Measurement and prediction of evaporation from forested and agricultural catchments

Agricultural Water Management, 8 (1984) 1--28

1

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands MEASUREMENT

AND

AGRICULTURAL

J.B.

PREDICTION

OF

EVAPORATION

FROM

FORESTED

AND

CATCHMENTS

Stewart

Institute of Ht]drology, Crowmarsh Gifford, Wallingford, Oxon OXlO 8BB (Great Britain) ABSTRACT

The workshop on Land and Stream Salinity held in Perth, Western Australia in November 1980 recommended that remedies to the salinity problems of south-western Australia should be sought in the recharge areas, as opposed to the treatment of the saline discharge areas. In the climate of t h a t a r e a , v e g e t a t i o n c o u l d e v a p o r a t e a t much h i g h e r r a t e s t h a n o c c u r s a t p r e s e n t , e i t h e r from t h e a g r i c u l t u r a l c r o p s o r from t h e n a t i v e f o r e s t s ; i n t r o d u c t i o n o f such v e g e t a t i o n c o u l d t h e r e f o r e lower t h e w a t e r t a b l e and r e d u c e saline seepages. C a l c u l a t i o n s show t h a t t o a c h i e v e a s i g n i f i c a n t d e c r e a s e i n t h e h e i g h t o f t h e w a t e r t a b l e would r e q u i r e t h a t e i t h e r t h e r e p l a c e m e n t v e g e t a t i o n had a v e r y h i g h r a t e o f e v a p o r a t i o n o r a l a r g e p r o p o r t i o n o f t h e a g r i c u l t u r a l a r e a had t o be c o n v e r t e d t o t h e new v e g e t a t i o n . The major d i f f e r e n c e s i n m e t e o r o l o g i c a l and b i o l o g i c a l c h a r a c t e r i s t i c s between f o r e s t s and a g r i c u l t u r a l c r o p s a r e h i g h l i g h t e d . Methods o f m e a s u r i n g evaporation are briefly reviewed and compared. It i s e m p h a s i s e d that methods of measuring evaporation from large areas and over periods of years can only be used to determine the total evaporation. Whereas physically based methods used for smaller areas and shorter periods can measure the individual evaporation components and provide a sound basis for prediction of evaporation. It is concluded that the Monteith-Penman equation is most suitable for predicting the effects of changes in land use management. The necessary data collection and analysis for use with this equation are discussed.

INTRODUCTION

In by

t h e w h e a t b e l t of W e s t e r n

annual

agricultural

hydrological problem. as

the

perennial

crops

Australia

has

the

resulted

in

r e p l a c e m e n t of n a t i v e f o r e s t the

d e v e l o p m e n t of

a

major

It h a s b e e n o b s e r v e d o v e r t h e l a s t h a l f c e n t u r y t h a t ,

deep-rooted

native

vegetation

has

been

r e m o v e d a n d s h a l l o w - r o o t e d a n n u a l c r o p s g r o w n in i t s p l a c e ,

progressively the water table

h a s r i s e n a n d now h a s r e a c h e d t h e s u r f a c e in m a n y p l a c e s (Dimmock e t a l . , 1974;

Nulsen and Henschke,

shallow d e p r e s s i o n s ,

1981).

Also s a l i n e g r o u n d w a t e r h a s a p p e a r e d in

where high evaporation rates have rapidly generated the

f o r m a t i o n of s a l t p a n s .

With f u r t h e r

the

longer

land,

which

is

no

r i s e s in t h e w a t e r t a b l e t h e e x t e n t of

suitable

for

agriculture,

has

continued

to

i n c r e a s e a t t h e r a t e o f 7800 h a y r "1 s i n c e 1955 (Hillman, 1981). Since there this

area,

is no e v i d e n c e o f a c h a n g e in t h e i n p u t of p r e c i p i t a t i o n to

it is t h o u g h t

that

the

rise

in t h e w a t e r t a b l e is solely d u e to a

d e c r e a s e in e v a p o r a t i o n as a r e s u l t of t h e c h a n g e in v e g e t a t i o n . 0378-3774/84/$03.00

© 1984 Elsevier Science Publishers B.V.

Besides the

a g r i c u l t u r a l crops h a v i n g shallower roots than the n a t i v e v e g e t a t i o n , they are also annual r a t h e r of

the

r ai n y

than p e r e n n i a l species, and p a r t i c u l a r l y at the b e g i n n i n g

season

in

May,

there

is

i n u f f i ci en t

vegetation

to

prevent

considerable drainage to the water table. Further

ch a n g e s

in

the

occurring.

For some y e a r s

marginata),

has been u n d e r

leading to 'die b a c k ' .

remaining

areas of native v e g e t a t i o n are still

the predominant Eucalyptus species, J a r r a h attack from a f u n g u s

Phytophthora

(E_.

cinnamomi,

It has been estimated that 200,000 ha of J a r r a h f o r e s t s

h a v e already been a f f e c t e d (CSIRO,

1981).

bau x i t e mining which is continuing.

Also t h e r e has been open cast

The worked out areas are then r e p l a n t e d

with some species of n a t i v e a n d / o r exotic v e g e t a t i o n . To discuss a

Land

and

the salinity problems of the wheatbelt of Western Australia,

Stream

November 1980.

Salinity

Seminar and

Workshop

was

held

in the A u g u s t 1981 is s u e of A g r i c u l t u r a l Water Management. conclusions was

in

P er t h

in

The p a p e r s p r e s e n t e d at this Seminar have been p u b l i s h e d a 'general

endorsement

salinity problems in the r e c h a r g e areas

of the

One of the main

policy to seek solutions to

as opposed to the treatment of the

saline d i s c h a r g e a r e a s ' . T h e r e are a number of possible ways of c a r r y i n g out this recommendation with the object of lowering the water table in these a r e a s , o r p r e v e n t i n g the o c c u r r e n c e of saline s e e p s .

thereby reducing

Borehole pumping and artificial

d r a i n a g e are two of the possible e n g i n e e r i n g solutions, whereas i n c r e a s i n g the e v ap o r at i o n in t h e s e areas is a n o t h e r possible solution.

In this p a p e r only

the last solution will be c o n s i d e r e d in detail. Peck and Hurle (1973) have estimated the i n c r e a s e in annual r e c h a r g e r e s u l t i n g from the removal of the n a t i v e v e g e t a t i o n .

T h e y have found that on

a v e r a g e it is g e n e r a l l y small o of the o r d e r of 23 to 65 ram y r "1 r e l a t i v e to the annual rate of potential evaporation which is g r e a t e r than 1500 mm y r "1. Peck (1977) concluded from this that it was physically possible to p r e v e n t or e v e n reclaim d r y l a n d salinity in this region by a l t e r i n g the land management p r a c t i c e s so that the evaporation from the region is i n c r e a s e d . To a p p r e c i a t e the magnitude of the r e q u i r e d ch an g es in land u s e , it is u s e f u l to c a r r y out some calculations with estimated values of the e v a p o r a t i o n . These

calculations

can

show what

proportion

of the

c o n v e r t e d to v e g e t a t i o n with a h i g h e r evaporation r a t e , e v a p o r a t i o n from the

region

total area

has

to be

so that the a v e r a g e

is g r e a t enough to cause the lowering of the

w a t e r table t h a t is d e s i r e d . To p r e v e n t the w a t e r table r i s i n g any f u r t h e r the a v e r a g e e v a p o r a t i o n , E¢o, from the region must be equal to: E~

=

Ec + W

(1)

where E c = the evaporation from the existing crops, W = the depth of water which is generating the current rise in the water table.

X mm

If it is decided to try and lower the water table by an amount equal to -I yr , the average regional evaporation E N must be equal to: EN

=

Ec + W + X

(2)

where the subscript N refers to the n u m b e r of years it will take to lower the water table to the level it was Y years ago. N

=

N is given by:

YW/X

(3)

If portion A of the total area is r e p l a n t e d with vegetation h a v i n g a higher rate of evaportion Eh then E~ = AooEh + (1 - Aoo)Ec

(4)

EN

(5)

or =

ANE h + (I - AN)E c

Using Eqns. (i) and (2) given respectively: Am

=

W/(E h - E c)

(6)

AN

=

(W + X)/(E h - mc)

(7)

For these calculations, values of the rainfall, inflow to the water table and the crop evaporation were estimated from the data given by

Peck and

Hurle (1973), Hfllman (1981) and R.A. Nulsen (personal communication, 1981). It was assumed that the net inflow causing rises in the water table was proportional to the annual rainfall, and that runoff could be neglected. For Fig. i, an annual rainfall P of 450 m m , table W 405 m m

a net inflow to the water

of 45 m m yr -1 and an evaporation from the existing crops E of -I c was used. For Fig. 2, two sets of climatic data were used: P

yr

of 600 and 300 m m , respectively.

W of 60 and 30 m m

yr -1 and E c of 540 and 270 m m yr -I

Figure 1 shows the curve for the proportion of the total area

with enhanced

evaporation required to hold the water table at its present

level (N = 0o) and also the curves required to return the water table to its level of 60 years ago within 30, 15 and 7½ years. curves

over

Australia.

Figure 2 compares similar

the range of climates where wheat is grown

in south-western

In the left hand part of Fig. 2, the results are presented against

the absolute rate of evaporation from the replacement vegetation;

whereas in

the right hand part of Fig. 2, they are presented against the rate of evaporation from the replacement vegetation relative to the evaporation from the

ID

20001

C

1500

E

£

1000

uJ

=0

500

0 t~

-I c

0

20

40

60

80

100

Percentage of total area converted Fi 8. 1.

Proportion of total area to be converted to vegetation with a high evaporation rate, Eh, (i) to keep the water table at its present level (N = ~); (ii~ to restore it to its level of 60 years ago after N = 30, 15, 7~ years. Assumed conditions ~ annual rainfall 450 mm; evaporation from existing crops 405 mm yr ; recharge to groundwater 45 mm yr

2000

i/ I\

~x ,\

~/ \\\

1500

4

- - P = 300 mm - - P = 600 mm

h p = 300& 600 mm

J 3 o

£m

1ooo

t "\~

"'.

N=15yr ~5

2

.o_

;

50o

o

0

20

40

60

80

100

o

2'o

8'o

lOO

Percentage of total area converted to vegetation with higher evaporation, Eh F i 8 . 2.

Proportion of total area to be converted to vegetation with a high evaporation rate, Eh, (i) to keep the water table at its present level (N = ~); (ii~ to restore it to its level of 60 years ago after N = 15 years. Two sets of assumed conditions - annual r~infall 600 mm; evaporation from ~xisting crops, E C , 540 am yr ; recharge . to ground-water 600 mm yr- ; recharge to groundwater 60 mm yr- and ann~al rainfall 300 mm; evaporation fromlexisting crops, Ec, 270 mm yr ; recharge the groundwater 30 mm yr

existing

crops.

In

the l a t t e r p a r t of the f i g u r e ,

the r e s u l t s

for the two

climates are identical b e c a u s e the assumed value of W is p r o p o r t i o n a l to the rainfall. The o v er al l r e s u l t from t h e s e calculations is to show that e i t h e r the p r o portion of the area c o n v e r t e d to enhance the evaporation has to be l a r g e , or the rate of evaporation from the replacement v e g e t a t i o n has to be h i g h ; more than twice the e v a p o r a ti o n from the e x i s t i n g c r o p s , water table is to be significantly r e d u c e d .

i.e.,

if the level of the

To maintain t h ese high r at es of

evaporation will r e q u i r e e i t h e r i r r i g a t i o n or the p l a n t i n g of a species of v e g e tation which can d i r e c t l y take up water from the s a t u r a t e d soft zone. An es s en t i al p a r t of any i n v e s t i g a t i o n into u s i n g ch an g es in land management to r e d u c e the o c c u r r e n c e of saline s e e p a g e s , is to measure the evaporation of the new v e g e t a t i o n in comparison to the e x i s t i n g v e g e t a t i o n . The main p u r p o s e of this p a p e r is the g e n e r a l evaluation of methods of m eas u r i n g

evaporation in relation to this r e q u i r e m e n t .

evaluation

the

meteorological and

Preparatory

biological c h a r a c t e r i s t i c s

of

to this

evaporating

s u r f a c e s will be d e s c r i b e d , and the c o n s t r a s t s between f o r e s t s and a g r i c u l t u r a l crops high-lighted.

The evaporation components will be c o n s i d e r e d s e p a r a t e l y

and, u s i n g data for s o u t h - w e s t e r n Australia, an estimate of the r e l a t i v e importance of the v a r io u s components will be made.

The final p a r t of the p a p e r

will be a review of the c u r r e n t methods of m easu r i n g and p r e d i c t i n g e v a p o ration.

CHARACTERISTICS OF EVAPORATING SURFACES E v a p o r a t i n g s u r f a c e s can be s e p a r a t e d , categories;

i.e.,

f o r c o n v e n i e n c e , into f o u r main

tall and s h o r t v e g e t a t i o n , bare soil and open water.

The

emphasis in this Section will be on those s u r f a c e c h a r a c t e r i s t i c s which account for

the

main d i f f e r e n c e s in evaporation between f o r e s t s ,

a g r i c u l t u r a l crops

and bare soft.

Meteorological Characteristics Radiational e n e r g y balance The r a t e of evaporation d e p e n d s primarily on the i n p u t of e n e r g y . radiational

e n e r g y i n p u t is usually divided into two main components:

The the

s h o r t wave radiation r e c e i v e d from the sun and sky and the long wave r ad i ation from the E a r t h ' s atmosphere. reflected

portion

of

the

emitted from the s u r f a c e .

shortwave

T h e s e two components are o f f - s e t by the radiation

and

the

l o n g - w a v e radiation

On the local scale the i n p u t radiation is i n d e p e n -

d e n t of t h e u n d e r l y i n g

surface;

balance

by

are

shortwave

influenced

radiation depends

cent for most agricultural 15 a n d

60 p e r

(Monteith,

t u r e of r o u g h fore

the

of t h e s u r f a c e .

on t h e a l b e d o of t h e s u r f a c e , about

The reflected

b e i n g a b o u t 25 p e r

10 p e r c e n t f o r f o r e s t s ,

soil d e p e n d i n g

on organic content

and between and wetness

As s h o w n in t h e n e x t s e c t i o n t h e d a y t i m e s u r f a c e t e m p e r a -

vegetation

daytime

agricultural

characteristics

crops,

cent for bare

1973).

b u t t h e o u t g o i n g c o m p o n e n t s of t h e r a d i a t i o n

the

will b e l o w e r t h a n t h a t of s m o o t h e r s u r f a c e s ,

long-wave

crops.

emmission

will

be

less

from f o r e s t s

there-

than

from

T h e e m m i s s i o n of b o t h s h o r t a n d l o n g - w a v e r a d i a t i o n will

t h e n b e l e s s d u r i n g t h e d a y from f o r e s t s t h a n f r o m a g r i c u l t u r a l t h e r a d i a t i o n i n p u t to a f o r e s t is g r e a t e r

crops,

a n d so

than for smoother surfaces.

Surface roughness A f t e r t h e i n p u t of e n e r g y , of e v a p o r a t i o n

the most important factor governing

is t h e e f f i c i e n c y of r e m o v a l of w a t e r v a p o u r f r o m t h e s u r f a c e .

For a given wind speed and vapour water

vapour

depends

on

the

blowing over the roughness bare

soil t h e

much

greater;

extremes.

turbulence for

The

surface

conductance

be

estimated

(Monteith,

and using

1 =

ga

d

constant

= the and

calculations

ra

least, crops,

transfer ga or its a

whilst over

s e t off b y Over

rough

the turbulence height

reciprocal

forests

formula

based

on

it will b e

between

the

atmosphere,

the

aerodynamic

the

wind

will b e b e t w e e n t h e s e

in t h e f r e e the

the

relatively smooth

coefficient for water vapour

some r e f e r e n c e

U of

=

resistance

logarithmic

wind

{in(z-d)/Zo}2 k2U

zero plane, = wind the

(1968) r e l a t i o n s h i p s for

turbulence

t h e r a t e of r e m o v a l of

ra

profile

1965) :

--

where

gradient

atmospheric

agricultural

evaporating

pressure

e l e m e n t s of t h e s u r f a c e .

will b e

integrated

aerodynamic can

the rate

z°

speed

the

=

at

aerodynamic

the

(8)

roughness

length,

reference

l e v e l z.

resistance

and

of d a n d z ° to t h e h e i g h t ,

four typical surfaces.

These

calculations

k

= yon

Karman's

Table 1 presents

conductance

using

Cowan's

h , of t h e r o u g h n e s s

elements

s h o w t h a t t h e r a t e of t r a n s f e r

of w a t e r v a p o u r i n c r e a s e s r a p i d l y as t h e s u r f a c e r o u g h n e s s

increases.

Now t h e f l u x of w a t e r v a p o u r is g i v e n b y :

E = g a A Aq z where

(9)

Aq

is the

difference

difference

of Az.

Therefore

in

specific humidity

measured

over

a height

for a given evaporation rate, Aq/Az must be

small, for

if g a is l a r g e f o r a p a r t i c u l a r

surface,

a n a r e a w h o s e v a l u e of g a is small.

over

forests

are

smaller

than

those

meteorological

conditions.

must

t h a n t h a t of o t h e r

be

less

Similarly the temperature

over

Therefore

c o m p a r e d to t h e v a l u e of A q / a z

smoother

the

surfaces

surface

surfaces

during

gradients

under

temperature

t h e same

of

a forest

the daytime and greater

at

night.

Table 1. The aerodynamic resistance and conductance for a wind speed of 2 m s"1 above bare soil, above grass, above agricultural crop and above forest.

Soil Grass Crop Forest

h (m)

d (m)

zo (m)

0.01 0.1 1 10

0.006 0.064 0.64 6.4

0.0013 0.013 0.13 1.3

The greater to t h a t dence

either

e f f i c i e n c y of t r a n s f e r

from

shorter

of t h e e v a p o r a t i o n

case

of t r a n s p i r a t i o n

related

z-d (m)

ra (s m "l )

2 2 3 5

160 75 29 5

of w a t e r v a p o u r from f o r e s t s c o m p a r e d

vegetation

or from bare

soil a f f e c t s t h e d e p e n -

rate on meteorological and surface

from v e g e t a t i o n ,

the surface

factors.

resistance

Whilst

for

evaporation

from

bare

soil,

the

r e l a t e d to t h e l e n g t h of t h e d i f f u s i o n p a t h w a y t h r o u g h For

aerodynamically

evaporation

VPD

0.006 0.013 0.03 0.18

In the

r s is p r i m a r i l y

to l e a f a r e a a n d t h e b i o l o g i c a l l y c o n t r o l l e d o p e n i n g a n d c l o s i n g of t h e

stomata.

the

qa ( m s "l )

of t h e

radiation equation

rate

to

n can

the

be

surfaces

is p r i m a r i l y

air passing

R

rough

over

separated

controlled

the

surface.

surface

The

into

with by

than

side

term

is

t h e soil.

the vapour

rather

energy

resistance

small a e r o d y n a m i c

right-hand

an

surface

by

resistances

pressure the

input

deficit of n e t

of t h e M o n t e i t h - P e n m a n

and

aerodynamic

term

as

follows :

hE =

where

hE

AR n A + y(1 + r s / r a)

= the

pressure

curve

( 0 . 6 7 mb

°c-l),

constant

latent at

pressure.

aerodynamic

terms

annual temperature

the

(10) A + y(1 + r s / r a)

heat

flux,

mean

temperature,

p = the Table for

pep(VPD)/r a +

density 2 shows

typical

short

A = the of a i r , the and

slope

of t h e

saturated

¥ = the

psychrometric

Cp = t h e

specific heat

difference

between

tall v e g e t a t i o n ,

the

using

vapour constant

of a i r

energy the

at and

average

and relative humidity index for Perth and Merredin given

in

the

Vol. 13

of

average

annual

net

For

the short

World

Survey

radiation

vegetation

for

under

than

energy

6 per

cent.

Climatology

whereas

From Eqn.

10 it c a n

be

deficit

affected

by

itself

the

evaporation

depends

proportion

rather

of

strongly the

than heating

and

the

1975).

term contributes

seen

that

on t h e

net

net

radiation

the air,

the

of t h e s u r f a c e

on t h e r a t i o of n e t r a d i a t i o n to v a p o u r p r e s s u r e

pressure

1971), (Paltridge,

f o r t h e tall v e g e t a t i o n it c o n t r i b u t e s

to t h e a e r o d y n a m i c t e r m is i n d e p e n d e n t

depends

(Gentilli, Australia

these conditions the energy

41 p e r c e n t of t h e e v a p o r a t i o n , less

of

south-western

r a t i o of t h e

resistance

deficit.

The vapour

radiation;

which

but

but

is u s e d

is also

to p r o m o t e

a n d h e n c e is a f f e c t e d b y t h e a v a i l -

a b i l i t y of soft w a t e r .

Table 2. The relative dependence of transpiration from short and tall vegetation on the net radiation, Rn, and vapour pressure deficit, VPD.

Vegetation

Temp

Rn

(°C)

VPD

ra

(Wm "2) (rob)

Energy term

Aerodynamic term (Wm "2) (Wm "2)

rs

( s m "l) (sin "1)

Latent heat flux (Wm "2)

short

18

112

8.5

50

75

49.2

69.8

119.0

tall

18

112

8.5

5

150

• 6.6

94.1

100.7

of w a t e r

vapour

the

evaporating

The

efficiency

surface

also

of t r a n s f e r

determines

the

degree

c h a n g e s in t h e s u r f a c e r e s i s t a n c e .

away from

of r e s p o n s e

of t h e

The transpiration

evaporation

rate

to

f r o m tall v e g e t a t i o n w i t h

a small a e r o d y n a m i c r e s i s t a n c e is m o r e s e n s i t i v e to a c h a n g e in t h i s biologically. controlled Eqn.

10

changes same

resistance to

calculate

the

in the surface

the

In general,

surface

resistance

of i n c r e a s i n g

resistance

to f o r e s t s . tation

short

vegetation,

used

case

this

Using

dependence

a n d tall v e g e t a t i o n . table

as f o r

the

on The

previous

(Monteith,

1981) so t h i s t a b l e s h o w s t h e e f f e c t

from 75 to 150 s m -1 w h i c h is m o r e a p p r o p r i a t e

calculations show that the transpiration

twice as s e n s i t i v e to a c h a n g e in s u r f a c e vegetation.

This

reduced

sensitivity,

from t h e tall v e g e resistance

in t h e

case

as t h a t of s h o r t

is c a u s e d b y t h e i n i t i a l l a r g e r e d u c t i o n in e v a p o r a t i o n b e i n g o f f s e t

by an increase the

in

vegetation. this

a r a b l e c r o p s w i t h a g o o d s u p p l y of soil w a t e r h a v e a l o w e r

in s u r f a c e

temperature

a n d h e n c e a n i n c r e a s e in t h e g r a d i e n t

of a t m o s p h e r i c h u m i d i t y d e f i c i t b e t w e e n In

short shows

as well as from 150 to 300 s m - 1 , w h i c h is m o r e a p p r o p r i a t e

These

is n e a r l y the

from

Table 3

for typical short

were

than forests

this resistance

to a r a b l e c r o p s ,

transpiration

evaporation,

clAmatological c o n d i t i o n s

table.

from

than

of a f o r e s t

the increase

t h e i n s i d e a n d o u t s i d e of t h e l e a v e s .

in s e n s i b l e h e a t f l u x to c o m p e n s a t e f o r

t h e r e d u c t i o n in e v a p o r a t i o n can be s e t up b y o n l y a v e r y small i n c r e a s e in s u r f a c e t e m p e r a t u r e a n d h e n c e a v e r y small i n c r e a s e in t h e g r a d i e n t of atmospheric deficit. For

the

same

reason

trees

can

reduce

their

evaporation

rate

by

i n c r e a s i n g t h e i r s u r f a c e r e s i s t a n c e w i t h o u t i n c u r r i n g t h e p e n a l t y of v e r y h i g h surface temperatures,

as can o c c u r with low v e g e t a t i o n u n d e r h i g h r a d i a t i o n ,

low e v a p o r a t i o n , c o n d i t i o n s .

Table 3. The effect of changes in the surface resistance on the evaporation from short and from tall vegetation.

Vegetation

Aerodynamic resistance (s m "1 )

Surface resistance (s m "1)

50 50 50

75 150 300 75 150 300

short

tall

5

5 5

Another atmosphere

result

is t h a t

atmospheric

of t h e h i g h understorey

humidity

rate

Latent heat flux (W m "2 )

Percentage reduction

119 89 59 185 101 53

of e x c h a n g e

25 34 45 47

between

forests

and

the

is likely to be e x p o s e d to air h a v i n g similar

d e f i c i t s to t h o s e

that the tree crowns experience,

and

h e n c e can make a s i g n i f i c a n t c o n t r i b u t i o n to t h e total e v a p o r a t i o n ( R o b e r t s et al.,

1980).

generated

Therefore primarily

i n p u t of e n e r g y .

evaporation

by

from this

the atmospheric

understorey

humidity

vegetation

deficit rather

will be

t h a n b y the

As an e x a m p l e , c o n s i d e r t y p i c a l s u n n y c o n d i t i o n s in summer

in T h e t f o r d F o r e s t ,

U.K.

U s i n g d a t a f o r 1300 to 1400 on 10 J u n e 1976, n e t

r a d i a t i o n was 368 W m -2 a n d a t m o s p h e r i c h u m i d i t y deficit a b o v e t h e f o r e s t was 8.2 g k g -1.

T h e a v e r a g e n e t r a d i a t i o n a b o v e the u n d e r s t o r e y

been

to

found

estimated ground

below

be

13 p e r

canopy

cent

net

of

that

radiation

above

the

was 48 W m -2.

forest

of b r a c k e n has canopy,

so t h e

At 1.25 m a b o v e t h e

the measured

a t m o s p h e r i c h u m i d i t y d e f i c i t was 8.6 g k g -1, w h e r e a s -2 f o r a r a d i a t i o n l e v e l of 48 W in t h e e x p e c t e d v a l u e would h a v e b e e n a b o u t 1 g k g -1. ration

from

Therefore, the

e v e n with a l a r g e a e r o d y n a m i c r e s i s t a n c e

bracken

is

dominated

rather than by the net radiation.

by

the

atmospheric

the e v a p o -

humidity

deficit

10

Biological C h a r a c t e r i s t i c s

Agricultural

crops

logically

simpler

majority

of c a s e s .

their

life

than for

cover

the

ground

cover

Pruitt,

1977). forests

forests, grass

rainfall

and

either

latitudes

or

native

are

evaporation

During

the first

from

generally

10 to

they

stage

80 p e r

are

and

are

due

decreases

to i n s u f f i c i e n t

and

The the

becomes

canopy

harsher.

comes from

in

as

Even

exotic

progressively much

of

in a r e a s of h i g h

environment

closure

the and

T h e m a j o r i t y of

becomes

to low t e m p e r a t u r e s

forests

the understorey

the

canopy

becomes In o p e n

growth

often have an understorey

rainfall or

variation

they

(Doorenbos

multi-storied.

to b e m o n o c u l t u r e s and

growing

of t h e i r cent

much more complex.

species

radiation,

altitudes.

forests,

environment

during

time

T h e c o m p l e x i t y of n a t i v e f o r e s t s is g r e a t e s t

high

harsher,

c o v e r of a n n u a l c r o p s v a r i e s of

the

shallow

bio-

the ground

numerous

which are intended

or ferns.

therefore

two

thirds

are

are

in t h e

typically

forests

They

and

rooting

ground.

have

monocultures

vegetation.

about

increases

In contrast native

generally

Though

span,

completely

are mixed

is

in h i g h

very

more

of t h e

great

open water

as

in the

lost by

a n d from t h e s o i l - l i t t e r l a y e r w h e n -

e v e r i t is w e t .

COMPONENTS OF THE EVAPORATION FROM VEGETATION Before considering the processes controlling evaporation it is useful to divide the evaporation into its three components as follows. E v a p o r a t i o n of I n t e r c e p t e d Whenever there branches occur.

of t h e

Precipitation

is w a t e r p r e s e n t

vegetation

Commonly t h e m o s t f r e q u e n t

w e t t i n g of t h e s u r f a c e s

on the surface

direct evaporation

can

s o u r c e of t h i s w a t e r is p r e c i p i t a t i o n ,

but

c a n also o c c u r as t h e

interception

of m i s t o r c l o u d d r o p l e t s .

surface

of

vegetation

physical

process

period

has

precipitation assumed

The

soil

and the

that

been

reaching

rather

involves

sum of t h e

of i n t e r c e p t e d

than fewer

soil

throughfall

precipitation

beneath factors

precipitation

measured the

r e s u l t of c o n d e n s a t i o n of dew o r

Since the evaporating

total intercepted

most frequently

to b e

evaporation

and/or

of e v a p o r a t i o n

soil e v a p o r a t i o n .

of t h e l e a v e s , s t e m s o r

a n d o n t h e soil o r l i t t e r ,

as

the

(which and

during

w a t e r is o n t h e

the

than

the or

over a storm or longer

difference in

surface,

transpiration

the

stemflow).

between

case

of

gross

forests

To m e a s u r e

individual rainstorms

is the

is m u c h

11 more difficult, b e c a u s e the wet conditions cause i n c r e a s e d i n s t r u m e n t a l e r r o r s and the r a t e s of evaporation u n d e r the cloudy conditions are low. Detailed studies of evaporation of i n t e r c e p t e d p r e c i p i t a t i o n from f o r e s t s in

the

UK have

shown

the

following.

Firstly,

on

average

the

rate

of

e v ap o rat i o n ex ce e d s that which could be s u p p o r t e d by the i n p u t of radiational e n e r g y alone ( S t e w a r t , 1977); were

considerably

Stewart,

1975).

whereas the rates of t r a n s p i r a t i o n from f o r e s t s

smaller than

the

input

of radiational e n e r g y

(Gash and

The additional e n e r g y is primarily supplied by a downward

flux of sensible h e a t,

since the e v a p o r a t i n g su r f ace is at a lower t e m p e r a t u r e

than

over

the

sensible Secondly,

air heat

passing has

been

the f o r e s t .

found

to be

On occasions, maintained

a downward flux of

for many

hours

on

end.

it has been found that the total i n t e r c e p t i o n a v e r a g e d o v e r many

rainstorms can be divided into r o u g h l y equal components ° from storms which do not s a t u r a t e the canopy, from the s a t u r a t e d canopy while rain is falling, and from the s a t u r a t e d canopy a f t e r rainfall has ceased (Gash, 1979). The rate of evaporation of i n t e r c e p t e d rainfall from a s a t u r a t e d canopy primarily

d ep en d s

on

the

aerodynamic c o n d u ct an ce and on the atmospheric

humidity close to the s u r f a c e .

The atmospheric humidity deficit is determined

by the i n t e r a c t i o n between the evaporation from the su r f ace and the t em p er a t u r e and humidity of the air p a s s i n g o v e r it.

For example, e n h a n c e d evapo-

ration due to an i n c r e a s e in wind speed o v e r the s u r f a c e ,

and a c o n s e q u e n t

i n c r e a s e in the aerodynamic c o n d u c t a n c e , will be rapidly o f f s e t by a r ed u ct i o n in the atmospheric humidity deficit as the enhanced evaporation r e s u l t s in a f u r t h e r r ed u ct i o n in the s u r f a c e t e m p e r a t u r e ( S t e w a r t , 1978). For v e g e t a t e d s u r f a c e s with smaller aerodynamic r o u g h n e s s than f o r e s t s , the r at es of evaporation of i n t e r c e p t e d p r e ci p i t at i o n will be lower in relation to the i n p u t of radiational e n e r g y and more similar to t h e i r r a t e s of t r a n s piration u n d e r the same levels of radiation in p u t ( S t e w a r t ,

1978).

Transpiration In p l a n t s , along

a

w a t e r p a s s e s from the soil to the air s u r r o u n d i n g the leaves

gradient

encountered.

of

water

potential,

the p r e s e n c e of p o r e s , the atmosphere.

still has air.

several

resistances

are

At the final stage of the liquid water pathway, water v a p o r i z e s

off the mesophyll cell walls and p a s s e s into

along which through

the r e s i s t a n c e s imposed by

the stomata, which r e s t r i c t the flow of water v a p o u r After passing through

the stomata the w at er v a p o u r

to overcome the aerodynamic r e s i s t a n c e in its p a s s a g e t h r o u g h the

A small amount of w a t e r v a p o u r can pass directly t h r o u g h the leaf wall,

the cuticle, b u t the q u a n t i t i e s are g e n e r a l l y r e g a r d e d as small ( e . g . , 1972), the c u t i c u l a r r e s i s t a n c e b e i n g v e r y l a r g e (Monteith, 1981).

Rutter,

12 The o p en i n g and closing of the stomata o c c u r s in r e s p o n s e to a number of

en v i ro n m en t al

v a r i a b le s

-

the

most important

being

light,

atmospheric

humidity deficit, soil mositure deficit and carbon dioxide c o n c e n t r a t i o n .

The

e x a c t mechanism of this r e s p o n s e is as y e t poorly u n d e r s t o o d and q u an t i f i ed . The minimum stomatal r e s i s t a n c e seems to depend on age and position of the leaves ( h e i g h t in the canopy,

s u n n y or s h a ded ) among o t h e r f act o r s ( J a r v i s ,

1976). Evaporation from Soil Evaporation from bare and soil f a c t o r s .

soil depends

on a combintation of meteorological

The e v a p o r a t io n d u r i n g the period immediately a f t e r rainfall

or i r r i g a t i o n o c c u r s at close to the potential rate

(ASCE, 1973).

A f t e r an

e v a p o r a t i v e loss of about 12 mm from sandy soils or 20 mm from h e a v i e r soils, the h y d r a u l i c c o n d u c t i v i t y of the soil s u r f a c e falls r a p i d l y , cau si n g a marked re d u ct i o n

in

its

evaporation

rate

(Winter,

1974).

Measurements

of

soil

moisture depletion b e n e a th

fields in C e n tr a l India a f t e r h a r v e s t i n g of wheat

and

that

other

crops

0.5 ram d -1

or

showed

le s s ,

when

the

the

evaporation

potential

from

ev ap o r at i o n

t h ese

dry

soils was

was

about

6 m m d -1

(Wallace et al. 1981). EVAPORATION FROM THE NATIVE FOREST AND WHEATLANDS OF SOUTHWESTERN AUSTRALIA To obtain components e v ap o rat i o n probably a r ea,

a better

feel for

transpiration,

the

relative

contributions

from n a t i v e f o r e s t and wheatlands,

worthwhile.

some v e r y

of the v a r i o u s

i n t e r c e p t i o n and soil evaporation to -

the total

even c r u d e calculations are

Given the s c a r c i t y of detailed measurements

gross

assumptions

had to be made;

for this

in p a r t i c u l a r it was

assumed that t h e r e was no r u n o f f from the area. For

the

wheatlands,

it was assumed that the t r a n s p i r a t i o n

E c can be

calculated from the Penman potential evaporation modified by the crop f act o r s k

g i v e n by Dor r e n b o s and P r u i t t (1977). It was assumed that the growth of c the wheat crop o c c u r r e d in the following s t a g e s : - initial s t a g e , 15-30 May; crop

development

S ep t em b er

and

stage,

late

1 June-10

season

stage,

July;

mid- season

21 September-31

stage,

October.

11 Ju l y - 2 0 During

the

period when the area was fallow the soil e v ap o r at i o n E s was also calculated from the potential e v a p o r a ti o n u s i n g soil factors k s which d e p e n d on the rate of potential ev a p o r a t io n and the f r e q u e n c y of significant r ai n , Doorenbos and P r u i t t

(1977).

as g i v e n by

D u r i n g periods of high p o t en t i al evaporation

and low rainfall, all the rainfall falling on the soft was assumed to e v a p o r a t e .

13

The

e f f e c t of i n t e r c e p t i o n

of r a i n f a l l

by

the

w h e a t is a s s u m e d to b e i n c o r -

porated into the crop factors. For the forest

a r e a it was a s s u m e d

c a n o p y was 50 p e r c e n t of t h e g r o u n d and

evaporates

evaporation

area,

that the forest canopy intercepts

20 p e r c e n t of t h e a n n u a l r a i n f a l l f a l l i n g o n it a n d t h a t soil

occurs

from

from t h e u n c o v e r e d from the forest

50 p e r

cent

of t h e

ground

area.

a r e a was d e t e r m i n e d as p r e v i o u s l y

are no crop factors

ration terms

t h a t t h e p r o j e c t e d a r e a of t h e f o r e s t

a v a i l a b l e from t h e l i t e r a t u r e

was t a k e n as t h e d i f f e r e n c e

(intercepted

The evaporation

described.

for forests,

Since there

the transpiration

b e t w e e n t h e sum of o t h e r e v a p o -

r a i n f a l l a n d soil e v a p o r a t i o n )

a n d t h e local p r e c i p i -

tation. According

to

Hlllman

(1981)

clearing

of

native

forests

for

farming

c o m m e n c e d in t h e 500 to 600 mm r a i n f a l l r e g i o n a n d d e v e l o p e d e a s t w a r d s i n t o the lower rainfall regions. two a r e a s

So t h e f o l l o w i n g c a l c u l a t i o n s w e r e c a r r i e d o u t f o r

- o n e w i t h a n a n n u a l r a i n f a l l of 600 mm a n d t h e o t h e r w i t h 300 ram.

Table 4 presents

t h e main c l i m a t o l o g i c a l d a t a u s e d f o r t h e c a l c u l a t i o n s .

monthly

distribution

o~sed

rainfall on

the

rainfall data

and for

frequency

Perth

of

significant

and Merredin.

The

rainfall

Penman

The were

potential

e v a p o r a t i o n was c a l c u l a t e d u s i n g m o n t h l y c l i m a t o l o g i c a l d a t a f o r P e r t h ( G e n t i l l i , 1971).

M~,uthly

wind

runs

for Merredin

calculations could not be carried out. evaporation

were

not

available

so t h e

Penman

H o w e v e r i t is t h o u g h t t h a t t h e P e n m a n

f o r M e r r e d i n s h o u l d b e s i m i l a r to t h a t of P e r t h ,

because the lower

wind speeds inland should compensate for the larger vapour pressure

deficits.

Table 4. Climatological data used for calculations of the evaporation components. Month

January February March April May June July August September October November December Annual

Penman ET (mm) 254 220 198 132 87 63 65 81 108 155 198 236 1799

High rainfall area Amount Frequency* (mm) (days) 7 12 19 37 84 120 118 89 46 36 19 13 600

* Mean interval between rainfalls.

>30 >30 30 20 8 3 3 4 8 20 30 >30

Low rainfall area Amount Frequency* (mm) (days) 7 12 16 20 40 47 52 40 21 18 16 11 300

>30 >30 >30 >30 16 6 6 8 16 >30 >30 >30

14

In

both

areas

it

was

assumed

that

the

net

b e n e a t h n a t i v e f o r e s t s was z e r o o v e r t h e y e a r . communication,

1981)

has

found

that

recharge

to g r o u n d w a t e r

While R . A . N u l s e n ( p e r s o n a l

the

d i f f e r e n c e in e v a p o r a t i o n °1 u n c l e a r e d a n d c l e a r e d a r e a s to b e of t h e o r d e r of 20 to 100 mm y r

between

Table 5. Calculated monthly and annual evaporation components (mm) for areas with an annual rainfall of 600 mm with native forests or wheat/fallow land usage. Forested area* soil evap. ks Es

Month

January February March April May June July August September October November December

Cropped area soil evap. ks

3 6 14 17 26 26 29 24 19 16 15 7

.15 .25 .06 .09 .09 .06 .35 .02 .15

crop evap. kc Ec

Es 7 12 19 34 26 0 0 0 0 0 19 13

.25 .06 n/a n/a n/a n/a n/a

0 0 0 0 26 52 73 93 120 90 0 0

.06 .82 1.12 1.15 1.11 .58

Annual totals Rainfall Soil evaporation Forest interception Forest transpiration Crop evaporation Total evaporation Drainage

600 204 60 336

600 130 454 584 16

600 0

50 per cent forest, 50 per cent bare soil.

The

results

show that

of t h e s e

calculations

are

given

here,

interception

insignificant. low

The

particularly

equation

(36)

transpiration 1973).

In

240 s m -1

in the

of

rainfall

by

estimated transpiration the

drier

6.

They

average

area.

surface

By

On t h e b a s i s of t h e a s s u m p t i o n s

these

sparse

from t h e

rearranging

resistance

can

native

forests

f o r e s t was f o u n d

be

the

was to b e

Monteith°Penman

calculated

when

the

a n d t h e climatological c o n d i t i o n s a r e k n o w n ( S t e w a r t a n d Thorn,

the and

wetter area for

the

consequent

this

drier

values appear very large, the

5 and

t h e e v a p o r a t i o n from b a r e soil was an i m p o r t a n t c o m p o n e n t of t h e

total evaporation, c o n t r i b u t i n g about a t h i r d . used

in T a b l e s

calculation

area

about

gave

an a v e r a g e

1200 s m -1.

At f i r s t

but remembering the sparseness

low leaf a r e a

index,

v a l u e of a b o u t sight

these

of t h e f o r e s t a n d

t h e s e v a l u e s do n o t imply e x c e p t i o n a l l y

h i g h v a l u e s of s t o m a t a l r e s i s t a n c e ( E q n .

(33)).

15

T h e e s t i m a t e s of t r a n s p i r a t i o n on the only

use

of p u b l i s h e d

crop

10 p e r

greater

about

drainage the

cent

in t h e w h e a t c r o p in t h e w e t t e r a r e a b a s e d

factors

with Penman potential evaporation

than

those

needed

to

give

the

( a s s u m i n g t h e e s t i m a t e s of soil e v a p o r a t i o n a r e c o r r e c t ) .

drier

area

twice

the

would

have

the

expected to b e

estimates

of t r a n s p i r a t i o n

values. less

than

To half

rectify the

from

this

values

the

are

expected

H o w e v e r in

wheat are more than

discrepancy

the

crop

factors

given by Doorenbos and Pruitt

(1977).

Table 6. Calculated monthly and annual evaporation components (mm) for areas with annual rainfall of 300 mm with native forest or wheat/fallow land usage. Forested area* soil evap. ks Es

Month

January February March April May June July August September October November Annual totals

Cropped area soil evap. ks

3 6 8 10 15 20 21 18 16 9 8

.35 .65 .65 .45 .03

Rainfall Soil evaporation Forest interception Forest transpiration Crop evaporation Total evaporation Drainage

crop evap. Es

kc

7 12 16 20 15 0 0 0 0 0 11

.35 n/a n/a n/a n/a n/a

Ec

.35 .66 1.12 1.15 1.11 .58

300 139 30 131

0 0 0 0 15 41 73 93 120 90 0

300 97 432 529 -229

300 0

* 50 per cent forest, 50 per cent bare soil.

METHODS OF MEASURING EVAPORATION Only here.

a brief

and Burtsaert the

summary

More e x t e n s i v e individual

limitations.

(1982). methods

of t h e m e t h o d s

reviews

have

Later papers in

more

been

currently prepared

presented

detail

a v a i l a b l e will b e g i v e n by Shuttleworth

(1979a)

a t t h e W o r k s h o p will d e s c r i b e

emphasising

their

advantages

and

16

Micrometeorological Methods

Aerodynamic technique Using

the

relationship

between

vertical humidity gradient, E

=

to

of

water

vapour

E and

the mean

(11) transfer

c o e f f i c i e n t f o r w a t e r v a p o u r a n d is u s u a l l y

to t h a t f o r m o m e n t u m Kin, w h i c h c a n b e d e r i v e d from m e a s u r e m e n t s

the mean vertical one

flux

-pK v 0 q / 0 z

w h e r e K v is t h e t u r b u l e n t related

the

~q/Sz

one

gradients

relationship

of w i n d s p e e d .

between

the

To allow f o r d e p a r t u r e s

two t r a n s f e r

coefficients,

f a c t o r ¢ v r e l a t e d to a t m o s p h e r i c s t a b i l i t y is i n t r o d u c e d ,

an

of

from a empirical

i.e.,

E = - PKm ~ ~v

(12)

E q u a t i o n (12) c a n also b e e x p r e s s e d

in t e r m s of r e s i s t a n c e s

C

E

p p - ~ {ew(Ts) - e } / ( r s + r a)

=

where ew(T s) = the saturated and e = vapour pressure

vapour pressure

at the surface temperature

T s,

of t h e a i r a t t h e r e f e r e n c e l e v e l z.

Energy budget T h e l a t e n t h e a t f l u x c a n b e o b t a i n e d from m e a s u r e m e n t s of t h e e n e r g y of t h e s u r f a c e , R

-

t h e o n e - d i m e n s i o n a l form of t h e e q u a t i o n hE

-

H

-

G -

S

-

P

=

budget

is:-

0

(14)

n

where H = sensible heat flux,

G = soil h e a t f l u x ,

in the air and the biomass between and

P = energy

with

the other

fully.

absorbed terms,

for photosynthesis.

certainly

S = c h a n g e in e n e r g y

t h e l e v e l s of m e a s u r e m e n t

over

a day,

stored

of G a n d R n ,

S a n d P a r e small c o m p a r e d and can be estimated success-

G is small ( a few p e r c e n t of R n ) u n d e r

a d e n s e c o v e r of v e g e t a t i o n

b u t c a n b e l a r g e o n a n h o u r l y b a s i s f o r b a r e soil, t h o u g h t h e n e t v a l u e of G over

24 h o u r s

is n e g l i g i b l e .

It can be measured

its importance.

For vegetated

R

into the sensible and latent heat fluxes.

n flux

is c o n v e r t e d can

be measured

technique i.e.,

directly

surfaces

or estimated depending

by

the

m o s t of t h e a b s o r b e d aerodynamic

or

radiant

on

energy

The sensible heat the

eddy

correlation

17

AE

=

R

-

H

-

G

-

S

-

(15)

P

n

30 H

=

pCp Km 3zz CH

(16)

or

H

=

- pcp(T s - T ) / r a

(17)

or

H

=

- pCp w'T'

(18)

with

where

30/3z=

mean

vertical

potential-temperature

factor relating the turbulent Ts

= surface

temperatue,

f l u c t u a t i o n of t e m p e r a t u r e Alternatively flux

to t h e

difference

heat

= temperature

H

and

temperature

at

can

height,

and

w' =

be obtained by measuring

C

_

p h

~_00

(19)

Aq

l a t e n t h e a t of v a p o u r i s a t i o n

R

the vertical

A0 a n d t h a t of s p e c i f i c h u m i d i t y Aq o v e r

of w a t e r E, c a n b e u s e d to a p p o r t i o n

a v a i l a b l e e n e r g y i n t o t h e two m a j o r c o m p o n e n t s ,

=

reference

i.e.,

hE

hE

OH = e m p i r i c a l

about its mean value. flux

in p o t e n t i a l

_

T

gradient,

c o e f f i c i e n t f o r h e a t to t h a t of m o m e n t u m ,

t h e B o w e n r a t i o 13, w h i c h is t h e r a t i o of t h e s e n s i b l e h e a t

latent

t h e same h e i g h t r a n g e ,

where

transfer

the

giving:

- G - S - P

n

(20)

1+~

Eddy correlation technique By mean

making

values

frequency

of

measurements

of

vertical

speed

to o b t a i n

the

wind

the

contributions

instantaneous w',

and

of

f r o m all t h e

fluctuations humidity

q'

about at

their

sufficient

s i g n i f i c a n t s i z e s of e d d y

a n d s u m m i n g t h e i r p r o d u c t o v e r a p e r i o d of h a l f a n h o u r o r m o r e , t h e f l u x of water vapour can be obtained from: E

=

-pw'q'

(21)

Water Balance Methods T h e w a t e r b a l a n c e of a n a r e a is g i v e n b y : P=Q+E+AS+D

(22)

18

where

P = precipitation,

content

of t h e soil,

size

the

of

area

Q = runoff,

E = evaporation,

AS = c h a n g e

a n d D = t h e d r a i n a g e to g r o u n d w a t e r .

and

the

time s c a l e o v e r

which

in w a t e r

Depending on the

the balance

is

determined

d i f f e r e n t m e t h o d s of m e a s u r i n g o r e s t i m a t i n g t h e c o m p o n e n t s c a n b e e m p l o y e d . W a t e r b a l a n c e of c a t c h m e n t a r e a For a catchment area, same

(usually

capacity')

times

at

b e t w e e n t i m e s w h e n t h e soil m o i s t u r e d e f i c i t is t h e

which

the

soil

profile

is

assumed

to

be

at

'field

t h e e v a p o r a t i o n is g i v e n b y : E

=

P

-

Q - D

(23)

and for a water-tight catchment this further reduces to E

=

P

-

Q

(24)

These calculations are usually only applicable to periods of one or more years.

For

shorter periods

the changes

in soil moisture storage become

important and have to be measured, usually using a neutron probe.

So again

for a water-tight catchment, E

=

P

-

Q - AS

(25)

Using these equations the total evaporation, i.e., the sum of the interception,

transpiration

and

soil evaporation,

is measured.

However

during

long dry periods Eqn. (20) reduces to: E = - Q - AS when

the measured

(26)

evaporation is then

evaporation components only. drainage

the sum

of transpiration and soil

In flat areas, where there is no runoff and the

to groundwater can be estimated or assumed negligible, over dry

periods, E=-AS Zero flux plane method In areas where measured

if the

drainage is not negligible the evaporation can still be

depth

of

the

zero

flux plane z z can

be

obtained from

measurements of the soil tension profile (Giesel et al., 1970), then the evaporation can be obtained from measurements of changes in soil moisture content between z z and the surface s i.e.,

19

Z

E

=

f s z - A.q dz

(28)

Lysimeters On

a much

smaller

scale,

a lysimeter

can be constructed

t h e c o m p o n e n t s of t h e w a t e r b a l a n c e c a n b e m e a s u r e d In the simplest version deficit

is

allowed

neglected,

- t h e d r a i n a g e l y s i m e t e r - no s i g n i f i c a n t soft m o i s t u r e

build

up

so

changes

in soil m o i s t u r e

content

can be

then E

where

to

f o r w h i c h all

with comparative ease.

=

P + I - D

(29)

I is the irrigation required to maintian a m i n i m u m soil moisture deficit.

For the most sophisticated systems,

the lysimeter is intermittently or contin-

uously weighed, then E

=

P

-

M - D

(30)

where M = change in mass of the lysimeter. The

most

sensitive lysimeters can measure

the evaporation over periods

as short as an hour, so that periods of evaporation of intercepted rainfall can be separated from periods of transpiration and soil evaporation.

Cut tree method The

evaporation

obtained amount

from i n d i v i d u a l

by cutting the trunk of w a t e r

which the trunk Ew

Wt r e q u i r e d sits,

under

trees

under

water (Roberts,

to m a i n t a i n

rainless conditions can be 1977) a n d m e a s u r i n g

then

(31)

= Wt

T h e cut tree generally experiences less resistance to water-uptake the uncut

the

a f i x e d w a t e r l e v e l in t h e t a n k in

than

trees, because any root resistance has been eliminated and so its

transpiration m a y be greater than soil-rooted trees.

Penman-Monteith Equation This equations at

the

equation (Eqns.

surface

e q u a t i o n is :

by

13, or

combining

14 a n d at

the

aerodynamic

flux

and

energy

budget

17), e l i m i n a t e s t h e n e e d f o r m e a s u r e m e n t s

several

heights

above

the

surface.

The

either

resulting

20

P) + pclo{ew(T ) - e } / r a

A(Rn - G - S KE

where

=

ew(T)

(32)

A + ~'(1 + r s / r a

= the

saturated

vapour

pressure

at

can

be measured

the

temperature

of

the

air, T. The

aerodynamic

or obtained

from m e a s u r e m e n t s

the literature. vegetation this

The

or

surface

can

directly using Eqn.

of t h e w i n d p r o f i l e o v e r t h e s u r f a c e

resistance

.off a r e w e t d u r i n g

resistance

Eqn.

resistance

be obtained

o r from

is z e r o w h e n all t h e s u r f a c e s

and shortly by

direct

after rainfall.

measurements

(35), of t h e

At other

using Eqn.

times

(33) o r

( 3 6 ) o r from t h e l i t e r a t u r e .

Sap Flow M e t h o d s Sap flow m e a s u r e m e n t s All t h e w a t e r t h a t is t r a n s p i r e d sap

flow.

A number

m o v e m e n t of t h e for

a pulse

separated Roberts,

of

sap.

The

of e i t h e r

points

the

have

been

developed

methods involve measurements

heat

on

from a t r e e h a s to p a s s u p t h e t r u n k

techniques or

radio-activity

tree

trunk

to p a s s

(Lassoie

et

to

measure

of t h e time t a k e n

between al.,

as this

two v e r t i c a l l y

1977;

Waring

and

1979).

Dendrometer measurements Another trunk

variation

diameter

measurements calibrated

by

on this

obtained into

those

measurements

method (Wronski,

by

technique a

of

is

dendrometer. transpiration,

to m e a s u r e

diurnal

To

convert

they

have

of s a p flow o r t r a n s p i r a t i o n

the to

be

variation

in

dendrometer previously

m a d e b y some o t h e r

1980).

Chamber Methods Porometry A small c h a m b e r is c l a m p e d o v e r a leaf o r a g r o u p of l e a v e s a n d t h e r a t e of flow of w a t e r from t h e p l a n t m a t e r i a l is m e a s u r e d . the resistance

p e r u n i t l e a f a r e a to t h e flow of w a t e r v a p o u r from t h e i n s i d e

of t h e l e a f to t h e a t m o s p h e r e , made the assumption be considered from :

By suitable calibration

the stomatal resistance

that the stomatal resistances

as a c t i n g in p a r a l l e l ,

r S T is o b t a i n e d .

Having

of t h e i n d i v i d u a l l e a v e s c a n

the canopy resistance

r c can be calculated

21

rST r

c

=

-LAI

(33)

where LAI is the total area of the leaves in the canopy p e r unit g r o u n d area. When the canopy is the only source of evaporation (none from bare soil or an u n d e r s t o r e y ) the canopy r e s i s t a n c e is the same as the su r f ace r e s i s t a n c e . Cuvettes By m e a s u r i n g the i n c r e a s e with time of the concentration of water v a p o u r in chambers ( c u v e t t e s ) enclosing shoots or small b r a n c h e s ,

the t r a n s p i r a t i o n

from the plant can be deduced. Ventilated chamber On a l a r g e r scale, whole t r e e s h a v e been enclosed in v e n t i l a t e d chambers and again the i n c r e a s e in water v a p o u r measured

to give the t r a n s p i r a t i o n

from the plant (Greenwood et a l . , 1981). O t h e r Methods B e n d i n g b r a n c h method To measure the rate of evaporation of i n t e r c e p t e d rainfall from individual b r a n c h e s or t r e e s ,

two methods h a v e been developed.

Hancock and C r o w t h e r

(1979) have d ev e l o p e d i n s t r u m e n t a t io n to measure the b e n d i n g of an individual b r a n c h as its mass i n c r e a s e s as the r e s u l t of water being s t o r e d on the leaves d u r i n g rain.

Besides indicating the total mass of water s t o r e d on the v e g e -

tation d u r i n g rainfall, the measurements g i v e the rate of d e c r e a s e in mass and therefore

the evaporation of the i n t e r c e p t e d p r eci p i t at i o n when rainfall and

dripping

cease.

This method is unable

to measure the evaporation d u r i n g

rainfall b ecau s e of the unknown gains and losses due to rainfall and d r i p p i n g respectively. y - r a y method An o t h er method of m e a s u r i n g the mass of w at er s t o r e d on the v e g e t a t i o n and

its

changes

over

s h o r t periods of time,

was developed u s i n g a y - r a y

a b s o r p t i o n system (Olszyczka and C r o w t h e r , 1981). Excised shoots A further precipitation

or

method

of m e a s u r i n g

transpiration

involves

either

the

measuring

ev ap o r at i o n the

of i n t e r c e p t e d

changes

of mass

of

22 excised shoots. an

B y c o m p a r i n g t h e r a t e s of loss of m a s s of a d r y s h o o t a n d of

artificially

cepted

wetted

shoot,

the

water and transpiration

be measured

(Rutter,

comparative

under

rates

of e v a p o r a t i o n

of i n t e r -

t h e same m e t e o r o l o g i c a l c o n d i t i o n s ,

can

1967).

COMPARISON OF METHODS OF MEASURING EVAPORATION It

is

accuracy

very

difficult,

often required

differences

if n o t

impossible,

by hydrologists.

of 20 p e r c e n t o r l e s s .

to m e a s u r e

Thus,

for measuring

have

able

we w o u l d

e f f e c t of r e p l a c i n g

still not

have

to t h e

if a t t h e b e g i n n i n g Of t h i s c e n t u r y

we h a d h a d t h e k n o w l e d g e a n d t e c h n i q u e s now,

evaporation

It is u n r e a l i s t i c to h o p e to m e a s u r e

been

e v a p o r a t i o n t h a t we

to p r e d i c t

t h e n a t i v e v e g e t a t i o n in s o u t h - w e s t e r n

quantitatively

the

Australia by wheat.

All t h a t c o u l d h a v e b e e n d o n e would h a v e b e e n to s u g g e s t

t h a t r e p l a c e m e n t of

perennial vegetation by annual crops would probably reduce the annual evaporation. T h e m e t h o d s of m e a s u r i n g should

be

divided

the

various

and

are

into

methods

not,

have

therefore,

between

primarily plants.

used

the surface

to f u l f i l v e r y

comparable.

One

water

section since

different objectives

s e t of m e t h o d s h a s b e e n

t h e t r a n s f e r of e n e r g y a n d

governing

and the atmosphere.

the

in t h e p r e v i o u s

before they are compared,

developed

the processes

to s t u d y

These

been

presented

categories

strictly

d e v e l o p e d to u n d e r s t a n d matter

evaporation

different

Another

s e t of m e t h o d s a r e

r e l a t i o n s of i n d i v i d u a l p l a n t s o r p a r t s

m e t h o d s also a r e o f t e n u s e d

to m e a s u r e

the evaporation

of

from

d i f f e r e n t p a r t s of t h e s y s t e m , f o r e x a m p l e from t h e t r e e s a n d f r o m t h e u n d e r storey

of a f o r e s t

to q u a n t i f y previous

separately.

the evaporation over a long period,

groups

methods

up

space.

I t is i m m e d i a t e l y to

to

of

minutes used

measurements

corner

months or years,

concerned

the methods

are

apparent

that,

in g e n e r a l ,

evaporation

over

short

of t h i s

time

classification

sight

that

of t h i s t a b l e a r e r e l e v a n t . the current

annual crops choose

primarily

T a b l e 7,

at first

with

whereas the

time

classified

by

scales

of

time a n d

the methods which are scales

are

confined

to

the

between

different

measure

vegetation over shorter

of t h e o b j e c t i v e s of t h e

t h e m e t h o d s in t h e t o p r i g h t

T h i s is c e r t a i n l y t r u e if i t is w i s h e d

if t h e aim is to m e a s u r e

increase

in e v a p o r a t i o n

by more water-demanding

which

and

only

s i t u a t i o n o r to c h e c k t h e e f f e c t of v e g e t a t i o n c h a n g e s

However,

time o r p r e d i c t

methods

are

o v e r small a r e a s a n d v i c e v e r s a .

in the long-term.

to

the

it a p p e a r s

to m e a s u r e short

In

a consideration

Workshop, hand

days.

measure

From

T h e l a s t s e t of m e t h o d s a r e p r i m a r i l y i n t e n d e d

types

evaporation

perennial of

from

it

is

individual

over a

b y r e p l a c e m e n t of

vegetation,

vegetation, the

the evaporation

caused

o r in p a r t i c u l a r necessary

components

time p e r i o d s a n d on s m a l l e r s p a c e s c a l e s .

to

use

of t h e

23

O

i i t

~7 i Q o

o

~

~7 t t t t

i i

i i

t

>

~

t

o

~7~7

~7~7

~ o~

o

~7

24

Methods of measuring evaporation can only be usefully compared when the specific objectives and locations of the studies have been decided upon. The relevance and accuracy of a particular method depends critically on the circumstances under which it used.

For example,

a lysimeter is very poor

for measuring the evaporation from a forest;

but for measuring some of the

components of the evaporation, it is ideal.

Similarly measurement of the

water balance of a catchment can give a very accurate determination of the average evaporation from the area over a year or longer period;

however

this method provides no information about the components that make up the total evaporation and therefore the measurements would provide a rather poor basis for modelling the effects of changes in vegetation. PREDICTION OF EVAPORATION

To d r a w u p p r o p o s a l s f o r a l a n d m a n a g e m e n t s c h e m e in o r d e r to a l l e v i a t e the

salinity

predict

problems

the

requirements, rather

of

evaporation it

is

south-western from

the

essential

to

Australia,

replacement

use

methods

with

than an empirical or statistical approach.

method

uses

the Monteith-Penman

equation

it

will b e

vegetation. a

necessary To

sound

At present

to

meet

these

physical

basis

the most suitable

with measurement

o r e s t i m a t e s of

the resistances. The Monteith-Penman equation treats single

surface.

particularly stringent

This

a

any evaporating

considerable

a r e a as if it was a

over-simplification

in t h e c a s e of a m u l t i - l a y e r e d f o r e s t c a n o p y . analysis,

However

is

the

Shuttleworth

theoretical

(1976)

advantages

has

developed

of t h e S h u t t l e w o r t h

of

reality ;

U s i n g a m u c h more a multi-layer

model.

model are more than

o f f s e t b y t h e p r a c t i c a l d i f f i c u l t i e s of d e v e l o p i n g s u f f i c i e n t l y a c c u r a t e m o d e l s of the resistances

f o r all t h e l e v e l s .

So in p r a c t i c e t h e s i m p l e r M o n t e i t h - P e n m a n

m o d e l is p r e f e r a b l e . To u s e t h e M o n t e i t h - P e n m a n e q u a t i o n f o r p r e d i c t i n g a particular

area requires

h o u r l y m e a s u r e m e n t s of t h e m e t e o r o l o g i c a l v a r i a b l e s

a n d v a l u e s of t h e r e s i s t a n c e s tation

and

discussed of

the

climate. earlier).

resistances

The

which are appropriate

resistances

depend

to t h e a r e a a n d i t s v e g e -

on a n u m b e r

of v a r i a b l e s ,

(as

To model t h e r e l a t i o n s h i p s w h i c h d e s c r i b e t h e d e p e n d e n c e on

these

variables,

the

resistances

factors must be measured over a representative There

t h e e v a p o r a t i o n from

and

the

controlling

r a n g e of c o n d i t i o n s .

a r e a n u m b e r of m e t h o d s of m e a s u r i n g t h e a e r o d y n a m i c r e s i s t a n c e .

T h e m o s t u s u a l m e t h o d is b y m a k i n g m e a s u r e m e n t s of t h e w i n d p r o f i l e o v e r a n extensive and

the

difficult

uniform surface roughness to m e a s u r e

length

and deriving values for the zero plane displacement for

accurately.

use

in E q n .

Also t h i s

8.

However

equation

d a n d zo a r e v e r y

t a k e s n o a c c o u n t of t h e

25

effects of stability.

Monteith (1965) points out that this results in the aero-

dynamic resistance being over-estimated in unstable conditions, during which most transpiration occurs.

Another method is to measure the evaporation

from a completely wet surface, then r

S

is zero and the aerodynamic resistance

can be determined from Eqn. (13) or from the rearranged Monteith-Penman equation, i . e . ,

pCp{ew(T) - e} ra

When

=

(A+y)XE - (Rn - G - S - P)

(34)

the evaporating surface is completely wet the temperature of the

surface is often less than the air temperature,

so the atmosphere is stable

and the aerodynamic resistance will again be over-estimated.

The only method

that

under

allows

conditions

measurements requires

of

the

measurements

aerodynamic of

the

resistance

sensible

heat flux by

unstable the eddy

correlation technique (see Eqn. 17) and of the surface temperature Ts, then ra

=

pCp(T s - T)/H

Measurements porometry

of the

measurements

(35)

surface

resistance

can

of stomatal resistance

either be obtained

or by measuring

from

the evapo-

ration and using the rearranged Monteith-Penman equation, i.e., =

rs

(Aft/'/ - 1)r a + pCp{ew(T) - e ) / ( y ~kE)

with values of the aerodynamic previous paragraph. minimised

by

The

using

resistance

(36)

determined

as described in the

effect of errors in aerodynamic resistance can be

the

same

method

to determine

ra for calculating

the

surface resistance from Eqn. (36) as for calculating the evaporation from the Monteith-Penman equation (32). CONCLUDING REMARKS

Over

the l a s t

understanding the

major

two d e c a d e s

there

has

been

a g r e a t i m p r o v e m e n t in the

of t h e e v a p o r a t i o n p r o c e s s and in p a r t i c u l a r which f a c t o r s h a v e

control

over

t y p e s of v e g e t a t i o n .

the

evaporation

and

how t h e s e

d i f f e r with v a r i o u s

F o r t e m p e r a t e climates t h e r e l a t i v e and a b s o l u t e m a g n i -

t u d e s of t h e c o m p o n e n t s of t h e e v a p o r a t i o n from d i f f e r e n t t y p e s of v e g e t a t i o n have

been

very

little i n f o r m a t i o n e v e n a b o u t t h e r e l a t i v e m a g n i t u d e s of t h e e v a p o r a t i o n

components. factors,

satisfactorily During

represented

quantified.

this

period

H o w e v e r f o r t r o p i c a l climates t h e r e it

also

became

appreciated

b y t h e s u r f a c e and a e r o d y n a m i c r e s i s t a n c e ,

that

is

surface

h a v e a much

26 greater

influence

implying that

on

there

the is

e v a p o r a t io n

considerable

than

had

variation

been

in

previously

thought,

evaporation from d i f f e r e n t

t y p e s of v e g e t a t i o n e x p e r i e n c i n g the same climate ( S h u t t l e w o r t h and Calder, 1979). To enable a land management scheme to alleviate the salinity problems of south-western

Australia to be chosen on scientific g r o u n d s ,

the evaporation

from d i f f e r e n t combinations of v e g e t a t i o n and areas wiU have to be calculated. This will r e q u i r e models d e s c r i b i n g ,

for example, the d e p e n d e n c e of su r f ace

resistance

on climatic and soil v a r i a b le s for all the t y p es of v e g e t a t i o n of

interest.

However s c r u t i n y of the world scientific l i t e r a t u r e shows that for

forests,

even in temperate climates, t h e r e are v e r y few suitable models (Gash

and S t e w a r t , Black,

1975;

1981);

for

Tan and Black,

1976;

tropical

there

forests

Calder, are

1977;

none.

Spittlehouse and

Therefore

the

first

r e q u i r e m e n t is to make measurements of the e v a p o r a t i o n , of the aerodynamic and

surfaces

resistances

and

of

the

concurrent

meteorological and

soil

conditions f o r all the major ty p e s of v e g e t a t i o n which may be grown in this region of Australia. be

d ev el o p ed and

S u b s e q u e n t l y , u s i n g this data b ase, suitable models can used

with

the

Monteith-Penman

equation

to p r e d i c t

the

evaporation. When the Ylonteith-Penman equation is used to p r e d i c t the evaporation from an area c o v e r e d by one type of v e g e t a t i o n , the t h eo r et i cal assumptions u s e d are well u n d e r s t o o d .

However to p r e d i c t the ev ap o r at i o n from a m i x t u r e

of v e g e t a t i o n , t h e r e is less c e r t a i n t y about how to use the r e s u l t s and models obtained

from

studies

of

individual

species.

measurements made on individual t r e e s ation

from

necessary

the to

complex

native

undertake

some

forests

calculating

the

e v a p o r a t io n

of

Western

th e o r e t ic a l

S h u t t l e w o r t h m u l t i - l a y e r models (1976, from areas

For

example,

how

should

be combined to estimate the e v a p o r -

1979b),

Australia?

studies,

starting

It may be from

the

to determine which method of

of e i t h e r

mixed v e g e t a t i o n

or from

combinations of v e g e t a t i o n and bare soil are valid. REFERENCES ASCE, 1973. Consumptive use of water and irrigation water requirements. American Society of Civil Engineers, New York, 215 pp. Brutsaert, W.H., 1982. Evaporation into the atmosphere. D. Reidel Publ. Co., Dordrecht, 299 pp. Calder, I.R., 1977. A model of transpiration and interception loss from a spruce forest in Plynlimon, central Wales. J. Hydrol., 33: 24-265. CSIRO, 1981. Institute of Earth Resources, Annual Report 1980/81. CSIRO: Melbourne, 111 pp. Cowan, I.C., 1968. Mass, heat and momentum exchange between stands of plants and their atmospheric environment. Q.J.R. Meteorol. Soc., 94: 523-544.

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28

Shuttleworth, W.J., 1979a. Evaporation. Institute of Hydrology Rep. No. 56, NERC, Swindon, 61 pp. Shuttleworth, W.J., 1979b. Below-canopy fluxes in a simplified onedimensional theoretical description of the vegetation-atmosphere interaction. Boundary-Layer Meteorol. 17: 315-331. Shuttleworth, W.J. and Calder, I.R., 1979. Has the Priestley-Taylor equation any relevance to forest evaporation? J. Appl. Met., 18: 639-646. Spittlehouse, D.L. and Black, T.A., 1981. A growing season water balance model applied to two Douglas fir stands. Water Resour. Res., 17: 1651-1656. Stewart, J.B., 1977. Evaporation from the wet canopy of a pine forest. Water Resour. Res., 13: 915-921. Stewart, J.B., 1978. A micrometeorological investigation into the factors controlling the evaporation from a forest. Ph.D. Thesis, Univ. of Reading, 211 pp. Stewart, J.B. and Thom, A.S., 1973. Energy budgets in pine forest. Q.J.R. Meteorol. Soc., 99: 154-170. Tan, C.S. and Black, T.A., 1976. Factors affecting the canopy resistance of a Douglas-fir forest. Boundary-Layer Meteorol., I0: 475-488. Wallace, J.S., Batchelor, C.H. and Hodnett, M.G., 1981. Crop evaporation and surface conductance calculated using soil moisture data from central India. Agric. Meteorol., 25: 83-96. Waring, R.H. and Roberts, J.M., 1979. Estimating water flux through stems of Scots pine with tritiated water and phosphorus-32. J. Exp. Bot., 30: 459-471. Winter, E.J., 1974. Water, soil and the plant. Macmillan Press, London, 141 pp. Wronski, E.B., 1980. Hydrometeorology and water relations of Pinus radiata. Ph.D. Thesis, Flinders Univ. of South Australia, 317 pp.