equation : all the preferred methods for esti-
mating daily evaporation recommended in this text are derived from the
Penman-Monteith equation, including:

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