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Formation of the Dry Surface Layer and Its Effect on Bare Soil Evaporation : Field Observation and Numerical Experiments

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i

c

h

u m

i

d

i

t

y

at

t

h

e s o

i

l

s n

c e,

h

・ t

h

e r e

l

a

t

i

v e

h

u m

i

d

i

t

y

at

t

h

e s o

il

st)r

f

a c e, a n

d

q

q

t

h

¢ s

p

e c

if

i

c

h

u m

i

d

i

t

y

o

f

atm o s

p h

e r e・

T i

e

h

s c an

b

e

gi

ve n

b y f

o

ll

o

i

w

n

g

e

q

u a

d

o n, a s

h

・ = e

(

V

r8

g

/

R

T

8

)

(

2

)

w

h

e r e

v

r・ an

d

T

' are

h

t

e m at

r

i

c

h

e a

d

an

d

te m

p

e r atu r e at

t

h

e s o

i

l

s u r

f

a c e,

g

i

s

t

h

e

g

r a

v

i

tat

i

o n a

l

a c c e

l

era

d

o n , an

d R

t

h

e

g

a s c o n s t a nt o

f

w ate r v a

p

o r.

D i

tm a

l

v ar

i

at

i

o n o

f

a血O S

P h

e

r

i

c c o n

d

i

t

i

o n s w a s not c ons

i

d

e r e

d

, a ri

d

c o n sta nt v

a

l

u e s o

f

atm o s

p

h

e

i

r

c c o n

d

i

t

i

o n s w e r e

g

ive n

i

n t

h

e

p

re s ent stu

d

y

.

T h

e

l

o w e r

b

o un

d

ar

y

c on

di

t

i

o n s w ere c onsta nt te m

p

e r atu r e a n

d

n o w a t e r

n

t )x at a

d

e

p

t

h

o

f

0

.

6

m .

T h i

s m o

d

e

l

c o n s

i

ste

d

o

f 1 4 4 l

a

y

e r s, a n

d h

a

d

v e r

y

fi

n e r e s o

l

ut

i

o n

(

m

i

n. ,

0

.

5

m m

)

n e a rt

h

e s o

n

s u r

f

a c e. Tab le

1

S

oil

h

y

d

r a t luCPa r a m ete r s

f

o r th e

t

h

r e e e x a mpleiS oils

S

oil m m e

e

d e, L X ” K d

(

m3

/

m3

) (

m3

/

♂)

(

m ll

)

(

-

)

(

n

/

s

)

Toyo t ) r aSa n

d

0

.4 1 2 0 .

0

2 3 9

3

.1 4 7.0 1.2

5

×1

0

-5

s

ilt Lo a m G .E .

3

0

.

3 9 6

0.1 3 1

0

.4 2

3

2 .

0 6

5 .7

3

×1

O

・7 Bei(N eto

f

aC layl o ・4 4

6

0.

0

0

.1

5

2 1.1 7

9

.4

9

×1 0 -9 I

d

ata

) m v a JI G e n u chte n

(

1

9 8 0

)

-1 0000

{ -10

00

ゥ ー 1

00

巾 ○ エ ー

1 0

O

i

1

-0 ・

0

.

0

1

..上_I”..

C

lay

S

a n

d

ヽ 、 -I ヽ 1 I

-丁

0

0

.

1

0

.

2

0

3

.

0

.

4

0

.

5

W

ate r c o n

t

e n

t

(

m 3

/

m3

)

F

i g

.2

S

o

il

m oistt m c

h

ar a cteristic c t lr V e Sfo r t h e

t

h

r e e s oils u s ed inthe n u m eri c al e xperi m e nts.

(4)

H y

d

rau

li

c

p

ro

p

e

r t

i

e s o

f

s o

il

s w ere

gi

ve n

b y

v an

G

enu c

h

t en

(

1

9

8

0

)

-s m o

d

e

l

T h

e

p

a r a m e t e r s

f

or

t

h

r e e exa m

p

l

e s o

il

s u s e

d

i

n

t

h

e

p

r e s e n t stu

d

y

l

a r e

li

s t e

d i

n

T

a

b l

e

1

, an

d

s o

il

m o

i

stu r e c

h

a r a cte

r

i

st

i

c c urv e s

f

or

t

h

o s e s o

il

s a r e s

h

o w n

i

n

F i g

u r e

2

.

A

s

an exa m

pl

e, c o m

p

uta t

i

o n

a

l

r e su

l

t s

f

o r s a n

d

are s

h

()w n

i

n

F i g

ur e

3

.

V

e r

y g

o o

d

s

i

m

il

a

r

i

t

i

e s w e r e re c o

g

n

i

z e

d b

etw ee n o

b

s er ve

d

a n

d

c o m

p

ute

d p

ro

f

i

l

e s o

f

w a t e r c o JlteJlt, re

l

at

i

ve

h

u m

i

d i

t

y

, a n

d

va

p

or

d

ens

i

t

y

, r es

p

ec t

i

v e

ly

.

T h

e se s

i

m

il

ar

i

t

i

e s

d

e m onstr ate t

h

e va

lid i

t

y

o

f

t

h

e m o

d

e

l

use

d

.

F

r o m c o m

p

a

d

s o n

b

e tw e e n t

h

e o

b

s er ve

d

a n

d

c o m

p

ut e

d

r e s u

l

ts, t

h

e

p

o s

i

t

i

o n o

f

a

i

n

fl

e c t

i

on

p

oin t

i

n w a te r co nte r lt

P

r o

fil

e w a s c ons

i d

ere・

d

t o

b

e cor r e s

p

o n

d

e

d

to

t

h

a t o

f th

e

b

o tto m

b

o u n

d

ar

y

o

f

t

h

e

D

S

L

.

I

t

i

s r e m ar

k

a

bl

e

t

h

a t t

h

e eva

p

ora ti n

g

∑o ne W a s

l

o c ate

d

at

t

h

e

i

n

f

l

e c t

i

on

p

ol nt

i

n w ate r c ontent

p

ro

fil

e an

d

t

h

a t

t

h

e eva

p

o r a ti n

g

∑o n e W a s e xtre m e

ly

t

h i

n.

'

o

o

'

i

l

:

.

3

K 惰

.

.

Fg

.

.

. .

W

a

t

e r c o n

t

e n

t

(

m 9

/

m 3

)

R

o

[

at

l

v e

h

u r n

Z dZt y

V

a

p

o r

d

e n s

Zt y

(

k g/

m 3

)

0

.

0 ∞

0

.

005

o

.

o

1

0

.=

o

015

. ⊂)

0

020

. 0025. I

d

JL J. I A ,

5

0

0

5

. 1 1.

5

N

orm a

ll

z ed w ate r

f

Tux

J

E

5

N

o r m a

]I

ze

d

e v a

p

o r 8t

Z

o n Fi

g

3

C

o m

p

uted v e rtic al pr ofile sfo r s a nd of

(

a

)

w ate r c o nte nt,

(

b

)

r eht

i

v eh umi

d

it

y

,

(

c

)

v a

p

o r de rl Sity,

(

d

)

1iq

d

(

J'

)

a n

d

v ap

r

(

Jv

)

w ate rflu x e s,a nd

(

e

)

e v apo r atio n 丘o r n e a ch calc u

l

atio n n ode. T

h

e w ate r8u x e s a nd e v ap o r ahO n a r e n O r r nal iz cd

b

y e v apo r atio n into ht e atm o sp he

.

0

.00

0

0

.

0

0 5

a

, o .

o1

0 ∫:

a

00 15・ 【】 0

0

.20 0

0

.2

5

I,

J

y

,

A

5

0

0

5

. 1

1

.

5

・o .

5

0

0

.

5

1

1

.

5

N

o r m a

]i

ze

d

w a

t

e r

fJ

u x

N

o r m a

I[

ze

d

e v a

p

o r a

ti

o n F

ig

・4

C

o m puted v e rt

i

cal pmfile s

f

o r clay of

(

a

)

l

i q

u

id

(

J

)

L a nd v a

p

o r

(

J

)

v w a(e f

f

l

u x e s, a n

d

(

b

)

e v ap o r a tio n 丘o m e a ch c a

l

c ulat

i

o n n o

d

e・T he w ate r

n

u x e s a n

d

e v apo r ati o n a r e n O r m al

i

z e

d

.

(5)

T h

e

f

e a tur e s

li k

e t

h

e a

b

ove w ere a

l

s o

f

o un

d

o u t

i

n t

h

e c o m

p

ut e

d

re su

l

t

f

or

l

o aJ n .

O

n t

h

e ot

h

e r

h

a n

d

,

f

o r c

l

a

y

, tw o m

j

a o r

p

o

i

nts o

f

d i

f

f

ere n c e c a n

b

e

f

o un

d

o ut

(

s e e

F

i

g

.

4

)

.

T h

e

f

i rst

i

s t

h

at t

h

e

d

e c r e a s e O

f liq

u

i

d

w ater

u x tow ar

d

s o

i

l

su r

f

a c e a r o un

d

t

h

e

b

otto m

b

oun

d

a r

y

o

f

t

h

e

D S L

w a s r e

l

at

i

v e

l y

g

e nt

l

e, a rL

d t

h

u s,t

h

e t

hi

c

k

n e s s o

f

t

h

e e v a

p

o r ati n

g

ZO n e W a s r e

l

at

i

v e

l

y

l

ar

g

e.

T

h

e s e c o n

d i

s t

h

at t

h

e va

p

or

fl

ux s

li g h

t

l y i

n c r e a s e

d

tow ar

d

s o

il

s u r

f

ac e a t t

h

e u

p pe

rr e

g

i o n O

f

t

h

e

D S L

, w

h

e r e n o

li q

u

i d

w ate rtr a n s

p

ort

⊃rn

d

e e

p

e r s o

il l

a

y

e r e x

i

ste

d

T h

e

r

lr St

P

Ol nt O

f diff

erenc e

i

s t

h

o u

g

h

t to

b

e

d

ue t o t

h

e

d iff

er e nc e o

f

t

h

e

p

ro

p

ert

i

e s o

f

t

h

e u nsat urate

d h y d

ra u

li

c c o n

d

uct

i

v lt

y

an

d

v a

p

or c o Tl

d

uct

i

v lt

y

b

e t w e e n c

l

a

y

a n

d

o (

h

e r t

y p

e s o

f

s o

il

(

s e e

F i g

5

)

.

F

or s a n

d

an

d l

o a m , t

h

e u n s aturate

d

h y d

r au

li

c c on

d

uct

i

v

i

t

y

ra

pi d l y d

e cre a s e

d

a

g

aln St t

h

e

d

e c r e a s e o

f

m at

r

i

c

h

e a

d

, w

hil

e

i

t

di d

n ot

f

o r c

l

a

y

.

T h

e r e

f

o r e,t

h

e

g

e n t

l

e s

h if t

i

n

g

o

f

w a t e r

n

u又c o m

p

o n e nts

f

o r c

l

a

y i

s t

h

o u

g

h

tto

b

e o w i n

g

tO

t

h

e

g

e nt

l

e

d

ec re a s e o

f i

t s

h y d

rau

li

c c o n

d

uct

i v

i

t

y

a

g a

i

nst m ab

i

c

h

e a

d

.

T h

e s e c o n

d p

o l nt O

f d

i

f f

e r e n c e

i

s r e

l

ate

d

to t

h

e r e t e nt

i

o n

p

r o

p

e rt

i

e s o

f

s o

il

s.

F

r o m t

h

e s o

i

l

m o

i

s tt)r e c

h

a r a c t e

r

i

st

i

c c u r v e s

(

F

i

g

.

2

)

,

t

h

e w ate r c o nte nts o

f

s a n

d

a n

d l

o a m

d

e m o n strateto

h

a v e a

l

r no stc onsta nt v

l

a

ue s a t

l

o w m a打

i

c-

h

e a

d l

e v e

l

, w

h

ere w ate rtr a n s

p

o rt

i

n

泣q

tl

i

d

p

h

a s e

i

s

d

iff

i

c u

l

tto

b

e m a

d

e.

O

n

t

h

e o t

h

e r

h

an

d

,

i

t

i

s s

h

o w n

t

h

at c

l

a

y

c a n r eta

i

n a

b

un

d

a nt w ate r ats a m e

l

e v e

l

o

f

m a

c

h

e a

d

.

T h

tl S,

i

t c an

b

e c o n c

l

u

d

e

d

t

h

at t

h

e s o

il

w ate r c a ll eVa

P

O r ate

o m

d

r

y

l

a

y

e r, w

h

e r e n o

li q

u

i

d

w ate r s u

p p ly

f

r o m u n

d

e r

l

n

g

s o

i

l

,

f

o r

hi

g

h

l y

r ete n

d

ve s o

il

s.

1 0

・1 3 ”)

>・■ lllJ

1 0

・14 O : ⊃ 1 コ ー= O O

1 0

・1 5 -0 .1 ・

1 0

・ 1

0 0 0

M

atr

l

c

h

e a

d

(

rr)

)

F i 普

.

5

R ela

d

o n ships

b

etw e e n c o nd u ct

i

v lty a nd m ab

i

c

h

e a

d f

o r s a n

d

(

th

i

nl

i

n e

)

,

l

o a m

(

m e

d

it l m lin e

)

a n

d

clay

(

血ick lin e

)

.B r o

k

e n a n

d

d

otte

d li

n e r epr e s e nt u n s atu r ate

d

h y

d

r a u

l

ic c o n

d

u cti vi ty a n

d

v apo r c o n

d

u ct

i

v lty, r e spe ctiv ely, a r l

d

s olid lin e 7 ePr e S e ntSthetotal of

t

h

o s e. ”

叫 ㌔ t l O

A

”) 1:1 +

o

D

e

t

a

5

[

0

[

e

d

,, E

l

(

0

&

/

m2

)

1

5 0

F i

g.

6

C

o m paris o n betw e e n e v ap o r atio n e stim ate

d b

y the s

i

m

plifi

e

d

m et

h

od wi 【

h

e

q

.

(

3

)

a n

d

that o

b

t

a

i

n e

d b

ythe

d

eta

il

ed m o

d

el

f

o r s a n

d

(

cir cle

)

, lo a m

(

sqt l a r e

)

a nd clay

(

p ln s

)

. Ev apo r atio n r ateiispr e s e nte

d

a slateinthe atflu x, whe r eI is th elate nthe atfo r v apod2 Rtio n.

A

s su m ln

g

t

h

a t t

h

e eva

p

ora tlJl

g

∑o n e C a n

b

e re

g

ar

d

e

d

a s a

h

o

r

i

zont a

l pl

a n e, an

d

t

h

a t r e

l

a t

i

ve

h

u m

id i

t

y

at t

h

e eva

p

o r a ti n

g

∑one

i

s e

q

u

a

l

to un lt

y

' t

h

e eva

p

o r at

i

on

-.

a t e c a n

b

e r e

p

r e s ente

d b y

f

o

ll

o wi n

g

e

q

u a

t

i

o n, a S

E

s ,

-a

(

3

)

(6)

w

h

ere

T

i

s

t

h

e t e m

p

e r a t ure a t

t

h

e ev a

p

ora

t

i

n

g

s u r

f

a c e, z・

t

h

e

d

e

p

t

h

o

f

eva

p

ora tin

g

S u r

f

a c e , w

h i

c

h i

s c orr e s

p

o n

d

e

d

to

t

h

e

t

h i

c

k

Jle S S O

f

t

h

e

D S

L

,a n

d D

y 't

h

e e

f

f

e c

d

ve

d

if

f

u

s

i

v lt

y

O

f

w ate r va

p

o r

i

n

t

h

e

I)

S L

.

F i g

u r e

6

s

h

o w s c o m

p

a

r

i

s o n o

f

h

t

e e v a

p

o r a

t

i

on e st

i

m ate

d

b

y t

h

e s

i

m

p li

f

i

e

d

m et

h

o

d

w

i

t

h

e

q

u at

i

o n

(

3

)

a n

d t

h

at o

b

t

a

i

ne

d b

y

t

h

e

d

et

a

i

l

e

d

m o

d

e

l

m e

m

i

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